Enviro Wiki - User contributions [en]

Articles

2026-05-07T17:14:24Z

Admin:

{| class="wikitable sortable"
|-
!Title!!First Author!!Linking Phrases
|-
|[[Groundwater Sampling - No-Purge/Passive]]||[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]||passive sampling, no purge sampling, grab samplers, diffusion samplers, sorptive samplers
|-
|[[ Long-Term Monitoring (LTM)]]||[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]||long-term monitoring, LTM, LTM objectives, LTM programs, LTM challenges
|-
|[[PFAS Monitored Retention (PMR) and PFAS Enhanced Retention (PER)]]
|[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]
|PFAS, MNA, natural attenuation
|-
|[[Sorption of Organic Contaminants]]||[[Richelle Allen-King|Allen-King, Richelle]]||
|-
|[[PFAS Transport and Fate]]
|[[Dr. Richard Anderson|Anderson, Richard, Ph.D.]]
|PFAS, fate and transport
|-
|[[Mass Flux and Mass Discharge]]||[[Dr. Michael Annable, P.E. |Annable, Michael, Ph.D., P.E.]]||source reduction
|-
|[[PFAS Toxicology and Risk Assessment]]
|[[Jennifer Arblaster|Arblaster, Jennifer]]
|PFAS, toxicology, risk assessment
|-
|[[Metal(loid)s - Small Arms Ranges]]|| Dr. Amanda Barker |[[Dr. Amanda Barker|Barker, Amanda, Ph.D.]]
|
|-
|[[Munitions Constituents – Photolysis|Munitions Constituents - Photolysis]]
|[[Dr. Warren Kadoya|Kadoya, Warren, Ph.D.]]
|
|-
|[[Munitions Constituents - Soil Sampling]]
|[[Dr. Samuel Beal|Beal, Samuel, Ph.D.]]
|
|-
|[[Vapor Intrusion – Sewers and Utility Tunnels as Preferential Pathways|Vapor Intrusion - Sewers and Utility Tunnels as Preferential Pathways]]
|[[Lila Beckley|Beckley, Lila]]
|vapor intrusion
|-
|[[Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill]]
|[[Dr. Barbara Bekins|Bekins, Barbara, Ph.D.]]||
|-
|[[Bioremediation - Anaerobic Secondary Water Quality Impacts]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||secondary impacts, water quality (in regards to anaerobic conditions)
|-
|[[Design Tool - Base Addition for ERD]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||aquifer acidity, base addition
|-
|[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation]]
|[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||
|-
|[[Low pH Inhibition of Reductive Dechlorination]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||low pH inhibition
|-
|[[In Situ Toxicity Identification Evaluation (iTIE)]]||[[Dr. G. Allen Burton |Burton, Allen, P.E.]]||toxicity evaluation
|-
|[[OPTically-based In-situ Characterization System (OPTICS)]]
|[[Dr. Grace Chang|Chang, Grace, Ph.D.]]
|
|-
|[[Munitions Constituents - Electrochemical Treatment]]
|[[Dr. Brian P. Chaplin|Chaplin, Brian, Ph.D.]]
|munitions constituents remediation
|-
|[[PFAS Sources]]
|[[Dr. Dora Chiang|Chiang, Dora, Ph.D.]]
|
|-
|[[Biodegradation - Cometabolic]]||[[Dr. Kung-Hui (Bella) Chu |Chu, Kung-Hui (Bella), Ph.D]]||cometabolic biodegradation
|-
|[[Munitions Constituents - Composting]]
|[[Harry Craig|Craig, Harry]]
|
|-
|[[Chemical Oxidation (In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||ISCO, chemical oxidation
|-
|[[Chemical Oxidation Oxidant Selection (In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||chemical oxidant, oxidant (in regards to ISCO)
|-
|[[Chemical Oxidation Design Considerations(In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||screening, design, implementation, oxidant delivery (in regards to ISCO)
|-
|[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]||[[Dr. Rula Deeb |Deeb, Rula, Ph.D.]]||PFAS, perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS)
|-
|[[Metal and Metalloid Contaminants]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||metal contaminant(s), metalloid contaminant(s), metal(s), metalloid(s),
|-
|[[Metals and Metalloids - Mobility in Groundwater]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||metal mobility, aqueous speciation, adsorption, precipitation, colloidal transport (in regards to metals)
|-
|[[Monitored Natural Attenuation (MNA) of Metal and Metalloids]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||MNA, attenuation of metal(s), natural attenuation processes, attenuation (in regards to metals)
|-
|[[Metal and Metalloids - Remediation]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||remediation, in situ technologies, contaminant removal (in regards to metals)
|-
|[[pH Buffering in Aquifers]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||ph buffer, natural pH buffer, engineered pH buffer
|-
|[[Munitions Constituents - Sorption]]||[[Dr. Katerina Dontsova |Dontsova, Katerina, Ph.D.]]||energetics, sorption
|-
|[[Biodegradation - Hydrocarbons]]||[[Dr. Elizabeth Edwards |Edwards, Elizabeth, Ph.D.]]||hydrocarbon, biodegradation
|-
|[[Source Zone Modeling]]||[[Dr. Ron Falta |Falta, Ron, Ph.D.]]||source zone modeling
|-
|[[Plume Response Modeling]]||[[Dr. Ron Falta |Falta, Ron, Ph.D.]]||plume response modeling
|-
|[[REMChlor - MD]]
|[[Dr. Ron Falta|Falta, Ron, Ph.D.]]
|
|-
|[[In Situ Groundwater Treatment with Activated Carbon]]
|[[Dr. Dimin Fan|Fan, Dimin, Ph.D.]]
|
|-
|[[LNAPL Remediation Technologies]]||[[Dr. Shahla Farhat |Farhat, Shahla, Ph.D.]]||
|-
|[[Sustainable Remediation]]||[[Paul Favara |Favara, Paul]]||sustainable remediation, social, economic and environmental impacts
|-
|[[Munitions Constituents]]||[[Dr. Kevin Finneran |Finneran, Kevin, Ph.D.]]||Explosives, energetics, insensitive munitions
|-
|[[Biodegradation - Reductive Processes]]||[[Dr. David Freedman |Freedman, David, Ph.D.]]||biotic reduction, biotic reductive processes, hydrogenolysis, dihaloelimination, coupling, organohalide respiration
|-
|[[Remediation of Stormwater Runoff Contaminated by Munition Constituents|Munitions Constituents - Remediation of Stormwater Runoff]]||Fuller, Mark, Ph.D.||energetics, insensitive munitions, stormwater runoff
|-
|[[Subgrade Biogeochemical Reactor (SBGR)]]||[[Jeff Gamlin |Gamlin, Jeff]]||SBGR, subgrade biogeochemical reactor, bioreactor
|-
|[[Thermal Remediation - Smoldering]]||[[Dr. Jason Gerhard |Gerhard, Jason, Ph.D.]]||smouldering remediation, self-sustaining treatment for active remediation, STAR
|-
|[[Contaminated Sediments - Introduction]]
|[[Dr. Upal Ghosh|Ghosh, Upal, Ph.D.]]
|
|-
|[[Contaminated Sediment Risk Assessment]]
|[[Richard Wenning|Wenning, Richard]]
|
|-
|[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
|[[Dr. Upal Ghosh|Ghosh, Upal, Ph.D.]]
|
|-
|[[PFAS Ex Situ Water Treatment]]
|[[Dr. Scott Grieco |Grieco, Scott, Ph.D.]]
|
|-
|[[Stream Restoration]]
|[[Dr. Natalie Griffiths|Griffiths, Natalie, Ph.D.]]
|
|-
|[[Passive Sampling of Sediments]]
|[[Dr. Philip M. Gschwend|Gschwend, Philip]]
|
|-
|[[Phytoplankton (Algae) Blooms]]
|[[Dr. Nathan Hall|Hall, Nathan]]
|
|-
|[[PFAS Soil Remediation Technologies]]||[[James_Hatton |Hatton, Jim]]||PFAS, Soil source zones
|-
|[[Proteomics and Proteogenomics]]
|[[Dr. Kate Kucharzyk|Kucharzyk, Kate, Ph.D.]]
|
|-
|[[N-nitrosodimethylamine (NDMA)]]
|[[Paul Hatzinger|Hatzinger, Paul, Ph.D.]]
|
|-
|[[Alternative Endpoints]]||[[Elisabeth Hawley |Hawley, Elisabeth]]||management of complex sites
|-
|[[Thermal Remediation]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation, in situ thermal
|-
|[[Thermal Remediation - Steam]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||Steam Enhanced Extraction
|-
|[[Thermal Remediation - Electrical Resistance Heating]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||Electrical Resistance Heating
|-
|[[Thermal Conduction Heating (TCH)]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal desorption
|-
|[[Thermal Remediation - Combined Remedies]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation
|-
|[[Thermal Conduction Heating for Treatment of PFAS-Impacted Soil]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation, PFAS
|-
|[[Munitions Constituents- TREECS™ Fate and Risk Modeling|Munitions Constituents - TREECS™ Fate and Risk Modeling]]||[[Dr. Billy E. Johnson |Johnson, Billy, Ph.D.]]||munitions constituents fate and transport modeling, TREECS
|-
|[[Munitions Constituents - Alkaline Degradation]]
|[[Jared Johnson|Johnson, Jared]]
|
|-
|[[Assessing Vapor Intrusion (VI) Impacts in Neighborhoods with Groundwater Contaminated by Chlorinated Volatile Organic Chemicals (CVOCs)|Vapor Intrusion - Assessing VI Impacts in Neighborhoods with Groundwater Contaminated CVOCs]]
|[[Dr. Paul C. Johnson|Johnson, Paul, Ph.D.]]
|vapor intrusion
|-
|[[Munitions Constituents - IM Toxicology]]||-----||insensitive explosives, insensitive munitions, IMX-101, IMX
|-
|[[Landfarming]]
|[[Dr. Roopa Kamath|Kamath, Roopa, Ph.D.]]
|
|-
|[[NAPL Mobility]]
|[[Andrew Kirkman|Kirkman, Andrew]]
|
|-
|[[Perchlorate]]||[[Thomas Krug |Krug, Thomas]]||perchlorate
|-
|[[Injection Techniques for Liquid Amendments]]||[[Thomas Krug |Krug, Thomas]]||amendment injection
|-
|[[Transition of Aqueous Film Forming Foam (AFFF) Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances (PFAS)]]
|[[Dr. Johnsie Ray Lang|Lang, Johnsie Ray, Ph.D.]]||PFAS
|-
|[[Characterization Methods – Hydraulic Conductivity]]
|[[Dr. Gaisheng Liu|Liu, Gaisheng, Ph.D.]]
|
|-
|[[Compound Specific Isotope Analysis (CSIA)]]||[[Dr. Barbara Sherwood Lollar, F.R.S.C. |Lollar, Barbara S., FRSC]]||Compound Specific Isotope Analysis (CSIA)
|-
|[[Passive Sampling of Munitions Constituents|Munitions Constituents - Passive Sampling]]
|[[Dr. Guilherme Lotufo|Lotufo, Guilerme, Ph.D.]]
|
|-
|[[Vapor Intrusion (VI)]]||[[Chris Lutes |Lutes, Chris]]||vapor intrusion
|-
|[[Bioremediation - Anaerobic]]||[[Leah MacKinnon, M.A.Sc., P. Eng.|MacKinnon, Leah]]||anaerobic bioremediation
|-
|[[Bioremediation - Anaerobic Design Considerations]]||[[Leah MacKinnon, M.A.Sc., P. Eng.|MacKinnon, Leah]]||
|-
|[[Biodegradation - 1,4-Dioxane]]
|[[Dr. Shaily Mahendra|Mahendra, Shaily, Ph.D.]]
|
|-
|[[Direct Push (DP) Technology]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push, DP, DP machines, DP technology
|-
|[[Direct Push Sampling]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push sampling, soil sampling, groundwater sampling, well installation, soil vapor sampling (in regards to DP)
|-
|[[Direct Push Logging]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push logging, Cone Penetration Testing, CPT, Electrical Conductivity, EC, Hydraulic Profiling Tool, HPT, Membrane Interface Probe, MIP, Optical Imaging Profiler, OIP
|-
|[[Remediation Performance Assessment at Chlorinated Solvent Sites]]||[[Travis McGuire|McGuire, Travis]]||multi-site studies
|-
|[[LNAPL Conceptual Site Models]]
|[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]
|
|-
|[[Long-Term Monitoring (LTM) - Data Analysis]]||[[Dr. Thomas McHugh |McHugh, Thomas, Ph.D.]]||data analysis, analysis methods (in regards to LTM)
|-
|[[Long-Term Monitoring (LTM) - Data Variability]]||[[Dr. Thomas McHugh |McHugh, Thomas, Ph.D.]]||data variability (in regards to LTM), LTM evaluation
|-
|[[Munitions Constituents - Abiotic Reduction]]||[[Dr. Jimmy Murillo-Gelvez |Murillo-Gelvez, Jimmy, Ph.D.]] ||
|-
|[[Supercritical Water Oxidation (SCWO)]]
|Nagar, Kobe
|
|-
|[[Matrix Diffusion]]
|[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]
|
|-
|[[Groundwater Flow and Solute Transport]]||[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]||groundwater flow, advection, dispersion, diffusion, molecular diffusion, mechanical dispersion
|-
|[[Molecular Biological Tools - MBTs]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||MBT, Molecular Biological Tool(s)
|-
|[[Quantitative Polymerase Chain Reaction (qPCR)]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||qPCR, Polymerase Chain Reaction
|-
|[[Sediment Capping]]
|[[Dr. Danny Reible|Reible, Danny]]
|
|-
|[[Stable Isotope Probing (SIP)]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||SIP, Stable Isotope Probing
|-
|[[Metagenomics]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||metagenomics
|-
|[[Natural Source Zone Depletion (NSZD)]]||[[Tom Palaia |Palaia, Tom]]||natural source zone depletion, NSZD
|-
|[[Amendment Distribution in Low Conductivity Materials]]||[[Dr. Stephen Richardson |Richardson, Stephen, Ph.D.]]||
|-
|[[Polycyclic Aromatic Hydrocarbons (PAHs)]]||[[Dr. Stephen Richardson |Richardson, Stephen, Ph.D.]]||polycyclic aromatic hydrocarbons, PAH(s)
|-
|[[Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)]]
|[[Florent Risacher|Risacher, Florent, M.Sc.]]
|
|-
|[[1,2,3-Trichloropropane]]||[[Dr. Alexandra Salter-Blanc |Salter-Blanc, Alexandra, Ph.D.]]||TCP, trichloropropane
|-
|[[Zerovalent Iron (ZVI) (Chemical Reduction - ISCR)]]||[[Dr. Alexandra Salter-Blanc |Salter-Blanc, Alexandra, Ph.D.]]||ZVI
|-
|[[Mercury in Sediments]]
|[[Dr. Grace Schwartz|Schwartz, Grace, Ph.D.]]
|
|-
|[[Munitions Constituents – Sample Extraction and Analytical Techniques|Munitions Constituents - Sample Extraction and Analytical Techniques]]
|[[Dr. Austin Scircle|Scircle, Austin]]
|
|-
|[[Geophysical Methods]]||[[Dr. Lee Slater |Slater, Lee, Ph.D.]]||geophysics
|-
|[[Geophysical Methods - Case Studies]]||[[Dr. Lee Slater |Slater, Lee, Ph.D.]]||geophysics
|-
|[[PFAS Treatment by Anion Exchange]]
|[[Dr. Timothy J. Strathmann|Strathmann, Timothy, Ph.D.]]
|PFAS
|-
|[[Munitions Constituents - Dissolution]]||[[Dr. Susan Taylor |Taylor, Susan, Ph.D.]]||explosive(s), dissolution
|-
|[[PFAS Treatment by Electrical Discharge Plasma]]
|[[Dr. Selma Mededovic Thagard|Thagard, Selma Mededovic, Ph.D.]]
|PFAS
|-
|[[Chemical Reduction (In Situ - ISCR)]]||[[Dr. Paul Tratnyek |Tratnyek, Paul, Ph.D.]]||In Situ Chemical Reduction, ISCR
|-
|[[Injection Techniques - Viscosity Modification]]||[[Michael Truex |Truex, Michael]]||viscosity, viscosity modifiers, viscosity modification
|-
|[[Soil Vapor Extraction (SVE)]]||[[Michael Truex |Truex, Michael]]||soil vapor extraction, SVE
|-
|[[Munitions Constituents - Deposition]]||[[Michael R. Walsh, P.E., M.E.|Walsh, Michael, P.E.]]||explosive deposition, energetics deposition
|-
|[[Vapor Intrusion - Separation Distances from Petroleum Sources]]
|[[Dr. James Weaver|Weaver, James, Ph.D.]]
|vapor intrusion
|-
|[[Zerovalent Iron Permeable Reactive Barriers]]
|[[Dr. Richard Wilkin|Wilkin, Rick, Ph.D.]]
|
|-
|[[Monitored Natural Attenuation (MNA)]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, In Situ MNA, natural attenuation, natural attenuation processes
|-
|[[Monitored Natural Attenuation (MNA) of Fuels]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, natural attenuation, attenuate (when used in context related to petroleum hydrocarbons and fuel components)
|-
|[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, natural attenuation, attenuate (when used in context related to chlorinated solvents)
|-
|[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
|[[Dr. John Wilson|Wilson, John, Ph.D.]]
|MNA, natural attenuation
|-
|[[Chlorinated Solvents]]||[[Dr. Bilgen Yuncu, P.E. |Yuncu, Bilgen, Ph.D., P.E.]]||chlorinated solvents
|-
|[[Petroleum Hydrocarbons (PHCs)]]
|[[Dr. Bilgen Yuncu, P.E.|Yuncu, Bilgen, Ph.D., P.E.]]
|Petroleum Hydrocarbons (PHCs)
|-
|[[Photoactivated Reductive Defluorination - PFAS Destruction]]
|[[Dr. Suzanne Witt|Witt, Suzanne, Ph.D.]]
|PFAS destruction
|-
|[[Hydrogeophysical Methods for Characterization and Monitoring of Groundwater-Surface Water Exchanges]]
|[[Dr. Lee Slater|Slater, Lee, Ph.D.]]
|geophysics, hydrogeophysical methods
|-
|[[Hydrothermal Alkaline Treatment (HALT)]]
|[[Dr. Brian Pinkard|Pinkard, Brian]]
|
|-
|[[1,4-Dioxane]]
|[[Matthew Zenker|Zenker, Matthew]]
|
|-
|[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]
|[[Dr. John F. Stults|Stults, Dr. John]]
|PFAS, vadose zone, lysimeter, field investigation
|-
|[[PFAS Destruction by Ultraviolet/Sulfite Treatment]]||[[Dr. Yida Fang |Fang, Yida, Ph.D.]]||PFAS destruction
|
|-
|[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
|[[Dani Tran|Tran, Dani]]||MNA, natural attenuation, natural attenuation processes, chlorinated solvents
|-
|}

Articles

2026-05-07T17:12:11Z

Admin:

Monitored Natural Attenuation - Transitioning from Active Remedies

2026-05-07T17:08:14Z

Admin:

Many contaminated sites use active remedies such as pump-and-treat or ''in situ'' remediation to clean up impacted groundwater. Natural attenuation processes such as natural degradation or [[Dispersion and Diffusion | hydrodynamic dispersion]] also contribute to the cleanup. As remediation progresses, a point is often reached when the time required to reach the remedial objectives using the active remedy is roughly the same as the time required if the active remedy is shut down, and the continuing remediation of the site is provided by natural attenuation processes alone. From that point forward, the extra effort and expense of the active remedy provides no benefit over natural attenuation, and it may be appropriate to transition the site to [[Monitored Natural Attenuation (MNA)]]. This article deals with currently available tools and approaches that can be used to support a decision to transition from active remediation to MNA.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s)''':

*[[Alternative Endpoints]]
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Plume Response Modeling]]
*[[REMChlor - MD]]
*[[Source Zone Modeling]]

'''Contributor(s):''' [[Dr. John Wilson]] and [[Dr. David Adamson, P.E.]]

'''Key Resource(s)''':

*[//www.enviro.wiki/images/1/10/2002-Newell-Calculation_and_Use_of_First-Order_Rate_Constants_for_Monitored_Natural_Attenuation_Studies.pdf Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies]<ref name="Newell2002">Newell, C.J., Rifai, H.S., Wilson, J.T., Connor, J.A., Aziz, J.A., Suarez, M.P., 2002. Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies. 28p. EPA/540/S-02/500. [//www.enviro.wiki/images/1/10/2002-Newell-Calculation_and_Use_of_First-Order_Rate_Constants_for_Monitored_Natural_Attenuation_Studies.pdf Report.pdf]</ref>

*[https://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS) Version 2.3.3]<ref name="Widdowson2008">Widdowson, M.A., Mendez, E., Chapelle, F.H., Casey, C.C., 2008. Natural Attenuation Software (NAS) Version 2.3.3. Virginia Polytechnic Institute and State University, the United States Geological Survey, and the United States Naval Facilities Engineering Command. NAS webpage: https://www.nas.cee.vt.edu/index.php See also: https://toxics.usgs.gov/highlights/nas_2.2.0/index.html</ref>

*[//www.enviro.wiki/images/3/39/2002-Aziz-Biochlor_Natural_Attenuation_Decision_Support_System_Vs_2.2.pdf BIOCHLOR Natural Attenuation Support System, Version 2.2]<ref name="Aziz2002">Aziz, C.E., Newell, C.J. and Gonzales, J.R., 2002. BIOCHLOR Natural Attenuation Decision Support System Version 2.2 User’s Manual Addendum. Groundwater Services, Inc., Houston, Texas for the Air Force Center for Environmental Excellence.[//www.enviro.wiki/images/3/39/2002-Aziz-Biochlor_Natural_Attenuation_Decision_Support_System_Vs_2.2.pdf Report.pdf] Available at: https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system</ref>

*[https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 BioPIC User's Guide and Tool Website]<ref name="BioPIC2021">Danko, A., Adamson, D., Newell, C., Wilson, J., Wilson, B., Freedman, D., Lebrón, C., 2021. Quick BioPIC User’s Guide, ESTCP Project ER-201730. [https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 Project Website]   [//www.enviro.wiki/images/c/c9/ER-201730_BioPIC_User%27s_Guide.pdf User’s Guide]</ref>

*[https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Transition Assessment Teaching Assistant (TA2) Tool Website]<ref name="TATA2024">Adamson, D.T., Newell, C.J., Hort, H.M, Wilson, J.T., 2024. TA2: The SERDP Transition Assessment Teaching Assistant. Strategic Environmental Research and Development Program (SERDP) Project ER20-1429. [https://serdp-estcp.mil/projects/details/350cbc0b-893a-43a6-8a0c-c9c057bacac0/er20-1429-project-overview Project Website]  [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Online Tool]</ref>

==Introduction==
Many active remedies are effective at treating higher concentrations of contaminants, but as the contaminant concentrations decrease, the rate of cleanup may slow before the site reaches the cleanup goal. At some sites, the rate of cleanup may slow until it is not significantly different from the rate of cleanup provided by the natural attenuation processes that occur at the site. At other sites, the concentration of contaminants in water produced by a pumping system is below the cleanup goal, but the concentration in monitoring wells in the source area are still above the goal. At some sites, active treatment has stopped further expansion of the plume toward a receptor, and concentrations are declining over time throughout the plume, but back diffusion is sustaining concentrations in the plume that are above the cleanup goal.

In 2013, a significant National Research Council (NRC) report noted that despite years of effort and considerable investment, many sites “will require long-term management that could extend for decades or longer”<ref name="NRC2013">National Research Council (NRC), 2013. Alternatives for Managing the Nation's Complex Contaminated Groundwater Sites. Committee on Future Options for Management in the Nation's Subsurface Remediation Effort, Water Science, Technology Board, Division on Earth and Life Studies, NRC. National Academies Press, 422 pages, ISBN 978-0-309-27874-4 [https://doi.org/10.17226/14668 doi: 10.17226/14668]. [//www.enviro.wiki/images/4/48/NRC2013.pdf Report.pdf]</ref>. The authors of the report discussed the need for developments that can aid in “transition from active remediation to more passive strategies and provide more cost-effective and protective long-term management of complex sites”<ref name="NRC2013" />.

The United States Environmental Protection Agency<ref> U.S. Environmental Protection Agency (USEPA), 1999. Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites. OSWER Directive 9200.4-17P. 39pp.[//www.enviro.wiki/images/a/aa/1999_USEPA-_Use_of_monitored_natural_attenuation_at_superfund.pdf Report.pdf]</ref> allows the use of [[Monitored Natural Attenuation (MNA) | monitored natural attenuation (MNA)]] to attain the cleanup goals when the site-specific remediation objectives can be attained within a time frame that is reasonable compared to that offered by other more active methods. Many CERCLA<ref>U.S. Environmental Protection Agency (USEPA), 2019. Summary of the Comprehensive Environmental Response, Compensation, and Liability Act (Superfund) https://www.epa.gov/laws-regulations/summary-comprehensive-environmental-response-compensation-and-liability-act</ref> and RCRA<ref>U.S. Environmental Protection Agency (USEPA), 2019. Resource Conservation and Recovery Act (RCRA) Laws and Regulations https://www.epa.gov/rcra</ref> sites take advantage of this policy. An active remedy is typically used initially to treat high concentrations of contaminants followed by MNA to treat the lower concentrations that remain.

Unfortunately, there is no well-established approach to determine when it is appropriate to discontinue the active remedy. The NRC report<ref name="NRC2013" /> emphasized the use of more rigorous evaluations of existing data to support these efforts. This can include a quantitative assessment of the performance of active remedies (e.g., evidence of asymptotic performance) as well as documenting site conditions that may be contributing to these performance limitations. Importantly, it also identifies alternative approaches for managing the site, which could include MNA if the natural attenuation processes can meaningfully contribute to the achievement of site cleanup objectives.

This article reviews available tools and approaches to evaluate a transition to MNA. The tools and approaches depend on calculations of rate constants for natural attenuation with distance in flowing groundwater or rate constants for attenuation over time in individual monitoring wells.

==Background on Rate Constants==
[[File:Wilson1w2Fig1.png|thumb|400px| Figure 1. Attenuation of Trichloroethene (TCE) over time in a monitoring well at a site in Michigan. The concentration vs. time rate constant is 0.326 per year and largely represents the rate of the attenuation of the source of contaminants in the aquifer.]]
At sites where a transition to MNA is being considered, a key step is estimating attenuation rate constants and understanding how they are extracted from monitoring data. A general formula to describe the rate of a chemical reaction is:

:{|
|-
|'''Equation 1:'''|| ||<big>''r = k [C]''</big>'' m''
|-
|where:
|-
| ||''r''    ||is the rate of the reaction,
|-
| ||''k''||is the rate constant,
|-
| ||''C''||is the concentration of the chemical undergoing the reaction, and
|-
|the exponent    ||''m''||is the order of the reaction.
|}

When the rate of the reaction is proportional to the concentration of the contaminant, the value of ''m'' is 1. Therefore, the reaction is described as a first-order reaction, and the rate constant is described as a first-order rate constant. In Equation 1, concentration could go up or down, but ''k'' is a constant of proportionality for the rate of increase in concentration. The rate constant for attenuation is the negative of ''k''. If the rate of degradation is a fixed value regardless of concentration, the value of ''m'' is 0, and degradation is a zero-order process.

Natural attenuation of concentrations over time in monitoring wells is frequently described by a first-order rate constant, and natural biological or abiotic degradation of contaminants in flowing groundwater is typically also described by a first-order rate constant. Figure 1 provides an example of monitoring data that is described by a first-order rate constant.

The rate constant for attenuation over time in a single well and the rate constant for attenuation with distance along a flow path in an aquifer describe different situations that are controlled by different processes. ''Attenuation over time'' in a well is largely controlled by the rate of attenuation of the source of contamination in the aquifer. ''Attenuation with distance'' along a flow path includes attenuation of concentrations in the source along with contributions from biological degradation processes, abiotic degradation processes and hydrodynamic dispersion of the contaminated groundwater into clean groundwater<ref name="Newell2002" />.

The first-order rate constant for attenuation over time in a single well is commonly referred to as ''kpoint''<ref name="Newell2002" />. A time series chart in Microsoft EXCEL of the concentrations of a contaminant (''y'' axis) on the date of sampling (''x'' axis) can be used to extract a value for ''kpoint''. Select the data, then insert an exponential trend line and display the equation on the chart. The value of ''kpoint'' can also be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Note that the rate constants extracted in EXCEL are constants for the rate of change, not the rate of attenuation. Take the negative of the rate of change to get ''kpoint''. In the example in Figure 1, the unit of time on the X axis is years, and the value of ''kpoint'' is 0.326 per year.

Attenuation versus distance rate coefficients describe a bulk attenuation rate including both degradation and non-destructive processes such as dispersion. To extract values for rate constants for degradation alone, it is necessary to calibrate a groundwater flow and transport model to the data at the site. The model is calibrated with values for the hydrogeological properties of the aquifer (effective porosity, hydraulic gradient, hydraulic conductivity, hydrodynamic dispersion and the organic carbon content of the aquifer matrix). After the hydrogeological properties of the aquifer are fixed in the model, the most appropriate values for the degradation rate constants are the values that produce the best fit between the contaminant concentrations that are predicted by the model and the contaminant monitoring data at the site.

There are a number of reasons why natural attenuation processes are better described as first-order relationship instead of zero-order or some other order. The attenuation over time in a monitoring well tracks the attenuation over time of the source of contamination that sustains the plume<ref name="Newell2002" />. Sites go through a lifecycle, and attenuation of sources at mature sites is often a first-order process<ref>Sale, T., Newell, C., Stroo, H., Hinchee, R. and Johnson, P., 2008. Frequently Asked Questions Regarding Management of Chlorinated Solvents in Soils and Groundwater. Environmental Security Technology Certification Program (ESTCP, Project ER-200530), Department of Defense (DoD), Arlington, VA. [//www.enviro.wiki/images/c/cb/2008-Sale-Frequently_Asked_Questions_Regarding_Management_of_Chlorinated_Solvent_in_Soils_and_Groundwater.pdf Report.pdf]   Project Overview Website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200530</ref>. If a chlorinated solvent site is mature, the contamination in the source area that was originally present as nonaqueous phase liquids (NAPL) has been redistributed and is now sequestered in a sorbed phase to aquifer solids or has diffused into non-transmissive portions of the aquifer matrix. Transfer of contaminants back into the more transmissive portions of the aquifer occurs by diffusion along a fixed path length, and the rate of transfer is controlled by the concentration of the contaminant remaining in the source material. Because the rate of transfer is proportional to the concentration of contaminant in the source material, attenuation of the source is a first-order process. These processes are discussed in more detail in [[Source Zone Modeling]].

Degradation processes are also usually first order. Abiotic reactions are almost always first order with respect to the concentration of the target chemical. Biodegradation reactions are zero order at high concentrations because the available enzymes are saturated with substrate, but are first order at lower concentrations that are typical of natural attenuation conditions in groundwater.

==Goals for MNA at Sites==

The information necessary to evaluate whether a site can be transitioned to MNA depends on the goal for MNA at the site. For many cleanup actions, the goal is to confine contamination within a waste management area where the contamination is left in place, in which case the cleanup goal applies to point-of-compliance wells that are outside the waste management area. For other cleanup actions, the entire site must be cleaned up, in which case the cleanup goal applies to any monitoring well on the site. The time by which the goal is to be attained is specified at CERCLA sites in the Record of Decision (the ROD). At RCRA sites, the time allowed for the cleanup to be attained may be specified in the permit.

==When the Goal Applies to Point-of-Compliance Wells==
Consider the following framework for evaluating a transition to MNA:

#Use a computer model to extract rate constants for the natural degradation of the contaminant that occurred in groundwater at the site before the active remedy was installed.
#Assume that the same rate constants will apply after the active remedy is no longer in operation. Note that this assumption may not be valid if the active remedy changes the geochemistry of the aquifer in the flow path to the point-of-compliance well.
#Calibrate a computer groundwater flow and transport model with the hydrogeological properties of the aquifer that pertain after the active remedy is no longer in operation, the concentration of contaminant after the active remedy, and the rate constants for natural degradation that are assumed to apply after the active remedy.
#Use the computer model to project the concentrations of the contaminant at the point-of-compliance well over time.
#If the concentrations at the point-of-compliance wells are predicted to be less than the goal before the specified date, that is a quantitative line of evidence in support of a transition to MNA.

There are several computer applications that are particularly useful to extract rate constants at a site from monitoring data that were collected before the active remedy was installed. For example, [https://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS)]<ref name="Widdowson2008" />, [https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system BIOCHLOR]<ref name="Aziz2002" /> and [https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 BioPIC]<ref name="BioPIC2021" /> can be downloaded from the internet at no cost. Another recent example, the [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ TA2 Tool]<ref name="TATA2024" />, is discussed in detail later in this article.

[[File:Wilson1w2Fig2.png|thumb|left|400px| Figure 2. Example calibration of NAS to natural attenuation of total BTEX at a site (Figure 17 of NAS User’s Manual).]]
[[File:Wilson1w2Fig3.png|thumb|400px| Figure 3. The data input screen for BIOCHLOR before remediation with cis-1,2-Dichloroethene (DCE) and vinyl chloride (VC) source concentrations of 500 and 87 mg/L respectively at the source when the release first occurred.]]
[[File:Wilson1w2Fig4.png|thumb|left|400px| Figure 4. Output of the RUN CENTERLINE simulation in BIOCHLOR comparing the fit between the simulation and the field data for vinyl chloride before an active remedy was implemented]]
[[File:Wilson1w2Fig5.png|thumb|400px| Figure 5. Output of the RUN CENTERLINE simulation of conditions after an active remedy was implemented with a source concentration of 1.1 mg/L, projecting the concentration of vinyl chloride at a distance corresponding to a point-of-compliance well.]]
 
In NAS, the user inputs the hydrogeological data, the distance of wells along the flow path, and the concentrations of contaminants in the wells. The NAS application extracts rate constants and makes projections at the point-of-compliance. With NAS, it is possible to extract different rate constants for specific geochemical environments along the flow path.

Figure 2 provides an example calibration of NAS. The concentrations in the monitoring wells used to calibrate the model are compared to the simulation provided by the model. The values of the rate constants that are extracted from the field data are available in the “Output” tab under “Data and Results Table.”

Figure 3 depicts the input screen for BIOCHLOR. The user inputs the hydrogeological parameters, the first-order rate constants (1st Order Decay Coefficient), the distribution of the wells along the flow path, and the concentrations of contaminants in the wells. The model is set up for conditions that apply before the installation of the active remedy.

BIOCHLOR does not automatically fit the rate constants to the field data. Instead, the user examines the output of the model, and adjusts the rate constants until they provide the best fit between the model prediction and the monitoring data for wells at the site. This comparison is illustrated in Figure 4.

If the distance from the source well to the point-of-compliance well is set as the “Modeled Area Length” in Section 5 of the input screen, the “Run Centerline” output will provide the projected concentrations at that length. Assume the distance from the source well to the point-of-compliance well is 250 feet. The projected concentration in Figure 3 of vinyl chloride at a point-of-compliance well is 0.042 mg/L. If the regulatory goal were the federal drinking water maximum contaminant level (MCL)<ref>U. S. Environmental Protection Agency (USEPA), 2009. National Primary Drinking Water Regulations. EPA 816-F-09-004. [//www.enviro.wiki/images/a/ae/2009-USEPA-national_Primary_Drinking_Water_Regulations.pdf Report.pdf]</ref> of 0.002 mg/L, the projected concentration would exceed the goal, and MNA would not be adequate as a remedy.

For the sake of illustration, assume that an active remedy has been implemented, and the concentrations in the source well are 5.4 mg/L for DCE and 1.1 mg/L for vinyl chloride. To evaluate whether it is now appropriate to transition to MNA, BIOCHLOR could be calibrated with these concentrations to predict concentrations in the point-of-compliance well. (See Figure 5). In this example, the projected concentration at the point-of-compliance well does meet the goal.

Some active remedies are subject to rebound. If this is the case, the evaluation should begin at the point in time when it is clear that the trend in concentrations is downward.

A new EXCEL-based tool that does many of the same basic calculations as BIOCHLOR was recently developed as part of an update to the BioPIC<ref name="BioPIC2021" /> decision support software. This tool, the MNA Rate Constant Estimator, extracts rate constants from concentration versus distance data for a variety of different chemicals, including chlorinated ethenes (e.g., PCE and TCE), chlorinated ethanes (e.g., 1,1,1-TCA), and 1,4-dioxane. This tool was developed to run using current versions of EXCEL, whereas BIOCHLOR must be run using older versions of EXCEL that may be unavailable to many users. The MNA Rate Constant Estimator can be used to estimate degradation rate constants and/or predict plume footprints over time. Consequently, it is a useful addition to the BioPIC decision framework for understanding if MNA is appropriate remedy for a site, and it can also be helpful for estimating rate constants as part of a transition assessment.

==When the Goal Applies to All the Wells==
At sites where a concentration-based cleanup goal must be achieved at all wells, each well at the site is evaluated independently, and the rate constant that is applicable is the rate constant for attenuation over time in the well (''kpoint''). To evaluate whether the region in an aquifer that is sampled by a particular monitoring well is ready to transition to MNA, it is necessary to have monitoring data from a period of time before the remedy was implemented. This data is used to extract a value for ''kpoint'' in the aquifer under natural attenuation conditions. The evaluation of a transition to MNA will assume that the same value for ''kpoint'' will apply after the active remedy is complete. This assumption may not be appropriate if the active remedy caused a permanent change in the geochemistry of the aquifer. The assumption is usually appropriate for pump-and-treat remedies.

If ''kpoint'' before implementation of the active remedy describes the time course of natural attenuation after the active remedy is completed, the time required to attain the cleanup goal is predicted from the following:

:{|
| || || rowspan="2" |<big>''ln (''</big>
| style="border-style:solid; border-width: 0px 0px 1px 0px" |''Cgoal''
| rowspan="2" |<big>'')''</big>||                                                                       
|-
|'''Equation 2:'''        ||''<big>t =''    
| style="vertical-align:top;" |''Ccurrent''||
|-
| || || colspan="3" style="text-align:center; border-style:solid; border-width: 1px 0 0 0" |''<big>-k</big>point''||
|-
|where:
|-
| ||''Ccurrent''    || colspan="5" |is the current concentration after active remediation,
|-
| ||''Cgoal''|| colspan="5" |is the cleanup goal, and
|-
| ||''t''|| colspan="5" |is the time required for concentrations to attenuate from ''Ccurrent'' to ''Cgoal.''
|}

If the value of ''t'' estimated using Equation 2 is less than the difference between the current date and the date specified by the site stakeholders to attain the goal, that is evidence in support of a transition to MNA.

Some active remedies are subject to contaminant concentration rebound. If this is the case, the evaluation should use a value of ''C''current that is attained after the rebound has stabilized.

This approach depends on a robust value for ''kpoint''. It is worthwhile to do a sensitivity analysis on ''kpoint'' where the lower 95% or 90% confidence interval on ''kpoint'' is used in Equation 2 to see if that changes the outcome of the evaluation. The confidence intervals can be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Wilson<ref name="Wilson2011">Wilson, J.T. 2011. An Approach for Evaluating the Progress of Natural Attenuation in Groundwater. EPA 600-R-11-204. [//www.enviro.wiki/images/e/e3/Wilson-2011-An_Approach_for_Evaluating_Progress.pdf Report.pdf]</ref> provides detailed discussion of the use of linear regression to extract ''kpoint'' and confidence intervals on ''kpoint''. Wilson<ref name="Wilson2011" /> also discusses the use of goodness-of-fit tests to determine if there is evidence that a first-order rate equation is not the best fit to the monitoring data, and as a result the use of Equation 2 would not be appropriate. The TA2 Tool<ref name="TATA2024" /> also has the capability to calculate ''kpoint'' with a user-specified confidence interval, as described below.

At many sites, there is no specified date when the cleanup goal must be attained. In this situation, the monitoring data can be evaluated to determine if the current rate of attenuation under the active remedy is faster than the rate of natural attenuation before the active remedy was installed. The monitoring data can be examined to identify a time interval when the benefit of the active remedy has approached an asymptote. A second value of for ''kpoint'' can be extracted for that time interval. The two values for ''kpoint'' can be evaluated statistically to see if the current rate is faster at some appropriate level of confidence. If there is no statistical evidence that the rate of attenuation is faster, that determination can support a decision to transition to MNA.
[[File:Wilson1w2Fig6.png|thumb|400px| Figure 6. Example calibration of NAS to predict the reduced concentration at the source that is necessary to meet the remediation goal at a point-of-compliance well (Figure 19 of NAS User’s Manual).]]

==Extent of Treatment Necessary to Transition to MNA==
There are several computer applications that can predict the extent of treatment that must be achieved by the active remedy before it is worthwhile to evaluate the site for transition to MNA. For example, based on the distribution of contamination along the flow path, the NAS application will automatically predict a reduced concentration at the source well that will bring concentrations to the goal in the point-of-compliance well (Figure 6). A table that opens under the “DOS/TOS” tab provides the “Time of Equilibration” required to meet the goal at the reduced concentration. Modules in NAS allow the user to evaluate the effect of various pump-and-treat and source removal scenarios on the time required to attain the goal at the point-of-compliance well.

The [[REMChlor - MD | REMChlor-MD]]<ref name="Falta2018">Falta, R.W., Farhat, S.K., Newell, C.J. and Lynch, K., 2018. A Practical Approach for Modeling Matrix Diffusion Effects in REMChlor. SERDP/ESTCP Project ER-201426 [//www.enviro.wiki/images/0/0b/2018-Falta-REMChlor_Modeling_Matrix_Diffusion_Effects.pdf Report.pdf]   Website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201426</ref> and [https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel REMFuel]<ref name="Falta2012">Falta, R.W., Ahsanuzzaman, A.N., Stacy, M.B., Earle, R.C. and Wilson, J.T., 2012. Remediation Evaluation Model for Fuel Hydrocarbons (REMFuel). Users Manual Version 1.0. U.S. Environmental Protection Agency. EPA/600/R-12/028. [//www.enviro.wiki/images/6/67/2012-Falta-REMFuel_Remediation_Evaluation-Model_for_Fuel_hydrocarbons_users_manual.PDF Report.pdf]   Website: https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel</ref> models are flexible screening tools that allow a simultaneous evaluation of the extent of treatment provided by (1) source removal, (2) ''in situ'' remediation of the contaminated groundwater, or (3) natural attenuation processes in three discrete intervals along the flow path and three discrete time periods. Both [[REMChlor - MD | REMChlor-MD]]<ref name="Falta2018" /> and [https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel REMFuel]<ref name="Falta2012" /> can be downloaded from the internet at no cost. Liang ''et al.''<ref>Liang, H., Falta, R.W., Newell, C.J., Farhat, S.K., Rao, P.S. and Basu, N., 2010. Decision & Management Tools for DNAPL Sites: Optimization of Chlorinated Solvent Source and Plume Remediation Considering Uncertainty. SERDP/ESTCP Project ER-200704. [//www.enviro.wiki/images/c/ce/2010-Liang-Decision_and_Management_Tools_for_DNAPL_sites-ER-200704-FR.pdf Report.pdf]   Project Overview Website: https://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200704/(language)/eng-US</ref> provide a modeling program that uses Monte Carlo simulations to evaluate the effects of the uncertainties in the modeling parameters on the predictions of REMChlor-MD.

==The Transition Assessment Teaching Assistant (TA2) Tool==
[[File:Wilson1w2Fig7.png|thumb|500px| Figure 7. Home Page for TA2 Tool. Users can click on buttons to access various modules that are designed to answer specific questions or research relevant topics.]]
[[File:Wilson1w2Fig8.png|thumb|500px| Figure 8. Example of an asymptote analysis using concentration versus time data in Tool 1 of the TA2 Tool. The source attenuation rate and corresponding remediation timeframe can be estimated for different monitoring periods.]]
A learning and decision-making tool was recently released as part of [https://serdp-estcp.mil/ Strategic Environmental Development and Research Program (SERDP)] Project [https://serdp-estcp.mil/projects/details/350cbc0b-893a-43a6-8a0c-c9c057bacac0/er20-1429-project-overview ER-201429] to help stakeholders gather information for the purposes of a site-specific transition assessment. This free software, the [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Transition Assessment Teaching Assistant (TA2) Tool]<ref name="TATA2024" />, was developed using the elements identified in the 2013 NRC report<ref name="NRC2013" /> as the critical learning objectives for end users.

The Tool is a web-based app that includes a collection of individual modules designed to answer specific questions or research relevant topics (Figure 7). The Tool has been developed as an R Shiny app (version 1.8.0)<ref> Chang, W., Cheng, J., Allaire, J., Sievert, C., Schloerke, B., Xie, Y., Allen, J., McPherson, J., Dipert, A., Borges, B., 2023. shiny: Web Application Framework for R. R package version 1.8.0, https://github.com/rstudio/shiny, https://shiny.posit.co/</ref>, which is an interactive platform using R programming to perform all quantitative functions. The user can then view the results in a simple interface that easily accommodates plots, charts, and various mapping features in a Web browser. The Tool is free and does not require the user to install R software.

The modules within the TA2 Tool include:

'''Five Quantitative Tools''' that focus on assessing asymptotic groundwater concentrations from monitoring data, evaluating plume stability, estimating remediation timeframes after a hypothetical source removal project, forecasting remediation performance if a technology is applied in the field, or projecting concentrations at downgradient points of compliance.

For example, Tool 1 in the TA2 Tool uses concentration versus time data from monitoring wells to estimate attenuation rate constants (''kpoint'') and evaluate if asymptotic conditions are present at particular locations or across the site. This helps to assess whether performance has plateaued at wells where a pump-and-treat system or other active treatment is in place. The user has the option to choose a “change point” within the monitoring record to determine if the attenuation rate has changed over time (e.g., once most of the accessible mass has been removed) (Figure 8). The user can either use visual interpretation to manually select the date when this apparent change occurred or have the date selected automatically using a binary segmentation protocol that is incorporated into the tool. The tool will calculate a rate for both the early period and a rate for the later period (after the change point), and then go through five different lines of evidence for asymptotic behavior (e.g., are the two rates of attenuation significantly different?). The user can then use the collective results as a technical justification demonstrating that the performance of the active remedy has plateaued as the first step in the transition assessment. The tool will also estimate the time to reach a user-specified cleanup goal if the overall attenuation rate (or the attenuation rate in the later period) were to continue.

Another module (Tool 5) focuses on evaluating sites where the concentration goal applies at a downgradient point of compliance, which is a key criterion for sites where MNA is being used as part of a risk-based strategy. The tool includes several different options to estimate a site-specific attenuation rate constant, including data from the pre-remediation period when natural attenuation processes were the sole means for reducing concentrations. Attenuation rate constants are then used to project the concentration versus distance from the contaminant source. Based on the predicted concentration at the downgradient point of compliance, the user can then see if the natural assimilative capacity along the aquifer flow path is sufficient to achieve the concentration goal in the absence of active treatment. For example, in the tab labeled “Use Pre-Remediation Rate Constant”, the logarithms of the concentrations from the period before active treatment began are plotted against the distance from the source well. The slope of the regression line is the rate constant for natural attenuation (including the contributions of degradation and dispersion). This rate constant can then be used to project the concentration moving downgradient from the well of concern after the end of active treatment. Similar approaches are provided within Tool 5 for using rate constants estimated from lab-based testing or derived from post-remediation data (after steady state has been reestablished).

'''Four Qualitative Tools''' provide information on matrix diffusion, enhanced attenuation options, geologic heterogeneity, and related research on transition assessments. Many of these modules are based on the current understanding of the role of matrix diffusion in influencing long-term concentration trends and remedial performance at contaminated groundwater sites. This includes summaries of different modeling options for better quantifying the effects of matrix diffusion. Sites impacted by matrix diffusion are generally challenging to treat using active remedies and thus are better candidates for less intensive management strategies that focus on reducing mass discharge rates, stabilizing the plume, and protecting potential downgradient receptors. As a result, matrix diffusion is critical to understanding and quantifying how natural attenuation processes are contributing to concentration trends.

'''One Summary Tool''' (Tool 10) compiles metrics from the other tools into a “Remediation Transition Assessment Index” (RTAI) and provides additional guidance on conducting site-specific transition assessments. The RTAI is a simple metric with a value from 1 to 5, where higher values reflect greater persistence of contamination due to matrix diffusion and other site-specific factors. An RTAI value is assigned to each of the results from the different tools that have been completed by the user. An RTAI of 5 suggests that the site is a strong candidate for transitioning to MNA or enhanced attenuation approaches, while a site with an RTAI value of 1 is a poor candidate. The user can assign an overall RTAI for the site based on the preponderance of evidence after reviewing the RTAI values generated by each tool, or calculate a site RTAI based on simple averaging, weighting, or other methods.

Tool 10 also contains a flowchart and a checklist for performing site-specific transition assessments that start with evaluating relevant bright line criteria, such as (1) can the concentration goals be met at the point of compliance by MNA; and (2) is the remediation timeframe for MNA reasonable and/or similar to the timeframe if source remediation were used. This checklist ensures that the user has gathered all relevant information that would be needed to support a technically rigorous site-specific Transition Assessment.

The TA2Tool provides a framework for remedial decision makers to evaluate different types of sites, including those where active treatment (e.g., pump and treat) is in use, as well as sites where future active source zone remediation is being considered. It also includes a description of enhanced MNA alternatives for sites where MNA alone may not be sufficient to control risk. As shown in Figure 8, the tool can be used to answer specific questions that have a primarily quantitative basis or to provide focused qualitative information for researching specific topics. Users can engage with just the modules that might be pertinent to assessment of an individual site, or they can go through all the modules to perform a more thorough, step-by-step analysis of the relevant issues for their site.

==Summary==
Tools and approaches are available that can be adapted to determine when a site is ready to transition from active remedy to MNA. However, these tools and approaches have not been applied for this purpose at a significant number of sites, and at the present time, they are not generally accepted by regulatory authorities. There is an opportunity to establish and implement a logical and consistent framework that can be widely implemented to evaluate sites for transition from active remedy to MNA.

==References==
<references />

==See Also==

*[http://dx.doi.org/10.1007/978-1-4614-6922-3 Newell, C.J., Kueper, B.H., Wilson, J.T., Johnson, P.C., 2014. Natural Attenuation of Chlorinated Solvent Source Zones. In: Chlorinated Solvent Source Zone Remediation, Editors: Kueper, B.H., Stroo, H.F., Vogel, C.M., Ward. SERDP ESTCP Environmental Remediation Technology, vol 7. Springer, New York, NY. pgs. 459-508. doi: 10.1007/978-1-4614-6922-3]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436/ER-200436/(language)/eng-US Kram, Mark, and Widdowson, Mark, 2008. Estimating Cleanup Times Associated with Combining Source-Area Remediation with Monitored Natural Attenuation. ESTCP ER-200436]

Monitored Natural Attenuation - Transitioning from Active Remedies

2026-05-07T17:07:57Z

Admin:

Many contaminated sites use active remedies such as pump-and-treat or ''in situ'' remediation to clean up impacted groundwater. Natural attenuation processes such as natural degradation or [[Dispersion and Diffusion | hydrodynamic dispersion]] also contribute to the cleanup. As remediation progresses, a point is often reached when the time required to reach the remedial objectives using the active remedy is roughly the same as the time required if the active remedy is shut down, and the continuing remediation of the site is provided by natural attenuation processes alone. From that point forward, the extra effort and expense of the active remedy provides no benefit over natural attenuation, and it may be appropriate to transition the site to [[Monitored Natural Attenuation (MNA)]]. This article deals with currently available tools and approaches that can be used to support a decision to transition from active remediation to MNA.
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'''Related Article(s)''':

*[[Alternative Endpoints]]
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Plume Response Modeling]]
*[[REMChlor - MD]]
*[[Source Zone Modeling]]

'''Contributor(s):''' *[[Dr. John Wilson]] and *[[Dr. David Adamson, P.E.]]

'''Key Resource(s)''':

*[//www.enviro.wiki/images/1/10/2002-Newell-Calculation_and_Use_of_First-Order_Rate_Constants_for_Monitored_Natural_Attenuation_Studies.pdf Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies]<ref name="Newell2002">Newell, C.J., Rifai, H.S., Wilson, J.T., Connor, J.A., Aziz, J.A., Suarez, M.P., 2002. Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies. 28p. EPA/540/S-02/500. [//www.enviro.wiki/images/1/10/2002-Newell-Calculation_and_Use_of_First-Order_Rate_Constants_for_Monitored_Natural_Attenuation_Studies.pdf Report.pdf]</ref>

*[https://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS) Version 2.3.3]<ref name="Widdowson2008">Widdowson, M.A., Mendez, E., Chapelle, F.H., Casey, C.C., 2008. Natural Attenuation Software (NAS) Version 2.3.3. Virginia Polytechnic Institute and State University, the United States Geological Survey, and the United States Naval Facilities Engineering Command. NAS webpage: https://www.nas.cee.vt.edu/index.php See also: https://toxics.usgs.gov/highlights/nas_2.2.0/index.html</ref>

*[//www.enviro.wiki/images/3/39/2002-Aziz-Biochlor_Natural_Attenuation_Decision_Support_System_Vs_2.2.pdf BIOCHLOR Natural Attenuation Support System, Version 2.2]<ref name="Aziz2002">Aziz, C.E., Newell, C.J. and Gonzales, J.R., 2002. BIOCHLOR Natural Attenuation Decision Support System Version 2.2 User’s Manual Addendum. Groundwater Services, Inc., Houston, Texas for the Air Force Center for Environmental Excellence.[//www.enviro.wiki/images/3/39/2002-Aziz-Biochlor_Natural_Attenuation_Decision_Support_System_Vs_2.2.pdf Report.pdf] Available at: https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system</ref>

*[https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 BioPIC User's Guide and Tool Website]<ref name="BioPIC2021">Danko, A., Adamson, D., Newell, C., Wilson, J., Wilson, B., Freedman, D., Lebrón, C., 2021. Quick BioPIC User’s Guide, ESTCP Project ER-201730. [https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 Project Website]   [//www.enviro.wiki/images/c/c9/ER-201730_BioPIC_User%27s_Guide.pdf User’s Guide]</ref>

*[https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Transition Assessment Teaching Assistant (TA2) Tool Website]<ref name="TATA2024">Adamson, D.T., Newell, C.J., Hort, H.M, Wilson, J.T., 2024. TA2: The SERDP Transition Assessment Teaching Assistant. Strategic Environmental Research and Development Program (SERDP) Project ER20-1429. [https://serdp-estcp.mil/projects/details/350cbc0b-893a-43a6-8a0c-c9c057bacac0/er20-1429-project-overview Project Website]  [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Online Tool]</ref>

==Introduction==
Many active remedies are effective at treating higher concentrations of contaminants, but as the contaminant concentrations decrease, the rate of cleanup may slow before the site reaches the cleanup goal. At some sites, the rate of cleanup may slow until it is not significantly different from the rate of cleanup provided by the natural attenuation processes that occur at the site. At other sites, the concentration of contaminants in water produced by a pumping system is below the cleanup goal, but the concentration in monitoring wells in the source area are still above the goal. At some sites, active treatment has stopped further expansion of the plume toward a receptor, and concentrations are declining over time throughout the plume, but back diffusion is sustaining concentrations in the plume that are above the cleanup goal.

In 2013, a significant National Research Council (NRC) report noted that despite years of effort and considerable investment, many sites “will require long-term management that could extend for decades or longer”<ref name="NRC2013">National Research Council (NRC), 2013. Alternatives for Managing the Nation's Complex Contaminated Groundwater Sites. Committee on Future Options for Management in the Nation's Subsurface Remediation Effort, Water Science, Technology Board, Division on Earth and Life Studies, NRC. National Academies Press, 422 pages, ISBN 978-0-309-27874-4 [https://doi.org/10.17226/14668 doi: 10.17226/14668]. [//www.enviro.wiki/images/4/48/NRC2013.pdf Report.pdf]</ref>. The authors of the report discussed the need for developments that can aid in “transition from active remediation to more passive strategies and provide more cost-effective and protective long-term management of complex sites”<ref name="NRC2013" />.

The United States Environmental Protection Agency<ref> U.S. Environmental Protection Agency (USEPA), 1999. Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites. OSWER Directive 9200.4-17P. 39pp.[//www.enviro.wiki/images/a/aa/1999_USEPA-_Use_of_monitored_natural_attenuation_at_superfund.pdf Report.pdf]</ref> allows the use of [[Monitored Natural Attenuation (MNA) | monitored natural attenuation (MNA)]] to attain the cleanup goals when the site-specific remediation objectives can be attained within a time frame that is reasonable compared to that offered by other more active methods. Many CERCLA<ref>U.S. Environmental Protection Agency (USEPA), 2019. Summary of the Comprehensive Environmental Response, Compensation, and Liability Act (Superfund) https://www.epa.gov/laws-regulations/summary-comprehensive-environmental-response-compensation-and-liability-act</ref> and RCRA<ref>U.S. Environmental Protection Agency (USEPA), 2019. Resource Conservation and Recovery Act (RCRA) Laws and Regulations https://www.epa.gov/rcra</ref> sites take advantage of this policy. An active remedy is typically used initially to treat high concentrations of contaminants followed by MNA to treat the lower concentrations that remain.

Unfortunately, there is no well-established approach to determine when it is appropriate to discontinue the active remedy. The NRC report<ref name="NRC2013" /> emphasized the use of more rigorous evaluations of existing data to support these efforts. This can include a quantitative assessment of the performance of active remedies (e.g., evidence of asymptotic performance) as well as documenting site conditions that may be contributing to these performance limitations. Importantly, it also identifies alternative approaches for managing the site, which could include MNA if the natural attenuation processes can meaningfully contribute to the achievement of site cleanup objectives.

This article reviews available tools and approaches to evaluate a transition to MNA. The tools and approaches depend on calculations of rate constants for natural attenuation with distance in flowing groundwater or rate constants for attenuation over time in individual monitoring wells.

==Background on Rate Constants==
[[File:Wilson1w2Fig1.png|thumb|400px| Figure 1. Attenuation of Trichloroethene (TCE) over time in a monitoring well at a site in Michigan. The concentration vs. time rate constant is 0.326 per year and largely represents the rate of the attenuation of the source of contaminants in the aquifer.]]
At sites where a transition to MNA is being considered, a key step is estimating attenuation rate constants and understanding how they are extracted from monitoring data. A general formula to describe the rate of a chemical reaction is:

:{|
|-
|'''Equation 1:'''|| ||<big>''r = k [C]''</big>'' m''
|-
|where:
|-
| ||''r''    ||is the rate of the reaction,
|-
| ||''k''||is the rate constant,
|-
| ||''C''||is the concentration of the chemical undergoing the reaction, and
|-
|the exponent    ||''m''||is the order of the reaction.
|}

When the rate of the reaction is proportional to the concentration of the contaminant, the value of ''m'' is 1. Therefore, the reaction is described as a first-order reaction, and the rate constant is described as a first-order rate constant. In Equation 1, concentration could go up or down, but ''k'' is a constant of proportionality for the rate of increase in concentration. The rate constant for attenuation is the negative of ''k''. If the rate of degradation is a fixed value regardless of concentration, the value of ''m'' is 0, and degradation is a zero-order process.

Natural attenuation of concentrations over time in monitoring wells is frequently described by a first-order rate constant, and natural biological or abiotic degradation of contaminants in flowing groundwater is typically also described by a first-order rate constant. Figure 1 provides an example of monitoring data that is described by a first-order rate constant.

The rate constant for attenuation over time in a single well and the rate constant for attenuation with distance along a flow path in an aquifer describe different situations that are controlled by different processes. ''Attenuation over time'' in a well is largely controlled by the rate of attenuation of the source of contamination in the aquifer. ''Attenuation with distance'' along a flow path includes attenuation of concentrations in the source along with contributions from biological degradation processes, abiotic degradation processes and hydrodynamic dispersion of the contaminated groundwater into clean groundwater<ref name="Newell2002" />.

The first-order rate constant for attenuation over time in a single well is commonly referred to as ''kpoint''<ref name="Newell2002" />. A time series chart in Microsoft EXCEL of the concentrations of a contaminant (''y'' axis) on the date of sampling (''x'' axis) can be used to extract a value for ''kpoint''. Select the data, then insert an exponential trend line and display the equation on the chart. The value of ''kpoint'' can also be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Note that the rate constants extracted in EXCEL are constants for the rate of change, not the rate of attenuation. Take the negative of the rate of change to get ''kpoint''. In the example in Figure 1, the unit of time on the X axis is years, and the value of ''kpoint'' is 0.326 per year.

Attenuation versus distance rate coefficients describe a bulk attenuation rate including both degradation and non-destructive processes such as dispersion. To extract values for rate constants for degradation alone, it is necessary to calibrate a groundwater flow and transport model to the data at the site. The model is calibrated with values for the hydrogeological properties of the aquifer (effective porosity, hydraulic gradient, hydraulic conductivity, hydrodynamic dispersion and the organic carbon content of the aquifer matrix). After the hydrogeological properties of the aquifer are fixed in the model, the most appropriate values for the degradation rate constants are the values that produce the best fit between the contaminant concentrations that are predicted by the model and the contaminant monitoring data at the site.

There are a number of reasons why natural attenuation processes are better described as first-order relationship instead of zero-order or some other order. The attenuation over time in a monitoring well tracks the attenuation over time of the source of contamination that sustains the plume<ref name="Newell2002" />. Sites go through a lifecycle, and attenuation of sources at mature sites is often a first-order process<ref>Sale, T., Newell, C., Stroo, H., Hinchee, R. and Johnson, P., 2008. Frequently Asked Questions Regarding Management of Chlorinated Solvents in Soils and Groundwater. Environmental Security Technology Certification Program (ESTCP, Project ER-200530), Department of Defense (DoD), Arlington, VA. [//www.enviro.wiki/images/c/cb/2008-Sale-Frequently_Asked_Questions_Regarding_Management_of_Chlorinated_Solvent_in_Soils_and_Groundwater.pdf Report.pdf]   Project Overview Website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200530</ref>. If a chlorinated solvent site is mature, the contamination in the source area that was originally present as nonaqueous phase liquids (NAPL) has been redistributed and is now sequestered in a sorbed phase to aquifer solids or has diffused into non-transmissive portions of the aquifer matrix. Transfer of contaminants back into the more transmissive portions of the aquifer occurs by diffusion along a fixed path length, and the rate of transfer is controlled by the concentration of the contaminant remaining in the source material. Because the rate of transfer is proportional to the concentration of contaminant in the source material, attenuation of the source is a first-order process. These processes are discussed in more detail in [[Source Zone Modeling]].

Degradation processes are also usually first order. Abiotic reactions are almost always first order with respect to the concentration of the target chemical. Biodegradation reactions are zero order at high concentrations because the available enzymes are saturated with substrate, but are first order at lower concentrations that are typical of natural attenuation conditions in groundwater.

==Goals for MNA at Sites==

The information necessary to evaluate whether a site can be transitioned to MNA depends on the goal for MNA at the site. For many cleanup actions, the goal is to confine contamination within a waste management area where the contamination is left in place, in which case the cleanup goal applies to point-of-compliance wells that are outside the waste management area. For other cleanup actions, the entire site must be cleaned up, in which case the cleanup goal applies to any monitoring well on the site. The time by which the goal is to be attained is specified at CERCLA sites in the Record of Decision (the ROD). At RCRA sites, the time allowed for the cleanup to be attained may be specified in the permit.

==When the Goal Applies to Point-of-Compliance Wells==
Consider the following framework for evaluating a transition to MNA:

#Use a computer model to extract rate constants for the natural degradation of the contaminant that occurred in groundwater at the site before the active remedy was installed.
#Assume that the same rate constants will apply after the active remedy is no longer in operation. Note that this assumption may not be valid if the active remedy changes the geochemistry of the aquifer in the flow path to the point-of-compliance well.
#Calibrate a computer groundwater flow and transport model with the hydrogeological properties of the aquifer that pertain after the active remedy is no longer in operation, the concentration of contaminant after the active remedy, and the rate constants for natural degradation that are assumed to apply after the active remedy.
#Use the computer model to project the concentrations of the contaminant at the point-of-compliance well over time.
#If the concentrations at the point-of-compliance wells are predicted to be less than the goal before the specified date, that is a quantitative line of evidence in support of a transition to MNA.

There are several computer applications that are particularly useful to extract rate constants at a site from monitoring data that were collected before the active remedy was installed. For example, [https://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS)]<ref name="Widdowson2008" />, [https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system BIOCHLOR]<ref name="Aziz2002" /> and [https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 BioPIC]<ref name="BioPIC2021" /> can be downloaded from the internet at no cost. Another recent example, the [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ TA2 Tool]<ref name="TATA2024" />, is discussed in detail later in this article.

[[File:Wilson1w2Fig2.png|thumb|left|400px| Figure 2. Example calibration of NAS to natural attenuation of total BTEX at a site (Figure 17 of NAS User’s Manual).]]
[[File:Wilson1w2Fig3.png|thumb|400px| Figure 3. The data input screen for BIOCHLOR before remediation with cis-1,2-Dichloroethene (DCE) and vinyl chloride (VC) source concentrations of 500 and 87 mg/L respectively at the source when the release first occurred.]]
[[File:Wilson1w2Fig4.png|thumb|left|400px| Figure 4. Output of the RUN CENTERLINE simulation in BIOCHLOR comparing the fit between the simulation and the field data for vinyl chloride before an active remedy was implemented]]
[[File:Wilson1w2Fig5.png|thumb|400px| Figure 5. Output of the RUN CENTERLINE simulation of conditions after an active remedy was implemented with a source concentration of 1.1 mg/L, projecting the concentration of vinyl chloride at a distance corresponding to a point-of-compliance well.]]
 
In NAS, the user inputs the hydrogeological data, the distance of wells along the flow path, and the concentrations of contaminants in the wells. The NAS application extracts rate constants and makes projections at the point-of-compliance. With NAS, it is possible to extract different rate constants for specific geochemical environments along the flow path.

Figure 2 provides an example calibration of NAS. The concentrations in the monitoring wells used to calibrate the model are compared to the simulation provided by the model. The values of the rate constants that are extracted from the field data are available in the “Output” tab under “Data and Results Table.”

Figure 3 depicts the input screen for BIOCHLOR. The user inputs the hydrogeological parameters, the first-order rate constants (1st Order Decay Coefficient), the distribution of the wells along the flow path, and the concentrations of contaminants in the wells. The model is set up for conditions that apply before the installation of the active remedy.

BIOCHLOR does not automatically fit the rate constants to the field data. Instead, the user examines the output of the model, and adjusts the rate constants until they provide the best fit between the model prediction and the monitoring data for wells at the site. This comparison is illustrated in Figure 4.

If the distance from the source well to the point-of-compliance well is set as the “Modeled Area Length” in Section 5 of the input screen, the “Run Centerline” output will provide the projected concentrations at that length. Assume the distance from the source well to the point-of-compliance well is 250 feet. The projected concentration in Figure 3 of vinyl chloride at a point-of-compliance well is 0.042 mg/L. If the regulatory goal were the federal drinking water maximum contaminant level (MCL)<ref>U. S. Environmental Protection Agency (USEPA), 2009. National Primary Drinking Water Regulations. EPA 816-F-09-004. [//www.enviro.wiki/images/a/ae/2009-USEPA-national_Primary_Drinking_Water_Regulations.pdf Report.pdf]</ref> of 0.002 mg/L, the projected concentration would exceed the goal, and MNA would not be adequate as a remedy.

For the sake of illustration, assume that an active remedy has been implemented, and the concentrations in the source well are 5.4 mg/L for DCE and 1.1 mg/L for vinyl chloride. To evaluate whether it is now appropriate to transition to MNA, BIOCHLOR could be calibrated with these concentrations to predict concentrations in the point-of-compliance well. (See Figure 5). In this example, the projected concentration at the point-of-compliance well does meet the goal.

Some active remedies are subject to rebound. If this is the case, the evaluation should begin at the point in time when it is clear that the trend in concentrations is downward.

A new EXCEL-based tool that does many of the same basic calculations as BIOCHLOR was recently developed as part of an update to the BioPIC<ref name="BioPIC2021" /> decision support software. This tool, the MNA Rate Constant Estimator, extracts rate constants from concentration versus distance data for a variety of different chemicals, including chlorinated ethenes (e.g., PCE and TCE), chlorinated ethanes (e.g., 1,1,1-TCA), and 1,4-dioxane. This tool was developed to run using current versions of EXCEL, whereas BIOCHLOR must be run using older versions of EXCEL that may be unavailable to many users. The MNA Rate Constant Estimator can be used to estimate degradation rate constants and/or predict plume footprints over time. Consequently, it is a useful addition to the BioPIC decision framework for understanding if MNA is appropriate remedy for a site, and it can also be helpful for estimating rate constants as part of a transition assessment.

==When the Goal Applies to All the Wells==
At sites where a concentration-based cleanup goal must be achieved at all wells, each well at the site is evaluated independently, and the rate constant that is applicable is the rate constant for attenuation over time in the well (''kpoint''). To evaluate whether the region in an aquifer that is sampled by a particular monitoring well is ready to transition to MNA, it is necessary to have monitoring data from a period of time before the remedy was implemented. This data is used to extract a value for ''kpoint'' in the aquifer under natural attenuation conditions. The evaluation of a transition to MNA will assume that the same value for ''kpoint'' will apply after the active remedy is complete. This assumption may not be appropriate if the active remedy caused a permanent change in the geochemistry of the aquifer. The assumption is usually appropriate for pump-and-treat remedies.

If ''kpoint'' before implementation of the active remedy describes the time course of natural attenuation after the active remedy is completed, the time required to attain the cleanup goal is predicted from the following:

:{|
| || || rowspan="2" |<big>''ln (''</big>
| style="border-style:solid; border-width: 0px 0px 1px 0px" |''Cgoal''
| rowspan="2" |<big>'')''</big>||                                                                       
|-
|'''Equation 2:'''        ||''<big>t =''    
| style="vertical-align:top;" |''Ccurrent''||
|-
| || || colspan="3" style="text-align:center; border-style:solid; border-width: 1px 0 0 0" |''<big>-k</big>point''||
|-
|where:
|-
| ||''Ccurrent''    || colspan="5" |is the current concentration after active remediation,
|-
| ||''Cgoal''|| colspan="5" |is the cleanup goal, and
|-
| ||''t''|| colspan="5" |is the time required for concentrations to attenuate from ''Ccurrent'' to ''Cgoal.''
|}

If the value of ''t'' estimated using Equation 2 is less than the difference between the current date and the date specified by the site stakeholders to attain the goal, that is evidence in support of a transition to MNA.

Some active remedies are subject to contaminant concentration rebound. If this is the case, the evaluation should use a value of ''C''current that is attained after the rebound has stabilized.

This approach depends on a robust value for ''kpoint''. It is worthwhile to do a sensitivity analysis on ''kpoint'' where the lower 95% or 90% confidence interval on ''kpoint'' is used in Equation 2 to see if that changes the outcome of the evaluation. The confidence intervals can be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Wilson<ref name="Wilson2011">Wilson, J.T. 2011. An Approach for Evaluating the Progress of Natural Attenuation in Groundwater. EPA 600-R-11-204. [//www.enviro.wiki/images/e/e3/Wilson-2011-An_Approach_for_Evaluating_Progress.pdf Report.pdf]</ref> provides detailed discussion of the use of linear regression to extract ''kpoint'' and confidence intervals on ''kpoint''. Wilson<ref name="Wilson2011" /> also discusses the use of goodness-of-fit tests to determine if there is evidence that a first-order rate equation is not the best fit to the monitoring data, and as a result the use of Equation 2 would not be appropriate. The TA2 Tool<ref name="TATA2024" /> also has the capability to calculate ''kpoint'' with a user-specified confidence interval, as described below.

At many sites, there is no specified date when the cleanup goal must be attained. In this situation, the monitoring data can be evaluated to determine if the current rate of attenuation under the active remedy is faster than the rate of natural attenuation before the active remedy was installed. The monitoring data can be examined to identify a time interval when the benefit of the active remedy has approached an asymptote. A second value of for ''kpoint'' can be extracted for that time interval. The two values for ''kpoint'' can be evaluated statistically to see if the current rate is faster at some appropriate level of confidence. If there is no statistical evidence that the rate of attenuation is faster, that determination can support a decision to transition to MNA.
[[File:Wilson1w2Fig6.png|thumb|400px| Figure 6. Example calibration of NAS to predict the reduced concentration at the source that is necessary to meet the remediation goal at a point-of-compliance well (Figure 19 of NAS User’s Manual).]]

==Extent of Treatment Necessary to Transition to MNA==
There are several computer applications that can predict the extent of treatment that must be achieved by the active remedy before it is worthwhile to evaluate the site for transition to MNA. For example, based on the distribution of contamination along the flow path, the NAS application will automatically predict a reduced concentration at the source well that will bring concentrations to the goal in the point-of-compliance well (Figure 6). A table that opens under the “DOS/TOS” tab provides the “Time of Equilibration” required to meet the goal at the reduced concentration. Modules in NAS allow the user to evaluate the effect of various pump-and-treat and source removal scenarios on the time required to attain the goal at the point-of-compliance well.

The [[REMChlor - MD | REMChlor-MD]]<ref name="Falta2018">Falta, R.W., Farhat, S.K., Newell, C.J. and Lynch, K., 2018. A Practical Approach for Modeling Matrix Diffusion Effects in REMChlor. SERDP/ESTCP Project ER-201426 [//www.enviro.wiki/images/0/0b/2018-Falta-REMChlor_Modeling_Matrix_Diffusion_Effects.pdf Report.pdf]   Website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201426</ref> and [https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel REMFuel]<ref name="Falta2012">Falta, R.W., Ahsanuzzaman, A.N., Stacy, M.B., Earle, R.C. and Wilson, J.T., 2012. Remediation Evaluation Model for Fuel Hydrocarbons (REMFuel). Users Manual Version 1.0. U.S. Environmental Protection Agency. EPA/600/R-12/028. [//www.enviro.wiki/images/6/67/2012-Falta-REMFuel_Remediation_Evaluation-Model_for_Fuel_hydrocarbons_users_manual.PDF Report.pdf]   Website: https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel</ref> models are flexible screening tools that allow a simultaneous evaluation of the extent of treatment provided by (1) source removal, (2) ''in situ'' remediation of the contaminated groundwater, or (3) natural attenuation processes in three discrete intervals along the flow path and three discrete time periods. Both [[REMChlor - MD | REMChlor-MD]]<ref name="Falta2018" /> and [https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel REMFuel]<ref name="Falta2012" /> can be downloaded from the internet at no cost. Liang ''et al.''<ref>Liang, H., Falta, R.W., Newell, C.J., Farhat, S.K., Rao, P.S. and Basu, N., 2010. Decision & Management Tools for DNAPL Sites: Optimization of Chlorinated Solvent Source and Plume Remediation Considering Uncertainty. SERDP/ESTCP Project ER-200704. [//www.enviro.wiki/images/c/ce/2010-Liang-Decision_and_Management_Tools_for_DNAPL_sites-ER-200704-FR.pdf Report.pdf]   Project Overview Website: https://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200704/(language)/eng-US</ref> provide a modeling program that uses Monte Carlo simulations to evaluate the effects of the uncertainties in the modeling parameters on the predictions of REMChlor-MD.

==The Transition Assessment Teaching Assistant (TA2) Tool==
[[File:Wilson1w2Fig7.png|thumb|500px| Figure 7. Home Page for TA2 Tool. Users can click on buttons to access various modules that are designed to answer specific questions or research relevant topics.]]
[[File:Wilson1w2Fig8.png|thumb|500px| Figure 8. Example of an asymptote analysis using concentration versus time data in Tool 1 of the TA2 Tool. The source attenuation rate and corresponding remediation timeframe can be estimated for different monitoring periods.]]
A learning and decision-making tool was recently released as part of [https://serdp-estcp.mil/ Strategic Environmental Development and Research Program (SERDP)] Project [https://serdp-estcp.mil/projects/details/350cbc0b-893a-43a6-8a0c-c9c057bacac0/er20-1429-project-overview ER-201429] to help stakeholders gather information for the purposes of a site-specific transition assessment. This free software, the [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Transition Assessment Teaching Assistant (TA2) Tool]<ref name="TATA2024" />, was developed using the elements identified in the 2013 NRC report<ref name="NRC2013" /> as the critical learning objectives for end users.

The Tool is a web-based app that includes a collection of individual modules designed to answer specific questions or research relevant topics (Figure 7). The Tool has been developed as an R Shiny app (version 1.8.0)<ref> Chang, W., Cheng, J., Allaire, J., Sievert, C., Schloerke, B., Xie, Y., Allen, J., McPherson, J., Dipert, A., Borges, B., 2023. shiny: Web Application Framework for R. R package version 1.8.0, https://github.com/rstudio/shiny, https://shiny.posit.co/</ref>, which is an interactive platform using R programming to perform all quantitative functions. The user can then view the results in a simple interface that easily accommodates plots, charts, and various mapping features in a Web browser. The Tool is free and does not require the user to install R software.

The modules within the TA2 Tool include:

'''Five Quantitative Tools''' that focus on assessing asymptotic groundwater concentrations from monitoring data, evaluating plume stability, estimating remediation timeframes after a hypothetical source removal project, forecasting remediation performance if a technology is applied in the field, or projecting concentrations at downgradient points of compliance.

For example, Tool 1 in the TA2 Tool uses concentration versus time data from monitoring wells to estimate attenuation rate constants (''kpoint'') and evaluate if asymptotic conditions are present at particular locations or across the site. This helps to assess whether performance has plateaued at wells where a pump-and-treat system or other active treatment is in place. The user has the option to choose a “change point” within the monitoring record to determine if the attenuation rate has changed over time (e.g., once most of the accessible mass has been removed) (Figure 8). The user can either use visual interpretation to manually select the date when this apparent change occurred or have the date selected automatically using a binary segmentation protocol that is incorporated into the tool. The tool will calculate a rate for both the early period and a rate for the later period (after the change point), and then go through five different lines of evidence for asymptotic behavior (e.g., are the two rates of attenuation significantly different?). The user can then use the collective results as a technical justification demonstrating that the performance of the active remedy has plateaued as the first step in the transition assessment. The tool will also estimate the time to reach a user-specified cleanup goal if the overall attenuation rate (or the attenuation rate in the later period) were to continue.

Another module (Tool 5) focuses on evaluating sites where the concentration goal applies at a downgradient point of compliance, which is a key criterion for sites where MNA is being used as part of a risk-based strategy. The tool includes several different options to estimate a site-specific attenuation rate constant, including data from the pre-remediation period when natural attenuation processes were the sole means for reducing concentrations. Attenuation rate constants are then used to project the concentration versus distance from the contaminant source. Based on the predicted concentration at the downgradient point of compliance, the user can then see if the natural assimilative capacity along the aquifer flow path is sufficient to achieve the concentration goal in the absence of active treatment. For example, in the tab labeled “Use Pre-Remediation Rate Constant”, the logarithms of the concentrations from the period before active treatment began are plotted against the distance from the source well. The slope of the regression line is the rate constant for natural attenuation (including the contributions of degradation and dispersion). This rate constant can then be used to project the concentration moving downgradient from the well of concern after the end of active treatment. Similar approaches are provided within Tool 5 for using rate constants estimated from lab-based testing or derived from post-remediation data (after steady state has been reestablished).

'''Four Qualitative Tools''' provide information on matrix diffusion, enhanced attenuation options, geologic heterogeneity, and related research on transition assessments. Many of these modules are based on the current understanding of the role of matrix diffusion in influencing long-term concentration trends and remedial performance at contaminated groundwater sites. This includes summaries of different modeling options for better quantifying the effects of matrix diffusion. Sites impacted by matrix diffusion are generally challenging to treat using active remedies and thus are better candidates for less intensive management strategies that focus on reducing mass discharge rates, stabilizing the plume, and protecting potential downgradient receptors. As a result, matrix diffusion is critical to understanding and quantifying how natural attenuation processes are contributing to concentration trends.

'''One Summary Tool''' (Tool 10) compiles metrics from the other tools into a “Remediation Transition Assessment Index” (RTAI) and provides additional guidance on conducting site-specific transition assessments. The RTAI is a simple metric with a value from 1 to 5, where higher values reflect greater persistence of contamination due to matrix diffusion and other site-specific factors. An RTAI value is assigned to each of the results from the different tools that have been completed by the user. An RTAI of 5 suggests that the site is a strong candidate for transitioning to MNA or enhanced attenuation approaches, while a site with an RTAI value of 1 is a poor candidate. The user can assign an overall RTAI for the site based on the preponderance of evidence after reviewing the RTAI values generated by each tool, or calculate a site RTAI based on simple averaging, weighting, or other methods.

Tool 10 also contains a flowchart and a checklist for performing site-specific transition assessments that start with evaluating relevant bright line criteria, such as (1) can the concentration goals be met at the point of compliance by MNA; and (2) is the remediation timeframe for MNA reasonable and/or similar to the timeframe if source remediation were used. This checklist ensures that the user has gathered all relevant information that would be needed to support a technically rigorous site-specific Transition Assessment.

The TA2Tool provides a framework for remedial decision makers to evaluate different types of sites, including those where active treatment (e.g., pump and treat) is in use, as well as sites where future active source zone remediation is being considered. It also includes a description of enhanced MNA alternatives for sites where MNA alone may not be sufficient to control risk. As shown in Figure 8, the tool can be used to answer specific questions that have a primarily quantitative basis or to provide focused qualitative information for researching specific topics. Users can engage with just the modules that might be pertinent to assessment of an individual site, or they can go through all the modules to perform a more thorough, step-by-step analysis of the relevant issues for their site.

==Summary==
Tools and approaches are available that can be adapted to determine when a site is ready to transition from active remedy to MNA. However, these tools and approaches have not been applied for this purpose at a significant number of sites, and at the present time, they are not generally accepted by regulatory authorities. There is an opportunity to establish and implement a logical and consistent framework that can be widely implemented to evaluate sites for transition from active remedy to MNA.

==References==
<references />

==See Also==

*[http://dx.doi.org/10.1007/978-1-4614-6922-3 Newell, C.J., Kueper, B.H., Wilson, J.T., Johnson, P.C., 2014. Natural Attenuation of Chlorinated Solvent Source Zones. In: Chlorinated Solvent Source Zone Remediation, Editors: Kueper, B.H., Stroo, H.F., Vogel, C.M., Ward. SERDP ESTCP Environmental Remediation Technology, vol 7. Springer, New York, NY. pgs. 459-508. doi: 10.1007/978-1-4614-6922-3]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436/ER-200436/(language)/eng-US Kram, Mark, and Widdowson, Mark, 2008. Estimating Cleanup Times Associated with Combining Source-Area Remediation with Monitored Natural Attenuation. ESTCP ER-200436]

Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions

2026-05-07T17:07:21Z

Admin:

The U.S. Department of Defense (DoD) faces many challenges in restoring aquifers at contaminated sites, often due to back-diffusion of tetrachloroethene (PCE) and trichloroethene (TCE) from low-permeability clay zones. The uptake, storage, and subsequent long-term release of these dissolved contaminants from clays are key processes in understanding the longevity, intensity, and risks associated with many persistent chlorinated ethene groundwater plumes. Although naturally occurring abiotic and biotic dechlorination processes in clays may reduce stored contaminant mass and significantly aid natural attenuation, no standardized field method currently exists to verify or quantify these reactions. It is critical to remediation design efforts to demonstrate and validate a cost-effective ''in situ'' approach for assessing these dechlorination processes using first-order rate constants. An approach was developed and applied across eight DoD sites to support Remedial Project Managers (RPMs) and regulators in evaluating natural attenuation potential in clay-rich environments.
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'''Related Article(s):'''

*[[Chlorinated Solvents]]
*[[Matrix Diffusion]]
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| Monitored Natural Attenuation of Chlorinated Solvents]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
*[[REMChlor - MD]]

'''Contributor(s):''' [[Dani Tran]], [[Dr. Charles Schaefer]], and [[Dr. Charles Werth]]

'''Key Resource(s):'''
*Schaefer, C.E, Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils<ref name="SchaeferEtAl2025"/>

==Introduction==
Cost-effective methods are needed to verify the occurrence of natural dechlorination processes and quantify their dechlorination rates in clays under ambient in situ conditions in order to reliably predict their long-term influence on plume longevity and mass discharge. However, accurately determining these rates is challenging due to slow reaction kinetics, the transient nature of transformation products, and the interplay of biotic and abiotic mechanisms within the clay matrix or at clay-sand interfaces. Tools capable of quantifying these reactions and assessing their role in mitigating plume persistence would be a significant aid for long-term site management.

For reductive abiotic dechlorination under anoxic conditions, a 1% hydrochloric acid (HCl) extraction of a sample of native clay coupled with X-ray diffraction (XRD) data can be used as a screening level tool to estimate reductive dechlorination rate constants. These rate constants can be inserted into fate and transport models such as [[REMChlor - MD]]<ref>Falta, R., and Wang, W., 2017. A semi-analytical method for simulating matrix diffusion in numerical transport models. Journal of Contaminant Hydrology, 197, pp. 39-49. [https://doi.org/10.1016/j.jconhyd.2016.12.007 doi: 10.1016/j.jconhyd.2016.12.007]  [[Media: FaltaWang2017.pdf | Open Access Manuscript]]</ref><ref>Kulkarni, P.R., Adamson, D.T., Popovic, J., Newell, C.J., 2022. Modeling a well-charactized perfluorooctane sulfate (PFOS) source and plume using the REMChlor-MD model to account for matrix diffusion. Journal of Contaminant Hydrology, 247, Article 103986. [https://doi.org/10.1016/j.jconhyd.2022.103986 doi: 10.1016/j.jconhyd.2022.103986]  [[Media: KulkarniEtAl2022.pdf | Open Access Manuscript]]</ref> to quantify abiotic dechlorination impacts within clay aquitards on chlorinated solvent plumes. Thus, determination of the abiotic reductive dechlorination rate constant for a particular clayey soil can be readily utilized to provide a more accurate assessment of aquifer cleanup timeframes for groundwater plumes that are being sustained by contaminant back-diffusion.

==Recommended Approach==
[[File: TranFig1.png | thumb | 500 px | Figure 1: First-order rate constants for abiotic reductive dechlorination of TCE under anaerobic conditions. Circles are data from Schaefer ''et al.'', 2021<ref>Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2021. Abiotic dechlorination in the presence of ferrous minerals. Journal of Contaminant Hydrology, 241, 103839. [https://doi.org/10.1016/j.jconhyd.2021.103839 doi: 10.1016/j.jconhyd.2021.103839]  [[Media: SchaeferEtAl2021.pdf | Open Access Manuscript]]</ref>, filled squares from Schaefer ''et al.'', 2018<ref name="SchaeferEtAl2018"/>, and Schaefer ''et al.'', 2017<ref>Schaefer, C.E., Ho., Gurr, C., Berns, E., Werth, C., 2017. Abiotic dechlorination of chlorinated ethenes in natural clayey soils: impacts of mineralogy and temperature. Journal of Contaminant Hydrology, 206, pp. 10-17. [https://doi.org/10.1016/j.jconhyd.2017.09.007 doi: 10.1016/j.jconhyd.2017.09.007]  [[Media: SchaeferEtAl2017.pdf | Open Access Manuscript]]</ref>, and open squares from Schaefer ''et al.'', 2025<ref name="SchaeferEtAl2025"/>. ]]
[[File: TranFig2.png | thumb | 600 px | Figure 2: Flowchart diagram of field screening procedures]]
The recommended approach builds upon the methodology and findings of a recent study<ref name="SchaeferEtAl2025">Schaefer, C.E., Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils. Groundwater Monitoring and Remediation, 45(2), pp. 31-39. [https://doi.org/10.1111/gwmr.12709 doi: 10.1111/gwmr.12709]</ref>, emphasizing field-based and analytical techniques to quantify abiotic first-order reductive dechlorination rate constants for PCE and TCE in clayey soils under anoxic conditions. Key components of this evaluation are listed below:
#Zone Identification: The focus of the investigation should be to delineate clayey zones adjacent to hydraulically conductive zones.
#Ferrous Mineral Quantification: Assess ferrous mineral context in clay via 1% HCl extraction at ambient temperature over a 10-minute interval.
#Mineralogical Characterization: Conduct XRD analysis with the specific intent of identifying the presence of pyrite and biotite.
#Reduced Gas Analysis: Measurement of reduced gases such as acetylene, ethene, and ethane concentrations in clay samples. Gas-tight sampling devices (e.g., En Core® soil samplers by En Novative Technologies, Inc.) should be used to ensure sample integrity during collection and transport.

Clay samples should be collected within a few centimeters of the high-permeability interface, with optional additional sampling further inward. For mineralogical analysis, a defined interval may be collected and subsequently subsampled. To preserve sample integrity, exposure to air should be minimized during collection, transport, and handling. Homogenization should occur within an anaerobic chamber, and if subsamples are required for external analysis, they must be shipped in gas-tight, anaerobic containers.

Estimation of the abiotic reductive first-order rate constant for PCE and TCE is based on the “reactive” ferrous content in the clay. Reactive ferrous content (Fe(II)r) is estimated as shown in Equation 1:

::'''Equation 1:'''       <big>''Fe(II)r = DA + XRDpyr - XRDbiotite''</big>

where ''DA'' is the ferrous content from the dilute acid (1% HCl) extraction, ''XRDpyr'' is the pyrite content from XRD analysis, and ''XRDbiotite'' is the biotite content from XRD analysis<ref name="SchaeferEtAl2025"/>.

Abiotic dechlorination is unlikely to contribute to mitigating contaminant back-diffusion when reactive ferrous iron (Fe(II)r) concentrations are below 100 mg/kg (Figure 1). For Fe(II)r above 100 mg/kg, the first-order rate constant for PCE and TCE reductive dechlorination can be estimated using the correlation shown in Figure 1<ref name="SchaeferEtAl2018">Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2018. Mechanisms for abiotic dechlorination of trichloroethene by ferrous minerals under oxic and anoxic conditions in natural sediments. Environmental Science and Technology, 52(23), pp.13747-13755. [https://doi.org/10.1021/acs.est.8b04108 doi: 10.1021/acs.est.8b04108]</ref><ref>Borden, R.C., Cha, K.Y., 2021. Evaluating the impact of back diffusion on groundwater cleanup time. Journal of Contaminant Hydrology, 243, Article 103889. [https://doi.org/10.1016/j.jconhyd.2021.103889 doi: 10.1016/j.jconhyd.2021]  [[Media: BordenCha2021.pdf | Open Access Manuscript]]</ref>. The rate constant exhibits a strong positive correlation with the logarithm of reactive Fe(II) content (Pearson’s ''r'' = 0.82), with a slope of 4.7 × 10⁻⁸ L g⁻¹ d⁻¹ (log mg kg⁻¹)⁻¹.

Figure 2 presents a decision flowchart designed to evaluate the significance and extent of abiotic reductive dechlorination. By applying Equation 1 to the dilute acid extractable Fe(II) plus measured mineral species data from clay samples, the reactive ferrous iron content (Fe(II)r) can be quantified, enabling a streamlined assessment of the extent to which abiotic processes are contributing to the mitigation of contaminant back-diffusion.

If Fe(II)r is ≥ 100 mg/kg, a first-order dechlorination rate constant can be estimated and subsequently used within a contaminant fate and transport model. However, if acetylene is detected in the clay, even with Fe(II)r less than 100 mg/kg, then bench-scale testing using methods similar to those described in a recent study<ref name="SchaeferEtAl2025"/> is recommended, as such results would likely be inconsistent with those shown in Figure 1, suggesting some other mechanism might be involved, or that the system mineralogy might be more complex than anticipated. Even if Fe(II)r ≥ 100 mg/kg, confirmatory bench-scale testing may be conducted for additional verification and to refine estimation of the abiotic dechlorination rate constant.

==Summary and Recommendations==
The approach outlined above is intended to serve as a generalized guide for practitioners and site managers to cost-effectively determine the extent to which beneficial abiotic reductive dechlorination reactions are likely occurring in low permeability (e.g., clayey) zones. This approach may be contraindicated if co-contaminants are present. It is currently unclear whether other classes of potentially reactive chemicals, such as trinitrotoluene (TNT) or chlorinated ethanes, could interact competitively with PCE and TCE.

In addition, it remains unclear how other classes of compounds such as per- and polyfluoroalkyl substances (PFAS) may interact or sorb with ferrous minerals and potentially inhibit abiotic dechlorination reactions. Coupling these recommended activities with conventional site investigation tasks would provide an opportunity to perform many of the up-front screening activities with minimal additional project costs. It is important to note that the guidance proposed herein pertains to particularly low permeability media. Sites with complex or varying lithology, where the mineralogy and/or redox conditions may vary, might require evaluation of multiple samples to provide appropriate site-wide information.
 

==References==
<references />

==See Also==
*[https://serdp-estcp.mil/projects/details/a7e3f7b5-ed82-4591-adaa-6196ff33dd60 ESTCP Project ER20-5031 – In Situ Verification and Quantification of Naturally Occurring Dechlorination Rates in Clays: Demonstrating Processes that Mitigate Back-Diffusion and Plume Persistence]

Matrix Diffusion

2026-05-07T17:07:00Z

Admin:

Matrix diffusion occurs when dissolved groundwater contaminants present in zones with greater hydraulic conductivity (''K'') are transported by [[wikipedia:Molecular diffusion | molecular diffusion]] into lower ''K'' zones, slowing the rate of contaminant migration in the high ''K'' zone. However, once the contaminant source is eliminated, contaminants diffuse back out of low ''K'' zones, slowing the cleanup rate in the high ''K'' zone. In some cases, matrix diffusion can maintain contaminant concentrations in more permeable zones at greater than target cleanup goals for decades or potentially even centuries after the primary sources have been addressed<ref name="Chapman2005">Chapman, S.W. and Parker, B.L., 2005. Plume persistence due to aquitard back diffusion following dense nonaqueous phase liquid source removal or isolation. Water Resources Research, 41(12), Report W12411. [https://doi.org/10.1029/2005WR004224 DOI: 10.1029/2005WR004224] [//www.enviro.wiki/images/a/a0/Chapman2005.pdf Report.pdf] Free access article from [https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2005WR004224 American Geophysical Union]</ref>. Field and laboratory results have illustrated the importance of this process. Analytical and numerical modeling tools are available for evaluating matrix diffusion.
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'''Related Article(s):'''
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Groundwater Flow and Solute Transport]]
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Plume Response Modeling]]
*[[REMChlor - MD]]

'''Contributor(s):''' [[Dr. Charles Newell, P.E.|Dr. Charles Newell]] and [[Dr. Robert Borden, P.E.|Dr. Robert Borden]]

'''Key Resource(s):'''

*[https://www.serdp-estcp.org/content/download/23838/240653/file/ER-1740 Management of Contaminants Stored in Low Permeability Zones – A State of the Science Review]<ref name="Sale2013">Sale, T., Parker, B.L., Newell, C.J. and Devlin, J.F., 2013. Management of Contaminants Stored in Low Permeability Zones – A State of the Science Review. Strategic Environmental Research and Development Program (SERDP) Project ER-1740. [//www.enviro.wiki/images/2/23/Sale2013ER-1740.pdf Report.pdf] Website: [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-1740 ER-1740]</ref>

==Introduction==
[[File:NewellMatrixDiffFig1.PNG | thumb |500px| Figure 1. Diffusion of a dissolved solute (chlorinated solvent) into lower ''K'' zones during loading period, followed by diffusion back out into higher ''K'' zones once the source is removed <ref name="Sale2007">Sale, T.C., Illangasekare, T.H., Zimbron, J., Rodriguez, D., Wilking, B., and Marinelli, F., 2007. AFCEE Source Zone Initiative. Air Force Center for Environmental Excellence, Brooks City-Base, San Antonio, TX. [https://www.enviro.wiki/images/0/08/AFCEE-2007-Sale.pdf Report.pdf]</ref>]]
Matrix Diffusion can have major impacts on solute migration in groundwater and on cleanup time following source removal. As a groundwater plume advances downgradient, dissolved contaminants are transported by [[Wikipedia: Molecular diffusion | molecular diffusion]] from zones with larger hydraulic conductivity (''K'') into lower ''K'' zones, slowing the rate of contaminant migration in the high ''K'' zone. However, once the contaminant source is eliminated, contaminants diffuse out of low ''K'' zones, slowing the cleanup rate in the high ''K'' zone (Figure 1). This process, termed ‘back diffusion’, can greatly extend cleanup times.

The impacts of back diffusion on aquifer cleanup have been examined in controlled laboratory experiments by several investigators<ref name="Doner2008">Doner, L.A., 2008. Tools to resolve water quality benefits of upgradient contaminant flux reduction. Master’s Thesis, Department of Civil and Environmental Engineering, Colorado State University.</ref><ref name="Yang2015">Yang, M., Annable, M.D. and Jawitz, J.W., 2015. Back Diffusion from Thin Low Permeability Zones. Environmental Science and Technology, 49(1), pp. 415-422. [https://doi.org/10.1021/es5045634 DOI: 10.1021/es5045634] Free download available from: [https://www.researchgate.net/publication/269189924_Back_Diffusion_from_Thin_Low_Permeability_Zones ResearchGate]</ref><ref name="Yang2016">Yang, M., Annable, M.D. and Jawitz, J.W., 2016. Solute source depletion control of forward and back diffusion through low-permeability zones. Journal of Contaminant Hydrology, 193, pp. 54-62. [https://doi.org/10.1016/j.jconhyd.2016.09.004 DOI: 10.1016/j.jconhyd.2016.09.004] Free download available from: [https://www.researchgate.net/profile/Minjune_Yang/publication/308004091_Solute_source_depletion_control_of_forward_and_back_diffusion_through_low-permeability_zones/links/5a2ed2c44585155b6179f489/Solute-source-depletion-control-of-forward-and-back-diffusion-through-low-permeability-zones.pdf ResearchGate]</ref><ref name="Tatti2018">Tatti, F., Papini, M.P., Sappa, G., Raboni, M., Arjmand, F., and Viotti, P., 2018. Contaminant back-diffusion from low-permeability layers as affected by groundwater velocity: A laboratory investigation by box model and image analysis. Science of The Total Environment, 622, pp. 164-171. [https://doi.org/10.1016/j.scitotenv.2017.11.347 DOI: 10.1016/j.scitotenv.2017.11.347]</ref>. The video in Figure 2 shows the results of a 122-day tracer test in a laboratory flow cell (sand tank)<ref name="Doner2008" />. The flow cell contained several clay zones (''K'' = 10-8 cm/s) surrounded by sand (''K'' = 0.02 cm/s). During the loading period, water containing a green fluorescent tracer migrated from left to right with the water flowing through the flow cell, while also diffusing into the clay. After 22 days, the fluorescent tracer is eliminated from the feed, and most of the green tracer is quickly flushed from the tank’s sandy zones. However, small amounts of tracer continue to diffuse out of the clay layers for over 100 days. This illustrates how back diffusion of contaminants out of low ''K'' zones can maintain low contaminant concentrations long after the contaminant source as been eliminated.

[[File: GreenTank.mp4 | thumb |500px| Figure 2. Video of dye tank simulation of matrix diffusion]]
In some cases, matrix diffusion can maintain contaminant concentrations in more permeable zones above target cleanup goals for decades or potentially even centuries after the primary sources have been addressed. At a site impacted by [[Wikipedia: Dense non-aqueous phase liquid | Dense Non-Aqueous Phase Liquids (DNAPL)]], [[Chlorinated Solvents | trichloroethene (TCE)]] concentrations in downgradient wells declined by roughly an order-of-magnitude (OoM) when the upgradient source area was isolated with sheet piling. However, after this initial decline, TCE concentrations appeared to plateau or decline more slowly, consistent with back diffusion from an underlying aquitard. Numerical simulations indicated that back diffusion would cause TCE concentrations in downgradient wells at the site to remain above target cleanup levels for centuries<ref name="Chapman2005" />.

One other implication of matrix diffusion is that plume migration is attenuated by the loss of contaminants into low permeability zones, leading to slower plume migration compared to a case where no matrix diffusion occurs. This phenomena was observed as far back as 1985 when Sudicky et al. observed that “A second consequence of the solute-storage effect offered by transverse diffusion into low-permeability layers is a rate of migration of the frontal portion of a contaminant in the permeable layers that is less than the groundwater velocity.”<ref name="Sudicky1985"> Sudicky, E.A., Gillham, R.W., and Frind, E.O., 1985. Experimental Investigation of Solute Transport in Stratified Porous Media: 1. The Nonreactive Case. Water Resources Research, 21(7), pp. 1035-1041. [https://doi.org/10.1029/WR021i007p01035 DOI: 10.1029/WR021i007p01035]</ref> In cases where there is an attenuating source, matrix diffusion can also reduce the peak concentrations observed in downgradient monitoring wells. The attenuation caused by matrix diffusion may be particularly important for implementing [[Monitored Natural Attenuation (MNA)]] for contaminants that do not completely degrade, such as [[Metal and Metalloid Contaminants | heavy metals]] and [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]].

==SERPD/ESTCP Research==

The SERDP/ESTCP programs have funded several projects focusing on how matrix diffusion can impede progress towards reaching site closure, including:
{|
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*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-1740 SERDP Management of Contaminants Stored in Low Permeability Zones, A State-of-the-Science Review] <ref name="Sale2013" />
|-
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*[https://www.serdp-estcp.org/Tools-and-Training/Environmental-Restoration/Groundwater-Plume-Treatment/Matrix-Diffusion-Tool-Kit ESTCP Matrix Diffusion Toolkit]<ref name="Farhat2012">Farhat, S.K., Newell, C.J., Seyedabbasi, M.A., McDade, J.M., Mahler, N.T., Sale, T.C., Dandy, D.S. and Wahlberg, J.J., 2012. Matrix Diffusion Toolkit. Environmental Security Technology Certification Program (ESTCP) Project ER-201126. [//www.enviro.wiki/images/3/3b/Farhat2012ER-201126UsersManual.pdf User’s Manual.pdf] Website: [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201126 ER-201126]</ref>
|-
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*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200530 ESTCP Decision Guide]<ref>Sale, T. and Newell, C., 2011. A Guide for Selecting Remedies for Subsurface Releases of Chlorinated Solvents. Environmental Security Technology Certification Program (ESTCP) Project ER-200530. [//www.enviro.wiki/images/6/6d/Sale2011ER-200530.pdf Report.pdf] Website: [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200530 ER-200530]</ref>
|-
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*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201426 ESTCP REMChlor-MD: the USEPA’s REMChlor model with a new matrix diffusion term for the plume]<ref name="Farhat2018">Farhat, S. K., Newell, C. J., Falta, R. W., and Lynch, K., 2018. A Practical Approach for Modeling Matrix Diffusion Effects in REMChlor. Environmental Security Technology Certification Program (ESTCP) Project ER-201426. [https://enviro.wiki/images/0/0b/2018-Falta-REMChlor_Modeling_Matrix_Diffusion_Effects.pdf User’s Manual.pdf] Website: [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201426 ER-201426]</ref>
|}

==Transport Modeling==
Several different modeling approaches have been developed to simulate the diffusive transport of dissolved solutes into and out of lower ''K'' zones<ref>Falta, R.W., and Wang, W., 2017. A semi-analytical method for simulating matrix diffusion in numerical transport models. Journal of Contaminant Hydrology, 197, pp. 39-49. [https://doi.org/10.1016/j.jconhyd.2016.12.007 DOI: 10.1016/j.jconhyd.2016.12.007]</ref><ref>Muskus, N. and Falta, R.W., 2018. Semi-analytical method for matrix diffusion in heterogeneous and fractured systems with parent-daughter reactions. Journal of Contaminant Hydrology, 218, pp. 94-109. [https://doi.org/10.1016/j.jconhyd.2018.10.002 DOI: 10.1016/j.jconhyd.2018.10.002]</ref>. The [https://www.serdp-estcp.org/Tools-and-Training/Environmental-Restoration/Groundwater-Plume-Treatment/Matrix-Diffusion-Tool-Kit Matrix Diffusion Toolkit]<ref name="Farhat2012" /> is a Microsoft Excel based tool for simulating forward and back diffusion using two different analytical models<ref name="Parker1994">Parker, B.L., Gillham, R.W., and Cherry, J.A., 1994. Diffusive Disappearance of Immiscible Phase Organic Liquids in Fractured Geologic Media. Groundwater, 32(5), pp. 805-820. [https://doi.org/10.1111/j.1745-6584.1994.tb00922.x DOI: 10.1111/j.1745-6584.1994.tb00922.x]</ref><ref>Sale, T.C., Zimbron, J.A., and Dandy, D.S., 2008. Effects of reduced contaminant loading on downgradient water quality in an idealized two-layer granular porous media. Journal of Contaminant Hydrology, 102(1), pp. 72-85. [https://doi.org/10.1016/j.jconhyd.2008.08.002 DOI: 10.1016/j.jconhyd.2008.08.002]</ref>. Numerical models including [https://en.wikipedia.org/wiki/MODFLOW MODFLOW]/[https://xmswiki.com/wiki/GMS:MT3DMS MT3DMS]<ref name="Zheng1999">Zheng, C. and Wang, P.P., 1999. MT3DMS: A Modular Three-Dimensional Multispecies Transport Model for Simulation of Advection, Dispersion, and Chemical Reactions of Contaminants in Groundwater Systems; Documentation and User’s Guide. Contract Report SERDP-99-1 U.S. Army Engineer Research and Development Center, Vicksburg, MS. [https://www.enviro.wiki/images/3/32/Mt3dmanual.pdf User’s Guide.pdf] [https://xmswiki.com/wiki/GMS:MT3DMS MT3DMS website]</ref> have been shown to be effective in simulating back diffusion processes and can accurately predict concentration changes over 3 orders-of-magnitude in heterogeneous sand tank experiments<ref>Chapman, S.W., Parker, B.L., Sale, T.C., Doner, L.A., 2012. Testing high resolution numerical models for analysis of contaminant storage and release from low permeability zones. Journal of Contaminant Hydrology, 136, pp. 106-116. [https://doi.org/10.1016/j.jconhyd.2012.04.006 DOI: 10.1016/j.jconhyd.2012.04.006]</ref>. However, numerical models require a fine vertical discretization with short time steps to accurately simulate back diffusion, greatly increasing computation times<ref>Farhat, S.K., Adamson, D.T., Gavaskar, A.R., Lee, S.A., Falta, R.W. and Newell, C.J., 2020. Vertical Discretization Impact in Numerical Modeling of Matrix Diffusion in Contaminated Groundwater. Groundwater Monitoring and Remediation, 40(2), pp. 52-64. [https://doi.org/10.1111/gwmr.12373 DOI: 10.1111/gwmr.12373]</ref>. These issues can be addressed by incorporating a local 1-D model domain within a general 3D numerical model<ref>Carey, G.R., Chapman, S.W., Parker, B.L. and McGregor, R., 2015. Application of an Adapted Version of MT3DMS for Modeling Back‐Diffusion Remediation Timeframes. Remediation, 25(4), pp. 55-79. [https://doi.org/10.1002/rem.21440 DOI: 10.1002/rem.21440]</ref>.

The [[REMChlor - MD]] toolkit is capable of simulating matrix diffusion in groundwater contaminant plumes by using a semi-analytical method for estimating mass transfer between high and low permeability zones that provides computationally accurate predictions, with much shorter run times than traditional fine grid numerical models<ref name="Farhat2018" />.

==Impacts on Breakthrough Curves==
[[File:ADRFig3.png | thumb| left |400px| Figure 3. Comparison of tracer breakthrough (upper graph) and cleanup curves (lower graph) from advection-dispersion based (gray lines) and advection-diffusion based (black lines) solute transport<ref name="ITRC2011">Interstate Technology and Regulatory Council (ITRC), 2011. Integrated DNAPL Site Strategy (IDSS-1), Integrated DNAPL Site Strategy Team, ITRC, Washington, DC. [https://www.enviro.wiki/images/d/d9/ITRC-2011-Integrated_DNAPL.pdf Report.pdf] Free download from: [https://itrcweb.org/GuidanceDocuments/IntegratedDNAPLStrategy_IDSSDoc/IDSS-1.pdf ITRC]</ref>.]]
The impacts of matrix diffusion on the initial breakthrough of the solute plume and on later cleanup are illustrated in Figure 3<ref name="ITRC2011" />. Using a traditional advection-dispersion model, the breakthrough curve for a pulse tracer injection appears as a bell-shaped ([[wikipedia:Gaussian function |Gaussian]]) curve (gray line on the right side of the upper graph) where the peak arrival time corresponds to the average groundwater velocity. Using an advection-diffusion approach, the breakthrough curve for a pulse injection is asymmetric (solid black line) with the peak tracer concentration arriving earlier than would be expected based on the average groundwater velocity, but with a long extended tail to the flushout curve.

The lower graph shows the predicted cleanup concentration profiles following complete elimination of a source area. The advection-dispersion model (gray line) predicts a clean-water front arriving at a time corresponding to the average groundwater velocity. The advection-diffusion model (black line) predicts that concentrations will start to decline more rapidly than expected (based on the average groundwater velocity) as clean water rapidly migrates through the highest-permeability strata. However, low but significant contaminant concentrations linger much longer (tailing) due to diffusive contaminant mass exchange between zones of high and low permeability. A similar response to source remediation is seen in models such as the sand tank experiment shown in Figure 2, and also in field observations of plume contaminant concentrations in heterogeneous aquifers.

 

==References==

<references />

==See Also==

*[https://www.youtube.com/watch?v=iLwsIjkVybU Matrix Diffusion Movie]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-1737 Impact of Clay-DNAPL Interactions on Transport and Storage of Chlorinated Solvents in Low Permeability Zones]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200320 Prediction of Groundwater Quality Improvement Down-Gradient of ''In Situ'' Permeable Treatment Barriers and Fully Remediated Source Zones]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201032 Determining Source Attenuation History to Support Closure by Natural Attenuation]
*[https://www.coursera.org/learn/natural-attenuation-of-groundwater-contaminants/lecture/2R7yh/matrix-diffusion-principles Coursera Matrix Diffusion Online Lecture]

Monitored Natural Attenuation - Transitioning from Active Remedies

2026-05-07T17:06:38Z

Admin:

Many contaminated sites use active remedies such as pump-and-treat or ''in situ'' remediation to clean up impacted groundwater. Natural attenuation processes such as natural degradation or [[Dispersion and Diffusion | hydrodynamic dispersion]] also contribute to the cleanup. As remediation progresses, a point is often reached when the time required to reach the remedial objectives using the active remedy is roughly the same as the time required if the active remedy is shut down, and the continuing remediation of the site is provided by natural attenuation processes alone. From that point forward, the extra effort and expense of the active remedy provides no benefit over natural attenuation, and it may be appropriate to transition the site to [[Monitored Natural Attenuation (MNA)]]. This article deals with currently available tools and approaches that can be used to support a decision to transition from active remediation to MNA.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s)''':

*[[Alternative Endpoints]]
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Plume Response Modeling]]
*[[REMChlor - MD]]
*[[Source Zone Modeling]]

'''Contributor(s):'''

*[[Dr. John Wilson]]
*[[Dr. David Adamson, P.E.]]

'''Key Resource(s)''':

*[//www.enviro.wiki/images/1/10/2002-Newell-Calculation_and_Use_of_First-Order_Rate_Constants_for_Monitored_Natural_Attenuation_Studies.pdf Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies]<ref name="Newell2002">Newell, C.J., Rifai, H.S., Wilson, J.T., Connor, J.A., Aziz, J.A., Suarez, M.P., 2002. Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies. 28p. EPA/540/S-02/500. [//www.enviro.wiki/images/1/10/2002-Newell-Calculation_and_Use_of_First-Order_Rate_Constants_for_Monitored_Natural_Attenuation_Studies.pdf Report.pdf]</ref>

*[https://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS) Version 2.3.3]<ref name="Widdowson2008">Widdowson, M.A., Mendez, E., Chapelle, F.H., Casey, C.C., 2008. Natural Attenuation Software (NAS) Version 2.3.3. Virginia Polytechnic Institute and State University, the United States Geological Survey, and the United States Naval Facilities Engineering Command. NAS webpage: https://www.nas.cee.vt.edu/index.php See also: https://toxics.usgs.gov/highlights/nas_2.2.0/index.html</ref>

*[//www.enviro.wiki/images/3/39/2002-Aziz-Biochlor_Natural_Attenuation_Decision_Support_System_Vs_2.2.pdf BIOCHLOR Natural Attenuation Support System, Version 2.2]<ref name="Aziz2002">Aziz, C.E., Newell, C.J. and Gonzales, J.R., 2002. BIOCHLOR Natural Attenuation Decision Support System Version 2.2 User’s Manual Addendum. Groundwater Services, Inc., Houston, Texas for the Air Force Center for Environmental Excellence.[//www.enviro.wiki/images/3/39/2002-Aziz-Biochlor_Natural_Attenuation_Decision_Support_System_Vs_2.2.pdf Report.pdf] Available at: https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system</ref>

*[https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 BioPIC User's Guide and Tool Website]<ref name="BioPIC2021">Danko, A., Adamson, D., Newell, C., Wilson, J., Wilson, B., Freedman, D., Lebrón, C., 2021. Quick BioPIC User’s Guide, ESTCP Project ER-201730. [https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 Project Website]   [//www.enviro.wiki/images/c/c9/ER-201730_BioPIC_User%27s_Guide.pdf User’s Guide]</ref>

*[https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Transition Assessment Teaching Assistant (TA2) Tool Website]<ref name="TATA2024">Adamson, D.T., Newell, C.J., Hort, H.M, Wilson, J.T., 2024. TA2: The SERDP Transition Assessment Teaching Assistant. Strategic Environmental Research and Development Program (SERDP) Project ER20-1429. [https://serdp-estcp.mil/projects/details/350cbc0b-893a-43a6-8a0c-c9c057bacac0/er20-1429-project-overview Project Website]  [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Online Tool]</ref>

==Introduction==
Many active remedies are effective at treating higher concentrations of contaminants, but as the contaminant concentrations decrease, the rate of cleanup may slow before the site reaches the cleanup goal. At some sites, the rate of cleanup may slow until it is not significantly different from the rate of cleanup provided by the natural attenuation processes that occur at the site. At other sites, the concentration of contaminants in water produced by a pumping system is below the cleanup goal, but the concentration in monitoring wells in the source area are still above the goal. At some sites, active treatment has stopped further expansion of the plume toward a receptor, and concentrations are declining over time throughout the plume, but back diffusion is sustaining concentrations in the plume that are above the cleanup goal.

In 2013, a significant National Research Council (NRC) report noted that despite years of effort and considerable investment, many sites “will require long-term management that could extend for decades or longer”<ref name="NRC2013">National Research Council (NRC), 2013. Alternatives for Managing the Nation's Complex Contaminated Groundwater Sites. Committee on Future Options for Management in the Nation's Subsurface Remediation Effort, Water Science, Technology Board, Division on Earth and Life Studies, NRC. National Academies Press, 422 pages, ISBN 978-0-309-27874-4 [https://doi.org/10.17226/14668 doi: 10.17226/14668]. [//www.enviro.wiki/images/4/48/NRC2013.pdf Report.pdf]</ref>. The authors of the report discussed the need for developments that can aid in “transition from active remediation to more passive strategies and provide more cost-effective and protective long-term management of complex sites”<ref name="NRC2013" />.

The United States Environmental Protection Agency<ref> U.S. Environmental Protection Agency (USEPA), 1999. Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites. OSWER Directive 9200.4-17P. 39pp.[//www.enviro.wiki/images/a/aa/1999_USEPA-_Use_of_monitored_natural_attenuation_at_superfund.pdf Report.pdf]</ref> allows the use of [[Monitored Natural Attenuation (MNA) | monitored natural attenuation (MNA)]] to attain the cleanup goals when the site-specific remediation objectives can be attained within a time frame that is reasonable compared to that offered by other more active methods. Many CERCLA<ref>U.S. Environmental Protection Agency (USEPA), 2019. Summary of the Comprehensive Environmental Response, Compensation, and Liability Act (Superfund) https://www.epa.gov/laws-regulations/summary-comprehensive-environmental-response-compensation-and-liability-act</ref> and RCRA<ref>U.S. Environmental Protection Agency (USEPA), 2019. Resource Conservation and Recovery Act (RCRA) Laws and Regulations https://www.epa.gov/rcra</ref> sites take advantage of this policy. An active remedy is typically used initially to treat high concentrations of contaminants followed by MNA to treat the lower concentrations that remain.

Unfortunately, there is no well-established approach to determine when it is appropriate to discontinue the active remedy. The NRC report<ref name="NRC2013" /> emphasized the use of more rigorous evaluations of existing data to support these efforts. This can include a quantitative assessment of the performance of active remedies (e.g., evidence of asymptotic performance) as well as documenting site conditions that may be contributing to these performance limitations. Importantly, it also identifies alternative approaches for managing the site, which could include MNA if the natural attenuation processes can meaningfully contribute to the achievement of site cleanup objectives.

This article reviews available tools and approaches to evaluate a transition to MNA. The tools and approaches depend on calculations of rate constants for natural attenuation with distance in flowing groundwater or rate constants for attenuation over time in individual monitoring wells.

==Background on Rate Constants==
[[File:Wilson1w2Fig1.png|thumb|400px| Figure 1. Attenuation of Trichloroethene (TCE) over time in a monitoring well at a site in Michigan. The concentration vs. time rate constant is 0.326 per year and largely represents the rate of the attenuation of the source of contaminants in the aquifer.]]
At sites where a transition to MNA is being considered, a key step is estimating attenuation rate constants and understanding how they are extracted from monitoring data. A general formula to describe the rate of a chemical reaction is:

:{|
|-
|'''Equation 1:'''|| ||<big>''r = k [C]''</big>'' m''
|-
|where:
|-
| ||''r''    ||is the rate of the reaction,
|-
| ||''k''||is the rate constant,
|-
| ||''C''||is the concentration of the chemical undergoing the reaction, and
|-
|the exponent    ||''m''||is the order of the reaction.
|}

When the rate of the reaction is proportional to the concentration of the contaminant, the value of ''m'' is 1. Therefore, the reaction is described as a first-order reaction, and the rate constant is described as a first-order rate constant. In Equation 1, concentration could go up or down, but ''k'' is a constant of proportionality for the rate of increase in concentration. The rate constant for attenuation is the negative of ''k''. If the rate of degradation is a fixed value regardless of concentration, the value of ''m'' is 0, and degradation is a zero-order process.

Natural attenuation of concentrations over time in monitoring wells is frequently described by a first-order rate constant, and natural biological or abiotic degradation of contaminants in flowing groundwater is typically also described by a first-order rate constant. Figure 1 provides an example of monitoring data that is described by a first-order rate constant.

The rate constant for attenuation over time in a single well and the rate constant for attenuation with distance along a flow path in an aquifer describe different situations that are controlled by different processes. ''Attenuation over time'' in a well is largely controlled by the rate of attenuation of the source of contamination in the aquifer. ''Attenuation with distance'' along a flow path includes attenuation of concentrations in the source along with contributions from biological degradation processes, abiotic degradation processes and hydrodynamic dispersion of the contaminated groundwater into clean groundwater<ref name="Newell2002" />.

The first-order rate constant for attenuation over time in a single well is commonly referred to as ''kpoint''<ref name="Newell2002" />. A time series chart in Microsoft EXCEL of the concentrations of a contaminant (''y'' axis) on the date of sampling (''x'' axis) can be used to extract a value for ''kpoint''. Select the data, then insert an exponential trend line and display the equation on the chart. The value of ''kpoint'' can also be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Note that the rate constants extracted in EXCEL are constants for the rate of change, not the rate of attenuation. Take the negative of the rate of change to get ''kpoint''. In the example in Figure 1, the unit of time on the X axis is years, and the value of ''kpoint'' is 0.326 per year.

Attenuation versus distance rate coefficients describe a bulk attenuation rate including both degradation and non-destructive processes such as dispersion. To extract values for rate constants for degradation alone, it is necessary to calibrate a groundwater flow and transport model to the data at the site. The model is calibrated with values for the hydrogeological properties of the aquifer (effective porosity, hydraulic gradient, hydraulic conductivity, hydrodynamic dispersion and the organic carbon content of the aquifer matrix). After the hydrogeological properties of the aquifer are fixed in the model, the most appropriate values for the degradation rate constants are the values that produce the best fit between the contaminant concentrations that are predicted by the model and the contaminant monitoring data at the site.

There are a number of reasons why natural attenuation processes are better described as first-order relationship instead of zero-order or some other order. The attenuation over time in a monitoring well tracks the attenuation over time of the source of contamination that sustains the plume<ref name="Newell2002" />. Sites go through a lifecycle, and attenuation of sources at mature sites is often a first-order process<ref>Sale, T., Newell, C., Stroo, H., Hinchee, R. and Johnson, P., 2008. Frequently Asked Questions Regarding Management of Chlorinated Solvents in Soils and Groundwater. Environmental Security Technology Certification Program (ESTCP, Project ER-200530), Department of Defense (DoD), Arlington, VA. [//www.enviro.wiki/images/c/cb/2008-Sale-Frequently_Asked_Questions_Regarding_Management_of_Chlorinated_Solvent_in_Soils_and_Groundwater.pdf Report.pdf]   Project Overview Website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200530</ref>. If a chlorinated solvent site is mature, the contamination in the source area that was originally present as nonaqueous phase liquids (NAPL) has been redistributed and is now sequestered in a sorbed phase to aquifer solids or has diffused into non-transmissive portions of the aquifer matrix. Transfer of contaminants back into the more transmissive portions of the aquifer occurs by diffusion along a fixed path length, and the rate of transfer is controlled by the concentration of the contaminant remaining in the source material. Because the rate of transfer is proportional to the concentration of contaminant in the source material, attenuation of the source is a first-order process. These processes are discussed in more detail in [[Source Zone Modeling]].

Degradation processes are also usually first order. Abiotic reactions are almost always first order with respect to the concentration of the target chemical. Biodegradation reactions are zero order at high concentrations because the available enzymes are saturated with substrate, but are first order at lower concentrations that are typical of natural attenuation conditions in groundwater.

==Goals for MNA at Sites==

The information necessary to evaluate whether a site can be transitioned to MNA depends on the goal for MNA at the site. For many cleanup actions, the goal is to confine contamination within a waste management area where the contamination is left in place, in which case the cleanup goal applies to point-of-compliance wells that are outside the waste management area. For other cleanup actions, the entire site must be cleaned up, in which case the cleanup goal applies to any monitoring well on the site. The time by which the goal is to be attained is specified at CERCLA sites in the Record of Decision (the ROD). At RCRA sites, the time allowed for the cleanup to be attained may be specified in the permit.

==When the Goal Applies to Point-of-Compliance Wells==
Consider the following framework for evaluating a transition to MNA:

#Use a computer model to extract rate constants for the natural degradation of the contaminant that occurred in groundwater at the site before the active remedy was installed.
#Assume that the same rate constants will apply after the active remedy is no longer in operation. Note that this assumption may not be valid if the active remedy changes the geochemistry of the aquifer in the flow path to the point-of-compliance well.
#Calibrate a computer groundwater flow and transport model with the hydrogeological properties of the aquifer that pertain after the active remedy is no longer in operation, the concentration of contaminant after the active remedy, and the rate constants for natural degradation that are assumed to apply after the active remedy.
#Use the computer model to project the concentrations of the contaminant at the point-of-compliance well over time.
#If the concentrations at the point-of-compliance wells are predicted to be less than the goal before the specified date, that is a quantitative line of evidence in support of a transition to MNA.

There are several computer applications that are particularly useful to extract rate constants at a site from monitoring data that were collected before the active remedy was installed. For example, [https://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS)]<ref name="Widdowson2008" />, [https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system BIOCHLOR]<ref name="Aziz2002" /> and [https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 BioPIC]<ref name="BioPIC2021" /> can be downloaded from the internet at no cost. Another recent example, the [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ TA2 Tool]<ref name="TATA2024" />, is discussed in detail later in this article.

[[File:Wilson1w2Fig2.png|thumb|left|400px| Figure 2. Example calibration of NAS to natural attenuation of total BTEX at a site (Figure 17 of NAS User’s Manual).]]
[[File:Wilson1w2Fig3.png|thumb|400px| Figure 3. The data input screen for BIOCHLOR before remediation with cis-1,2-Dichloroethene (DCE) and vinyl chloride (VC) source concentrations of 500 and 87 mg/L respectively at the source when the release first occurred.]]
[[File:Wilson1w2Fig4.png|thumb|left|400px| Figure 4. Output of the RUN CENTERLINE simulation in BIOCHLOR comparing the fit between the simulation and the field data for vinyl chloride before an active remedy was implemented]]
[[File:Wilson1w2Fig5.png|thumb|400px| Figure 5. Output of the RUN CENTERLINE simulation of conditions after an active remedy was implemented with a source concentration of 1.1 mg/L, projecting the concentration of vinyl chloride at a distance corresponding to a point-of-compliance well.]]
 
In NAS, the user inputs the hydrogeological data, the distance of wells along the flow path, and the concentrations of contaminants in the wells. The NAS application extracts rate constants and makes projections at the point-of-compliance. With NAS, it is possible to extract different rate constants for specific geochemical environments along the flow path.

Figure 2 provides an example calibration of NAS. The concentrations in the monitoring wells used to calibrate the model are compared to the simulation provided by the model. The values of the rate constants that are extracted from the field data are available in the “Output” tab under “Data and Results Table.”

Figure 3 depicts the input screen for BIOCHLOR. The user inputs the hydrogeological parameters, the first-order rate constants (1st Order Decay Coefficient), the distribution of the wells along the flow path, and the concentrations of contaminants in the wells. The model is set up for conditions that apply before the installation of the active remedy.

BIOCHLOR does not automatically fit the rate constants to the field data. Instead, the user examines the output of the model, and adjusts the rate constants until they provide the best fit between the model prediction and the monitoring data for wells at the site. This comparison is illustrated in Figure 4.

If the distance from the source well to the point-of-compliance well is set as the “Modeled Area Length” in Section 5 of the input screen, the “Run Centerline” output will provide the projected concentrations at that length. Assume the distance from the source well to the point-of-compliance well is 250 feet. The projected concentration in Figure 3 of vinyl chloride at a point-of-compliance well is 0.042 mg/L. If the regulatory goal were the federal drinking water maximum contaminant level (MCL)<ref>U. S. Environmental Protection Agency (USEPA), 2009. National Primary Drinking Water Regulations. EPA 816-F-09-004. [//www.enviro.wiki/images/a/ae/2009-USEPA-national_Primary_Drinking_Water_Regulations.pdf Report.pdf]</ref> of 0.002 mg/L, the projected concentration would exceed the goal, and MNA would not be adequate as a remedy.

For the sake of illustration, assume that an active remedy has been implemented, and the concentrations in the source well are 5.4 mg/L for DCE and 1.1 mg/L for vinyl chloride. To evaluate whether it is now appropriate to transition to MNA, BIOCHLOR could be calibrated with these concentrations to predict concentrations in the point-of-compliance well. (See Figure 5). In this example, the projected concentration at the point-of-compliance well does meet the goal.

Some active remedies are subject to rebound. If this is the case, the evaluation should begin at the point in time when it is clear that the trend in concentrations is downward.

A new EXCEL-based tool that does many of the same basic calculations as BIOCHLOR was recently developed as part of an update to the BioPIC<ref name="BioPIC2021" /> decision support software. This tool, the MNA Rate Constant Estimator, extracts rate constants from concentration versus distance data for a variety of different chemicals, including chlorinated ethenes (e.g., PCE and TCE), chlorinated ethanes (e.g., 1,1,1-TCA), and 1,4-dioxane. This tool was developed to run using current versions of EXCEL, whereas BIOCHLOR must be run using older versions of EXCEL that may be unavailable to many users. The MNA Rate Constant Estimator can be used to estimate degradation rate constants and/or predict plume footprints over time. Consequently, it is a useful addition to the BioPIC decision framework for understanding if MNA is appropriate remedy for a site, and it can also be helpful for estimating rate constants as part of a transition assessment.

==When the Goal Applies to All the Wells==
At sites where a concentration-based cleanup goal must be achieved at all wells, each well at the site is evaluated independently, and the rate constant that is applicable is the rate constant for attenuation over time in the well (''kpoint''). To evaluate whether the region in an aquifer that is sampled by a particular monitoring well is ready to transition to MNA, it is necessary to have monitoring data from a period of time before the remedy was implemented. This data is used to extract a value for ''kpoint'' in the aquifer under natural attenuation conditions. The evaluation of a transition to MNA will assume that the same value for ''kpoint'' will apply after the active remedy is complete. This assumption may not be appropriate if the active remedy caused a permanent change in the geochemistry of the aquifer. The assumption is usually appropriate for pump-and-treat remedies.

If ''kpoint'' before implementation of the active remedy describes the time course of natural attenuation after the active remedy is completed, the time required to attain the cleanup goal is predicted from the following:

:{|
| || || rowspan="2" |<big>''ln (''</big>
| style="border-style:solid; border-width: 0px 0px 1px 0px" |''Cgoal''
| rowspan="2" |<big>'')''</big>||                                                                       
|-
|'''Equation 2:'''        ||''<big>t =''    
| style="vertical-align:top;" |''Ccurrent''||
|-
| || || colspan="3" style="text-align:center; border-style:solid; border-width: 1px 0 0 0" |''<big>-k</big>point''||
|-
|where:
|-
| ||''Ccurrent''    || colspan="5" |is the current concentration after active remediation,
|-
| ||''Cgoal''|| colspan="5" |is the cleanup goal, and
|-
| ||''t''|| colspan="5" |is the time required for concentrations to attenuate from ''Ccurrent'' to ''Cgoal.''
|}

If the value of ''t'' estimated using Equation 2 is less than the difference between the current date and the date specified by the site stakeholders to attain the goal, that is evidence in support of a transition to MNA.

Some active remedies are subject to contaminant concentration rebound. If this is the case, the evaluation should use a value of ''C''current that is attained after the rebound has stabilized.

This approach depends on a robust value for ''kpoint''. It is worthwhile to do a sensitivity analysis on ''kpoint'' where the lower 95% or 90% confidence interval on ''kpoint'' is used in Equation 2 to see if that changes the outcome of the evaluation. The confidence intervals can be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Wilson<ref name="Wilson2011">Wilson, J.T. 2011. An Approach for Evaluating the Progress of Natural Attenuation in Groundwater. EPA 600-R-11-204. [//www.enviro.wiki/images/e/e3/Wilson-2011-An_Approach_for_Evaluating_Progress.pdf Report.pdf]</ref> provides detailed discussion of the use of linear regression to extract ''kpoint'' and confidence intervals on ''kpoint''. Wilson<ref name="Wilson2011" /> also discusses the use of goodness-of-fit tests to determine if there is evidence that a first-order rate equation is not the best fit to the monitoring data, and as a result the use of Equation 2 would not be appropriate. The TA2 Tool<ref name="TATA2024" /> also has the capability to calculate ''kpoint'' with a user-specified confidence interval, as described below.

At many sites, there is no specified date when the cleanup goal must be attained. In this situation, the monitoring data can be evaluated to determine if the current rate of attenuation under the active remedy is faster than the rate of natural attenuation before the active remedy was installed. The monitoring data can be examined to identify a time interval when the benefit of the active remedy has approached an asymptote. A second value of for ''kpoint'' can be extracted for that time interval. The two values for ''kpoint'' can be evaluated statistically to see if the current rate is faster at some appropriate level of confidence. If there is no statistical evidence that the rate of attenuation is faster, that determination can support a decision to transition to MNA.
[[File:Wilson1w2Fig6.png|thumb|400px| Figure 6. Example calibration of NAS to predict the reduced concentration at the source that is necessary to meet the remediation goal at a point-of-compliance well (Figure 19 of NAS User’s Manual).]]

==Extent of Treatment Necessary to Transition to MNA==
There are several computer applications that can predict the extent of treatment that must be achieved by the active remedy before it is worthwhile to evaluate the site for transition to MNA. For example, based on the distribution of contamination along the flow path, the NAS application will automatically predict a reduced concentration at the source well that will bring concentrations to the goal in the point-of-compliance well (Figure 6). A table that opens under the “DOS/TOS” tab provides the “Time of Equilibration” required to meet the goal at the reduced concentration. Modules in NAS allow the user to evaluate the effect of various pump-and-treat and source removal scenarios on the time required to attain the goal at the point-of-compliance well.

The [[REMChlor - MD | REMChlor-MD]]<ref name="Falta2018">Falta, R.W., Farhat, S.K., Newell, C.J. and Lynch, K., 2018. A Practical Approach for Modeling Matrix Diffusion Effects in REMChlor. SERDP/ESTCP Project ER-201426 [//www.enviro.wiki/images/0/0b/2018-Falta-REMChlor_Modeling_Matrix_Diffusion_Effects.pdf Report.pdf]   Website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201426</ref> and [https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel REMFuel]<ref name="Falta2012">Falta, R.W., Ahsanuzzaman, A.N., Stacy, M.B., Earle, R.C. and Wilson, J.T., 2012. Remediation Evaluation Model for Fuel Hydrocarbons (REMFuel). Users Manual Version 1.0. U.S. Environmental Protection Agency. EPA/600/R-12/028. [//www.enviro.wiki/images/6/67/2012-Falta-REMFuel_Remediation_Evaluation-Model_for_Fuel_hydrocarbons_users_manual.PDF Report.pdf]   Website: https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel</ref> models are flexible screening tools that allow a simultaneous evaluation of the extent of treatment provided by (1) source removal, (2) ''in situ'' remediation of the contaminated groundwater, or (3) natural attenuation processes in three discrete intervals along the flow path and three discrete time periods. Both [[REMChlor - MD | REMChlor-MD]]<ref name="Falta2018" /> and [https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel REMFuel]<ref name="Falta2012" /> can be downloaded from the internet at no cost. Liang ''et al.''<ref>Liang, H., Falta, R.W., Newell, C.J., Farhat, S.K., Rao, P.S. and Basu, N., 2010. Decision & Management Tools for DNAPL Sites: Optimization of Chlorinated Solvent Source and Plume Remediation Considering Uncertainty. SERDP/ESTCP Project ER-200704. [//www.enviro.wiki/images/c/ce/2010-Liang-Decision_and_Management_Tools_for_DNAPL_sites-ER-200704-FR.pdf Report.pdf]   Project Overview Website: https://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200704/(language)/eng-US</ref> provide a modeling program that uses Monte Carlo simulations to evaluate the effects of the uncertainties in the modeling parameters on the predictions of REMChlor-MD.

==The Transition Assessment Teaching Assistant (TA2) Tool==
[[File:Wilson1w2Fig7.png|thumb|500px| Figure 7. Home Page for TA2 Tool. Users can click on buttons to access various modules that are designed to answer specific questions or research relevant topics.]]
[[File:Wilson1w2Fig8.png|thumb|500px| Figure 8. Example of an asymptote analysis using concentration versus time data in Tool 1 of the TA2 Tool. The source attenuation rate and corresponding remediation timeframe can be estimated for different monitoring periods.]]
A learning and decision-making tool was recently released as part of [https://serdp-estcp.mil/ Strategic Environmental Development and Research Program (SERDP)] Project [https://serdp-estcp.mil/projects/details/350cbc0b-893a-43a6-8a0c-c9c057bacac0/er20-1429-project-overview ER-201429] to help stakeholders gather information for the purposes of a site-specific transition assessment. This free software, the [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Transition Assessment Teaching Assistant (TA2) Tool]<ref name="TATA2024" />, was developed using the elements identified in the 2013 NRC report<ref name="NRC2013" /> as the critical learning objectives for end users.

The Tool is a web-based app that includes a collection of individual modules designed to answer specific questions or research relevant topics (Figure 7). The Tool has been developed as an R Shiny app (version 1.8.0)<ref> Chang, W., Cheng, J., Allaire, J., Sievert, C., Schloerke, B., Xie, Y., Allen, J., McPherson, J., Dipert, A., Borges, B., 2023. shiny: Web Application Framework for R. R package version 1.8.0, https://github.com/rstudio/shiny, https://shiny.posit.co/</ref>, which is an interactive platform using R programming to perform all quantitative functions. The user can then view the results in a simple interface that easily accommodates plots, charts, and various mapping features in a Web browser. The Tool is free and does not require the user to install R software.

The modules within the TA2 Tool include:

'''Five Quantitative Tools''' that focus on assessing asymptotic groundwater concentrations from monitoring data, evaluating plume stability, estimating remediation timeframes after a hypothetical source removal project, forecasting remediation performance if a technology is applied in the field, or projecting concentrations at downgradient points of compliance.

For example, Tool 1 in the TA2 Tool uses concentration versus time data from monitoring wells to estimate attenuation rate constants (''kpoint'') and evaluate if asymptotic conditions are present at particular locations or across the site. This helps to assess whether performance has plateaued at wells where a pump-and-treat system or other active treatment is in place. The user has the option to choose a “change point” within the monitoring record to determine if the attenuation rate has changed over time (e.g., once most of the accessible mass has been removed) (Figure 8). The user can either use visual interpretation to manually select the date when this apparent change occurred or have the date selected automatically using a binary segmentation protocol that is incorporated into the tool. The tool will calculate a rate for both the early period and a rate for the later period (after the change point), and then go through five different lines of evidence for asymptotic behavior (e.g., are the two rates of attenuation significantly different?). The user can then use the collective results as a technical justification demonstrating that the performance of the active remedy has plateaued as the first step in the transition assessment. The tool will also estimate the time to reach a user-specified cleanup goal if the overall attenuation rate (or the attenuation rate in the later period) were to continue.

Another module (Tool 5) focuses on evaluating sites where the concentration goal applies at a downgradient point of compliance, which is a key criterion for sites where MNA is being used as part of a risk-based strategy. The tool includes several different options to estimate a site-specific attenuation rate constant, including data from the pre-remediation period when natural attenuation processes were the sole means for reducing concentrations. Attenuation rate constants are then used to project the concentration versus distance from the contaminant source. Based on the predicted concentration at the downgradient point of compliance, the user can then see if the natural assimilative capacity along the aquifer flow path is sufficient to achieve the concentration goal in the absence of active treatment. For example, in the tab labeled “Use Pre-Remediation Rate Constant”, the logarithms of the concentrations from the period before active treatment began are plotted against the distance from the source well. The slope of the regression line is the rate constant for natural attenuation (including the contributions of degradation and dispersion). This rate constant can then be used to project the concentration moving downgradient from the well of concern after the end of active treatment. Similar approaches are provided within Tool 5 for using rate constants estimated from lab-based testing or derived from post-remediation data (after steady state has been reestablished).

'''Four Qualitative Tools''' provide information on matrix diffusion, enhanced attenuation options, geologic heterogeneity, and related research on transition assessments. Many of these modules are based on the current understanding of the role of matrix diffusion in influencing long-term concentration trends and remedial performance at contaminated groundwater sites. This includes summaries of different modeling options for better quantifying the effects of matrix diffusion. Sites impacted by matrix diffusion are generally challenging to treat using active remedies and thus are better candidates for less intensive management strategies that focus on reducing mass discharge rates, stabilizing the plume, and protecting potential downgradient receptors. As a result, matrix diffusion is critical to understanding and quantifying how natural attenuation processes are contributing to concentration trends.

'''One Summary Tool''' (Tool 10) compiles metrics from the other tools into a “Remediation Transition Assessment Index” (RTAI) and provides additional guidance on conducting site-specific transition assessments. The RTAI is a simple metric with a value from 1 to 5, where higher values reflect greater persistence of contamination due to matrix diffusion and other site-specific factors. An RTAI value is assigned to each of the results from the different tools that have been completed by the user. An RTAI of 5 suggests that the site is a strong candidate for transitioning to MNA or enhanced attenuation approaches, while a site with an RTAI value of 1 is a poor candidate. The user can assign an overall RTAI for the site based on the preponderance of evidence after reviewing the RTAI values generated by each tool, or calculate a site RTAI based on simple averaging, weighting, or other methods.

Tool 10 also contains a flowchart and a checklist for performing site-specific transition assessments that start with evaluating relevant bright line criteria, such as (1) can the concentration goals be met at the point of compliance by MNA; and (2) is the remediation timeframe for MNA reasonable and/or similar to the timeframe if source remediation were used. This checklist ensures that the user has gathered all relevant information that would be needed to support a technically rigorous site-specific Transition Assessment.

The TA2Tool provides a framework for remedial decision makers to evaluate different types of sites, including those where active treatment (e.g., pump and treat) is in use, as well as sites where future active source zone remediation is being considered. It also includes a description of enhanced MNA alternatives for sites where MNA alone may not be sufficient to control risk. As shown in Figure 8, the tool can be used to answer specific questions that have a primarily quantitative basis or to provide focused qualitative information for researching specific topics. Users can engage with just the modules that might be pertinent to assessment of an individual site, or they can go through all the modules to perform a more thorough, step-by-step analysis of the relevant issues for their site.

==Summary==
Tools and approaches are available that can be adapted to determine when a site is ready to transition from active remedy to MNA. However, these tools and approaches have not been applied for this purpose at a significant number of sites, and at the present time, they are not generally accepted by regulatory authorities. There is an opportunity to establish and implement a logical and consistent framework that can be widely implemented to evaluate sites for transition from active remedy to MNA.

==References==
<references />

==See Also==

*[http://dx.doi.org/10.1007/978-1-4614-6922-3 Newell, C.J., Kueper, B.H., Wilson, J.T., Johnson, P.C., 2014. Natural Attenuation of Chlorinated Solvent Source Zones. In: Chlorinated Solvent Source Zone Remediation, Editors: Kueper, B.H., Stroo, H.F., Vogel, C.M., Ward. SERDP ESTCP Environmental Remediation Technology, vol 7. Springer, New York, NY. pgs. 459-508. doi: 10.1007/978-1-4614-6922-3]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436/ER-200436/(language)/eng-US Kram, Mark, and Widdowson, Mark, 2008. Estimating Cleanup Times Associated with Combining Source-Area Remediation with Monitored Natural Attenuation. ESTCP ER-200436]

Biodegradation - Reductive Processes

2026-05-07T17:05:44Z

Admin:

Microbial removal of [[wikipedia: Halogen| halogens]] from [[wikipedia: Organic compound | organic compounds]] by reductive processes forms the basis for many types of bioremediation technologies. The process was discovered within the last several decades and our understanding of how microbes perform this activity has improved significantly. Advances in our understanding of the microbiology of reductive dehalogenation have led to improvements in documenting natural attenuation and implementation of biostimulation and bioaugmentation. As a general rule, bioremediation is a lower cost approach to treatment of halogenated solvents than competing processes based on physical or chemical techniques (e.g., [[Chemical Oxidation (In Situ - ISCO) | chemical oxidation]]). For practitioners, a working knowledge of microbial reductive processes is essential to successful application at hazardous waste sites.

<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s)''':

*[[Bioremediation - Anaerobic]]
*[[Chemical Reduction (In Situ - ISCR)]]
*[[Chlorinated Solvents]]
*[[REMChlor - MD]]

'''Contributor(s):''' [[Dr. David L. Freedman]]

'''Key Resource(s)''':

*[https://doi.org/10.1002/jctb.1567 Enhanced Anaerobic Bioremediation of Chlorinated Solvents: Environmental Factors Influencing Microbial Activity and Their Relevance under Field Conditions]<ref name="Aulenta2006">Aulenta, F., Majone, M. and Tandoi, V., 2006. Enhanced anaerobic bioremediation of chlorinated solvents: environmental factors influencing microbial activity and their relevance under field conditions. Journal of Chemical Technology and Biotechnology, 81(9), 1463-1474. [https://doi.org/10.1002/jctb.1567 doi: 10.1002/jctb.1567]</ref>.
*[https://doi.org/10.1007/978-3-662-49875-0 Organohalide Respiring Bacteria]<ref name="Adrian2016">Adrian, L. and Löffler, F., 2016. Organohalide Respiring Bacteria. 495 pgs. ISBN: 978-3-662-49873-6. [https://doi.org/10.1007/978-3-662-49875-0 doi: 10.1007/978-3-662-49875-0]</ref>

==Introduction==
Organic compounds with one or more halogens attached are referred to as halogenated organics. The halogens include [[wikipedia:Chlorine |chlorine (Cl)]], [[wikipedia: Bromine | bromine (Br)]], [[wikipedia: Fluorine | fluorine (F)]], and [[wikipedia: Iodine | iodine (I)]]. For example, [[wikipedia: Ethylene | ethene]] (C2H4) is an organic compound. When three of the four hydrogen atoms on ethene are replaced with chlorine, the resulting compound is [[wikipedia: Trichloroethylene | trichloroethene]] (TCE; C2HCl3).

The proliferation of halogenated organic compounds in the environment is a consequence of their widespread use in industrial activities. A critical part of many manufacturing processes involves removal of oil and grease from metal, fabrics, and other commodities. Because “like dissolves like,” a common way to remove oil and grease is to soak products in a nonpolar solvent. Non-halogenated hydrocarbons serve this purpose; however, accumulation of hydrocarbon vapors creates the risk of an explosion. Organic chemists solved this problem by adding halogens to the hydrocarbons, rendering them non-flammable. Use of halogenated organic compounds grew dramatically after World War II. With increased use came increased releases to the environment. Initially, halogenated solvents were thought to be inert in the environment and hence not to pose any risk to humans or wildlife. Gradually, the risks associated with chronic human exposure to halogenated solvents were revealed and concern grew about their fate in the environment. Most of the compounds on the United Nations list of persistent organic pollutants (first developed at the Stockholm Convention on Persistent Organic Pollutants) are halogenated organic compounds.

==Dehalogenation==
Microbes possess the ability to remove halogens from halogenated organic compounds. Dehalogenation may occur via a variety of reactions, including oxidation, reduction, or hydrolysis. The halogens are released as halides, i.e., Cl-, Br-, F-, and I-. The process of removing a halogen from a halogenated organic compound by a reductive reaction is referred to as [[wikipedia: Reductive dechlorination | reductive dehalogenation]]. Reductive reactions are ones in which electrons are transferred to the carbon-halogen bond, thereby lowering the oxidation state of the parent compound (example reactions below).

One of the earliest studies (1982) to demonstrate that microbes are capable of removing halogens from halogenated organic compounds used halobenzoates (e.g., 3-chlorobenzoate)<ref>Suflita, J.M., Horowitz, A., Shelton, D.R. and Tiedje, J.M., 1982. Dehalogenation: a novel pathway for the anaerobic biodegradation of haloaromatic compounds. Science, 218(4577), 1115-1117. [https://doi.org/10.1126/science.218.4577.1115 doi: 10.1126/science.218.4577.1115]</ref>. Knowledge about microbial dehalogenation has since grown considerably and now forms the basis for bioremediation, a commonly applied strategy to treat halogenated organic compounds found in soils, groundwater, and wastewater.

==Bioremediation==
As a general rule, bioremediation is a lower cost approach to treatment of halogenated solvents than competing processes based on physical or chemical techniques (e.g., [[Chemical Oxidation (In Situ - ISCO) | chemical oxidation]]). The discovery that microbes are capable of replacing the chlorines on tetrachloroethene (PCE) and TCE, the two most frequently encountered organic contaminants at hazardous waste sites, opened the door to development of current bioremediation strategies. Notably, there was a pause in interest when it was initially believed that the dechlorination process stopped at vinyl chloride (VC), which is more toxic than PCE, TCE, and dichloroethene (DCE) isomers. It was subsequently determined that microbial reduction of VC to ethene occurs<ref>Freedman, D.L. and Gossett, J.M., 1989. Biological reductive dechlorination of tetrachloroethylene and trichloroethylene to ethylene under methanogenic conditions. Applied and Environmental Microbiology, 55(9), 2144-2151. [//www.enviro.wiki/images/9/96/Freedman-1989-Biological_reductive_dechlorination.pdf Report pdf]</ref>. Ethene is an acceptable endpoint since it poses no human health risks at the concentrations typically found in groundwater.

Although humans are responsible for a large influx of halogenated organics into the environment as a consequence of improper handling and disposal practices, it has also come to light that there are natural sources of halogenated organic compounds; nearly 5,000 have been catalogued to date<ref>Gribble, G. W., 2010. Naturally Occurring Organohalogen Compounds - A Comprehensive Update. SpringerWien: New York. [https://doi.org/10.1007/978-3-211-99323-1 doi: 10.1007/978-3-211-99323-1]</ref>. For example, marine algae produce chloromethane as part of a chemical defense system to dissuade predation. Consequently, it is not too surprising that natural processes exist to break down halogenated organic compounds, and those processes have been harnessed to help clean up the excessive amounts released to the environment as a consequence of human activity.

There are several types of reaction pathways that involve reduction and dehalogenation, including [[wikipedia:Hydrogenolysis |hydrogenolysis]] and dihaloelimination<ref>Vogel, T.M., Criddle, C.S. and McCarty, P.L., 1987. ES&T critical reviews: transformations of halogenated aliphatic compounds. Environmental Science & Technology, 21(8), 722-736. [http://dx.doi.org/10.1021/es00162a001 doi:10.1021/es00162a001]</ref>. Here, we detail each of these pathways and the conditions under which each is likely to be prevalent. It should be noted that many of the reactions are also possible via abiotic processes (e.g., via reaction with [[Zerovalent Iron (ZVI) (Chemical Reduction - ISCR) |zerovalent iron (ZVI)]])<ref>Arnold, W.A. and Roberts, A.L., 2000. Pathways and kinetics of chlorinated ethylene and chlorinated acetylene reaction with Fe(0) particles. Environmental Science & Technology, 34(9), 1794-1805. [http://dx.doi.org/10.1021/es990884q doi:10.1021/es990884q]</ref>. The focus of this article is on biotic reductive processes.

==Hydrogenolysis==
[[wikipedia: Hydrogenolysis |Hydrogenolysis]] is the process by which a carbon—halogen bond is broken and hydrogen replaces the halogen substituent, resulting in release of a halide ion (R = organic compound, X = halide):
[[File:Freedman_BRP_EQ1.jpg|center|200 px|]]

The process requires an input of reducing power, represented in the above equation by 2e-, or two electron equivalents. Hydrogenolysis is the most frequently observed reductive pathway, and as such it is commonly referred to as reductive dehalogenation. Nevertheless, other pathways are also reductive and result in dehalogenation, so the more correct description of the above reaction is hydrogenolysis. This descriptor derives in part from the fact that hydrogen (H2) is often a source of the electron equivalents:
[[File:Freedman Article 1 Equation 1.PNG|150px|center|]]

Hydrogenolysis applies to any organohalide, yet it is most commonly associated with removal of chlorine from organic solvents and the term [[wikipedia: Reductive dechlorination | reductive dechlorination]] is often used synonymously (mostly amongst practitioners) to describe this reaction. As mentioned above, this is not quite correct, since other pathways are also reductive and result in removal of chlorine atoms, and the correct chemical mechanism is hydrogenolysis.

Hydrogenolysis applies to numerous categories of compounds; we highlight several of the major categories below. The process typically occurs via a respiratory process referred to as organohalide respiration (described below).

===Chlorinated Ethenes===
When applied to PCE, hydrogenolysis proceeds in 2e- steps through TCE, ''cis''- or ''trans''-1,2- DCE, VC, and ethene, which is also called ethylene (Fig. 1). ''cis''-DCE is the more frequently identified of the 1,2-DCE isomers. Nevertheless, ''trans''-DCE is predominant in some environments<ref>Griffin, B.M., Tiedje, J.M. and Löffler, F.E., 2004. Anaerobic microbial reductive dechlorination of tetrachloroethene to predominately trans-1, 2-dichloroethene. Environmental Science & Technology, 38(16), 4300-4303. [https://doi.org/10.1021/es035439g doi: 10.1021/es035439g]</ref>. 1,1-DCE is another possible dichloroethene isomer, but it is not typically formed during microbial TCE respiration. The presence of 1,1-DCE in the environment is most typically associated with prior contamination by 1,1,1-trichloroethane, which undergoes several types of transformation, including the abiotic process of dehydrohalogenation to 1,1-DCE.

In some locations, further reduction of ethene to ethane has been reported. This is not a dechlorination reaction, but it is important to mention because ethane may be the terminal product from hydrogenolysis of chlorinated ethenes. Monitoring ethane is recommended for establishing a complete assessment of the fate of chlorinated ethenes.

[[File:Freedman_BRP_Fig1.jpg|thumb|center|800 px|Figure 1. Stepwise reduction of PCE and TCE to ethene and ethane. cDCE = cis-1,2-dichloroethene; tDCE = trans-1,2-dichloroethene.]]
The oxidation state of carbon in an organic compound varies from -4 (e.g., in CH4) to +4 (e.g., in CO2). The lower the oxidation state, the easier it is for oxidation to occur, and vice versa.

The oxidation state of the carbon in PCE is +4. With each successive reduction step (via an input of 2e-), the oxidation state of the carbon decreases by 2, so that the carbon in TCE has an oxidation state of +2, DCE has 0, VC has -2, and ethene has -4. For this category of contaminants, the final step is critical from a remediation perspective, since VC is a known human carcinogen while ethene and athnae pose no human health risks at the concentrations typically found in groundwater.

===Chlorinated Ethanes===
Like chlorinated ethenes, chlorinated ethanes are reduced by hydrogenolysis. One of the most widely evaluated compounds is [[wikipedia:1,1,1-Trichloroethane|1,1,1-trichloroethane]], which undergoes sequential reduction to [[wikipedia: 1,1-Dichloroethane|1,1-dichloroethane]] and [[wikipedia: Chloroethane| chloroethane]]. Although further reduction to ethane is possible, it is a much slower reaction and chloroethane is typically regarded as the terminal product. This example serves to illustrate that hydrogenolysis does not always yield complete dechlorination.

Other commonly encountered chlorinated ethanes undergo hydrogenolysis, including reduction of [[wikipedia:1,2-Dichloroethane| 1,2-dichloroethane]] to chloroethane and [[wikipedia:1,2-Dichloropropane|1,2-dichloropropane]] to 1- or 2-chloropropane.

===Chlorinated Methanes===
[[wikipedia: Carbon tetrachloride|Carbon tetrachloride]] (tetrachloromethane) undergoes hydrogenolysis to [[wikipedia: Chloroform| chloroform]] (trichloromethane) and then methylene chloride (dichloromethane). These reactions are also catalyzed by reduced iron, which may be generated by iron-reducing bacteria.

Further reduction to chloromethane and methane is not commonly observed; other anaerobic biodegradation processes, such as organohalide fermentation, are more significant for dichloromethane and chloromethane.

===Chlorinated Aromatic Compounds===
Hydrogenolysis also occurs for chlorinated aromatic compounds, including chlorinated benzenes, [[wikipedia: Polychlorinated biphenyl| polychlorinated biphenyls (PCBs)]], chlorinated dioxins (e.g., 2,3,7,8-tetrachlorodibenzo-''p''-dioxin, or TCDD) and furans, and polychlorinated phenols (e.g. pentachlorophenol, or PCP). For these compounds, the pathways are more complicated, since reduction proceeds through multiple isomers, depending on which position on the ring that each specific chlorine atom is removed. Like chlorinated ethanes and methanes, hydrogenolysis of chlorinated aromatic compounds is rarely complete, and the rate of reduction often decreases with a decreasing number of chlorine-carbon bonds. An important exception has been identification of cultures that reduce chlorinated benzenes completely to benzene<ref>Fung, J.M., Weisenstein, B.P., Mack, E.E., Vidumsky, J.E., Ei, T.A. and Zinder, S.H., 2009. Reductive dehalogenation of dichlorobenzenes and monochlorobenzene to benzene in microcosms. Environmental Science & Technology, 43(7), 2302-2307. [https://doi.org/10.1021/es802131d doi: 10.1021/es802131d]</ref><ref>Nelson, J.L., Fung, J.M., Cadillo-Quiroz, H., Cheng, X. and Zinder, S.H., 2011. A role for ''Dehalobacter'' spp. in the reductive dehalogenation of dichlorobenzenes and monochlorobenzene. Environmental Science & Technology, 45(16), 6806-6813. [https://doi.org/10.1021/es200480k doi: 10.1021/es200480k]</ref>.

===Bromo- and Fluoro-Organic Compounds===
Hydrogenolysis of brominated organic compounds has been documented. Examples include reduction of 1,2-dibromoethane (ethylene dibromide) to bromoethane and reduction of polybrominated diphenyl ethers. Likewise, defluorination also occurs. For example, reduction of vinyl fluoride to ethene has been reported. Nevertheless, reductive defluorination is characterized by slow rates, if it occurs at all. This is consistent with the general expectation that when the rate-limiting step for a reaction is cleavage of the carbon halogen bond, the order of reactivity is:

<div align="center">C—I > C—Br > C—Cl > C—F<ref>Wackett, L.P., Logan, M.S., Blocki, F.A. and Bao-Li, C., 1992. A mechanistic perspective on bacterial metabolism of chlorinated methanes. Biodegradation, 3(1), 19-36. [https://doi.org/10.1007/bf00189633 doi: 10.1007/BF00189633]</ref></div>

For example, hydrogenolysis of trichlorofluoromethane (CFC-11) typically results in accumulation of dichloro- and chlorofluoro-methane; further hydrogenolysis occurs at a much slower rate, if at all. Microbial reductive defluorination of perflurooctanoic acid (PFOA; used in the manufacture of Teflon) has not been reported to any appreciable extent<ref>Liou, J.C., Szostek, B., DeRito, C.M. and Madsen, E.L., 2010. Investigating the biodegradability of perfluorooctanoic acid. Chemosphere, 80(2), 176-183. [http://dx.doi.org/10.1016/j.chemosphere.2010.03.009 doi: 10.1016/j.chemosphere.2010.03.009]</ref>.

==Dihaloelimination==
Dihaloelimination is the process by which two groups, e.g., a hydrogen and chlorine, are removed from adjacent carbon atoms, resulting in the formation of a double bond and release of two halide ions (X- = halide.):

[[File:Freedman A 1 Fig 3.PNG|center]]

Although less frequently encountered than hydrogenolysis, dihaloelimination is a critical pathway for several common groundwater contaminants, including reduction of 1,2-dichloroethane and 1,2-dibromoethane (more commonly referred to as ethylene dibromide, or EDB) to ethene<ref>Yu, R., Peethambaram, H.S., Falta, R.W., Verce, M.F., Henderson, J.K., Bagwell, C.E., Brigmon, R.L. and Freedman, D.L., 2013. Kinetics of 1,2-dichloroethane and 1,2-dibromoethane biodegradation in anaerobic enrichment cultures. Applied and Environmental Microbiology, 79(4), 1359-1367. [https://doi.org/10.1128/aem.02163-12 doi: 10.1128/AEM.02163-12]</ref>:

[[File:Freedman A 1 Fig 4.PNG|center|]]

In this example, the oxidation state of the carbon decreases from -2 in 1,2-dibromoethane to -4 in ethene, i.e., by 2 electrons. Thus, the process is both reductive and results in removal of halides. Other examples of dihaloelimination include reduction of 1- or 2-chloropropane to propene.

==Organohalide Respiration==
A variety of microbes have developed pathways to conserve the energy made available when breaking carbon-halogen bonds via reduction (i.e., hydrogenolysis and dihaloelimination), by using the halogenated organic compounds as terminal electron acceptors<ref name="Adrian2016" />. This has led to the notion that certain microbes are capable of “breathing” halogenated organics, analogous to respiration involving oxygen as the electron acceptor<ref name="McCarty1997">McCarty, P.L., 1997. Breathing with chlorinated solvents. Science, 276(5318), 1521-1522. [https://doi.org/10.1126/science.276.5318.1521 doi: 10.1126/science.276.5318.1521]</ref>. When halogenated organic compounds serve as terminal electron acceptors, the process is referred to as organohalide respiration. The discovery of this process adds to the extensive list of terminal electron acceptors that microbes are capable of exploiting. Notably, a few types of microbes are obligate halorespirers, meaning the only known terminal electron acceptors for the cells are halogenated organic compounds. Other types of microbes are facultative with respect to their use of halogenated organics as terminal electron acceptors.

Under some circumstances, microbes carry out reductive dehalogenation but are unable to conserve energy from the process. For example, several strains of microbes are able to use PCE, TCE and ''cis''-DCE as terminal electron acceptors, whereas reduction of VC to ethene is not a growth-linked respiratory process<ref>Maymó-Gatell, X., Anguish, T. and Zinder, S.H., 1999. Reductive dechlorination of chlorinated ethenes and 1, 2-dichloroethane by ''Dehalococcoides ethenogenes'' 195. Applied and Environmental Microbiology, 65(7), 3108-3113. [//www.enviro.wiki/images/2/22/Maymo-Gatell-1999-Reductive_dechlorination.pdf Report pdf]</ref><ref>Maymó-Gatell, X., Nijenhuis, I. and Zinder, S.H., 2001. Reductive dechlorination of cis-1,2-dichloroethene and vinyl chloride by ''Dehalococcoides ethenogenes''. Environmental Science & Technology, 35(3), 516-521. [https://doi.org/10.1021/es001285i doi: 10.1021/es001285i]</ref>. These cultures are able to reduce VC to ethene when growing with the other chlorinated ethenes as electron acceptors, but when provided with only VC, they are unable to grow. Under these circumstances, the transformation of VC to ethene is referred to as cometabolic, i.e., the transformation process is not linked to growth. In general, organohalide respiration occurs at a higher rate than reduction via cometabolism. Some microbes are capable of reducing VC to ethene via respiration, others are not.

Organohalide respiration appears to be a widely-distributed process in nature, even in some environments that have not previously been contaminated by human activity<ref>Krzmarzick, M.J., Crary, B.B., Harding, J.J., Oyerinde, O.O., Leri, A.C., Myneni, S.C. and Novak, P.J., 2012. Natural niche for organohalide-respiring ''Chloroflexi''. Applied and Environmental Microbiology, 78(2), 393-401. [https://doi.org/10.1128/aem.06510-11 doi: 10.1128/AEM.06510-11]</ref>. The widespread distribution of organohalide respiring microbes is likely related to the natural formation of halogenated compounds, which have been generated on the planet long before human activity increased the rate and amount of halogenated compounds released to the environment. Having the capacity to dehalogenate is essential in natural systems in which organohalide compounds are also synthesized.

The process of organohalide respiration is centered on reductive dehalogenases, an iron–sulfur and coronoid containing family of enzymes that break carbon-halogen bonds. Among the best characterized genes are the ones involved in reductive dehalogenation of PCE, TCE, ''cis''-DCE, and VC. Several microbes and enzymes are involved in each reduction step (Fig. 2).

[[File:Freedman_BRP_Fig2.jpg|700 px|thumb|center|Figure 2. Key genes in hydrogenolysis of chlorinated ethenes; M = metabolic, C = cometabolic]]

The identification of dehalogenases has progressed to the point that quantification of key genes (e.g., ''tceA'', ''bvcA'', ''vcrA'', and others) in environmental samples is now a routine part of assessing the capacity for reductive dehalogenation to occur in the environment.

[[File:Freedman Article 1 Figure 6.PNG|thumb|500 px|left|Figure 3. Schematic representation of a microbial cell carrying out organohalide respiration. Blue shape = the cell membrane; red oval = hydrogenase; yellow oval = electron carrier and proton translocation; orange oval = reductive dehalogenase; green shape = ATP synthase (modified from Jugder et al. (2016)<ref name="Jugder2016" />).]]
Figure 3 presents a simplified schematic for how energy may be conserved during organohalide respiration. Many details of the process still need to be resolved and likely vary among the growing list of organohalide respiring microbes<ref name="Jugder2016">Jugder, B.E., Ertan, H., Bohl, S., Lee, M., Marquis, C.P. and Manefield, M., 2016. Organohalide Respiring Bacteria and Reductive Dehalogenases: Key Tools in Organohalide Bioremediation. Frontiers in Microbiology, 7. [https://doi.org/10.3389/fmicb.2016.00249 doi: 10.3389/fmicb.2016.00249]</ref>. Reductive dehalogenases are a key component of the respiratory chain, which in the example shown culminates in development of a proton motive force and subsequent synthesis of adenosine triphosphate (ATP).

Among the various microbes capable of organohalide respiration, ''Dehalococcoides'' are the most frequently mentioned because of their capacity for complete reduction of chlorinated ethenes (i.e., PCE, TCE, DCEs, and VC) to ethene<ref>Löffler, F.E., Yan, J., Ritalahti, K.M., Adrian, L., Edwards, E.A., Konstantinidis, K.T., Müller, J.A., Fullerton, H., Zinder, S.H. and Spormann, A.M., 2013. Dehalococcoides mccartyi gen. nov., sp. nov., obligately organohalide-respiring anaerobic bacteria relevant to halogen cycling and bioremediation, belong to a novel bacterial class, Dehalococcoidia classis nov., order Dehalococcoidales ord. nov. and family Dehalococcoidaceae fam. nov., within the phylum Chloroflexi. International Journal of Systematic and Evolutionary Microbiology, 63(2), 625-635. [https://doi.org/10.1099/ijs.0.034926-0 doi: 10.1099/ijs.0.034926-0]</ref>. At this point, members of this genus are the only known that are capable of completely dechlorinating the chlorinated ethenes to ethene, and more specifically, the steps from ''cis''-DCE to VC, and VC to ethene. Their versatility extends to use of many other organohalides as terminal electron acceptors, including polychlorinated biphenyls, chlorinated ethanes, and chlorinated benzenes. ''Dehalococcoides'' use only hydrogen as an electron donor and acetate as a carbon source. Their limited electron donor use stands in contrast to other organohalide respiring microbes, which are able to use a variety of organic compounds as electron donors. Other key types of organohalide respiring microbes (that cannot generate ethene) include ''Dehalobacter'', ''Dehalogenimonas'', ''Desulfitobacterium'', ''Sulfurospirillum'', and a number of ''Deltaproteobacteria''<ref name="Adrian2016" /> 2. ''Dehalobacter'' includes microbes that respire chlorinated ethanes<ref>Sun, B., Griffin, B.M., Ayala-del-Rı́o, H.L., Hashsham, S.A. and Tiedje, J.M., 2002. Microbial dehalorespiration with 1, 1, 1-trichloroethane. Science, 298(5595), 1023-1025. [https://doi.org/10.1126/science.1074675 doi: 10.1126/science.1074675]</ref>, chlorinated ethenes<ref>Holliger, C., Hahn, D., Harmsen, H., Ludwig, W., Schumacher, W., Tindall, B., Vazquez, F., Weiss, N. and Zehnder, A.J., 1998. Dehalobacter restrictus gen. nov. and sp. nov., a strictly anaerobic bacterium that reductively dechlorinates tetra-and trichloroethene in an anaerobic respiration. Archives of Microbiology, 169(4), 313-321. [https://doi.org/10.1007/s002030050577 doi: 10.1007/s002030050577]</ref>, and chloroform<ref>Tang, S., Wang, P.H., Higgins, S.A., Löffler, F.E. and Edwards, E.A., 2016. Sister ''Dehalobacter'' Genomes reveal specialization in organohalide respiration and recent strain differentiation likely driven by chlorinated substrates. Frontiers in Microbiology, 7. [https://doi.org/10.3389/fmicb.2016.00100 doi: 10.3389/fmicb.2016.00100]</ref>.

==Environmental Conditions==
With few exceptions, reductive dehalogenation occurs under anoxic conditions<ref name="Adrian2016" />. With the exception of nitrate, reductive dehalogenation has been observed in the presence of other anaerobic terminal electron acceptors, including ferric iron and sulfate. The effect of iron and sulfate on the rate and extent of reductive dechlorination is a matter of some debate, with some observing that these compounds (or the reduced forms) are inhibitory (e.g., via competition for hydrogen or the toxicity of sulfide) while others have shown that iron reduction is beneficial to the process<ref name="Aulenta2006" /><ref>Wei, N. and Finneran, K.T., 2011. Influence of ferric iron on complete dechlorination of trichloroethylene (TCE) to ethene: Fe (III) reduction does not always inhibit complete dechlorination. Environmental Science & Technology, 45(17), 7422-7430. [https://doi.org/10.1021/es201501a doi 10.1021/es201501a]</ref>. An environment in which a community of anaerobes produces an excess of cobalamin (vitamin B12) is beneficial, since this coenzyme is an essential component of several dehalogenases<ref>Yan, J., Im, J., Yang, Y. and Löffler, F.E., 2013. Guided cobalamin biosynthesis supports ''Dehalococcoides mccartyi'' reductive dechlorination activity. Phil. Trans. R. Soc. B, 368(1616), 20120320. [https://doi.org/10.1098/rstb.2012.0320 doi: 10.1098/rstb.2012.0320]</ref>. Halogenated compounds can also be reduced in the presence of methanogens; however, methanogens compete for hydrogen as an electron donor and may at times limit the dechlorination reactions.

Circumneutral pH is considered to be optimum for complete reduction of chlorinated ethenes to ethene<ref>Robinson, C., Barry, D.A., McCarty, P.L., Gerhard, J.I. and Kouznetsova, I, 2009. pH control for enhanced reductive bioremediation of chlorinated solvent source zones. Science of the Total Environment, 407(16), 4560-4573. [https://doi.org/10.1016/j.scitotenv.2009.03.029 doi 10.1016/j.scitotenv.2009.03.029]</ref><ref>Vainberg, S., Condee, C.W. and Steffan, R.J., 2009. Large-scale production of bacterial consortia for remediation of chlorinated solvent-contaminated groundwater. Journal of Industrial Microbiology & Biotechnology, 36(9), 1189-1197. [https://doi.org/10.1007/s10295-009-0600-5 doi: 10.1007/s10295-009-0600-5]</ref>. However, organohalide respiring microbes other than ''Dehalococcoides'' tolerate lower pH levels (e.g., as low as 4). There is growing evidence to indicate that strains of ''Dehalococcoides'' exist that are also tolerant of pH levels below circumneutral. For example, the pH range for a commonly used bioaugmentation culture that includes ''Dehalococcoides'' is 5.8-6.3 ([http://siremlab.com/kb-1-kb-1-plus/ SIREM]). This is an important consideration for bioaugmentation in low pH aquifers, since there are significant challenges associates with adjusting groundwater pH.

==Significance==
Our understanding of the microbial processes that result in reductive removal of halogens from halogenated organic compounds has grown remarkably over the past three decades. The field has advanced from an assumption that halogenated organic compounds are non-biodegradable to our current understanding that not only do microbes perform dehalogenation reactions, but many do so via a growth-linked reductive, respiratory process (i.e. “breathing with chlorinated solvents”<ref name="McCarty1997" />). This underlying science has formed the basis for the practice of bioremediation, which has revolutionized the options available for cleaning up hazardous waste sites. Exciting new discoveries await that will open the door to biological treatment of emerging contaminants, as well as improvements in how to treat halogenated organics at complex sites where there are often mixtures of contaminants.

==References==

<references />

==See Also==

*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1167 Aerobic and Anaerobic Transformation of cis-DCE and VC: Steps for Reliable Remediation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1168 Characterization of the Aerobic Oxidation of cis-DCE and VC in Support of Bioremediation of Chloroethene-Contaminated Sites]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1169 Factors Affecting cis-DCE and VC Biological Transformation under Anaerobic Conditions]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1556 Characterization of Microbes Capable of Using Vinyl Chloride as a Sole Carbon and Energy Source by Anaerobic Oxidation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1557 Elucidation of the Mechanisms and Environmental Relevance of cis-Dichloroethene and Vinyl Chloride Biodegradation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1558 Microbial Dichloroethene and Vinyl Chloride Oxidation and the Fate of Ethene and Ethane Under Anoxic Conditions]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-199921 Push-Pull Tests for Evaluating the In-Situ Aerobic Treatment of Chlorinated Mixtures in Groundwater]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200516 Enhancing Natural Attenuation through Bioaugmentation with Aerobic Bacteria that Degrade cis-1,2-Dichloroethene]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201026/ER-201026 Incorporating Aerobic Processes into Remedies for Large Chlorinated Solvent Plumes]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200316 Enhanced Oxidative Bioremediation of cis-Dichloroethene and Vinyl Chloride Using Electron Shuttles]
*[https://www.coursera.org/learn/natural-attenuation-of-groundwater-contaminants/lecture/6UJTE/biodegradation-mechanisms-chlorinated-solvents-vs-hydrocarbons Online Lecture Course - Chlorinated Solvents Biodegradation]

Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions

2026-05-07T17:04:31Z

Admin:

The U.S. Department of Defense (DoD) faces many challenges in restoring aquifers at contaminated sites, often due to back-diffusion of tetrachloroethene (PCE) and trichloroethene (TCE) from low-permeability clay zones. The uptake, storage, and subsequent long-term release of these dissolved contaminants from clays are key processes in understanding the longevity, intensity, and risks associated with many persistent chlorinated ethene groundwater plumes. Although naturally occurring abiotic and biotic dechlorination processes in clays may reduce stored contaminant mass and significantly aid natural attenuation, no standardized field method currently exists to verify or quantify these reactions. It is critical to remediation design efforts to demonstrate and validate a cost-effective ''in situ'' approach for assessing these dechlorination processes using first-order rate constants. An approach was developed and applied across eight DoD sites to support Remedial Project Managers (RPMs) and regulators in evaluating natural attenuation potential in clay-rich environments.
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'''Related Article(s):'''

*[[Chlorinated Solvents]]
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| Monitored Natural Attenuation of Chlorinated Solvents]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
*[[Matrix Diffusion]]
*[[REMChlor - MD]]

'''Contributor(s):''' [[Dani Tran]], [[Dr. Charles Schaefer]], and [[Dr. Charles Werth]]

'''Key Resource(s):'''
*Schaefer, C.E, Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils<ref name="SchaeferEtAl2025"/>

==Introduction==
Cost-effective methods are needed to verify the occurrence of natural dechlorination processes and quantify their dechlorination rates in clays under ambient in situ conditions in order to reliably predict their long-term influence on plume longevity and mass discharge. However, accurately determining these rates is challenging due to slow reaction kinetics, the transient nature of transformation products, and the interplay of biotic and abiotic mechanisms within the clay matrix or at clay-sand interfaces. Tools capable of quantifying these reactions and assessing their role in mitigating plume persistence would be a significant aid for long-term site management.

For reductive abiotic dechlorination under anoxic conditions, a 1% hydrochloric acid (HCl) extraction of a sample of native clay coupled with X-ray diffraction (XRD) data can be used as a screening level tool to estimate reductive dechlorination rate constants. These rate constants can be inserted into fate and transport models such as [[REMChlor - MD]]<ref>Falta, R., and Wang, W., 2017. A semi-analytical method for simulating matrix diffusion in numerical transport models. Journal of Contaminant Hydrology, 197, pp. 39-49. [https://doi.org/10.1016/j.jconhyd.2016.12.007 doi: 10.1016/j.jconhyd.2016.12.007]  [[Media: FaltaWang2017.pdf | Open Access Manuscript]]</ref><ref>Kulkarni, P.R., Adamson, D.T., Popovic, J., Newell, C.J., 2022. Modeling a well-charactized perfluorooctane sulfate (PFOS) source and plume using the REMChlor-MD model to account for matrix diffusion. Journal of Contaminant Hydrology, 247, Article 103986. [https://doi.org/10.1016/j.jconhyd.2022.103986 doi: 10.1016/j.jconhyd.2022.103986]  [[Media: KulkarniEtAl2022.pdf | Open Access Manuscript]]</ref> to quantify abiotic dechlorination impacts within clay aquitards on chlorinated solvent plumes. Thus, determination of the abiotic reductive dechlorination rate constant for a particular clayey soil can be readily utilized to provide a more accurate assessment of aquifer cleanup timeframes for groundwater plumes that are being sustained by contaminant back-diffusion.

==Recommended Approach==
[[File: TranFig1.png | thumb | 500 px | Figure 1: First-order rate constants for abiotic reductive dechlorination of TCE under anaerobic conditions. Circles are data from Schaefer ''et al.'', 2021<ref>Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2021. Abiotic dechlorination in the presence of ferrous minerals. Journal of Contaminant Hydrology, 241, 103839. [https://doi.org/10.1016/j.jconhyd.2021.103839 doi: 10.1016/j.jconhyd.2021.103839]  [[Media: SchaeferEtAl2021.pdf | Open Access Manuscript]]</ref>, filled squares from Schaefer ''et al.'', 2018<ref name="SchaeferEtAl2018"/>, and Schaefer ''et al.'', 2017<ref>Schaefer, C.E., Ho., Gurr, C., Berns, E., Werth, C., 2017. Abiotic dechlorination of chlorinated ethenes in natural clayey soils: impacts of mineralogy and temperature. Journal of Contaminant Hydrology, 206, pp. 10-17. [https://doi.org/10.1016/j.jconhyd.2017.09.007 doi: 10.1016/j.jconhyd.2017.09.007]  [[Media: SchaeferEtAl2017.pdf | Open Access Manuscript]]</ref>, and open squares from Schaefer ''et al.'', 2025<ref name="SchaeferEtAl2025"/>. ]]
[[File: TranFig2.png | thumb | 600 px | Figure 2: Flowchart diagram of field screening procedures]]
The recommended approach builds upon the methodology and findings of a recent study<ref name="SchaeferEtAl2025">Schaefer, C.E., Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils. Groundwater Monitoring and Remediation, 45(2), pp. 31-39. [https://doi.org/10.1111/gwmr.12709 doi: 10.1111/gwmr.12709]</ref>, emphasizing field-based and analytical techniques to quantify abiotic first-order reductive dechlorination rate constants for PCE and TCE in clayey soils under anoxic conditions. Key components of this evaluation are listed below:
#Zone Identification: The focus of the investigation should be to delineate clayey zones adjacent to hydraulically conductive zones.
#Ferrous Mineral Quantification: Assess ferrous mineral context in clay via 1% HCl extraction at ambient temperature over a 10-minute interval.
#Mineralogical Characterization: Conduct XRD analysis with the specific intent of identifying the presence of pyrite and biotite.
#Reduced Gas Analysis: Measurement of reduced gases such as acetylene, ethene, and ethane concentrations in clay samples. Gas-tight sampling devices (e.g., En Core® soil samplers by En Novative Technologies, Inc.) should be used to ensure sample integrity during collection and transport.

Clay samples should be collected within a few centimeters of the high-permeability interface, with optional additional sampling further inward. For mineralogical analysis, a defined interval may be collected and subsequently subsampled. To preserve sample integrity, exposure to air should be minimized during collection, transport, and handling. Homogenization should occur within an anaerobic chamber, and if subsamples are required for external analysis, they must be shipped in gas-tight, anaerobic containers.

Estimation of the abiotic reductive first-order rate constant for PCE and TCE is based on the “reactive” ferrous content in the clay. Reactive ferrous content (Fe(II)r) is estimated as shown in Equation 1:

::'''Equation 1:'''       <big>''Fe(II)r = DA + XRDpyr - XRDbiotite''</big>

where ''DA'' is the ferrous content from the dilute acid (1% HCl) extraction, ''XRDpyr'' is the pyrite content from XRD analysis, and ''XRDbiotite'' is the biotite content from XRD analysis<ref name="SchaeferEtAl2025"/>.

Abiotic dechlorination is unlikely to contribute to mitigating contaminant back-diffusion when reactive ferrous iron (Fe(II)r) concentrations are below 100 mg/kg (Figure 1). For Fe(II)r above 100 mg/kg, the first-order rate constant for PCE and TCE reductive dechlorination can be estimated using the correlation shown in Figure 1<ref name="SchaeferEtAl2018">Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2018. Mechanisms for abiotic dechlorination of trichloroethene by ferrous minerals under oxic and anoxic conditions in natural sediments. Environmental Science and Technology, 52(23), pp.13747-13755. [https://doi.org/10.1021/acs.est.8b04108 doi: 10.1021/acs.est.8b04108]</ref><ref>Borden, R.C., Cha, K.Y., 2021. Evaluating the impact of back diffusion on groundwater cleanup time. Journal of Contaminant Hydrology, 243, Article 103889. [https://doi.org/10.1016/j.jconhyd.2021.103889 doi: 10.1016/j.jconhyd.2021]  [[Media: BordenCha2021.pdf | Open Access Manuscript]]</ref>. The rate constant exhibits a strong positive correlation with the logarithm of reactive Fe(II) content (Pearson’s ''r'' = 0.82), with a slope of 4.7 × 10⁻⁸ L g⁻¹ d⁻¹ (log mg kg⁻¹)⁻¹.

Figure 2 presents a decision flowchart designed to evaluate the significance and extent of abiotic reductive dechlorination. By applying Equation 1 to the dilute acid extractable Fe(II) plus measured mineral species data from clay samples, the reactive ferrous iron content (Fe(II)r) can be quantified, enabling a streamlined assessment of the extent to which abiotic processes are contributing to the mitigation of contaminant back-diffusion.

If Fe(II)r is ≥ 100 mg/kg, a first-order dechlorination rate constant can be estimated and subsequently used within a contaminant fate and transport model. However, if acetylene is detected in the clay, even with Fe(II)r less than 100 mg/kg, then bench-scale testing using methods similar to those described in a recent study<ref name="SchaeferEtAl2025"/> is recommended, as such results would likely be inconsistent with those shown in Figure 1, suggesting some other mechanism might be involved, or that the system mineralogy might be more complex than anticipated. Even if Fe(II)r ≥ 100 mg/kg, confirmatory bench-scale testing may be conducted for additional verification and to refine estimation of the abiotic dechlorination rate constant.

==Summary and Recommendations==
The approach outlined above is intended to serve as a generalized guide for practitioners and site managers to cost-effectively determine the extent to which beneficial abiotic reductive dechlorination reactions are likely occurring in low permeability (e.g., clayey) zones. This approach may be contraindicated if co-contaminants are present. It is currently unclear whether other classes of potentially reactive chemicals, such as trinitrotoluene (TNT) or chlorinated ethanes, could interact competitively with PCE and TCE.

In addition, it remains unclear how other classes of compounds such as per- and polyfluoroalkyl substances (PFAS) may interact or sorb with ferrous minerals and potentially inhibit abiotic dechlorination reactions. Coupling these recommended activities with conventional site investigation tasks would provide an opportunity to perform many of the up-front screening activities with minimal additional project costs. It is important to note that the guidance proposed herein pertains to particularly low permeability media. Sites with complex or varying lithology, where the mineralogy and/or redox conditions may vary, might require evaluation of multiple samples to provide appropriate site-wide information.
 

==References==
<references />

==See Also==
*[https://serdp-estcp.mil/projects/details/a7e3f7b5-ed82-4591-adaa-6196ff33dd60 ESTCP Project ER20-5031 – In Situ Verification and Quantification of Naturally Occurring Dechlorination Rates in Clays: Demonstrating Processes that Mitigate Back-Diffusion and Plume Persistence]

REMChlor - MD

2026-05-07T17:04:06Z

Admin:

REMChlor-MD is a free toolkit available for download and is capable of simulating matrix diffusion in groundwater contaminant plumes. It is a significant upgrade from REMChlor 1.0<ref>Falta, R.W., 2008. Methodology for comparing source and plume remediation alternatives. Groundwater, 46(2), pp.272-285. [https://doi.org/10.1111/j.1745-6584.2007.00416.x doi: 10.1111/j.1745-6584.2007.00416.x]</ref>, which does not include matrix diffusion in the plume. REMChlor-MD is useful for planning-level approximations of contamination extent and duration at sites where matrix diffusion is important. REMChlor-MD employes a semi-analytical method for simulating mass transfer between high and low permeability zones that provides computationally accurate predictions of concentration distributions and mass discharge in the higher permeability portions of an aquifer. Model run times for REMChlor-MD are much shorter than traditional fine grid numerical models.

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'''Related Article(s)''':

*[[Dispersion and Diffusion]]
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Matrix Diffusion]]
*[[Plume Response Modeling]]
*[[Source Zone Modeling]]

'''Contributor(s):''' [[Dr. Ron Falta]] and [[Kien Pham]]

'''Key Resource(s)''':

*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201426 REMChlor-MD Toolkit package]

==Matrix Diffusion of Dissolved Contaminants==
[[File:Falta1w2 Fig1.png|thumb|450px|Figure 1. Spread of CVOCs in the Subsurface]]
Dissolved contaminants, such as [[Chlorinated Solvents | chlorinated volatile organic compounds (CVOCs)]], are found in groundwater at many current and former industrial sites globally. Plumes of CVOCs dissolved in groundwater often originate when the chemicals enter the subsurface as dense non-aqueous phase liquids (DNAPL). Being heavier than water and immiscible, DNAPLs are particularly problematic because they can spread vertically into the aquifer, generating extensive contaminated groundwater plumes. In Figure 1, CVOCs in DNAPL form are shown in bold red, and the resulting dissolved groundwater plume is represented by the pink halo emanating from the DNAPLs. One of the most significant lessons learned about chlorinated solvents is that their groundwater plumes can persist even when the source DNAPLs have been depleted. Plume persistence in the absence of source materials has been attributed to a phenomenon called matrix diffusion. Matrix diffusion can affect any type of dissolved contaminant, particularly when dissolved concentrations are high relative to regulatory standards. In addition to CVOCs, matrix diffusion can occur with other dissolved contaminants including radionuclides, [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | per- and polyfluoroalkyl substances (PFAS)]], dissolved petroleum hydrocarbons such as benzene, toluene, ethylbenzene, and xylene (BTEX), and with solvent stabilizers such as [[1,4-Dioxane | 1,4-dioxane]].

The life cycle of a matrix diffusion site is separated into two periods. In the loading period, the [[Characterization Methods – Hydraulic Conductivity | low-permeability (low K)]] zones act as storage areas for the contaminant mass. Following clean-up actions in the transmissive (high K) zones, the concentration gradient is reversed. The unloading period begins when the contaminant mass previously stored in the low-permeability zones diffuses back out into the transmissive zones. In Figure 1, the blue arrows represent the forward and backward diffusion gradients into the surrounding materials. Because the pace of back diffusion can be significantly slower than the loading of the contaminant, the plume resulting from the back diffusing mass can last for decades to centuries<ref>Chapman, S.W. and Parker, B.L., 2005. Plume persistence due to aquitard back diffusion following dense nonaqueous phase liquid source removal or isolation. Water Resources Research, 41(12). [https://doi.org/10.1029/2005WR004224 doi: 10.1029/2005WR004224]</ref><ref>Parker, B.L., Chapman, S.W. and Guilbeault, M.A., 2008. Plume persistence caused by back diffusion from thin clay layers in a sand aquifer following TCE source-zone hydraulic isolation. Journal of Contaminant Hydrology, 102(1-2), pp.86-104. [https://doi.org/10.1016/j.jconhyd.2008.07.003 doi: 10.1016/j.jconhyd.2008.07.003]</ref>.

Given the prevalence and persistence of back diffusion plumes, modeling tools are needed that can accurately capture the dynamic progression of the contamination in both loading and unloading periods.

==Matrix Diffusion Modeling Options==
[[File:Falta1w2 Fig2.PNG|thumb|450px|Figure 2. A Typical Fine Grid Model Capable of Simulating Matrix Diffusion]]
There have been limited options available for simulating matrix diffusion effects at groundwater contamination sites. Available analytical models require simplifications that may overlook the unique complexities at actual remediation sites. Numerical models are often used to simulate solute transport, but require fine discretization to capture matrix diffusion concentration gradients, which occur at millimeters to centimeters scale. This very fine discretization (fine model grid) greatly increases the number of model cells required to simulate transport at field sites, greatly increasing computer model run times. Figure 2 shows a typical fine grid model containing almost 3 million gridblocks (cells).

Another type of numerical model of matrix diffusion is the dual-porosity model which is more computationally efficient because it uses a first order approximation to describe the mass transfer between the high K and low K zones. Dual-porosity models can be calibrated to match the effects of matrix diffusion on contaminant concentrations over short time periods. However, the first order mass transfer coefficient is time-dependent<ref>Guan, J., Molz, F.J., Zhou, Q., Liu, H.H. and Zheng, C., 2008. Behavior of the mass transfer coefficient during the MADE‐2 experiment: New insights. Water Resources Research, 44(2). [https://doi.org/10.1029/2007WR006120 doi: 10.1029/2007WR006120]</ref>, so models calibrated to early time data will not accurately simulate concentrations at a later time.

The REMChlor-MD model was developed under ESTCP project [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201426 ER-201426] to provide a practical alternative to previous approaches for modeling matrix diffusion.

==REMChlor-MD’s Modeling Approach==
REMChlor-MD employs a semi-analytical method adapted from a heat diffusivity estimation strategy used in petroleum engineering and applied here to the problem of chemical diffusion with first order decay kinetics<ref>Vinsome, P.K.W. and Westerveld, J., 1980. A simple method for predicting cap and base rock heat losses in'thermal reservoir simulators. Journal of Canadian Petroleum Technology, 19(03). Pp 87-90 [https://doi.org/10.2118/80-03-04 doi: 10.2118/80-03-04]</ref><ref name="Falta2017">Falta, R.W. and Wang, W., 2017. A semi-analytical method for simulating matrix diffusion in numerical transport models. Journal of contaminant hydrology, 197, pp.39-49. [https://doi.org/10.1016/j.jconhyd.2016.12.007 doi: 10.1016/j.jconhyd.2016.12.007]</ref>. This semi-analytical method utilizes a fitting function to describe the concentration of the chemical in the low K zones in, or adjacent to, each gridblock:

::'''Equation 1:'''    <big>''Cl''</big>''(zl,t)<big> = (C</big>t + Δt<big> + pzl + qzl2) e</big>-zl / d''

{|
|-
|where:
|-
|''Cl''||represents the concentration in the low K material;
|-
|''Ct + Δt''||is the concentration in the high K zone during the next numerical time-step;
|-
|''zl''||is the distance into the low K zone from the high K / low K interface;
|-
|''p'' and ''q''    ||are fitting parameters that can be solved for by enforcing the diffusion differential equation at the interface and mass conservation in the matrix gridblock; and
|-
|''d''||is the concentration penetration depth, which is defined in the REMChlor-MD User’s Manual in Appendix 1.
|}

Using the trial function, the diffusive mass flux can be computed analytically, which is the key time-saving component of the method. The only variable that requires numerical simulation is the concentration in the high-permeability parts of the aquifer.

There are three key geometrical model inputs to the semi-analytical method that can be used to model the diffusion process:

*''Vf'' is the volume fraction of the high-permeability materials in a gridblock;
*''Amd'' is the interfacial area between the high-K and low-K materials in a gridblock; and
*''L'' is the characteristic maximum diffusion length.

Because the volume of low K material in the gridblock should equal the product of the interfacial area and the characteristic maximum diffusion length, the third parameter (L) can be calculated from the first two using the volume balance equation for a gridblock of volume V:

::'''Equation 2:'''    <big>''Amd L = (1 – Vf) V''</big>

This semi-analytical method has undergone thorough testing and proved to be accurate in numerous scenarios<ref name="Falta2017" /><ref name="Muskus2018">Muskus, N. and Falta, R.W., 2018. Semi-analytical method for matrix diffusion in heterogeneous and fractured systems with parent-daughter reactions. Journal of Contaminant Hydrology, 218, pp.94-109. [https://doi.org/10.1016/j.jconhyd.2018.10.002 doi: 10.1016/j.jconhyd.2018.10.002]</ref>. The following section shows common test scenarios where the method can be used.

[[File:Falta1w2 Fig3.png|thumb|left|Figure 3. Concentration profiles in fractures at 1, 49, 51 and 100 years.]]
[[File:Falta1w2 Fig4.png|thumb|right|Figure 4: Simulated mass discharge at downstream edge of fine-grid MT3DMS and REMChlor-MD models.]]
[[File:Falta1w2 Fig5.png|thumb|left|Figure 5: Top view (''xy'') of concentration contours computed with the fine-grid numerical model (top) and with REMChlor-MD (bottom) at 30 years.]]
[[File:Falta1w2 Fig6.png|thumb|right|Figure 6: Top view (''xy'') of concentration contours computed with the fine-grid numerical model (top) and with REMChlor-MD (bottom) at 130 years.]]

==Modeling Examples==

===Matrix Diffusion in Fractured Media===
Contaminants in fractured rock media are often the most challenging to remediate because of the small volume of individual fractures and the large surface area of the matrix. Networks of fractures can store a large mass of contaminants and potentially become a secondary source over time.

The semi-analytical method was tested in a parallel fractured system, and the results were compared against an exact analytical solution<ref>Sudicky, E.A. and Frind, E.O., 1982. Contaminant transport in fractured porous media: Analytical solutions for a system of parallel fractures. Water Resources Research, 18(6), pp.1634-1642. [https://doi.org/10.1029/WR018i006p01634 doi: 10.1029/WR018i006p01634]</ref>. In this example, the fracture spacing was set at 2 m, and the fracture aperture was 100 ''µm''. A known concentration of trichloroethene (TCE) was introduced into the fractures at the upstream end of the model, where its concentration was held constant for 50 years. Then the contaminant source was completely removed, and clean water was flushed through the fractures for another 50 years. Figure 3 shows the concentration profiles in the fractures calculated using the semi-analytical method over 100 years of simulation time compared to the exact analytical solution.

In the 1-year profile, TCE concentrations in the fractures decreased rapidly with distance, since the steep concentration gradient results in rapid mass transfer from the fracture into the low K matrix. Toward the end of the loading period at 49 years, TCE concentration declines more gradually with distance, since TCE has now diffused into the adjoining low K matrix, reducing the concentration gradient and associated mass flux. At 51 years, one year after source removal, there was a decrease in TCE concentration near the inlet, but almost no effect downstream in the fractures due to back diffusion of TCE from the matrix to the fractures. After 50 years of clean water flushing, back diffusing TCE was still acting as a secondary source, reaching a peak concentration of approximately 16 mg/L. The profiles show a typical fractured system prior to and after source removal with matrix diffusion effects preventing the system from reaching clean-up level.

The match between the analytical solution and the semi-analytical method was good, particularly during the loading period. In the unloading period, the semi-analytical method overestimated the peak concentration somewhat, but it still captured the long tailing of concentration almost exactly.

[[File:Falta1w2 Fig7.png|thumb|right|400px| link=https://www.enviro.wiki/images/c/c5/Falta1w2_Fig7.mp4 | [//www.enviro.wiki/images/c/c5/Falta1w2_Fig7.mp4 Figure 7: Video tutorial demonstrating use of REMChlor-MD]]]

===Matrix Diffusion in Heterogeneous Media===
One of the motivations for REMChlor-MD was to develop an alternative to a computationally expensive fine grid model for simulating matrix diffusion. To evaluate the ability of the semi-analytical method incorporated into REMChlor-MD to accurately simulate back-diffusion, a heterogeneous fine grid model was developed<ref name="Muskus2018" /> using MT3DMS. A geostatistical model generated multiple realizations of the subsurface’s heterogeneity using synthetic borehole data. Using the heterogeneous material distribution of sand and clay, a fine grid flow and transport model, consisting of almost 3 million gridblocks, was created. A TCE source was present near the upgradient end of the model for 30 years and then it was completely removed. Following source removal, clean water was flushed through the system for another 200 years.

For comparison, a REMChlor-MD model was developed with a much coarser grid (about 14,000 gridblocks) where flow and transport parameters were comparable to the fine grid model. The mass discharge at the outlet of the models was used as a comparison criterion between the two models. Figure 4 shows a comparison of the computed mass discharge versus time at the downstream edge of the model. The mass discharge predicted by the 3-million gridblock MT2DMS simulation is shown as the yellow dots. The green line shows the REMChlor-MD prediction based on the geostatistical properties of the high and low K zones (characteristic maximum diffusion length, L= 1.85m). The REMChlor-MD match to the fine-grid MT3DMS model was slightly improved by reducing the characteristic maximum diffusion length (L= 1.5m). Both of these results are similar to the 3-million gridblock MT3DMS simulation result.

Plan view concentrations from the fine grid model and the semi-analytical model were extracted to generate contours of the TCE plume in Figures 5 and 6 after 30 and 130 years, respectively. The top panels of these figures were generated with the fine grid MT3DMS model, whereas the bottom panels were generated with the REMChlor-MD model. The fine grid contour has jagged edges because small changes in concentration were captured at a higher spatial resolution. In contrast, the semi-analytical contour was smoother because the concentration changes were interpolated over larger gridblocks. However, the overall shape and contours from the two models are similar. On a high performance 2018 workstation, the run time for the fine-grid MT3DMS model was many hours, while the REMChlor-MD model completed its run in a little over one minute.

==REMChlor-MD User Interface==
The semi-analytical method was programmed in FORTRAN, and a graphical user interface (GUI) was built using Visual Basic in Excel® to facilitate data inputs and output processing<ref>Farhat, S. K., Newell, C. J., Falta, R. W., and Lynch, K. (2018). REMChlor-MD toolkit user’s manual. ER-201426 [//www.enviro.wiki/images/0/0b/2018-Falta-REMChlor_Modeling_Matrix_Diffusion_Effects.pdf Report.pdf]</ref>. The Visual Basic REMChlor-MD interface calls the FORTRAN executable using a dynamic link library, and then processes the FORTRAN output files to produce various graphs. A short video tutorial demonstrating data entry and some of the capabilities of REMChlor-MD is shown in Figure 7. Additional detail is provided in an online [https://www.serdp-estcp.org/Tools-and-Training/Webinar-Series/02-07-2019 webinar]. The toolkit package, including a user’s manual is available free of charge and can be downloaded here ([https://www.serdp-estcp.org/content/download/48566/462193/file/REMChlorMD_64bit.zip 64bit version]) and ([https://www.serdp-estcp.org/content/download/48565/462183/file/REMChlorMD_32bit.zip 32 bit version]). Readers are referred to the [https://www.serdp-estcp.org/content/download/48433/460814/file/ER-201426%20REMChlor-MD%20User's%20Manual.pdf user manual] for a complete explanation of the program, its input parameters, outputs, and examples.

==References==

<references />

==See Also==

REMChlor - MD

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Admin:

Matrix Diffusion

2026-05-07T17:00:47Z

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Monitored Natural Attenuation - Transitioning from Active Remedies

2026-05-07T16:59:00Z

Admin:

Many contaminated sites use active remedies such as pump-and-treat or ''in situ'' remediation to clean up impacted groundwater. Natural attenuation processes such as natural degradation or [[Dispersion and Diffusion | hydrodynamic dispersion]] also contribute to the cleanup. As remediation progresses, a point is often reached when the time required to reach the remedial objectives using the active remedy is roughly the same as the time required if the active remedy is shut down, and the continuing remediation of the site is provided by natural attenuation processes alone. From that point forward, the extra effort and expense of the active remedy provides no benefit over natural attenuation, and it may be appropriate to transition the site to [[Monitored Natural Attenuation (MNA)]]. This article deals with currently available tools and approaches that can be used to support a decision to transition from active remediation to MNA.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s)''':

*[[Alternative Endpoints]]
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Plume Response Modeling]]
*[[REMChlor - MD]]
*[[Source Zone Modeling]]

'''Contributor(s):'''

*[[Dr. John Wilson]]
*[[Dr. David Adamson, P.E.]]

'''Key Resource(s)''':

*[//www.enviro.wiki/images/1/10/2002-Newell-Calculation_and_Use_of_First-Order_Rate_Constants_for_Monitored_Natural_Attenuation_Studies.pdf Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies]<ref name="Newell2002">Newell, C.J., Rifai, H.S., Wilson, J.T., Connor, J.A., Aziz, J.A., Suarez, M.P., 2002. Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies. 28p. EPA/540/S-02/500. [//www.enviro.wiki/images/1/10/2002-Newell-Calculation_and_Use_of_First-Order_Rate_Constants_for_Monitored_Natural_Attenuation_Studies.pdf Report.pdf]</ref>

*[https://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS) Version 2.3.3]<ref name="Widdowson2008">Widdowson, M.A., Mendez, E., Chapelle, F.H., Casey, C.C., 2008. Natural Attenuation Software (NAS) Version 2.3.3. Virginia Polytechnic Institute and State University, the United States Geological Survey, and the United States Naval Facilities Engineering Command. NAS webpage: https://www.nas.cee.vt.edu/index.php See also: https://toxics.usgs.gov/highlights/nas_2.2.0/index.html</ref>

*[//www.enviro.wiki/images/3/39/2002-Aziz-Biochlor_Natural_Attenuation_Decision_Support_System_Vs_2.2.pdf BIOCHLOR Natural Attenuation Support System, Version 2.2]<ref name="Aziz2002">Aziz, C.E., Newell, C.J. and Gonzales, J.R., 2002. BIOCHLOR Natural Attenuation Decision Support System Version 2.2 User’s Manual Addendum. Groundwater Services, Inc., Houston, Texas for the Air Force Center for Environmental Excellence.[//www.enviro.wiki/images/3/39/2002-Aziz-Biochlor_Natural_Attenuation_Decision_Support_System_Vs_2.2.pdf Report.pdf] Available at: https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system</ref>

*[https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 BioPIC User's Guide and Tool Website]<ref name="BioPIC2021">Danko, A., Adamson, D., Newell, C., Wilson, J., Wilson, B., Freedman, D., Lebrón, C., 2021. Quick BioPIC User’s Guide, ESTCP Project ER-201730. [https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 Project Website]   [//www.enviro.wiki/images/c/c9/ER-201730_BioPIC_User%27s_Guide.pdf User’s Guide]</ref>

*[https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Transition Assessment Teaching Assistant (TA2) Tool Website]<ref name="TATA2024">Adamson, D.T., Newell, C.J., Hort, H.M, Wilson, J.T., 2024. TA2: The SERDP Transition Assessment Teaching Assistant. Strategic Environmental Research and Development Program (SERDP) Project ER20-1429. [https://serdp-estcp.mil/projects/details/350cbc0b-893a-43a6-8a0c-c9c057bacac0/er20-1429-project-overview Project Website]  [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Online Tool]</ref>

==Introduction==
Many active remedies are effective at treating higher concentrations of contaminants, but as the contaminant concentrations decrease, the rate of cleanup may slow before the site reaches the cleanup goal. At some sites, the rate of cleanup may slow until it is not significantly different from the rate of cleanup provided by the natural attenuation processes that occur at the site. At other sites, the concentration of contaminants in water produced by a pumping system is below the cleanup goal, but the concentration in monitoring wells in the source area are still above the goal. At some sites, active treatment has stopped further expansion of the plume toward a receptor, and concentrations are declining over time throughout the plume, but back diffusion is sustaining concentrations in the plume that are above the cleanup goal.

In 2013, a significant National Research Council (NRC) report noted that despite years of effort and considerable investment, many sites “will require long-term management that could extend for decades or longer”<ref name="NRC2013">National Research Council (NRC), 2013. Alternatives for Managing the Nation's Complex Contaminated Groundwater Sites. Committee on Future Options for Management in the Nation's Subsurface Remediation Effort, Water Science, Technology Board, Division on Earth and Life Studies, NRC. National Academies Press, 422 pages, ISBN 978-0-309-27874-4 [https://doi.org/10.17226/14668 doi: 10.17226/14668]. [//www.enviro.wiki/images/4/48/NRC2013.pdf Report.pdf]</ref>. The authors of the report discussed the need for developments that can aid in “transition from active remediation to more passive strategies and provide more cost-effective and protective long-term management of complex sites”<ref name="NRC2013" />.

The United States Environmental Protection Agency<ref> U.S. Environmental Protection Agency (USEPA), 1999. Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites. OSWER Directive 9200.4-17P. 39pp.[//www.enviro.wiki/images/a/aa/1999_USEPA-_Use_of_monitored_natural_attenuation_at_superfund.pdf Report.pdf]</ref> allows the use of [[Monitored Natural Attenuation (MNA) | monitored natural attenuation (MNA)]] to attain the cleanup goals when the site-specific remediation objectives can be attained within a time frame that is reasonable compared to that offered by other more active methods. Many CERCLA<ref>U.S. Environmental Protection Agency (USEPA), 2019. Summary of the Comprehensive Environmental Response, Compensation, and Liability Act (Superfund) https://www.epa.gov/laws-regulations/summary-comprehensive-environmental-response-compensation-and-liability-act</ref> and RCRA<ref>U.S. Environmental Protection Agency (USEPA), 2019. Resource Conservation and Recovery Act (RCRA) Laws and Regulations https://www.epa.gov/rcra</ref> sites take advantage of this policy. An active remedy is typically used initially to treat high concentrations of contaminants followed by MNA to treat the lower concentrations that remain.

Unfortunately, there is no well-established approach to determine when it is appropriate to discontinue the active remedy. The NRC report<ref name="NRC2013" /> emphasized the use of more rigorous evaluations of existing data to support these efforts. This can include a quantitative assessment of the performance of active remedies (e.g., evidence of asymptotic performance) as well as documenting site conditions that may be contributing to these performance limitations. Importantly, it also identifies alternative approaches for managing the site, which could include MNA if the natural attenuation processes can meaningfully contribute to the achievement of site cleanup objectives.

This article reviews available tools and approaches to evaluate a transition to MNA. The tools and approaches depend on calculations of rate constants for natural attenuation with distance in flowing groundwater or rate constants for attenuation over time in individual monitoring wells.

==Background on Rate Constants==
[[File:Wilson1w2Fig1.png|thumb|400px| Figure 1. Attenuation of Trichloroethene (TCE) over time in a monitoring well at a site in Michigan. The concentration vs. time rate constant is 0.326 per year and largely represents the rate of the attenuation of the source of contaminants in the aquifer.]]
At sites where a transition to MNA is being considered, a key step is estimating attenuation rate constants and understanding how they are extracted from monitoring data. A general formula to describe the rate of a chemical reaction is:

:{|
|-
|'''Equation 1:'''|| ||<big>''r = k [C]''</big>'' m''
|-
|where:
|-
| ||''r''    ||is the rate of the reaction,
|-
| ||''k''||is the rate constant,
|-
| ||''C''||is the concentration of the chemical undergoing the reaction, and
|-
|the exponent    ||''m''||is the order of the reaction.
|}

When the rate of the reaction is proportional to the concentration of the contaminant, the value of ''m'' is 1. Therefore, the reaction is described as a first-order reaction, and the rate constant is described as a first-order rate constant. In Equation 1, concentration could go up or down, but ''k'' is a constant of proportionality for the rate of increase in concentration. The rate constant for attenuation is the negative of ''k''. If the rate of degradation is a fixed value regardless of concentration, the value of ''m'' is 0, and degradation is a zero-order process.

Natural attenuation of concentrations over time in monitoring wells is frequently described by a first-order rate constant, and natural biological or abiotic degradation of contaminants in flowing groundwater is typically also described by a first-order rate constant. Figure 1 provides an example of monitoring data that is described by a first-order rate constant.

The rate constant for attenuation over time in a single well and the rate constant for attenuation with distance along a flow path in an aquifer describe different situations that are controlled by different processes. ''Attenuation over time'' in a well is largely controlled by the rate of attenuation of the source of contamination in the aquifer. ''Attenuation with distance'' along a flow path includes attenuation of concentrations in the source along with contributions from biological degradation processes, abiotic degradation processes and hydrodynamic dispersion of the contaminated groundwater into clean groundwater<ref name="Newell2002" />.

The first-order rate constant for attenuation over time in a single well is commonly referred to as ''kpoint''<ref name="Newell2002" />. A time series chart in Microsoft EXCEL of the concentrations of a contaminant (''y'' axis) on the date of sampling (''x'' axis) can be used to extract a value for ''kpoint''. Select the data, then insert an exponential trend line and display the equation on the chart. The value of ''kpoint'' can also be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Note that the rate constants extracted in EXCEL are constants for the rate of change, not the rate of attenuation. Take the negative of the rate of change to get ''kpoint''. In the example in Figure 1, the unit of time on the X axis is years, and the value of ''kpoint'' is 0.326 per year.

Attenuation versus distance rate coefficients describe a bulk attenuation rate including both degradation and non-destructive processes such as dispersion. To extract values for rate constants for degradation alone, it is necessary to calibrate a groundwater flow and transport model to the data at the site. The model is calibrated with values for the hydrogeological properties of the aquifer (effective porosity, hydraulic gradient, hydraulic conductivity, hydrodynamic dispersion and the organic carbon content of the aquifer matrix). After the hydrogeological properties of the aquifer are fixed in the model, the most appropriate values for the degradation rate constants are the values that produce the best fit between the contaminant concentrations that are predicted by the model and the contaminant monitoring data at the site.

There are a number of reasons why natural attenuation processes are better described as first-order relationship instead of zero-order or some other order. The attenuation over time in a monitoring well tracks the attenuation over time of the source of contamination that sustains the plume<ref name="Newell2002" />. Sites go through a lifecycle, and attenuation of sources at mature sites is often a first-order process<ref>Sale, T., Newell, C., Stroo, H., Hinchee, R. and Johnson, P., 2008. Frequently Asked Questions Regarding Management of Chlorinated Solvents in Soils and Groundwater. Environmental Security Technology Certification Program (ESTCP, Project ER-200530), Department of Defense (DoD), Arlington, VA. [//www.enviro.wiki/images/c/cb/2008-Sale-Frequently_Asked_Questions_Regarding_Management_of_Chlorinated_Solvent_in_Soils_and_Groundwater.pdf Report.pdf]   Project Overview Website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200530</ref>. If a chlorinated solvent site is mature, the contamination in the source area that was originally present as nonaqueous phase liquids (NAPL) has been redistributed and is now sequestered in a sorbed phase to aquifer solids or has diffused into non-transmissive portions of the aquifer matrix. Transfer of contaminants back into the more transmissive portions of the aquifer occurs by diffusion along a fixed path length, and the rate of transfer is controlled by the concentration of the contaminant remaining in the source material. Because the rate of transfer is proportional to the concentration of contaminant in the source material, attenuation of the source is a first-order process. These processes are discussed in more detail in [[Source Zone Modeling]].

Degradation processes are also usually first order. Abiotic reactions are almost always first order with respect to the concentration of the target chemical. Biodegradation reactions are zero order at high concentrations because the available enzymes are saturated with substrate, but are first order at lower concentrations that are typical of natural attenuation conditions in groundwater.

==Goals for MNA at Sites==

The information necessary to evaluate whether a site can be transitioned to MNA depends on the goal for MNA at the site. For many cleanup actions, the goal is to confine contamination within a waste management area where the contamination is left in place, in which case the cleanup goal applies to point-of-compliance wells that are outside the waste management area. For other cleanup actions, the entire site must be cleaned up, in which case the cleanup goal applies to any monitoring well on the site. The time by which the goal is to be attained is specified at CERCLA sites in the Record of Decision (the ROD). At RCRA sites, the time allowed for the cleanup to be attained may be specified in the permit.

==When the Goal Applies to Point-of-Compliance Wells==
Consider the following framework for evaluating a transition to MNA:

#Use a computer model to extract rate constants for the natural degradation of the contaminant that occurred in groundwater at the site before the active remedy was installed.
#Assume that the same rate constants will apply after the active remedy is no longer in operation. Note that this assumption may not be valid if the active remedy changes the geochemistry of the aquifer in the flow path to the point-of-compliance well.
#Calibrate a computer groundwater flow and transport model with the hydrogeological properties of the aquifer that pertain after the active remedy is no longer in operation, the concentration of contaminant after the active remedy, and the rate constants for natural degradation that are assumed to apply after the active remedy.
#Use the computer model to project the concentrations of the contaminant at the point-of-compliance well over time.
#If the concentrations at the point-of-compliance wells are predicted to be less than the goal before the specified date, that is a quantitative line of evidence in support of a transition to MNA.

There are several computer applications that are particularly useful to extract rate constants at a site from monitoring data that were collected before the active remedy was installed. For example, [https://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS)]<ref name="Widdowson2008" />, [https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system BIOCHLOR]<ref name="Aziz2002" /> and [https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 BioPIC]<ref name="BioPIC2021" /> can be downloaded from the internet at no cost. Another recent example, the [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ TA2 Tool]<ref name="TATA2024" />, is discussed in detail later in this article.

[[File:Wilson1w2Fig2.png|thumb|left|400px| Figure 2. Example calibration of NAS to natural attenuation of total BTEX at a site (Figure 17 of NAS User’s Manual).]]
[[File:Wilson1w2Fig3.png|thumb|400px| Figure 3. The data input screen for BIOCHLOR before remediation with cis-1,2-Dichloroethene (DCE) and vinyl chloride (VC) source concentrations of 500 and 87 mg/L respectively at the source when the release first occurred.]]
[[File:Wilson1w2Fig4.png|thumb|left|400px| Figure 4. Output of the RUN CENTERLINE simulation in BIOCHLOR comparing the fit between the simulation and the field data for vinyl chloride before an active remedy was implemented]]
[[File:Wilson1w2Fig5.png|thumb|400px| Figure 5. Output of the RUN CENTERLINE simulation of conditions after an active remedy was implemented with a source concentration of 1.1 mg/L, projecting the concentration of vinyl chloride at a distance corresponding to a point-of-compliance well.]]
 
In NAS, the user inputs the hydrogeological data, the distance of wells along the flow path, and the concentrations of contaminants in the wells. The NAS application extracts rate constants and makes projections at the point-of-compliance. With NAS, it is possible to extract different rate constants for specific geochemical environments along the flow path.

Figure 2 provides an example calibration of NAS. The concentrations in the monitoring wells used to calibrate the model are compared to the simulation provided by the model. The values of the rate constants that are extracted from the field data are available in the “Output” tab under “Data and Results Table.”

Figure 3 depicts the input screen for BIOCHLOR. The user inputs the hydrogeological parameters, the first-order rate constants (1st Order Decay Coefficient), the distribution of the wells along the flow path, and the concentrations of contaminants in the wells. The model is set up for conditions that apply before the installation of the active remedy.

BIOCHLOR does not automatically fit the rate constants to the field data. Instead, the user examines the output of the model, and adjusts the rate constants until they provide the best fit between the model prediction and the monitoring data for wells at the site. This comparison is illustrated in Figure 4.

If the distance from the source well to the point-of-compliance well is set as the “Modeled Area Length” in Section 5 of the input screen, the “Run Centerline” output will provide the projected concentrations at that length. Assume the distance from the source well to the point-of-compliance well is 250 feet. The projected concentration in Figure 3 of vinyl chloride at a point-of-compliance well is 0.042 mg/L. If the regulatory goal were the federal drinking water maximum contaminant level (MCL)<ref>U. S. Environmental Protection Agency (USEPA), 2009. National Primary Drinking Water Regulations. EPA 816-F-09-004. [//www.enviro.wiki/images/a/ae/2009-USEPA-national_Primary_Drinking_Water_Regulations.pdf Report.pdf]</ref> of 0.002 mg/L, the projected concentration would exceed the goal, and MNA would not be adequate as a remedy.

For the sake of illustration, assume that an active remedy has been implemented, and the concentrations in the source well are 5.4 mg/L for DCE and 1.1 mg/L for vinyl chloride. To evaluate whether it is now appropriate to transition to MNA, BIOCHLOR could be calibrated with these concentrations to predict concentrations in the point-of-compliance well. (See Figure 5). In this example, the projected concentration at the point-of-compliance well does meet the goal.

Some active remedies are subject to rebound. If this is the case, the evaluation should begin at the point in time when it is clear that the trend in concentrations is downward.

A new EXCEL-based tool that does many of the same basic calculations as BIOCHLOR was recently developed as part of an update to the BioPIC<ref name="BioPIC2021" /> decision support software. This tool, the MNA Rate Constant Estimator, extracts rate constants from concentration versus distance data for a variety of different chemicals, including chlorinated ethenes (e.g., PCE and TCE), chlorinated ethanes (e.g., 1,1,1-TCA), and 1,4-dioxane. This tool was developed to run using current versions of EXCEL, whereas BIOCHLOR must be run using older versions of EXCEL that may be unavailable to many users. The MNA Rate Constant Estimator can be used to estimate degradation rate constants and/or predict plume footprints over time. Consequently, it is a useful addition to the BioPIC decision framework for understanding if MNA is appropriate remedy for a site, and it can also be helpful for estimating rate constants as part of a transition assessment.

==When the Goal Applies to All the Wells==
At sites where a concentration-based cleanup goal must be achieved at all wells, each well at the site is evaluated independently, and the rate constant that is applicable is the rate constant for attenuation over time in the well (''kpoint''). To evaluate whether the region in an aquifer that is sampled by a particular monitoring well is ready to transition to MNA, it is necessary to have monitoring data from a period of time before the remedy was implemented. This data is used to extract a value for ''kpoint'' in the aquifer under natural attenuation conditions. The evaluation of a transition to MNA will assume that the same value for ''kpoint'' will apply after the active remedy is complete. This assumption may not be appropriate if the active remedy caused a permanent change in the geochemistry of the aquifer. The assumption is usually appropriate for pump-and-treat remedies.

If ''kpoint'' before implementation of the active remedy describes the time course of natural attenuation after the active remedy is completed, the time required to attain the cleanup goal is predicted from the following:

:{|
| || || rowspan="2" |<big>''ln (''</big>
| style="border-style:solid; border-width: 0px 0px 1px 0px" |''Cgoal''
| rowspan="2" |<big>'')''</big>||                                                                       
|-
|'''Equation 2:'''        ||''<big>t =''    
| style="vertical-align:top;" |''Ccurrent''||
|-
| || || colspan="3" style="text-align:center; border-style:solid; border-width: 1px 0 0 0" |''<big>-k</big>point''||
|-
|where:
|-
| ||''Ccurrent''    || colspan="5" |is the current concentration after active remediation,
|-
| ||''Cgoal''|| colspan="5" |is the cleanup goal, and
|-
| ||''t''|| colspan="5" |is the time required for concentrations to attenuate from ''Ccurrent'' to ''Cgoal.''
|}

If the value of ''t'' estimated using Equation 2 is less than the difference between the current date and the date specified by the site stakeholders to attain the goal, that is evidence in support of a transition to MNA.

Some active remedies are subject to contaminant concentration rebound. If this is the case, the evaluation should use a value of ''C''current that is attained after the rebound has stabilized.

This approach depends on a robust value for ''kpoint''. It is worthwhile to do a sensitivity analysis on ''kpoint'' where the lower 95% or 90% confidence interval on ''kpoint'' is used in Equation 2 to see if that changes the outcome of the evaluation. The confidence intervals can be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Wilson<ref name="Wilson2011">Wilson, J.T. 2011. An Approach for Evaluating the Progress of Natural Attenuation in Groundwater. EPA 600-R-11-204. [//www.enviro.wiki/images/e/e3/Wilson-2011-An_Approach_for_Evaluating_Progress.pdf Report.pdf]</ref> provides detailed discussion of the use of linear regression to extract ''kpoint'' and confidence intervals on ''kpoint''. Wilson<ref name="Wilson2011" /> also discusses the use of goodness-of-fit tests to determine if there is evidence that a first-order rate equation is not the best fit to the monitoring data, and as a result the use of Equation 2 would not be appropriate. The TA2 Tool<ref name="TATA2024" /> also has the capability to calculate ''kpoint'' with a user-specified confidence interval, as described below.

At many sites, there is no specified date when the cleanup goal must be attained. In this situation, the monitoring data can be evaluated to determine if the current rate of attenuation under the active remedy is faster than the rate of natural attenuation before the active remedy was installed. The monitoring data can be examined to identify a time interval when the benefit of the active remedy has approached an asymptote. A second value of for ''kpoint'' can be extracted for that time interval. The two values for ''kpoint'' can be evaluated statistically to see if the current rate is faster at some appropriate level of confidence. If there is no statistical evidence that the rate of attenuation is faster, that determination can support a decision to transition to MNA.
[[File:Wilson1w2Fig6.png|thumb|400px| Figure 6. Example calibration of NAS to predict the reduced concentration at the source that is necessary to meet the remediation goal at a point-of-compliance well (Figure 19 of NAS User’s Manual).]]

==Extent of Treatment Necessary to Transition to MNA==
There are several computer applications that can predict the extent of treatment that must be achieved by the active remedy before it is worthwhile to evaluate the site for transition to MNA. For example, based on the distribution of contamination along the flow path, the NAS application will automatically predict a reduced concentration at the source well that will bring concentrations to the goal in the point-of-compliance well (Figure 6). A table that opens under the “DOS/TOS” tab provides the “Time of Equilibration” required to meet the goal at the reduced concentration. Modules in NAS allow the user to evaluate the effect of various pump-and-treat and source removal scenarios on the time required to attain the goal at the point-of-compliance well.

The [[REMChlor - MD | REMChlor-MD]]<ref name="Falta2018">Falta, R.W., Farhat, S.K., Newell, C.J. and Lynch, K., 2018. A Practical Approach for Modeling Matrix Diffusion Effects in REMChlor. SERDP/ESTCP Project ER-201426 [//www.enviro.wiki/images/0/0b/2018-Falta-REMChlor_Modeling_Matrix_Diffusion_Effects.pdf Report.pdf]   Website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201426</ref> and [https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel REMFuel]<ref name="Falta2012">Falta, R.W., Ahsanuzzaman, A.N., Stacy, M.B., Earle, R.C. and Wilson, J.T., 2012. Remediation Evaluation Model for Fuel Hydrocarbons (REMFuel). Users Manual Version 1.0. U.S. Environmental Protection Agency. EPA/600/R-12/028. [//www.enviro.wiki/images/6/67/2012-Falta-REMFuel_Remediation_Evaluation-Model_for_Fuel_hydrocarbons_users_manual.PDF Report.pdf]   Website: https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel</ref> models are flexible screening tools that allow a simultaneous evaluation of the extent of treatment provided by (1) source removal, (2) ''in situ'' remediation of the contaminated groundwater, or (3) natural attenuation processes in three discrete intervals along the flow path and three discrete time periods. Both [[REMChlor - MD | REMChlor-MD]]<ref name="Falta2018" /> and [https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel REMFuel]<ref name="Falta2012" /> can be downloaded from the internet at no cost. Liang ''et al.''<ref>Liang, H., Falta, R.W., Newell, C.J., Farhat, S.K., Rao, P.S. and Basu, N., 2010. Decision & Management Tools for DNAPL Sites: Optimization of Chlorinated Solvent Source and Plume Remediation Considering Uncertainty. SERDP/ESTCP Project ER-200704. [//www.enviro.wiki/images/c/ce/2010-Liang-Decision_and_Management_Tools_for_DNAPL_sites-ER-200704-FR.pdf Report.pdf]   Project Overview Website: https://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200704/(language)/eng-US</ref> provide a modeling program that uses Monte Carlo simulations to evaluate the effects of the uncertainties in the modeling parameters on the predictions of REMChlor-MD.

==The Transition Assessment Teaching Assistant (TA2) Tool==
[[File:Wilson1w2Fig7.png|thumb|500px| Figure 7. Home Page for TA2 Tool. Users can click on buttons to access various modules that are designed to answer specific questions or research relevant topics.]]
[[File:Wilson1w2Fig8.png|thumb|500px| Figure 8. Example of an asymptote analysis using concentration versus time data in Tool 1 of the TA2 Tool. The source attenuation rate and corresponding remediation timeframe can be estimated for different monitoring periods.]]
A learning and decision-making tool was recently released as part of [https://serdp-estcp.mil/ Strategic Environmental Development and Research Program (SERDP)] Project [https://serdp-estcp.mil/projects/details/350cbc0b-893a-43a6-8a0c-c9c057bacac0/er20-1429-project-overview ER-201429] to help stakeholders gather information for the purposes of a site-specific transition assessment. This free software, the [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Transition Assessment Teaching Assistant (TA2) Tool]<ref name="TATA2024" />, was developed using the elements identified in the 2013 NRC report<ref name="NRC2013" /> as the critical learning objectives for end users.

The Tool is a web-based app that includes a collection of individual modules designed to answer specific questions or research relevant topics (Figure 7). The Tool has been developed as an R Shiny app (version 1.8.0)<ref> Chang, W., Cheng, J., Allaire, J., Sievert, C., Schloerke, B., Xie, Y., Allen, J., McPherson, J., Dipert, A., Borges, B., 2023. shiny: Web Application Framework for R. R package version 1.8.0, https://github.com/rstudio/shiny, https://shiny.posit.co/</ref>, which is an interactive platform using R programming to perform all quantitative functions. The user can then view the results in a simple interface that easily accommodates plots, charts, and various mapping features in a Web browser. The Tool is free and does not require the user to install R software.

The modules within the TA2 Tool include:

'''Five Quantitative Tools''' that focus on assessing asymptotic groundwater concentrations from monitoring data, evaluating plume stability, estimating remediation timeframes after a hypothetical source removal project, forecasting remediation performance if a technology is applied in the field, or projecting concentrations at downgradient points of compliance.

For example, Tool 1 in the TA2 Tool uses concentration versus time data from monitoring wells to estimate attenuation rate constants (''kpoint'') and evaluate if asymptotic conditions are present at particular locations or across the site. This helps to assess whether performance has plateaued at wells where a pump-and-treat system or other active treatment is in place. The user has the option to choose a “change point” within the monitoring record to determine if the attenuation rate has changed over time (e.g., once most of the accessible mass has been removed) (Figure 8). The user can either use visual interpretation to manually select the date when this apparent change occurred or have the date selected automatically using a binary segmentation protocol that is incorporated into the tool. The tool will calculate a rate for both the early period and a rate for the later period (after the change point), and then go through five different lines of evidence for asymptotic behavior (e.g., are the two rates of attenuation significantly different?). The user can then use the collective results as a technical justification demonstrating that the performance of the active remedy has plateaued as the first step in the transition assessment. The tool will also estimate the time to reach a user-specified cleanup goal if the overall attenuation rate (or the attenuation rate in the later period) were to continue.

Another module (Tool 5) focuses on evaluating sites where the concentration goal applies at a downgradient point of compliance, which is a key criterion for sites where MNA is being used as part of a risk-based strategy. The tool includes several different options to estimate a site-specific attenuation rate constant, including data from the pre-remediation period when natural attenuation processes were the sole means for reducing concentrations. Attenuation rate constants are then used to project the concentration versus distance from the contaminant source. Based on the predicted concentration at the downgradient point of compliance, the user can then see if the natural assimilative capacity along the aquifer flow path is sufficient to achieve the concentration goal in the absence of active treatment. For example, in the tab labeled “Use Pre-Remediation Rate Constant”, the logarithms of the concentrations from the period before active treatment began are plotted against the distance from the source well. The slope of the regression line is the rate constant for natural attenuation (including the contributions of degradation and dispersion). This rate constant can then be used to project the concentration moving downgradient from the well of concern after the end of active treatment. Similar approaches are provided within Tool 5 for using rate constants estimated from lab-based testing or derived from post-remediation data (after steady state has been reestablished).

'''Four Qualitative Tools''' provide information on matrix diffusion, enhanced attenuation options, geologic heterogeneity, and related research on transition assessments. Many of these modules are based on the current understanding of the role of matrix diffusion in influencing long-term concentration trends and remedial performance at contaminated groundwater sites. This includes summaries of different modeling options for better quantifying the effects of matrix diffusion. Sites impacted by matrix diffusion are generally challenging to treat using active remedies and thus are better candidates for less intensive management strategies that focus on reducing mass discharge rates, stabilizing the plume, and protecting potential downgradient receptors. As a result, matrix diffusion is critical to understanding and quantifying how natural attenuation processes are contributing to concentration trends.

'''One Summary Tool''' (Tool 10) compiles metrics from the other tools into a “Remediation Transition Assessment Index” (RTAI) and provides additional guidance on conducting site-specific transition assessments. The RTAI is a simple metric with a value from 1 to 5, where higher values reflect greater persistence of contamination due to matrix diffusion and other site-specific factors. An RTAI value is assigned to each of the results from the different tools that have been completed by the user. An RTAI of 5 suggests that the site is a strong candidate for transitioning to MNA or enhanced attenuation approaches, while a site with an RTAI value of 1 is a poor candidate. The user can assign an overall RTAI for the site based on the preponderance of evidence after reviewing the RTAI values generated by each tool, or calculate a site RTAI based on simple averaging, weighting, or other methods.

Tool 10 also contains a flowchart and a checklist for performing site-specific transition assessments that start with evaluating relevant bright line criteria, such as (1) can the concentration goals be met at the point of compliance by MNA; and (2) is the remediation timeframe for MNA reasonable and/or similar to the timeframe if source remediation were used. This checklist ensures that the user has gathered all relevant information that would be needed to support a technically rigorous site-specific Transition Assessment.

The TA2Tool provides a framework for remedial decision makers to evaluate different types of sites, including those where active treatment (e.g., pump and treat) is in use, as well as sites where future active source zone remediation is being considered. It also includes a description of enhanced MNA alternatives for sites where MNA alone may not be sufficient to control risk. As shown in Figure 8, the tool can be used to answer specific questions that have a primarily quantitative basis or to provide focused qualitative information for researching specific topics. Users can engage with just the modules that might be pertinent to assessment of an individual site, or they can go through all the modules to perform a more thorough, step-by-step analysis of the relevant issues for their site.

==Summary==
Tools and approaches are available that can be adapted to determine when a site is ready to transition from active remedy to MNA. However, these tools and approaches have not been applied for this purpose at a significant number of sites, and at the present time, they are not generally accepted by regulatory authorities. There is an opportunity to establish and implement a logical and consistent framework that can be widely implemented to evaluate sites for transition from active remedy to MNA.

==References==
<references />

==See Also==

*[http://dx.doi.org/10.1007/978-1-4614-6922-3 Newell, C.J., Kueper, B.H., Wilson, J.T., Johnson, P.C., 2014. Natural Attenuation of Chlorinated Solvent Source Zones. In: Chlorinated Solvent Source Zone Remediation, Editors: Kueper, B.H., Stroo, H.F., Vogel, C.M., Ward. SERDP ESTCP Environmental Remediation Technology, vol 7. Springer, New York, NY. pgs. 459-508. doi: 10.1007/978-1-4614-6922-3]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436/ER-200436/(language)/eng-US Kram, Mark, and Widdowson, Mark, 2008. Estimating Cleanup Times Associated with Combining Source-Area Remediation with Monitored Natural Attenuation. ESTCP ER-200436]

Monitored Natural Attenuation - Transitioning from Active Remedies

2026-05-07T16:58:45Z

Admin:

Many contaminated sites use active remedies such as pump-and-treat or ''in situ'' remediation to clean up impacted groundwater. Natural attenuation processes such as natural degradation or [[Dispersion and Diffusion | hydrodynamic dispersion]] also contribute to the cleanup. As remediation progresses, a point is often reached when the time required to reach the remedial objectives using the active remedy is roughly the same as the time required if the active remedy is shut down, and the continuing remediation of the site is provided by natural attenuation processes alone. From that point forward, the extra effort and expense of the active remedy provides no benefit over natural attenuation, and it may be appropriate to transition the site to [[Monitored Natural Attenuation (MNA)]]. This article deals with currently available tools and approaches that can be used to support a decision to transition from active remediation to MNA.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s)''':

*[[Alternative Endpoints]]
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Plume Response Modeling]]
*[[REMChlor - MD]]
*[[Source Zone Modeling]]

'''Contributor(s):'''

*[[Dr. John Wilson]]
*[[Dr. David Adamson, P.E.]]

'''Key Resource(s)''':

*[//www.enviro.wiki/images/1/10/2002-Newell-Calculation_and_Use_of_First-Order_Rate_Constants_for_Monitored_Natural_Attenuation_Studies.pdf Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies]<ref name="Newell2002">Newell, C.J., Rifai, H.S., Wilson, J.T., Connor, J.A., Aziz, J.A., Suarez, M.P., 2002. Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies. 28p. EPA/540/S-02/500. [//www.enviro.wiki/images/1/10/2002-Newell-Calculation_and_Use_of_First-Order_Rate_Constants_for_Monitored_Natural_Attenuation_Studies.pdf Report.pdf]</ref>

*[https://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS) Version 2.3.3]<ref name="Widdowson2008">Widdowson, M.A., Mendez, E., Chapelle, F.H., Casey, C.C., 2008. Natural Attenuation Software (NAS) Version 2.3.3. Virginia Polytechnic Institute and State University, the United States Geological Survey, and the United States Naval Facilities Engineering Command. NAS webpage: https://www.nas.cee.vt.edu/index.php See also: https://toxics.usgs.gov/highlights/nas_2.2.0/index.html</ref>

*[//www.enviro.wiki/images/3/39/2002-Aziz-Biochlor_Natural_Attenuation_Decision_Support_System_Vs_2.2.pdf BIOCHLOR Natural Attenuation Support System, Version 2.2]<ref name="Aziz2002">Aziz, C.E., Newell, C.J. and Gonzales, J.R., 2002. BIOCHLOR Natural Attenuation Decision Support System Version 2.2 User’s Manual Addendum. Groundwater Services, Inc., Houston, Texas for the Air Force Center for Environmental Excellence.[//www.enviro.wiki/images/3/39/2002-Aziz-Biochlor_Natural_Attenuation_Decision_Support_System_Vs_2.2.pdf Report.pdf] Available at: https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system</ref>

*[https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 BioPIC User's Guide and Tool Website]<ref name="BioPIC2021">Danko, A., Adamson, D., Newell, C., Wilson, J., Wilson, B., Freedman, D., Lebrón, C., 2021. Quick BioPIC User’s Guide, ESTCP Project ER-201730. [https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 Project Website]   [//www.enviro.wiki/images/c/c9/ER-201730_BioPIC_User%27s_Guide.pdf User’s Guide]</ref>

*[https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Transition Assessment Teaching Assistant (TA2) Tool Website]<ref name="TATA2024">Adamson, D.T., Newell, C.J., Hort, H.M, Wilson, J.T., 2024. TA2: The SERDP Transition Assessment Teaching Assistant. Strategic Environmental Research and Development Program (SERDP) Project ER20-1429. [https://serdp-estcp.mil/projects/details/350cbc0b-893a-43a6-8a0c-c9c057bacac0/er20-1429-project-overview Project Website]  [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Online Tool]</ref>

==Introduction==
Many active remedies are effective at treating higher concentrations of contaminants, but as the contaminant concentrations decrease, the rate of cleanup may slow before the site reaches the cleanup goal. At some sites, the rate of cleanup may slow until it is not significantly different from the rate of cleanup provided by the natural attenuation processes that occur at the site. At other sites, the concentration of contaminants in water produced by a pumping system is below the cleanup goal, but the concentration in monitoring wells in the source area are still above the goal. At some sites, active treatment has stopped further expansion of the plume toward a receptor, and concentrations are declining over time throughout the plume, but back diffusion is sustaining concentrations in the plume that are above the cleanup goal.

In 2013, a significant National Research Council (NRC) report noted that despite years of effort and considerable investment, many sites “will require long-term management that could extend for decades or longer”<ref name="NRC2013">National Research Council (NRC), 2013. Alternatives for Managing the Nation's Complex Contaminated Groundwater Sites. Committee on Future Options for Management in the Nation's Subsurface Remediation Effort, Water Science, Technology Board, Division on Earth and Life Studies, NRC. National Academies Press, 422 pages, ISBN 978-0-309-27874-4 [https://doi.org/10.17226/14668 doi: 10.17226/14668]. [//www.enviro.wiki/images/4/48/NRC2013.pdf Report.pdf]</ref>. The authors of the report discussed the need for developments that can aid in “transition from active remediation to more passive strategies and provide more cost-effective and protective long-term management of complex sites”<ref name="NRC2013" />.

The United States Environmental Protection Agency<ref> U.S. Environmental Protection Agency (USEPA), 1999. Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites. OSWER Directive 9200.4-17P. 39pp.[//www.enviro.wiki/images/a/aa/1999_USEPA-_Use_of_monitored_natural_attenuation_at_superfund.pdf Report.pdf]</ref> allows the use of [[Monitored Natural Attenuation (MNA) | monitored natural attenuation (MNA)]] to attain the cleanup goals when the site-specific remediation objectives can be attained within a time frame that is reasonable compared to that offered by other more active methods. Many CERCLA<ref>U.S. Environmental Protection Agency (USEPA), 2019. Summary of the Comprehensive Environmental Response, Compensation, and Liability Act (Superfund) https://www.epa.gov/laws-regulations/summary-comprehensive-environmental-response-compensation-and-liability-act</ref> and RCRA<ref>U.S. Environmental Protection Agency (USEPA), 2019. Resource Conservation and Recovery Act (RCRA) Laws and Regulations https://www.epa.gov/rcra</ref> sites take advantage of this policy. An active remedy is typically used initially to treat high concentrations of contaminants followed by MNA to treat the lower concentrations that remain.

Unfortunately, there is no well-established approach to determine when it is appropriate to discontinue the active remedy. The NRC report<ref name="NRC2013" /> emphasized the use of more rigorous evaluations of existing data to support these efforts. This can include a quantitative assessment of the performance of active remedies (e.g., evidence of asymptotic performance) as well as documenting site conditions that may be contributing to these performance limitations. Importantly, it also identifies alternative approaches for managing the site, which could include MNA if the natural attenuation processes can meaningfully contribute to the achievement of site cleanup objectives.

This article reviews available tools and approaches to evaluate a transition to MNA. The tools and approaches depend on calculations of rate constants for natural attenuation with distance in flowing groundwater or rate constants for attenuation over time in individual monitoring wells.

==Background on Rate Constants==
[[File:Wilson1w2Fig1.png|thumb|400px| Figure 1. Attenuation of Trichloroethene (TCE) over time in a monitoring well at a site in Michigan. The concentration vs. time rate constant is 0.326 per year and largely represents the rate of the attenuation of the source of contaminants in the aquifer.]]
At sites where a transition to MNA is being considered, a key step is estimating attenuation rate constants and understanding how they are extracted from monitoring data. A general formula to describe the rate of a chemical reaction is:

:{|
|-
|'''Equation 1:'''|| ||<big>''r = k [C]''</big>'' m''
|-
|where:
|-
| ||''r''    ||is the rate of the reaction,
|-
| ||''k''||is the rate constant,
|-
| ||''C''||is the concentration of the chemical undergoing the reaction, and
|-
|the exponent    ||''m''||is the order of the reaction.
|}

When the rate of the reaction is proportional to the concentration of the contaminant, the value of ''m'' is 1. Therefore, the reaction is described as a first-order reaction, and the rate constant is described as a first-order rate constant. In Equation 1, concentration could go up or down, but ''k'' is a constant of proportionality for the rate of increase in concentration. The rate constant for attenuation is the negative of ''k''. If the rate of degradation is a fixed value regardless of concentration, the value of ''m'' is 0, and degradation is a zero-order process.

Natural attenuation of concentrations over time in monitoring wells is frequently described by a first-order rate constant, and natural biological or abiotic degradation of contaminants in flowing groundwater is typically also described by a first-order rate constant. Figure 1 provides an example of monitoring data that is described by a first-order rate constant.

The rate constant for attenuation over time in a single well and the rate constant for attenuation with distance along a flow path in an aquifer describe different situations that are controlled by different processes. ''Attenuation over time'' in a well is largely controlled by the rate of attenuation of the source of contamination in the aquifer. ''Attenuation with distance'' along a flow path includes attenuation of concentrations in the source along with contributions from biological degradation processes, abiotic degradation processes and hydrodynamic dispersion of the contaminated groundwater into clean groundwater<ref name="Newell2002" />.

The first-order rate constant for attenuation over time in a single well is commonly referred to as ''kpoint''<ref name="Newell2002" />. A time series chart in Microsoft EXCEL of the concentrations of a contaminant (''y'' axis) on the date of sampling (''x'' axis) can be used to extract a value for ''kpoint''. Select the data, then insert an exponential trend line and display the equation on the chart. The value of ''kpoint'' can also be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Note that the rate constants extracted in EXCEL are constants for the rate of change, not the rate of attenuation. Take the negative of the rate of change to get ''kpoint''. In the example in Figure 1, the unit of time on the X axis is years, and the value of ''kpoint'' is 0.326 per year.

Attenuation versus distance rate coefficients describe a bulk attenuation rate including both degradation and non-destructive processes such as dispersion. To extract values for rate constants for degradation alone, it is necessary to calibrate a groundwater flow and transport model to the data at the site. The model is calibrated with values for the hydrogeological properties of the aquifer (effective porosity, hydraulic gradient, hydraulic conductivity, hydrodynamic dispersion and the organic carbon content of the aquifer matrix). After the hydrogeological properties of the aquifer are fixed in the model, the most appropriate values for the degradation rate constants are the values that produce the best fit between the contaminant concentrations that are predicted by the model and the contaminant monitoring data at the site.

There are a number of reasons why natural attenuation processes are better described as first-order relationship instead of zero-order or some other order. The attenuation over time in a monitoring well tracks the attenuation over time of the source of contamination that sustains the plume<ref name="Newell2002" />. Sites go through a lifecycle, and attenuation of sources at mature sites is often a first-order process<ref>Sale, T., Newell, C., Stroo, H., Hinchee, R. and Johnson, P., 2008. Frequently Asked Questions Regarding Management of Chlorinated Solvents in Soils and Groundwater. Environmental Security Technology Certification Program (ESTCP, Project ER-200530), Department of Defense (DoD), Arlington, VA. [//www.enviro.wiki/images/c/cb/2008-Sale-Frequently_Asked_Questions_Regarding_Management_of_Chlorinated_Solvent_in_Soils_and_Groundwater.pdf Report.pdf]   Project Overview Website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200530</ref>. If a chlorinated solvent site is mature, the contamination in the source area that was originally present as nonaqueous phase liquids (NAPL) has been redistributed and is now sequestered in a sorbed phase to aquifer solids or has diffused into non-transmissive portions of the aquifer matrix. Transfer of contaminants back into the more transmissive portions of the aquifer occurs by diffusion along a fixed path length, and the rate of transfer is controlled by the concentration of the contaminant remaining in the source material. Because the rate of transfer is proportional to the concentration of contaminant in the source material, attenuation of the source is a first-order process. These processes are discussed in more detail in [[Source Zone Modeling]].

Degradation processes are also usually first order. Abiotic reactions are almost always first order with respect to the concentration of the target chemical. Biodegradation reactions are zero order at high concentrations because the available enzymes are saturated with substrate, but are first order at lower concentrations that are typical of natural attenuation conditions in groundwater.

==Goals for MNA at Sites==

The information necessary to evaluate whether a site can be transitioned to MNA depends on the goal for MNA at the site. For many cleanup actions, the goal is to confine contamination within a waste management area where the contamination is left in place, in which case the cleanup goal applies to point-of-compliance wells that are outside the waste management area. For other cleanup actions, the entire site must be cleaned up, in which case the cleanup goal applies to any monitoring well on the site. The time by which the goal is to be attained is specified at CERCLA sites in the Record of Decision (the ROD). At RCRA sites, the time allowed for the cleanup to be attained may be specified in the permit.

==When the Goal Applies to Point-of-Compliance Wells==
Consider the following framework for evaluating a transition to MNA:

#Use a computer model to extract rate constants for the natural degradation of the contaminant that occurred in groundwater at the site before the active remedy was installed.
#Assume that the same rate constants will apply after the active remedy is no longer in operation. Note that this assumption may not be valid if the active remedy changes the geochemistry of the aquifer in the flow path to the point-of-compliance well.
#Calibrate a computer groundwater flow and transport model with the hydrogeological properties of the aquifer that pertain after the active remedy is no longer in operation, the concentration of contaminant after the active remedy, and the rate constants for natural degradation that are assumed to apply after the active remedy.
#Use the computer model to project the concentrations of the contaminant at the point-of-compliance well over time.
#If the concentrations at the point-of-compliance wells are predicted to be less than the goal before the specified date, that is a quantitative line of evidence in support of a transition to MNA.

There are several computer applications that are particularly useful to extract rate constants at a site from monitoring data that were collected before the active remedy was installed. For example, [https://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS)]<ref name="Widdowson2008" />, [https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system BIOCHLOR]<ref name="Aziz2002" /> and [https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 BioPIC]<ref name="BioPIC2021" /> can be downloaded from the internet at no cost. Another recent example, the [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ TA2 Tool]<ref name="TATA2024" />, is discussed in detail later in this article.

[[File:Wilson1w2Fig2.png|thumb|left|400px| Figure 2. Example calibration of NAS to natural attenuation of total BTEX at a site (Figure 17 of NAS User’s Manual).]]
[[File:Wilson1w2Fig3.png|thumb|400px| Figure 3. The data input screen for BIOCHLOR before remediation with cis-1,2-Dichloroethene (DCE) and vinyl chloride (VC) source concentrations of 500 and 87 mg/L respectively at the source when the release first occurred.]]
[[File:Wilson1w2Fig4.png|thumb|left|400px| Figure 4. Output of the RUN CENTERLINE simulation in BIOCHLOR comparing the fit between the simulation and the field data for vinyl chloride before an active remedy was implemented]]
[[File:Wilson1w2Fig5.png|thumb|400px| Figure 5. Output of the RUN CENTERLINE simulation of conditions after an active remedy was implemented with a source concentration of 1.1 mg/L, projecting the concentration of vinyl chloride at a distance corresponding to a point-of-compliance well.]]
 
In NAS, the user inputs the hydrogeological data, the distance of wells along the flow path, and the concentrations of contaminants in the wells. The NAS application extracts rate constants and makes projections at the point-of-compliance. With NAS, it is possible to extract different rate constants for specific geochemical environments along the flow path.

Figure 2 provides an example calibration of NAS. The concentrations in the monitoring wells used to calibrate the model are compared to the simulation provided by the model. The values of the rate constants that are extracted from the field data are available in the “Output” tab under “Data and Results Table.”

Figure 3 depicts the input screen for BIOCHLOR. The user inputs the hydrogeological parameters, the first-order rate constants (1st Order Decay Coefficient), the distribution of the wells along the flow path, and the concentrations of contaminants in the wells. The model is set up for conditions that apply before the installation of the active remedy.

BIOCHLOR does not automatically fit the rate constants to the field data. Instead, the user examines the output of the model, and adjusts the rate constants until they provide the best fit between the model prediction and the monitoring data for wells at the site. This comparison is illustrated in Figure 4.

If the distance from the source well to the point-of-compliance well is set as the “Modeled Area Length” in Section 5 of the input screen, the “Run Centerline” output will provide the projected concentrations at that length. Assume the distance from the source well to the point-of-compliance well is 250 feet. The projected concentration in Figure 3 of vinyl chloride at a point-of-compliance well is 0.042 mg/L. If the regulatory goal were the federal drinking water maximum contaminant level (MCL)<ref>U. S. Environmental Protection Agency (USEPA), 2009. National Primary Drinking Water Regulations. EPA 816-F-09-004. [//www.enviro.wiki/images/a/ae/2009-USEPA-national_Primary_Drinking_Water_Regulations.pdf Report.pdf]</ref> of 0.002 mg/L, the projected concentration would exceed the goal, and MNA would not be adequate as a remedy.

For the sake of illustration, assume that an active remedy has been implemented, and the concentrations in the source well are 5.4 mg/L for DCE and 1.1 mg/L for vinyl chloride. To evaluate whether it is now appropriate to transition to MNA, BIOCHLOR could be calibrated with these concentrations to predict concentrations in the point-of-compliance well. (See Figure 5). In this example, the projected concentration at the point-of-compliance well does meet the goal.

Some active remedies are subject to rebound. If this is the case, the evaluation should begin at the point in time when it is clear that the trend in concentrations is downward.

A new EXCEL-based tool that does many of the same basic calculations as BIOCHLOR was recently developed as part of an update to the BioPIC<ref name="BioPIC2021" /> decision support software. This tool, the MNA Rate Constant Estimator, extracts rate constants from concentration versus distance data for a variety of different chemicals, including chlorinated ethenes (e.g., PCE and TCE), chlorinated ethanes (e.g., 1,1,1-TCA), and 1,4-dioxane. This tool was developed to run using current versions of EXCEL, whereas BIOCHLOR must be run using older versions of EXCEL that may be unavailable to many users. The MNA Rate Constant Estimator can be used to estimate degradation rate constants and/or predict plume footprints over time. Consequently, it is a useful addition to the BioPIC decision framework for understanding if MNA is appropriate remedy for a site, and it can also be helpful for estimating rate constants as part of a transition assessment.

==When the Goal Applies to All the Wells==
At sites where a concentration-based cleanup goal must be achieved at all wells, each well at the site is evaluated independently, and the rate constant that is applicable is the rate constant for attenuation over time in the well (''kpoint''). To evaluate whether the region in an aquifer that is sampled by a particular monitoring well is ready to transition to MNA, it is necessary to have monitoring data from a period of time before the remedy was implemented. This data is used to extract a value for ''kpoint'' in the aquifer under natural attenuation conditions. The evaluation of a transition to MNA will assume that the same value for ''kpoint'' will apply after the active remedy is complete. This assumption may not be appropriate if the active remedy caused a permanent change in the geochemistry of the aquifer. The assumption is usually appropriate for pump-and-treat remedies.

If ''kpoint'' before implementation of the active remedy describes the time course of natural attenuation after the active remedy is completed, the time required to attain the cleanup goal is predicted from the following:

:{|
| || || rowspan="2" |<big>''ln (''</big>
| style="border-style:solid; border-width: 0px 0px 1px 0px" |''Cgoal''
| rowspan="2" |<big>'')''</big>||                                                                       
|-
|'''Equation 2:'''        ||''<big>t =''    
| style="vertical-align:top;" |''Ccurrent''||
|-
| || || colspan="3" style="text-align:center; border-style:solid; border-width: 1px 0 0 0" |''<big>-k</big>point''||
|-
|where:
|-
| ||''Ccurrent''    || colspan="5" |is the current concentration after active remediation,
|-
| ||''Cgoal''|| colspan="5" |is the cleanup goal, and
|-
| ||''t''|| colspan="5" |is the time required for concentrations to attenuate from ''Ccurrent'' to ''Cgoal.''
|}

If the value of ''t'' estimated using Equation 2 is less than the difference between the current date and the date specified by the site stakeholders to attain the goal, that is evidence in support of a transition to MNA.

Some active remedies are subject to contaminant concentration rebound. If this is the case, the evaluation should use a value of ''C''current that is attained after the rebound has stabilized.

This approach depends on a robust value for ''kpoint''. It is worthwhile to do a sensitivity analysis on ''kpoint'' where the lower 95% or 90% confidence interval on ''kpoint'' is used in Equation 2 to see if that changes the outcome of the evaluation. The confidence intervals can be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Wilson<ref name="Wilson2011">Wilson, J.T. 2011. An Approach for Evaluating the Progress of Natural Attenuation in Groundwater. EPA 600-R-11-204. [//www.enviro.wiki/images/e/e3/Wilson-2011-An_Approach_for_Evaluating_Progress.pdf Report.pdf]</ref> provides detailed discussion of the use of linear regression to extract ''kpoint'' and confidence intervals on ''kpoint''. Wilson<ref name="Wilson2011" /> also discusses the use of goodness-of-fit tests to determine if there is evidence that a first-order rate equation is not the best fit to the monitoring data, and as a result the use of Equation 2 would not be appropriate. The TA2 Tool<ref name="TATA2024" /> also has the capability to calculate ''kpoint'' with a user-specified confidence interval, as described below.

At many sites, there is no specified date when the cleanup goal must be attained. In this situation, the monitoring data can be evaluated to determine if the current rate of attenuation under the active remedy is faster than the rate of natural attenuation before the active remedy was installed. The monitoring data can be examined to identify a time interval when the benefit of the active remedy has approached an asymptote. A second value of for ''kpoint'' can be extracted for that time interval. The two values for ''kpoint'' can be evaluated statistically to see if the current rate is faster at some appropriate level of confidence. If there is no statistical evidence that the rate of attenuation is faster, that determination can support a decision to transition to MNA.
[[File:Wilson1w2Fig6.png|thumb|400px| Figure 6. Example calibration of NAS to predict the reduced concentration at the source that is necessary to meet the remediation goal at a point-of-compliance well (Figure 19 of NAS User’s Manual).]]

==Extent of Treatment Necessary to Transition to MNA==
There are several computer applications that can predict the extent of treatment that must be achieved by the active remedy before it is worthwhile to evaluate the site for transition to MNA. For example, based on the distribution of contamination along the flow path, the NAS application will automatically predict a reduced concentration at the source well that will bring concentrations to the goal in the point-of-compliance well (Figure 6). A table that opens under the “DOS/TOS” tab provides the “Time of Equilibration” required to meet the goal at the reduced concentration. Modules in NAS allow the user to evaluate the effect of various pump-and-treat and source removal scenarios on the time required to attain the goal at the point-of-compliance well.

The [[REMChlor - MD | REMChlor-MD]]<ref name="Falta2018">Falta, R.W., Farhat, S.K., Newell, C.J. and Lynch, K., 2018. A Practical Approach for Modeling Matrix Diffusion Effects in REMChlor. SERDP/ESTCP Project ER-201426 [//www.enviro.wiki/images/0/0b/2018-Falta-REMChlor_Modeling_Matrix_Diffusion_Effects.pdf Report.pdf]   Website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201426</ref> and [https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel REMFuel]<ref name="Falta2012">Falta, R.W., Ahsanuzzaman, A.N., Stacy, M.B., Earle, R.C. and Wilson, J.T., 2012. Remediation Evaluation Model for Fuel Hydrocarbons (REMFuel). Users Manual Version 1.0. U.S. Environmental Protection Agency. EPA/600/R-12/028. [//www.enviro.wiki/images/6/67/2012-Falta-REMFuel_Remediation_Evaluation-Model_for_Fuel_hydrocarbons_users_manual.PDF Report.pdf]   Website: https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel</ref> models are flexible screening tools that allow a simultaneous evaluation of the extent of treatment provided by (1) source removal, (2) ''in situ'' remediation of the contaminated groundwater, or (3) natural attenuation processes in three discrete intervals along the flow path and three discrete time periods. Both [[REMChlor - MD | REMChlor-MD]]<ref name="Falta2018" /> and [https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel REMFuel]<ref name="Falta2012" /> can be downloaded from the internet at no cost. Liang ''et al.''<ref>Liang, H., Falta, R.W., Newell, C.J., Farhat, S.K., Rao, P.S. and Basu, N., 2010. Decision & Management Tools for DNAPL Sites: Optimization of Chlorinated Solvent Source and Plume Remediation Considering Uncertainty. SERDP/ESTCP Project ER-200704. [//www.enviro.wiki/images/c/ce/2010-Liang-Decision_and_Management_Tools_for_DNAPL_sites-ER-200704-FR.pdf Report.pdf]   Project Overview Website: https://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200704/(language)/eng-US</ref> provide a modeling program that uses Monte Carlo simulations to evaluate the effects of the uncertainties in the modeling parameters on the predictions of REMChlor-MD.

==The Transition Assessment Teaching Assistant (TA2) Tool==
[[File:Wilson1w2Fig7.png|thumb|500px| Figure 7. Home Page for TA2 Tool. Users can click on buttons to access various modules that are designed to answer specific questions or research relevant topics.]]
[[File:Wilson1w2Fig8.png|thumb|500px| Figure 8. Example of an asymptote analysis using concentration versus time data in Tool 1 of the TA2 Tool. The source attenuation rate and corresponding remediation timeframe can be estimated for different monitoring periods.]]
A learning and decision-making tool was recently released as part of [https://serdp-estcp.mil/ Strategic Environmental Development and Research Program (SERDP)] Project [https://serdp-estcp.mil/projects/details/350cbc0b-893a-43a6-8a0c-c9c057bacac0/er20-1429-project-overview ER-201429] to help stakeholders gather information for the purposes of a site-specific transition assessment. This free software, the [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Transition Assessment Teaching Assistant (TA2) Tool]<ref name="TATA2024" />, was developed using the elements identified in the 2013 NRC report<ref name="NRC2013" /> as the critical learning objectives for end users.

The Tool is a web-based app that includes a collection of individual modules designed to answer specific questions or research relevant topics (Figure 7). The Tool has been developed as an R Shiny app (version 1.8.0)<ref> Chang, W., Cheng, J., Allaire, J., Sievert, C., Schloerke, B., Xie, Y., Allen, J., McPherson, J., Dipert, A., Borges, B., 2023. shiny: Web Application Framework for R. R package version 1.8.0, https://github.com/rstudio/shiny, https://shiny.posit.co/</ref>, which is an interactive platform using R programming to perform all quantitative functions. The user can then view the results in a simple interface that easily accommodates plots, charts, and various mapping features in a Web browser. The Tool is free and does not require the user to install R software.

The modules within the TA2 Tool include:

'''Five Quantitative Tools''' that focus on assessing asymptotic groundwater concentrations from monitoring data, evaluating plume stability, estimating remediation timeframes after a hypothetical source removal project, forecasting remediation performance if a technology is applied in the field, or projecting concentrations at downgradient points of compliance.

For example, Tool 1 in the TA2 Tool uses concentration versus time data from monitoring wells to estimate attenuation rate constants (''kpoint'') and evaluate if asymptotic conditions are present at particular locations or across the site. This helps to assess whether performance has plateaued at wells where a pump-and-treat system or other active treatment is in place. The user has the option to choose a “change point” within the monitoring record to determine if the attenuation rate has changed over time (e.g., once most of the accessible mass has been removed) (Figure 8). The user can either use visual interpretation to manually select the date when this apparent change occurred or have the date selected automatically using a binary segmentation protocol that is incorporated into the tool. The tool will calculate a rate for both the early period and a rate for the later period (after the change point), and then go through five different lines of evidence for asymptotic behavior (e.g., are the two rates of attenuation significantly different?). The user can then use the collective results as a technical justification demonstrating that the performance of the active remedy has plateaued as the first step in the transition assessment. The tool will also estimate the time to reach a user-specified cleanup goal if the overall attenuation rate (or the attenuation rate in the later period) were to continue.

Another module (Tool 5) focuses on evaluating sites where the concentration goal applies at a downgradient point of compliance, which is a key criterion for sites where MNA is being used as part of a risk-based strategy. The tool includes several different options to estimate a site-specific attenuation rate constant, including data from the pre-remediation period when natural attenuation processes were the sole means for reducing concentrations. Attenuation rate constants are then used to project the concentration versus distance from the contaminant source. Based on the predicted concentration at the downgradient point of compliance, the user can then see if the natural assimilative capacity along the aquifer flow path is sufficient to achieve the concentration goal in the absence of active treatment. For example, in the tab labeled “Use Pre-Remediation Rate Constant”, the logarithms of the concentrations from the period before active treatment began are plotted against the distance from the source well. The slope of the regression line is the rate constant for natural attenuation (including the contributions of degradation and dispersion). This rate constant can then be used to project the concentration moving downgradient from the well of concern after the end of active treatment. Similar approaches are provided within Tool 5 for using rate constants estimated from lab-based testing or derived from post-remediation data (after steady state has been reestablished).

'''Four Qualitative Tools''' provide information on matrix diffusion, enhanced attenuation options, geologic heterogeneity, and related research on transition assessments. Many of these modules are based on the current understanding of the role of matrix diffusion in influencing long-term concentration trends and remedial performance at contaminated groundwater sites. This includes summaries of different modeling options for better quantifying the effects of matrix diffusion. Sites impacted by matrix diffusion are generally challenging to treat using active remedies and thus are better candidates for less intensive management strategies that focus on reducing mass discharge rates, stabilizing the plume, and protecting potential downgradient receptors. As a result, matrix diffusion is critical to understanding and quantifying how natural attenuation processes are contributing to concentration trends.

'''One Summary Tool''' (Tool 10) compiles metrics from the other tools into a “Remediation Transition Assessment Index” (RTAI) and provides additional guidance on conducting site-specific transition assessments. The RTAI is a simple metric with a value from 1 to 5, where higher values reflect greater persistence of contamination due to matrix diffusion and other site-specific factors. An RTAI value is assigned to each of the results from the different tools that have been completed by the user. An RTAI of 5 suggests that the site is a strong candidate for transitioning to MNA or enhanced attenuation approaches, while a site with an RTAI value of 1 is a poor candidate. The user can assign an overall RTAI for the site based on the preponderance of evidence after reviewing the RTAI values generated by each tool, or calculate a site RTAI based on simple averaging, weighting, or other methods.

Tool 10 also contains a flowchart and a checklist for performing site-specific transition assessments that start with evaluating relevant bright line criteria, such as (1) can the concentration goals be met at the point of compliance by MNA; and (2) is the remediation timeframe for MNA reasonable and/or similar to the timeframe if source remediation were used. This checklist ensures that the user has gathered all relevant information that would be needed to support a technically rigorous site-specific Transition Assessment.

The TA2Tool provides a framework for remedial decision makers to evaluate different types of sites, including those where active treatment (e.g., pump and treat) is in use, as well as sites where future active source zone remediation is being considered. It also includes a description of enhanced MNA alternatives for sites where MNA alone may not be sufficient to control risk. As shown in Figure 8, the tool can be used to answer specific questions that have a primarily quantitative basis or to provide focused qualitative information for researching specific topics. Users can engage with just the modules that might be pertinent to assessment of an individual site, or they can go through all the modules to perform a more thorough, step-by-step analysis of the relevant issues for their site.

==Summary==
Tools and approaches are available that can be adapted to determine when a site is ready to transition from active remedy to MNA. However, these tools and approaches have not been applied for this purpose at a significant number of sites, and at the present time, they are not generally accepted by regulatory authorities. There is an opportunity to establish and implement a logical and consistent framework that can be widely implemented to evaluate sites for transition from active remedy to MNA.

==References==
<references />

==See Also==

*[http://dx.doi.org/10.1007/978-1-4614-6922-3 Newell, C.J., Kueper, B.H., Wilson, J.T., Johnson, P.C., 2014. Natural Attenuation of Chlorinated Solvent Source Zones. In: Chlorinated Solvent Source Zone Remediation, Editors: Kueper, B.H., Stroo, H.F., Vogel, C.M., Ward. SERDP ESTCP Environmental Remediation Technology, vol 7. Springer, New York, NY. pgs. 459-508. doi: 10.1007/978-1-4614-6922-3]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436/ER-200436/(language)/eng-US Kram, Mark, and Widdowson, Mark, 2008. Estimating Cleanup Times Associated with Combining Source-Area Remediation with Monitored Natural Attenuation. ESTCP ER-200436]

Monitored Natural Attenuation (MNA) of Chlorinated Solvents

2026-05-07T16:56:24Z

Admin:

[[Monitored Natural Attenuation (MNA)]] is a common remedy for contamination of [[Chlorinated Solvents |chlorinated solvents]] in groundwater. Chlorinated solvents are susceptible to many natural processes that can attenuate their concentrations in groundwater including biological degradation, abiotic degradation, sorption, dispersion, and volatilization. Typically, MNA is used for plumes with low dissolved concentrations or in peripheral areas of plumes away from areas with non-aqueous phase liquid (NAPL) or other materials that serve as the source of groundwater contamination.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''
*[[Chlorinated Solvents]]
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Monitored Natural Attenuation (MNA) of Fuels| Monitored Natural Attenuation of Fuels]]
*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| Monitored Natural Attenuation of Metal and Metalloids]]
*[[Natural Source Zone Depletion (NSZD)]]

'''CONTRIBUTOR(S):''' [[Dr. John Wilson]] 

'''Key Resource(s):'''
*[[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Identification & Characterization Methods for Reactive Minerals Responsible for Natural Attenuation of Chlorinated Organic Compounds in Ground Water]]<ref name="HE2009">He, Y., Su, C., Wilson, J., Wilkin, R., Adair, C., Lee, T., Bradley, P. and Ferrey, M., 2009. Identification and characterization methods for reactive minerals responsible for natural attenuation of chlorinated organic compounds in ground water. U.S. Environmental Protection Agency. [[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Report pdf]]</ref>

==Introduction==
[[Chlorinated Solvents |Chlorinated solvents]] and their transformation products are among the most abundant contaminants in groundwater. In 2006, the United States Geological Survey published results from a systematic survey of volatile organic chemicals in drinking water wells<ref>Zogorski, J.S., Carter, J.M., Ivahnenko, T., Lapham, W.W., Moran, M.J., Rowe, B.L., Squillace, P.J., Toccalino, P.L., 2006. The quality of our Nation’s waters - Volatile organic compounds in the nation’s ground water and drinking-water supply wells. US Geological Survey Circular, 1292, 101. [[Media:Zogorski-2006-_Volatile_organic_compounds_in_the_nations_ground_water_and_wells.pdf|Report pdf]]</ref> in the USA. Approximately 12% of wells contained detectable concentrations of tetrachloromethane ([[wikipedia: Chloroform | chloroform]]), 5% contained [[wikipedia: Tetrachloroethylene | tetrachloroethene (PCE)]], 4% contained [[wikipedia: Trichloroethylene | trichloroethene (TCE)]], 2% contained [[wikipedia: 1,1,1-Trichloroethane | 1,1,1-trichloroethane (1,1,1-TCA)]], and 2% contained [[wikipedia: 1,1-Dichloroethane | 1,1-dichloroethane (1,1-DCA)]].

[[Monitored Natural Attenuation (MNA) | Monitored Natural Attenuation (MNA)]] is one remedy that is available for contamination from chlorinated solvents in groundwater. Natural processes that can attenuate the concentrations of chlorinated solvents in groundwater include biological degradation, abiotic degradation, sorption, dispersion into ground adjacent to the contaminant plume, and volatilization to soil gas above the groundwater. At most sites where MNA has been selected as a remedy, or part of a remedy, the chlorinated solvents have been shown to be degrading in groundwater.

==Biodegradation==
The prospects for degradation of selected chlorinated solvents and their transformation products in groundwater are good (Table 1).

[[File:Wilson 3 Table1.JPG|thumbnail|600 px|left|Table 1. Summary of the prospects for degradation of selected chlorinated solvents and their transformation products in groundwater<ref>Lawrence, S.J., 2006. Description, properties, and degradation of selected volatile organic compounds detected in ground water--A review of selected literature (No. 2006-1338). [[Media:Lawrence-2006-Description_properties_degradation_of_VOCs.pdf|Report pdf]]</ref><ref name="HE2009"/>]]
Biodegradation can occur under both aerobic and anaerobic conditions. Under aerobic conditions, the chlorinated solvent can act as a source of food for the microorganisms (referred to as direct biodegradation in Table 1). Degradation can also be a fortuitous reaction that does not provide any benefit to the microorganisms. The fortuitous reaction is called a cometabolism or cooxidation. The fortuitous reaction is most commonly carried out by an oxygenase enzyme that is produced by the microorganisms in order to allow them to degrade some other compound.

When the chlorinated solvent is degraded as a food source, the population of active organisms and the rate of degradation will increase over time. If the degradation is fortuitous, the bacteria do not grow as a result of degrading the chlorinated solvent, and the rate constant does not increase over time.

The prospects for direct aerobic biodegradation of chlorinated alkenes depends on the extent of chlorination. PCE and TCE do not support growth under aerobic conditions, cis-dichloroethene<ref>Cox, E., 2012. Elucidation of the mechanisms and environmental relevance of cis-dichloroethene and vinyl chloride biodegradation. ER-1557. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1557/ER-1557 ER-1557]</ref> (c-DCE) can be degraded in aerobic groundwater, and vinyl chloride (VC) is readily degradable in many aerobic groundwaters.

Many samples of groundwater contain microorganisms that express oxygenase enzymes and can cometabolize PCE, TCE or dichloroethene (DCE)<ref>ITRC. 2011. Enzyme Activity Probes EMD Team Fact Sheet. [http://www.itrcweb.org/documents/team_emd/EAP_Fact_Sheet.pdf Fact Sheet]</ref>. However, the specific contribution of these organisms to MNA is not well understood<ref>Looney, B., 2010. Incorporating Aerobic Processes into Remedies for Large Chlorinated Solvent Plumes. ER-201026. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201026/ER-201026 ER-201026]</ref>, and studies are trying to define their contribution<ref>Wiedemeier, T.H., 2015. Providing Additional Support for MNA by Including Quantitative Lines of Evidence for Abiotic Degradation and Cometabolic Oxidation of Chlorinated Ethylenes. ER-201584. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201584/ER-201584 ER-201584]</ref>.

Under anaerobic conditions, the chlorinated solvents act as an electron acceptor. In such cases, electron donors may be in the form of naturally occurring, bioavailable organic carbon, or possibly from comingled plumes of petroleum hydrocarbons. The chlorinated solvents function in bacterial metabolism in the same fashion as oxygen functions in human metabolism. The chlorinated solvents are essentially something for the bacteria to breath in the absence of other electron acceptors such as oxygen, nitrate, or sulfate.

[[File:Wilson 3 Fig1.png|thumbnail|450 px|right|Figure 1. Degradation chlorinated alkenes to ethene.]]
In anaerobic groundwater, when conditions are favorable, chlorinated alkenes can undergo a sequential reductive dehalogenation where a chlorine atom is replaced with a hydrogen atom. Degradation proceeds from PCE to TCE, then to DCE, then to VC and finally to ethene (Fig. 1). The minimal geochemical conditions<ref>Wiedemeier, T.H., Swanson, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T., Wilson, B.H., Kampbell, D.H., Haas, P.E., Hansen, J.E., Chapelle, F.H., 1998. Technical protocol for evaluating natural attenuation of chlorinated solvents in ground water. EPA-600-R-98-128. [[Media:Wiedemeier-1998-Technical_Protocol_for_Evaluating_Natuaral_Attenuation.pdf|Report pdf]]</ref> that must be taken into account include pH, oxidation-reduction potential (ORP), dissolved oxygen (DO) concentration, total organic carbon (TOC) and competing electron acceptors including oxygen, nitrate, sulfate and ferric iron.

PCE and TCE can be used as an electron acceptor by a wide variety of bacteria<ref>Nyer, E.K., Payne, F., Sutherson, S., 2003. Discussion of environment vs. bacteria or let's play,‘name that bacteria’. Groundwater Monitoring & Remediation, 23(2), 32-48. [http://dx.doi.org/10.1111/j.1745-6592.2003.tb00665.x doi: 10.1111/j.1745-6592.2003.tb00665.x]</ref>. The bacteria can degrade PCE or TCE as far as DCE. The only organisms that can degrade DCE to VC and then degrade VC to the harmless end product ethene are stains of ''Dehalococcoides mccartyi''<ref>Löffler, F.E., Ritalahti, K.M., Zinder, S.H., 2013. Dehalococcoides and reductive dechlorination of chlorinated solvents. Bioaugmentation for groundwater remediation, ed. H.F. Stroo, Leeson, A., Ward, C.H. Springer, New York, NY. pgs. 39-88. ISBN: 978-1-4614-4114-4 ISBN 978-1-4614-4115-1. [http://dx.doi.org/10.1007/978-1-4614-4115-1 doi: 10.1007/978-1-4614-4115-1]</ref>.

The degradation of chlorinated alkanes in anaerobic groundwater is more complicated (Fig. 2). Chlorinated alkanes can undergo a sequential reductive dehalogenation. In addition, they can undergo the loss of a hydrogen and a chlorine atom to form an alkene (a dehydrochlorination) or the loss of two chlorine atoms to form an alkene (a dichloroelimination).

Three reactions have been demonstrated for 1,1,1-TCA in groundwater (Fig. 2)<ref>Scheutz, C., Durant, N.D., Hansen, M.H., Bjerg, P.L., 2011. Natural and enhanced anaerobic degradation of 1,1,1-trichloroethane and its degradation products in the subsurface–a critical review. Water Research, 45(9), 2701-2723. [http://dx.doi.org/10.1016/j.watres.2011.02.027 doi:10.1016/j.watres.2011.02.027]</ref>. It can undergo an abiotic hydrolysis reaction to produce acetate, an abiotic dehydrochlorination to produce 1,1-DCE, and a biological reductive dechlorination reaction to 1,1-DCA and then chloroethane. In addition to being reduced to chloroethane, 1,1-DCA can undergo a dichloroelimination reaction<ref>Lollar, B.S., Hirschorn, S., Mundle, S.O., Grostern, A., Edwards, E.A., Lacrampe-Couloume, G., 2010. Insights into enzyme kinetics of chloroethane biodegradation using compound specific stable isotopes. Environmental Science & Technology, 44(19), 7498-7503. [http://dx.doi.org/10.1021/es101330r doi: 10.1021/es101330r]</ref> to produce ethene.

[[File:Wilson 3 Fig2.PNG|thumbnail|400 px|left|Figure 2. Degradation of Chlorinated alkanes to ethane.]]

The degradation of 1,1,2-TCA follows a similar pattern (Fig. 2). One strain of ''Desulfitobacterium'' has been shown to dechlorinate 1,1,2-TCA to 1,2-DCA and chloroethane<ref>Zhao, S., Ding, C., He, J., 2015. Detoxification of 1,1,2-trichloroethane to ethene by desulfitobacterium and identification of its functional reductase gene. PloS One, 10(4), p.e0119507. [http://dx.doi.org/10.1371/journal.pone.0119507 doi:10.1371/journal.pone.0119507]</ref> through a sequential reductive dehalogenation. Certain strains of ''Dehalogenimonas'' go through a dichloroelimination reaction<ref>Bowman, K.S., Nobre, M.F., da Costa, M.S., Rainey, F.A. and Moe, W.M., 2013. Dehalogenimonas alkenigignens sp. nov., a chlorinated-alkane-dehalogenating bacterium isolated from groundwater. International Journal of Systematic and Evolutionary Microbiology, 63(4), 1492-1498. [http://dx.doi.org/10.1099/ijs.0.045054-0 doi: 10.1099/ijs.0.045054-0]</ref> to dechlorinate 1,1,2-TCA to VC and 1,2-DCA to ethene. A strain of ''Dehalobacter'' can also dechlorinate 1,2-DCA to ethene<ref>Grostern, A., Edwards, E.A., 2009. Characterization of a Dehalobacter coculture that dechlorinates 1,2-dichloroethane to ethene and identification of the putative reductive dehalogenase gene. Applied and Environmental Microbiology, 75(9), 2684-2693. [http://dx.doi.org/10.1128/aem.02037-08 doi: 10.1128/AEM.02037-08]</ref>.

A quantitative framework (BioPIC)<ref name="Lebron2015">Lebron, C. A., Wiedemeier, T. H., Wilson, J.T., Löffler, F.E., Hinchee, R.E., Singletary, M.A., 2015. Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites. ER-201129. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129/ER-201129 ER-201129]</ref> is now available that allows an evaluation of the rate constant for anaerobic biological degradation of cDCE and VC based on the abundance of gene markers for ''Dehalococcoides mccartyi''. The relationships between the rate constants for degradation of the chlorinated alkanes and abundance of gene copies of ''Dehalobacter'', ''Dehalogenimonas'' and other active bacteria are still being explored.

==Abiotic Degradation==
Chlorinated solvents can chemically react with a number of iron minerals in aquifers<ref name="HE2009"/>. The most important of these are magnetite, iron mono-sulfide, and pyrite.

Iron sulfide minerals form as a consequence of sulfate reduction in groundwater. The sulfide produced from sulfate reduction will react with Iron (III) minerals to form iron mono-sulfide. Over time the iron mono-sulfide will react with excess sulfide to produce pyrite.

The reactions of the chlorinated alkanes with the iron sulfide minerals is a sequential reductive dechlorination. However, the reaction of iron sulfide minerals with chlorinated alkenes is more complex (Fig. 3). Reductive dechlorination and dichloroelimination can proceed at the same time.

[[File:Wilson 3 Fig3.png|thumbnail|250 px|right|Figure 3. Degradation of chlorinated alkenes carried out by iron sulfide minerals.]]

The U.S. EPA regulates the maximum concentration of contaminants that are allowed in water that is supplied as drinking water. These U.S. EPA regulations are referred to as the Maximum Contaminant Level<ref>U.S. Environmental Protection Agency, 2016. Table of Regulated Drinking Water Contaminants. [http://www.epa.gov/your-drinking-water/table-regulated-drinking-water-contaminants Table of Regulated Drinking Water]</ref> or MCL. There are MCLs for the transformation products of reductive dechlorination (the DCEs and VC) and these products will be included in the target list of analytes in any conventional monitoring program. The products of dichloroelimination do not have MCLs and are not usually on the target list of analytes for conventional monitoring.

If the major pathway of abiotic degradation is dichloroelimination, then conventional monitoring will fail to recognize the contribution of abiotic degradation on iron sulfide minerals. However, the stable isotopes of carbon in chlorinated solvents are strongly fractionated during abiotic degradation on iron sulfide minerals. [[Compound Specific Isotope Analysis (CSIA) | Compound Specific Isotope Analysis (CSIA)]]<ref>Hunkeler, D., Meckenstock, R. U., Sherwood Lollar, B., Schmidt, T.C., Wilson, J.T., 2008. A Guide for Assessing Biodegradation and Source Identification of Organic Groundwater Contaminants Using Compound Specific Isotope Analysis (CSIA). U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-08/148. [[Media:Hunkeler-2008-A_Guide.pdf|Report pdf]]</ref> can be a useful tool to recognize abiotic degradation of chlorinated alkenes on iron sulfide minerals.

Magnetite is often present in unconsolidated glacial aquifers and aquifers that form in sediments that are shed by uplands composed of granite or other igneous rocks. Magnetite reacts readily with the chlorinated alkenes. The actual chemical interactions on magnetite are not well understood (Fig. 4). The ultimate degradation products are oxidized organic compounds and carbon dioxide<ref>Darlington, R., Rectanus, H., 2015. Biogeochemical Transformation Handbook. TR-NAVFAC EXWC-EV-1601, 41 pgs. [[Media:Darlington-2015-Biogeochem_Transformation_Handbook.pdf|Report pdf]]</ref>.

[[File:Wilson 3 Fig4.png|thumbnail|300 px|left|Figure 4. Degradation of chlorinated alkenes carried out by magnetite.]]

==Footprints==
Most plumes have some contribution of anaerobic sequential reductive dechlorination. As a result, the primary contaminant and the transformation products of reductive dechlorination are present in the groundwater. The highest concentrations of the primary contaminant will be near the source of contamination, and the flow of groundwater carries the transformation products further downgradient from the source (Figure 5).

Many plumes of chlorinated solvents also have a contribution of abiotic degradation. As a result, the intermediate degradation products (such as DCE) do not accumulate to stoichiometric concentrations. There is an appearance that degradation of the cDCE has stalled, when in fact it is actively degrading, but not to vinyl chloride (Fig. 5). A quantitative framework<ref name="Lebron2015"/> is now available that allows an evaluation of the contribution of abiotic degradation on magnetite based on the magnetic susceptibility of the sediment, and the contribution of abiotic degradation on pyrite based on the extent of sulfate reduction and the geochemistry of the groundwater.
[[File:Wilson 3 Fig5.png|thumbnail|400 px|center|Figure 5. Comparison of a chlorinated alkenes plume undergoing biodegradation alone vs. biodegradation with abiotic degradation.]]

==Tools and Databases for Chlorinated Solvent MNA==
The Scenarios Evaluation Tool for Chlorinated Solvent MNA<ref>Truex, M.J., Newell, C.J., Looney, B.B, Vangelas, K., 2006. Scenarios evaluation tool for chlorinated solvent MNA. Savannah River National Laboratory, Aiken, South Carolina. WSRC-STI-2006-0096. [[Media:Truex-2006-Scenarios_Evaluation_Tool_for_Chlorinated_Solvent_MNA.pdf|Report pdf]]</ref> was designed to provide a structure where the MNA methods and decision logic are linked together in one of 13 different “scenarios” or site types. Based on site data (e.g. Table 2), one selects which of the 13 scenarios best fits their site or portion of a site. Then one goes to the description of that scenario to learn which attenuation reactions are likely to be active, how to design a MNA monitoring program, whether MNA will work, and other relevant factors.
[[File:Wilson 3 Table2.png|thumbnail|600 px|center|Table 2. Key elements of the scenarios tool for chlorinated solvent MNA.]]

A data mining study of MNA at 45 chlorinated solvent sites<ref>McGuire, T.M., Newell, C.J., Looney, B.B., Vangelas, K.M., 2003. Historical and retrospective survey of monitored natural attenuation: A line of inquiry supporting monitored natural attenuation and enhanced passive remediation of chlorinated solvents. Westinghouse Savannah River Company, Aiken, SC. [[Media:McGuire-2003-Historical_and_Retrospective_Survey_of_MNA.pdf|Report pdf]]</ref> provides some interesting information about plume sources, strength, and size (Fig. 6).

[[File:Wilson 3 Fig6.png|thumbnail|500 px|center|Figure 6. Plume characteristics evaluation of 45 chlorinated solvent sites.]]

The performance of MNA was evaluated<ref>McGuire, T., 2016. Development of an Expanded, High-Reliability Cost and Performance Database for In-Situ Remediation Technologies. ESTCP Project No. ER-201120. [https://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201120/ER-201120 ER-201120]</ref> by comparing the change in concentrations of chlorinated organic compounds in wells in the source zone of plumes from the beginning to the end of an MNA monitoring period (Fig. 7).

[[File:Wilson 3 Fig7.png|thumbnail|500 px|center|Figure 7. Each dot represents an individual project, showing the geometric mean of the concentration at the beginning of the monitoring record (X-axis) and at the end of the monitoring record (Y-axis). The median duration of MNA monitoring for these 45 sites was 8.7 years and ranged from 4.1 to 15 years.]]

One study evaluated the change in source concentration over time at 23 chlorinated solvent sites by calculating concentration vs. time decay rates for source zone wells<ref>Newell, C.J., Cowie, I., McGuire, T.M., McNab Jr, W.W., 2006. Multiyear temporal changes in chlorinated solvent concentrations at 23 monitored natural attenuation sites. Journal of Environmental Engineering, American Society of Environmental Engineers, 132(6), 653-663. [http://dx.doi.org/10.1061/(asce)0733-9372(2006)132:6(653) doi: 10.1061/(asce)0733-9372(2006)132:6(653)]</ref>. The authors concluded, “If the median point decay rates from these sites are maintained over a 20 year period, the resulting reduction in concentration will be similar to the reported reduction in source zone concentrations achieved by active in situ source remediation technologies (typical project length: 1–2 years)".

As part of the development process for the chlorinated solvent natural attenuation model<ref>Aziz, C.E., Smith, A.P., Newell, C.J., Gonzales, J.R., 2000. BIOCHLOR Chlorinated solvent plume database report. Air Force Center for Environmental Excellence, Texas. [[Media:Aziz-2000-BIOCHLOR-plume-database.pdf|Report pdf]]</ref> BIOCHLOR, 24 chlorinated solvent plumes were studied in detail. Key findings included:

*TCE and c-DCE had median plume lengths of 1215 ft and 1205 ft, respectively.
*Chlorinated ethene plume lengths were moderately correlated with seepage velocity and source width (Fig. 8).
*First order decay rates ranged between 1 and 2 per year for the chlorinated ethane plumes.

[[File:Wilson 3 Fig8.png|thumbnail|900 px|center|Figure 8. Effect of estimated source size and groundwater seepage velocity on plume length.]]

==Summary==
MNA is an important remediation technology at some chlorinated solvent sites. There are numerous reactions, both biotic and abiotic, that can act on different chlorinated solvent compounds. Several tools and databases are available to help understand how chlorinated solvent plumes behave and to design and implement appropriate MNA programs.

==References==

<references/>

==See Also==
*[[Media:EPA-MNA-Chlorinated-Organics-Symposium.pdf|Proceedings of the Symposium on Natural Attenuation of Chlorinated Organics in Ground Water]]
*[[Media:AFCEE-Natural_Attenuation-Chlorinated_Solvents-1999.pdf|Natural Attenuation of Chlorinated Solvents Performance and Cost Results From Multiple Air Force Demonstration Sites]]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-1348 Using Advanced Analysis Approaches to Complete Long-Term Evaluations of Natural Attenuation Processes on the Remediation of Dissolved Chlorinated Solvent Contamination]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-1349/ER-1349 Integrated Protocol for Assessment of Long-Term Sustainability of Monitored Natural Attenuation of Chlorinated Solvent Plumes]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200019 Impact of Landfill Closure Designs on Long-Term Natural Attenuation of Chlorinated Hydrocarbons]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436 Estimating Cleanup Times Associated with Combining Source-Area Remediation with Monitored Natural Attenuation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200708/ER-200708 Use of Enzyme Probes for Estimation of Trichloroethene Degradation Rates and Acceptance of Monitored Natural Attenuation ]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200824/ER-200824 Verification of Methods for Assessing the Sustainability of Monitored Natural Attenuation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129 Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201211/ER-201211 Frequently Asked Questions about Monitored Natural Attenuation in the 21st Century]
*[https://www.coursera.org/learn/natural-attenuation-of-groundwater-contaminants/lecture/kBe2j/abiotic-degradation-principles Online Lecture Course - Abiotic Degradation]

Monitored Natural Attenuation (MNA) of Chlorinated Solvents

2026-05-07T16:56:03Z

Admin:

[[Monitored Natural Attenuation (MNA)]] is a common remedy for contamination of [[Chlorinated Solvents |chlorinated solvents]] in groundwater. Chlorinated solvents are susceptible to many natural processes that can attenuate their concentrations in groundwater including biological degradation, abiotic degradation, sorption, dispersion, and volatilization. Typically, MNA is used for plumes with low dissolved concentrations or in peripheral areas of plumes away from areas with non-aqueous phase liquid (NAPL) or other materials that serve as the source of groundwater contamination.
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'''Related Article(s):'''
*[[Chlorinated Solvents]]
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Monitored Natural Attenuation (MNA) of Fuels| Monitored Natural Attenuation of Fuels]]
*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| Monitored Natural Attenuation of Metal and Metalloids]]
*[[Natural Source Zone Depletion (NSZD)]]

'''CONTRIBUTOR(S):''' [[Dr. John Wilson]] 

'''Key Resource(s):'''
*[[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Identification & Characterization Methods for Reactive Minerals Responsible for Natural Attenuation of Chlorinated Organic Compounds in Ground Water]]<ref name="HE2009">He, Y., Su, C., Wilson, J., Wilkin, R., Adair, C., Lee, T., Bradley, P. and Ferrey, M., 2009. Identification and characterization methods for reactive minerals responsible for natural attenuation of chlorinated organic compounds in ground water. U.S. Environmental Protection Agency. [[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Report pdf]]</ref>

==Introduction==
[[Chlorinated Solvents |Chlorinated solvents]] and their transformation products are among the most abundant contaminants in groundwater. In 2006, the United States Geological Survey published results from a systematic survey of volatile organic chemicals in drinking water wells<ref>Zogorski, J.S., Carter, J.M., Ivahnenko, T., Lapham, W.W., Moran, M.J., Rowe, B.L., Squillace, P.J., Toccalino, P.L., 2006. The quality of our Nation’s waters - Volatile organic compounds in the nation’s ground water and drinking-water supply wells. US Geological Survey Circular, 1292, 101. [[Media:Zogorski-2006-_Volatile_organic_compounds_in_the_nations_ground_water_and_wells.pdf|Report pdf]]</ref> in the USA. Approximately 12% of wells contained detectable concentrations of tetrachloromethane ([[wikipedia: Chloroform | chloroform]]), 5% contained [[wikipedia: Tetrachloroethylene | tetrachloroethene (PCE)]], 4% contained [[wikipedia: Trichloroethylene | trichloroethene (TCE)]], 2% contained [[wikipedia: 1,1,1-Trichloroethane | 1,1,1-trichloroethane (1,1,1-TCA)]], and 2% contained [[wikipedia: 1,1-Dichloroethane | 1,1-dichloroethane (1,1-DCA)]].

[[Monitored Natural Attenuation (MNA) | Monitored Natural Attenuation (MNA)]] is one remedy that is available for contamination from chlorinated solvents in groundwater. Natural processes that can attenuate the concentrations of chlorinated solvents in groundwater include biological degradation, abiotic degradation, sorption, dispersion into ground adjacent to the contaminant plume, and volatilization to soil gas above the groundwater. At most sites where MNA has been selected as a remedy, or part of a remedy, the chlorinated solvents have been shown to be degrading in groundwater.

==Biodegradation==
The prospects for degradation of selected chlorinated solvents and their transformation products in groundwater are good (Table 1).

[[File:Wilson 3 Table1.JPG|thumbnail|600 px|left|Table 1. Summary of the prospects for degradation of selected chlorinated solvents and their transformation products in groundwater<ref>Lawrence, S.J., 2006. Description, properties, and degradation of selected volatile organic compounds detected in ground water--A review of selected literature (No. 2006-1338). [[Media:Lawrence-2006-Description_properties_degradation_of_VOCs.pdf|Report pdf]]</ref><ref name="HE2009"/>]]
Biodegradation can occur under both aerobic and anaerobic conditions. Under aerobic conditions, the chlorinated solvent can act as a source of food for the microorganisms (referred to as direct biodegradation in Table 1). Degradation can also be a fortuitous reaction that does not provide any benefit to the microorganisms. The fortuitous reaction is called a cometabolism or cooxidation. The fortuitous reaction is most commonly carried out by an oxygenase enzyme that is produced by the microorganisms in order to allow them to degrade some other compound.

When the chlorinated solvent is degraded as a food source, the population of active organisms and the rate of degradation will increase over time. If the degradation is fortuitous, the bacteria do not grow as a result of degrading the chlorinated solvent, and the rate constant does not increase over time.

The prospects for direct aerobic biodegradation of chlorinated alkenes depends on the extent of chlorination. PCE and TCE do not support growth under aerobic conditions, cis-dichloroethene<ref>Cox, E., 2012. Elucidation of the mechanisms and environmental relevance of cis-dichloroethene and vinyl chloride biodegradation. ER-1557. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1557/ER-1557 ER-1557]</ref> (c-DCE) can be degraded in aerobic groundwater, and vinyl chloride (VC) is readily degradable in many aerobic groundwaters.

Many samples of groundwater contain microorganisms that express oxygenase enzymes and can cometabolize PCE, TCE or dichloroethene (DCE)<ref>ITRC. 2011. Enzyme Activity Probes EMD Team Fact Sheet. [http://www.itrcweb.org/documents/team_emd/EAP_Fact_Sheet.pdf Fact Sheet]</ref>. However, the specific contribution of these organisms to MNA is not well understood<ref>Looney, B., 2010. Incorporating Aerobic Processes into Remedies for Large Chlorinated Solvent Plumes. ER-201026. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201026/ER-201026 ER-201026]</ref>, and studies are trying to define their contribution<ref>Wiedemeier, T.H., 2015. Providing Additional Support for MNA by Including Quantitative Lines of Evidence for Abiotic Degradation and Cometabolic Oxidation of Chlorinated Ethylenes. ER-201584. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201584/ER-201584 ER-201584]</ref>.

Under anaerobic conditions, the chlorinated solvents act as an electron acceptor. In such cases, electron donors may be in the form of naturally occurring, bioavailable organic carbon, or possibly from comingled plumes of petroleum hydrocarbons. The chlorinated solvents function in bacterial metabolism in the same fashion as oxygen functions in human metabolism. The chlorinated solvents are essentially something for the bacteria to breath in the absence of other electron acceptors such as oxygen, nitrate, or sulfate.

[[File:Wilson 3 Fig1.png|thumbnail|450 px|right|Figure 1. Degradation chlorinated alkenes to ethene.]]
In anaerobic groundwater, when conditions are favorable, chlorinated alkenes can undergo a sequential reductive dehalogenation where a chlorine atom is replaced with a hydrogen atom. Degradation proceeds from PCE to TCE, then to DCE, then to VC and finally to ethene (Fig. 1). The minimal geochemical conditions<ref>Wiedemeier, T.H., Swanson, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T., Wilson, B.H., Kampbell, D.H., Haas, P.E., Hansen, J.E., Chapelle, F.H., 1998. Technical protocol for evaluating natural attenuation of chlorinated solvents in ground water. EPA-600-R-98-128. [[Media:Wiedemeier-1998-Technical_Protocol_for_Evaluating_Natuaral_Attenuation.pdf|Report pdf]]</ref> that must be taken into account include pH, oxidation-reduction potential (ORP), dissolved oxygen (DO) concentration, total organic carbon (TOC) and competing electron acceptors including oxygen, nitrate, sulfate and ferric iron.

PCE and TCE can be used as an electron acceptor by a wide variety of bacteria<ref>Nyer, E.K., Payne, F., Sutherson, S., 2003. Discussion of environment vs. bacteria or let's play,‘name that bacteria’. Groundwater Monitoring & Remediation, 23(2), 32-48. [http://dx.doi.org/10.1111/j.1745-6592.2003.tb00665.x doi: 10.1111/j.1745-6592.2003.tb00665.x]</ref>. The bacteria can degrade PCE or TCE as far as DCE. The only organisms that can degrade DCE to VC and then degrade VC to the harmless end product ethene are stains of ''Dehalococcoides mccartyi''<ref>Löffler, F.E., Ritalahti, K.M., Zinder, S.H., 2013. Dehalococcoides and reductive dechlorination of chlorinated solvents. Bioaugmentation for groundwater remediation, ed. H.F. Stroo, Leeson, A., Ward, C.H. Springer, New York, NY. pgs. 39-88. ISBN: 978-1-4614-4114-4 ISBN 978-1-4614-4115-1. [http://dx.doi.org/10.1007/978-1-4614-4115-1 doi: 10.1007/978-1-4614-4115-1]</ref>.

The degradation of chlorinated alkanes in anaerobic groundwater is more complicated (Fig. 2). Chlorinated alkanes can undergo a sequential reductive dehalogenation. In addition, they can undergo the loss of a hydrogen and a chlorine atom to form an alkene (a dehydrochlorination) or the loss of two chlorine atoms to form an alkene (a dichloroelimination).

Three reactions have been demonstrated for 1,1,1-TCA in groundwater (Fig. 2)<ref>Scheutz, C., Durant, N.D., Hansen, M.H., Bjerg, P.L., 2011. Natural and enhanced anaerobic degradation of 1,1,1-trichloroethane and its degradation products in the subsurface–a critical review. Water Research, 45(9), 2701-2723. [http://dx.doi.org/10.1016/j.watres.2011.02.027 doi:10.1016/j.watres.2011.02.027]</ref>. It can undergo an abiotic hydrolysis reaction to produce acetate, an abiotic dehydrochlorination to produce 1,1-DCE, and a biological reductive dechlorination reaction to 1,1-DCA and then chloroethane. In addition to being reduced to chloroethane, 1,1-DCA can undergo a dichloroelimination reaction<ref>Lollar, B.S., Hirschorn, S., Mundle, S.O., Grostern, A., Edwards, E.A., Lacrampe-Couloume, G., 2010. Insights into enzyme kinetics of chloroethane biodegradation using compound specific stable isotopes. Environmental Science & Technology, 44(19), 7498-7503. [http://dx.doi.org/10.1021/es101330r doi: 10.1021/es101330r]</ref> to produce ethene.

[[File:Wilson 3 Fig2.PNG|thumbnail|400 px|left|Figure 2. Degradation of Chlorinated alkanes to ethane.]]

The degradation of 1,1,2-TCA follows a similar pattern (Fig. 2). One strain of ''Desulfitobacterium'' has been shown to dechlorinate 1,1,2-TCA to 1,2-DCA and chloroethane<ref>Zhao, S., Ding, C., He, J., 2015. Detoxification of 1,1,2-trichloroethane to ethene by desulfitobacterium and identification of its functional reductase gene. PloS One, 10(4), p.e0119507. [http://dx.doi.org/10.1371/journal.pone.0119507 doi:10.1371/journal.pone.0119507]</ref> through a sequential reductive dehalogenation. Certain strains of ''Dehalogenimonas'' go through a dichloroelimination reaction<ref>Bowman, K.S., Nobre, M.F., da Costa, M.S., Rainey, F.A. and Moe, W.M., 2013. Dehalogenimonas alkenigignens sp. nov., a chlorinated-alkane-dehalogenating bacterium isolated from groundwater. International Journal of Systematic and Evolutionary Microbiology, 63(4), 1492-1498. [http://dx.doi.org/10.1099/ijs.0.045054-0 doi: 10.1099/ijs.0.045054-0]</ref> to dechlorinate 1,1,2-TCA to VC and 1,2-DCA to ethene. A strain of ''Dehalobacter'' can also dechlorinate 1,2-DCA to ethene<ref>Grostern, A., Edwards, E.A., 2009. Characterization of a Dehalobacter coculture that dechlorinates 1,2-dichloroethane to ethene and identification of the putative reductive dehalogenase gene. Applied and Environmental Microbiology, 75(9), 2684-2693. [http://dx.doi.org/10.1128/aem.02037-08 doi: 10.1128/AEM.02037-08]</ref>.

A quantitative framework (BioPIC)<ref name="Lebron2015">Lebron, C. A., Wiedemeier, T. H., Wilson, J.T., Löffler, F.E., Hinchee, R.E., Singletary, M.A., 2015. Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites. ER-201129. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129/ER-201129 ER-201129]</ref> is now available that allows an evaluation of the rate constant for anaerobic biological degradation of cDCE and VC based on the abundance of gene markers for ''Dehalococcoides mccartyi''. The relationships between the rate constants for degradation of the chlorinated alkanes and abundance of gene copies of ''Dehalobacter'', ''Dehalogenimonas'' and other active bacteria are still being explored.

==Abiotic Degradation==
Chlorinated solvents can chemically react with a number of iron minerals in aquifers<ref name="HE2009"/>. The most important of these are magnetite, iron mono-sulfide, and pyrite.

Iron sulfide minerals form as a consequence of sulfate reduction in groundwater. The sulfide produced from sulfate reduction will react with Iron (III) minerals to form iron mono-sulfide. Over time the iron mono-sulfide will react with excess sulfide to produce pyrite.

The reactions of the chlorinated alkanes with the iron sulfide minerals is a sequential reductive dechlorination. However, the reaction of iron sulfide minerals with chlorinated alkenes is more complex (Fig. 3). Reductive dechlorination and dichloroelimination can proceed at the same time.

[[File:Wilson 3 Fig3.png|thumbnail|250 px|right|Figure 3. Degradation of chlorinated alkenes carried out by iron sulfide minerals.]]

The U.S. EPA regulates the maximum concentration of contaminants that are allowed in water that is supplied as drinking water. These U.S. EPA regulations are referred to as the Maximum Contaminant Level<ref>U.S. Environmental Protection Agency, 2016. Table of Regulated Drinking Water Contaminants. [http://www.epa.gov/your-drinking-water/table-regulated-drinking-water-contaminants Table of Regulated Drinking Water]</ref> or MCL. There are MCLs for the transformation products of reductive dechlorination (the DCEs and VC) and these products will be included in the target list of analytes in any conventional monitoring program. The products of dichloroelimination do not have MCLs and are not usually on the target list of analytes for conventional monitoring.

If the major pathway of abiotic degradation is dichloroelimination, then conventional monitoring will fail to recognize the contribution of abiotic degradation on iron sulfide minerals. However, the stable isotopes of carbon in chlorinated solvents are strongly fractionated during abiotic degradation on iron sulfide minerals. [[Compound Specific Isotope Analysis (CSIA) | Compound Specific Isotope Analysis (CSIA)]]<ref>Hunkeler, D., Meckenstock, R. U., Sherwood Lollar, B., Schmidt, T.C., Wilson, J.T., 2008. A Guide for Assessing Biodegradation and Source Identification of Organic Groundwater Contaminants Using Compound Specific Isotope Analysis (CSIA). U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-08/148. [[Media:Hunkeler-2008-A_Guide.pdf|Report pdf]]</ref> can be a useful tool to recognize abiotic degradation of chlorinated alkenes on iron sulfide minerals.

Magnetite is often present in unconsolidated glacial aquifers and aquifers that form in sediments that are shed by uplands composed of granite or other igneous rocks. Magnetite reacts readily with the chlorinated alkenes. The actual chemical interactions on magnetite are not well understood (Fig. 4). The ultimate degradation products are oxidized organic compounds and carbon dioxide<ref>Darlington, R., Rectanus, H., 2015. Biogeochemical Transformation Handbook. TR-NAVFAC EXWC-EV-1601, 41 pgs. [[Media:Darlington-2015-Biogeochem_Transformation_Handbook.pdf|Report pdf]]</ref>.

[[File:Wilson 3 Fig4.png|thumbnail|300 px|left|Figure 4. Degradation of chlorinated alkenes carried out by magnetite.]]

==Footprints==
Most plumes have some contribution of anaerobic sequential reductive dechlorination. As a result, the primary contaminant and the transformation products of reductive dechlorination are present in the groundwater. The highest concentrations of the primary contaminant will be near the source of contamination, and the flow of groundwater carries the transformation products further downgradient from the source (Figure 5).

Many plumes of chlorinated solvents also have a contribution of abiotic degradation. As a result, the intermediate degradation products (such as DCE) do not accumulate to stoichiometric concentrations. There is an appearance that degradation of the cDCE has stalled, when in fact it is actively degrading, but not to vinyl chloride (Fig. 5). A quantitative framework<ref name="Lebron2015"/> is now available that allows an evaluation of the contribution of abiotic degradation on magnetite based on the magnetic susceptibility of the sediment, and the contribution of abiotic degradation on pyrite based on the extent of sulfate reduction and the geochemistry of the groundwater.
[[File:Wilson 3 Fig5.png|thumbnail|400 px|center|Figure 5. Comparison of a chlorinated alkenes plume undergoing biodegradation alone vs. biodegradation with abiotic degradation.]]

==Tools and Databases for Chlorinated Solvent MNA==
The Scenarios Evaluation Tool for Chlorinated Solvent MNA<ref>Truex, M.J., Newell, C.J., Looney, B.B, Vangelas, K., 2006. Scenarios evaluation tool for chlorinated solvent MNA. Savannah River National Laboratory, Aiken, South Carolina. WSRC-STI-2006-0096. [[Media:Truex-2006-Scenarios_Evaluation_Tool_for_Chlorinated_Solvent_MNA.pdf|Report pdf]]</ref> was designed to provide a structure where the MNA methods and decision logic are linked together in one of 13 different “scenarios” or site types. Based on site data (e.g. Table 2), one selects which of the 13 scenarios best fits their site or portion of a site. Then one goes to the description of that scenario to learn which attenuation reactions are likely to be active, how to design a MNA monitoring program, whether MNA will work, and other relevant factors.
[[File:Wilson 3 Table2.png|thumbnail|600 px|center|Table 2. Key elements of the scenarios tool for chlorinated solvent MNA.]]

A data mining study of MNA at 45 chlorinated solvent sites<ref>McGuire, T.M., Newell, C.J., Looney, B.B., Vangelas, K.M., 2003. Historical and retrospective survey of monitored natural attenuation: A line of inquiry supporting monitored natural attenuation and enhanced passive remediation of chlorinated solvents. Westinghouse Savannah River Company, Aiken, SC. [[Media:McGuire-2003-Historical_and_Retrospective_Survey_of_MNA.pdf|Report pdf]]</ref> provides some interesting information about plume sources, strength, and size (Fig. 6).

[[File:Wilson 3 Fig6.png|thumbnail|500 px|center|Figure 6. Plume characteristics evaluation of 45 chlorinated solvent sites.]]

The performance of MNA was evaluated<ref>McGuire, T., 2016. Development of an Expanded, High-Reliability Cost and Performance Database for In-Situ Remediation Technologies. ESTCP Project No. ER-201120. [https://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201120/ER-201120 ER-201120]</ref> by comparing the change in concentrations of chlorinated organic compounds in wells in the source zone of plumes from the beginning to the end of an MNA monitoring period (Fig. 7).

[[File:Wilson 3 Fig7.png|thumbnail|500 px|center|Figure 7. Each dot represents an individual project, showing the geometric mean of the concentration at the beginning of the monitoring record (X-axis) and at the end of the monitoring record (Y-axis). The median duration of MNA monitoring for these 45 sites was 8.7 years and ranged from 4.1 to 15 years.]]

One study evaluated the change in source concentration over time at 23 chlorinated solvent sites by calculating concentration vs. time decay rates for source zone wells<ref>Newell, C.J., Cowie, I., McGuire, T.M., McNab Jr, W.W., 2006. Multiyear temporal changes in chlorinated solvent concentrations at 23 monitored natural attenuation sites. Journal of Environmental Engineering, American Society of Environmental Engineers, 132(6), 653-663. [http://dx.doi.org/10.1061/(asce)0733-9372(2006)132:6(653) doi: 10.1061/(asce)0733-9372(2006)132:6(653)]</ref>. The authors concluded, “If the median point decay rates from these sites are maintained over a 20 year period, the resulting reduction in concentration will be similar to the reported reduction in source zone concentrations achieved by active in situ source remediation technologies (typical project length: 1–2 years)".

As part of the development process for the chlorinated solvent natural attenuation model<ref>Aziz, C.E., Smith, A.P., Newell, C.J., Gonzales, J.R., 2000. BIOCHLOR Chlorinated solvent plume database report. Air Force Center for Environmental Excellence, Texas. [[Media:Aziz-2000-BIOCHLOR-plume-database.pdf|Report pdf]]</ref> BIOCHLOR, 24 chlorinated solvent plumes were studied in detail. Key findings included:

*TCE and c-DCE had median plume lengths of 1215 ft and 1205 ft, respectively.
*Chlorinated ethene plume lengths were moderately correlated with seepage velocity and source width (Fig. 8).
*First order decay rates ranged between 1 and 2 per year for the chlorinated ethane plumes.

[[File:Wilson 3 Fig8.png|thumbnail|900 px|center|Figure 8. Effect of estimated source size and groundwater seepage velocity on plume length.]]

==Summary==
MNA is an important remediation technology at some chlorinated solvent sites. There are numerous reactions, both biotic and abiotic, that can act on different chlorinated solvent compounds. Several tools and databases are available to help understand how chlorinated solvent plumes behave and to design and implement appropriate MNA programs.

==References==

<references/>

==See Also==
*[[Media:EPA-MNA-Chlorinated-Organics-Symposium.pdf|Proceedings of the Symposium on Natural Attenuation of Chlorinated Organics in Ground Water]]
*[[Media:AFCEE-Natural_Attenuation-Chlorinated_Solvents-1999.pdf|Natural Attenuation of Chlorinated Solvents Performance and Cost Results From Multiple Air Force Demonstration Sites]]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-1348 Using Advanced Analysis Approaches to Complete Long-Term Evaluations of Natural Attenuation Processes on the Remediation of Dissolved Chlorinated Solvent Contamination]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-1349/ER-1349 Integrated Protocol for Assessment of Long-Term Sustainability of Monitored Natural Attenuation of Chlorinated Solvent Plumes]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200019 Impact of Landfill Closure Designs on Long-Term Natural Attenuation of Chlorinated Hydrocarbons]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436 Estimating Cleanup Times Associated with Combining Source-Area Remediation with Monitored Natural Attenuation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200708/ER-200708 Use of Enzyme Probes for Estimation of Trichloroethene Degradation Rates and Acceptance of Monitored Natural Attenuation ]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200824/ER-200824 Verification of Methods for Assessing the Sustainability of Monitored Natural Attenuation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129 Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201211/ER-201211 Frequently Asked Questions about Monitored Natural Attenuation in the 21st Century]
*[https://www.coursera.org/learn/natural-attenuation-of-groundwater-contaminants/lecture/kBe2j/abiotic-degradation-principles Online Lecture Course - Abiotic Degradation]

Monitored Natural Attenuation (MNA) of Chlorinated Solvents

2026-05-07T16:55:37Z

Admin:

[[Monitored Natural Attenuation (MNA)]] is a common remedy for contamination of [[Chlorinated Solvents |chlorinated solvents]] in groundwater. Chlorinated solvents are susceptible to many natural processes that can attenuate their concentrations in groundwater including biological degradation, abiotic degradation, sorption, dispersion, and volatilization. Typically, MNA is used for plumes with low dissolved concentrations or in peripheral areas of plumes away from areas with non-aqueous phase liquid (NAPL) or other materials that serve as the source of groundwater contamination.
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'''Related Article(s):'''
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Monitored Natural Attenuation (MNA) of Fuels| Monitored Natural Attenuation of Fuels]]
*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| Monitored Natural Attenuation of Metal and Metalloids]]
*[[Chlorinated Solvents]]
*[[Natural Source Zone Depletion (NSZD)]]

'''CONTRIBUTOR(S):''' [[Dr. John Wilson]] 

'''Key Resource(s):'''
*[[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Identification & Characterization Methods for Reactive Minerals Responsible for Natural Attenuation of Chlorinated Organic Compounds in Ground Water]]<ref name="HE2009">He, Y., Su, C., Wilson, J., Wilkin, R., Adair, C., Lee, T., Bradley, P. and Ferrey, M., 2009. Identification and characterization methods for reactive minerals responsible for natural attenuation of chlorinated organic compounds in ground water. U.S. Environmental Protection Agency. [[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Report pdf]]</ref>

==Introduction==
[[Chlorinated Solvents |Chlorinated solvents]] and their transformation products are among the most abundant contaminants in groundwater. In 2006, the United States Geological Survey published results from a systematic survey of volatile organic chemicals in drinking water wells<ref>Zogorski, J.S., Carter, J.M., Ivahnenko, T., Lapham, W.W., Moran, M.J., Rowe, B.L., Squillace, P.J., Toccalino, P.L., 2006. The quality of our Nation’s waters - Volatile organic compounds in the nation’s ground water and drinking-water supply wells. US Geological Survey Circular, 1292, 101. [[Media:Zogorski-2006-_Volatile_organic_compounds_in_the_nations_ground_water_and_wells.pdf|Report pdf]]</ref> in the USA. Approximately 12% of wells contained detectable concentrations of tetrachloromethane ([[wikipedia: Chloroform | chloroform]]), 5% contained [[wikipedia: Tetrachloroethylene | tetrachloroethene (PCE)]], 4% contained [[wikipedia: Trichloroethylene | trichloroethene (TCE)]], 2% contained [[wikipedia: 1,1,1-Trichloroethane | 1,1,1-trichloroethane (1,1,1-TCA)]], and 2% contained [[wikipedia: 1,1-Dichloroethane | 1,1-dichloroethane (1,1-DCA)]].

[[Monitored Natural Attenuation (MNA) | Monitored Natural Attenuation (MNA)]] is one remedy that is available for contamination from chlorinated solvents in groundwater. Natural processes that can attenuate the concentrations of chlorinated solvents in groundwater include biological degradation, abiotic degradation, sorption, dispersion into ground adjacent to the contaminant plume, and volatilization to soil gas above the groundwater. At most sites where MNA has been selected as a remedy, or part of a remedy, the chlorinated solvents have been shown to be degrading in groundwater.

==Biodegradation==
The prospects for degradation of selected chlorinated solvents and their transformation products in groundwater are good (Table 1).

[[File:Wilson 3 Table1.JPG|thumbnail|600 px|left|Table 1. Summary of the prospects for degradation of selected chlorinated solvents and their transformation products in groundwater<ref>Lawrence, S.J., 2006. Description, properties, and degradation of selected volatile organic compounds detected in ground water--A review of selected literature (No. 2006-1338). [[Media:Lawrence-2006-Description_properties_degradation_of_VOCs.pdf|Report pdf]]</ref><ref name="HE2009"/>]]
Biodegradation can occur under both aerobic and anaerobic conditions. Under aerobic conditions, the chlorinated solvent can act as a source of food for the microorganisms (referred to as direct biodegradation in Table 1). Degradation can also be a fortuitous reaction that does not provide any benefit to the microorganisms. The fortuitous reaction is called a cometabolism or cooxidation. The fortuitous reaction is most commonly carried out by an oxygenase enzyme that is produced by the microorganisms in order to allow them to degrade some other compound.

When the chlorinated solvent is degraded as a food source, the population of active organisms and the rate of degradation will increase over time. If the degradation is fortuitous, the bacteria do not grow as a result of degrading the chlorinated solvent, and the rate constant does not increase over time.

The prospects for direct aerobic biodegradation of chlorinated alkenes depends on the extent of chlorination. PCE and TCE do not support growth under aerobic conditions, cis-dichloroethene<ref>Cox, E., 2012. Elucidation of the mechanisms and environmental relevance of cis-dichloroethene and vinyl chloride biodegradation. ER-1557. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1557/ER-1557 ER-1557]</ref> (c-DCE) can be degraded in aerobic groundwater, and vinyl chloride (VC) is readily degradable in many aerobic groundwaters.

Many samples of groundwater contain microorganisms that express oxygenase enzymes and can cometabolize PCE, TCE or dichloroethene (DCE)<ref>ITRC. 2011. Enzyme Activity Probes EMD Team Fact Sheet. [http://www.itrcweb.org/documents/team_emd/EAP_Fact_Sheet.pdf Fact Sheet]</ref>. However, the specific contribution of these organisms to MNA is not well understood<ref>Looney, B., 2010. Incorporating Aerobic Processes into Remedies for Large Chlorinated Solvent Plumes. ER-201026. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201026/ER-201026 ER-201026]</ref>, and studies are trying to define their contribution<ref>Wiedemeier, T.H., 2015. Providing Additional Support for MNA by Including Quantitative Lines of Evidence for Abiotic Degradation and Cometabolic Oxidation of Chlorinated Ethylenes. ER-201584. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201584/ER-201584 ER-201584]</ref>.

Under anaerobic conditions, the chlorinated solvents act as an electron acceptor. In such cases, electron donors may be in the form of naturally occurring, bioavailable organic carbon, or possibly from comingled plumes of petroleum hydrocarbons. The chlorinated solvents function in bacterial metabolism in the same fashion as oxygen functions in human metabolism. The chlorinated solvents are essentially something for the bacteria to breath in the absence of other electron acceptors such as oxygen, nitrate, or sulfate.

[[File:Wilson 3 Fig1.png|thumbnail|450 px|right|Figure 1. Degradation chlorinated alkenes to ethene.]]
In anaerobic groundwater, when conditions are favorable, chlorinated alkenes can undergo a sequential reductive dehalogenation where a chlorine atom is replaced with a hydrogen atom. Degradation proceeds from PCE to TCE, then to DCE, then to VC and finally to ethene (Fig. 1). The minimal geochemical conditions<ref>Wiedemeier, T.H., Swanson, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T., Wilson, B.H., Kampbell, D.H., Haas, P.E., Hansen, J.E., Chapelle, F.H., 1998. Technical protocol for evaluating natural attenuation of chlorinated solvents in ground water. EPA-600-R-98-128. [[Media:Wiedemeier-1998-Technical_Protocol_for_Evaluating_Natuaral_Attenuation.pdf|Report pdf]]</ref> that must be taken into account include pH, oxidation-reduction potential (ORP), dissolved oxygen (DO) concentration, total organic carbon (TOC) and competing electron acceptors including oxygen, nitrate, sulfate and ferric iron.

PCE and TCE can be used as an electron acceptor by a wide variety of bacteria<ref>Nyer, E.K., Payne, F., Sutherson, S., 2003. Discussion of environment vs. bacteria or let's play,‘name that bacteria’. Groundwater Monitoring & Remediation, 23(2), 32-48. [http://dx.doi.org/10.1111/j.1745-6592.2003.tb00665.x doi: 10.1111/j.1745-6592.2003.tb00665.x]</ref>. The bacteria can degrade PCE or TCE as far as DCE. The only organisms that can degrade DCE to VC and then degrade VC to the harmless end product ethene are stains of ''Dehalococcoides mccartyi''<ref>Löffler, F.E., Ritalahti, K.M., Zinder, S.H., 2013. Dehalococcoides and reductive dechlorination of chlorinated solvents. Bioaugmentation for groundwater remediation, ed. H.F. Stroo, Leeson, A., Ward, C.H. Springer, New York, NY. pgs. 39-88. ISBN: 978-1-4614-4114-4 ISBN 978-1-4614-4115-1. [http://dx.doi.org/10.1007/978-1-4614-4115-1 doi: 10.1007/978-1-4614-4115-1]</ref>.

The degradation of chlorinated alkanes in anaerobic groundwater is more complicated (Fig. 2). Chlorinated alkanes can undergo a sequential reductive dehalogenation. In addition, they can undergo the loss of a hydrogen and a chlorine atom to form an alkene (a dehydrochlorination) or the loss of two chlorine atoms to form an alkene (a dichloroelimination).

Three reactions have been demonstrated for 1,1,1-TCA in groundwater (Fig. 2)<ref>Scheutz, C., Durant, N.D., Hansen, M.H., Bjerg, P.L., 2011. Natural and enhanced anaerobic degradation of 1,1,1-trichloroethane and its degradation products in the subsurface–a critical review. Water Research, 45(9), 2701-2723. [http://dx.doi.org/10.1016/j.watres.2011.02.027 doi:10.1016/j.watres.2011.02.027]</ref>. It can undergo an abiotic hydrolysis reaction to produce acetate, an abiotic dehydrochlorination to produce 1,1-DCE, and a biological reductive dechlorination reaction to 1,1-DCA and then chloroethane. In addition to being reduced to chloroethane, 1,1-DCA can undergo a dichloroelimination reaction<ref>Lollar, B.S., Hirschorn, S., Mundle, S.O., Grostern, A., Edwards, E.A., Lacrampe-Couloume, G., 2010. Insights into enzyme kinetics of chloroethane biodegradation using compound specific stable isotopes. Environmental Science & Technology, 44(19), 7498-7503. [http://dx.doi.org/10.1021/es101330r doi: 10.1021/es101330r]</ref> to produce ethene.

[[File:Wilson 3 Fig2.PNG|thumbnail|400 px|left|Figure 2. Degradation of Chlorinated alkanes to ethane.]]

The degradation of 1,1,2-TCA follows a similar pattern (Fig. 2). One strain of ''Desulfitobacterium'' has been shown to dechlorinate 1,1,2-TCA to 1,2-DCA and chloroethane<ref>Zhao, S., Ding, C., He, J., 2015. Detoxification of 1,1,2-trichloroethane to ethene by desulfitobacterium and identification of its functional reductase gene. PloS One, 10(4), p.e0119507. [http://dx.doi.org/10.1371/journal.pone.0119507 doi:10.1371/journal.pone.0119507]</ref> through a sequential reductive dehalogenation. Certain strains of ''Dehalogenimonas'' go through a dichloroelimination reaction<ref>Bowman, K.S., Nobre, M.F., da Costa, M.S., Rainey, F.A. and Moe, W.M., 2013. Dehalogenimonas alkenigignens sp. nov., a chlorinated-alkane-dehalogenating bacterium isolated from groundwater. International Journal of Systematic and Evolutionary Microbiology, 63(4), 1492-1498. [http://dx.doi.org/10.1099/ijs.0.045054-0 doi: 10.1099/ijs.0.045054-0]</ref> to dechlorinate 1,1,2-TCA to VC and 1,2-DCA to ethene. A strain of ''Dehalobacter'' can also dechlorinate 1,2-DCA to ethene<ref>Grostern, A., Edwards, E.A., 2009. Characterization of a Dehalobacter coculture that dechlorinates 1,2-dichloroethane to ethene and identification of the putative reductive dehalogenase gene. Applied and Environmental Microbiology, 75(9), 2684-2693. [http://dx.doi.org/10.1128/aem.02037-08 doi: 10.1128/AEM.02037-08]</ref>.

A quantitative framework (BioPIC)<ref name="Lebron2015">Lebron, C. A., Wiedemeier, T. H., Wilson, J.T., Löffler, F.E., Hinchee, R.E., Singletary, M.A., 2015. Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites. ER-201129. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129/ER-201129 ER-201129]</ref> is now available that allows an evaluation of the rate constant for anaerobic biological degradation of cDCE and VC based on the abundance of gene markers for ''Dehalococcoides mccartyi''. The relationships between the rate constants for degradation of the chlorinated alkanes and abundance of gene copies of ''Dehalobacter'', ''Dehalogenimonas'' and other active bacteria are still being explored.

==Abiotic Degradation==
Chlorinated solvents can chemically react with a number of iron minerals in aquifers<ref name="HE2009"/>. The most important of these are magnetite, iron mono-sulfide, and pyrite.

Iron sulfide minerals form as a consequence of sulfate reduction in groundwater. The sulfide produced from sulfate reduction will react with Iron (III) minerals to form iron mono-sulfide. Over time the iron mono-sulfide will react with excess sulfide to produce pyrite.

The reactions of the chlorinated alkanes with the iron sulfide minerals is a sequential reductive dechlorination. However, the reaction of iron sulfide minerals with chlorinated alkenes is more complex (Fig. 3). Reductive dechlorination and dichloroelimination can proceed at the same time.

[[File:Wilson 3 Fig3.png|thumbnail|250 px|right|Figure 3. Degradation of chlorinated alkenes carried out by iron sulfide minerals.]]

The U.S. EPA regulates the maximum concentration of contaminants that are allowed in water that is supplied as drinking water. These U.S. EPA regulations are referred to as the Maximum Contaminant Level<ref>U.S. Environmental Protection Agency, 2016. Table of Regulated Drinking Water Contaminants. [http://www.epa.gov/your-drinking-water/table-regulated-drinking-water-contaminants Table of Regulated Drinking Water]</ref> or MCL. There are MCLs for the transformation products of reductive dechlorination (the DCEs and VC) and these products will be included in the target list of analytes in any conventional monitoring program. The products of dichloroelimination do not have MCLs and are not usually on the target list of analytes for conventional monitoring.

If the major pathway of abiotic degradation is dichloroelimination, then conventional monitoring will fail to recognize the contribution of abiotic degradation on iron sulfide minerals. However, the stable isotopes of carbon in chlorinated solvents are strongly fractionated during abiotic degradation on iron sulfide minerals. [[Compound Specific Isotope Analysis (CSIA) | Compound Specific Isotope Analysis (CSIA)]]<ref>Hunkeler, D., Meckenstock, R. U., Sherwood Lollar, B., Schmidt, T.C., Wilson, J.T., 2008. A Guide for Assessing Biodegradation and Source Identification of Organic Groundwater Contaminants Using Compound Specific Isotope Analysis (CSIA). U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-08/148. [[Media:Hunkeler-2008-A_Guide.pdf|Report pdf]]</ref> can be a useful tool to recognize abiotic degradation of chlorinated alkenes on iron sulfide minerals.

Magnetite is often present in unconsolidated glacial aquifers and aquifers that form in sediments that are shed by uplands composed of granite or other igneous rocks. Magnetite reacts readily with the chlorinated alkenes. The actual chemical interactions on magnetite are not well understood (Fig. 4). The ultimate degradation products are oxidized organic compounds and carbon dioxide<ref>Darlington, R., Rectanus, H., 2015. Biogeochemical Transformation Handbook. TR-NAVFAC EXWC-EV-1601, 41 pgs. [[Media:Darlington-2015-Biogeochem_Transformation_Handbook.pdf|Report pdf]]</ref>.

[[File:Wilson 3 Fig4.png|thumbnail|300 px|left|Figure 4. Degradation of chlorinated alkenes carried out by magnetite.]]

==Footprints==
Most plumes have some contribution of anaerobic sequential reductive dechlorination. As a result, the primary contaminant and the transformation products of reductive dechlorination are present in the groundwater. The highest concentrations of the primary contaminant will be near the source of contamination, and the flow of groundwater carries the transformation products further downgradient from the source (Figure 5).

Many plumes of chlorinated solvents also have a contribution of abiotic degradation. As a result, the intermediate degradation products (such as DCE) do not accumulate to stoichiometric concentrations. There is an appearance that degradation of the cDCE has stalled, when in fact it is actively degrading, but not to vinyl chloride (Fig. 5). A quantitative framework<ref name="Lebron2015"/> is now available that allows an evaluation of the contribution of abiotic degradation on magnetite based on the magnetic susceptibility of the sediment, and the contribution of abiotic degradation on pyrite based on the extent of sulfate reduction and the geochemistry of the groundwater.
[[File:Wilson 3 Fig5.png|thumbnail|400 px|center|Figure 5. Comparison of a chlorinated alkenes plume undergoing biodegradation alone vs. biodegradation with abiotic degradation.]]

==Tools and Databases for Chlorinated Solvent MNA==
The Scenarios Evaluation Tool for Chlorinated Solvent MNA<ref>Truex, M.J., Newell, C.J., Looney, B.B, Vangelas, K., 2006. Scenarios evaluation tool for chlorinated solvent MNA. Savannah River National Laboratory, Aiken, South Carolina. WSRC-STI-2006-0096. [[Media:Truex-2006-Scenarios_Evaluation_Tool_for_Chlorinated_Solvent_MNA.pdf|Report pdf]]</ref> was designed to provide a structure where the MNA methods and decision logic are linked together in one of 13 different “scenarios” or site types. Based on site data (e.g. Table 2), one selects which of the 13 scenarios best fits their site or portion of a site. Then one goes to the description of that scenario to learn which attenuation reactions are likely to be active, how to design a MNA monitoring program, whether MNA will work, and other relevant factors.
[[File:Wilson 3 Table2.png|thumbnail|600 px|center|Table 2. Key elements of the scenarios tool for chlorinated solvent MNA.]]

A data mining study of MNA at 45 chlorinated solvent sites<ref>McGuire, T.M., Newell, C.J., Looney, B.B., Vangelas, K.M., 2003. Historical and retrospective survey of monitored natural attenuation: A line of inquiry supporting monitored natural attenuation and enhanced passive remediation of chlorinated solvents. Westinghouse Savannah River Company, Aiken, SC. [[Media:McGuire-2003-Historical_and_Retrospective_Survey_of_MNA.pdf|Report pdf]]</ref> provides some interesting information about plume sources, strength, and size (Fig. 6).

[[File:Wilson 3 Fig6.png|thumbnail|500 px|center|Figure 6. Plume characteristics evaluation of 45 chlorinated solvent sites.]]

The performance of MNA was evaluated<ref>McGuire, T., 2016. Development of an Expanded, High-Reliability Cost and Performance Database for In-Situ Remediation Technologies. ESTCP Project No. ER-201120. [https://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201120/ER-201120 ER-201120]</ref> by comparing the change in concentrations of chlorinated organic compounds in wells in the source zone of plumes from the beginning to the end of an MNA monitoring period (Fig. 7).

[[File:Wilson 3 Fig7.png|thumbnail|500 px|center|Figure 7. Each dot represents an individual project, showing the geometric mean of the concentration at the beginning of the monitoring record (X-axis) and at the end of the monitoring record (Y-axis). The median duration of MNA monitoring for these 45 sites was 8.7 years and ranged from 4.1 to 15 years.]]

One study evaluated the change in source concentration over time at 23 chlorinated solvent sites by calculating concentration vs. time decay rates for source zone wells<ref>Newell, C.J., Cowie, I., McGuire, T.M., McNab Jr, W.W., 2006. Multiyear temporal changes in chlorinated solvent concentrations at 23 monitored natural attenuation sites. Journal of Environmental Engineering, American Society of Environmental Engineers, 132(6), 653-663. [http://dx.doi.org/10.1061/(asce)0733-9372(2006)132:6(653) doi: 10.1061/(asce)0733-9372(2006)132:6(653)]</ref>. The authors concluded, “If the median point decay rates from these sites are maintained over a 20 year period, the resulting reduction in concentration will be similar to the reported reduction in source zone concentrations achieved by active in situ source remediation technologies (typical project length: 1–2 years)".

As part of the development process for the chlorinated solvent natural attenuation model<ref>Aziz, C.E., Smith, A.P., Newell, C.J., Gonzales, J.R., 2000. BIOCHLOR Chlorinated solvent plume database report. Air Force Center for Environmental Excellence, Texas. [[Media:Aziz-2000-BIOCHLOR-plume-database.pdf|Report pdf]]</ref> BIOCHLOR, 24 chlorinated solvent plumes were studied in detail. Key findings included:

*TCE and c-DCE had median plume lengths of 1215 ft and 1205 ft, respectively.
*Chlorinated ethene plume lengths were moderately correlated with seepage velocity and source width (Fig. 8).
*First order decay rates ranged between 1 and 2 per year for the chlorinated ethane plumes.

[[File:Wilson 3 Fig8.png|thumbnail|900 px|center|Figure 8. Effect of estimated source size and groundwater seepage velocity on plume length.]]

==Summary==
MNA is an important remediation technology at some chlorinated solvent sites. There are numerous reactions, both biotic and abiotic, that can act on different chlorinated solvent compounds. Several tools and databases are available to help understand how chlorinated solvent plumes behave and to design and implement appropriate MNA programs.

==References==

<references/>

==See Also==
*[[Media:EPA-MNA-Chlorinated-Organics-Symposium.pdf|Proceedings of the Symposium on Natural Attenuation of Chlorinated Organics in Ground Water]]
*[[Media:AFCEE-Natural_Attenuation-Chlorinated_Solvents-1999.pdf|Natural Attenuation of Chlorinated Solvents Performance and Cost Results From Multiple Air Force Demonstration Sites]]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-1348 Using Advanced Analysis Approaches to Complete Long-Term Evaluations of Natural Attenuation Processes on the Remediation of Dissolved Chlorinated Solvent Contamination]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-1349/ER-1349 Integrated Protocol for Assessment of Long-Term Sustainability of Monitored Natural Attenuation of Chlorinated Solvent Plumes]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200019 Impact of Landfill Closure Designs on Long-Term Natural Attenuation of Chlorinated Hydrocarbons]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436 Estimating Cleanup Times Associated with Combining Source-Area Remediation with Monitored Natural Attenuation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200708/ER-200708 Use of Enzyme Probes for Estimation of Trichloroethene Degradation Rates and Acceptance of Monitored Natural Attenuation ]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200824/ER-200824 Verification of Methods for Assessing the Sustainability of Monitored Natural Attenuation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129 Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201211/ER-201211 Frequently Asked Questions about Monitored Natural Attenuation in the 21st Century]
*[https://www.coursera.org/learn/natural-attenuation-of-groundwater-contaminants/lecture/kBe2j/abiotic-degradation-principles Online Lecture Course - Abiotic Degradation]

Monitored Natural Attenuation (MNA) of Chlorinated Solvents

2026-05-07T16:55:24Z

Admin:

[[Monitored Natural Attenuation (MNA)]] is a common remedy for contamination of [[Chlorinated Solvents |chlorinated solvents]] in groundwater. Chlorinated solvents are susceptible to many natural processes that can attenuate their concentrations in groundwater including biological degradation, abiotic degradation, sorption, dispersion, and volatilization. Typically, MNA is used for plumes with low dissolved concentrations or in peripheral areas of plumes away from areas with non-aqueous phase liquid (NAPL) or other materials that serve as the source of groundwater contamination.
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'''Related Article(s):'''
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Monitored Natural Attenuation (MNA) of Fuels| Monitored Natural Attenuation of Fuels]]
*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| Monitored Natural Attenuation (MNA) of Metal and Metalloids]]
*[[Chlorinated Solvents]]
*[[Natural Source Zone Depletion (NSZD)]]

'''CONTRIBUTOR(S):''' [[Dr. John Wilson]] 

'''Key Resource(s):'''
*[[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Identification & Characterization Methods for Reactive Minerals Responsible for Natural Attenuation of Chlorinated Organic Compounds in Ground Water]]<ref name="HE2009">He, Y., Su, C., Wilson, J., Wilkin, R., Adair, C., Lee, T., Bradley, P. and Ferrey, M., 2009. Identification and characterization methods for reactive minerals responsible for natural attenuation of chlorinated organic compounds in ground water. U.S. Environmental Protection Agency. [[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Report pdf]]</ref>

==Introduction==
[[Chlorinated Solvents |Chlorinated solvents]] and their transformation products are among the most abundant contaminants in groundwater. In 2006, the United States Geological Survey published results from a systematic survey of volatile organic chemicals in drinking water wells<ref>Zogorski, J.S., Carter, J.M., Ivahnenko, T., Lapham, W.W., Moran, M.J., Rowe, B.L., Squillace, P.J., Toccalino, P.L., 2006. The quality of our Nation’s waters - Volatile organic compounds in the nation’s ground water and drinking-water supply wells. US Geological Survey Circular, 1292, 101. [[Media:Zogorski-2006-_Volatile_organic_compounds_in_the_nations_ground_water_and_wells.pdf|Report pdf]]</ref> in the USA. Approximately 12% of wells contained detectable concentrations of tetrachloromethane ([[wikipedia: Chloroform | chloroform]]), 5% contained [[wikipedia: Tetrachloroethylene | tetrachloroethene (PCE)]], 4% contained [[wikipedia: Trichloroethylene | trichloroethene (TCE)]], 2% contained [[wikipedia: 1,1,1-Trichloroethane | 1,1,1-trichloroethane (1,1,1-TCA)]], and 2% contained [[wikipedia: 1,1-Dichloroethane | 1,1-dichloroethane (1,1-DCA)]].

[[Monitored Natural Attenuation (MNA) | Monitored Natural Attenuation (MNA)]] is one remedy that is available for contamination from chlorinated solvents in groundwater. Natural processes that can attenuate the concentrations of chlorinated solvents in groundwater include biological degradation, abiotic degradation, sorption, dispersion into ground adjacent to the contaminant plume, and volatilization to soil gas above the groundwater. At most sites where MNA has been selected as a remedy, or part of a remedy, the chlorinated solvents have been shown to be degrading in groundwater.

==Biodegradation==
The prospects for degradation of selected chlorinated solvents and their transformation products in groundwater are good (Table 1).

[[File:Wilson 3 Table1.JPG|thumbnail|600 px|left|Table 1. Summary of the prospects for degradation of selected chlorinated solvents and their transformation products in groundwater<ref>Lawrence, S.J., 2006. Description, properties, and degradation of selected volatile organic compounds detected in ground water--A review of selected literature (No. 2006-1338). [[Media:Lawrence-2006-Description_properties_degradation_of_VOCs.pdf|Report pdf]]</ref><ref name="HE2009"/>]]
Biodegradation can occur under both aerobic and anaerobic conditions. Under aerobic conditions, the chlorinated solvent can act as a source of food for the microorganisms (referred to as direct biodegradation in Table 1). Degradation can also be a fortuitous reaction that does not provide any benefit to the microorganisms. The fortuitous reaction is called a cometabolism or cooxidation. The fortuitous reaction is most commonly carried out by an oxygenase enzyme that is produced by the microorganisms in order to allow them to degrade some other compound.

When the chlorinated solvent is degraded as a food source, the population of active organisms and the rate of degradation will increase over time. If the degradation is fortuitous, the bacteria do not grow as a result of degrading the chlorinated solvent, and the rate constant does not increase over time.

The prospects for direct aerobic biodegradation of chlorinated alkenes depends on the extent of chlorination. PCE and TCE do not support growth under aerobic conditions, cis-dichloroethene<ref>Cox, E., 2012. Elucidation of the mechanisms and environmental relevance of cis-dichloroethene and vinyl chloride biodegradation. ER-1557. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1557/ER-1557 ER-1557]</ref> (c-DCE) can be degraded in aerobic groundwater, and vinyl chloride (VC) is readily degradable in many aerobic groundwaters.

Many samples of groundwater contain microorganisms that express oxygenase enzymes and can cometabolize PCE, TCE or dichloroethene (DCE)<ref>ITRC. 2011. Enzyme Activity Probes EMD Team Fact Sheet. [http://www.itrcweb.org/documents/team_emd/EAP_Fact_Sheet.pdf Fact Sheet]</ref>. However, the specific contribution of these organisms to MNA is not well understood<ref>Looney, B., 2010. Incorporating Aerobic Processes into Remedies for Large Chlorinated Solvent Plumes. ER-201026. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201026/ER-201026 ER-201026]</ref>, and studies are trying to define their contribution<ref>Wiedemeier, T.H., 2015. Providing Additional Support for MNA by Including Quantitative Lines of Evidence for Abiotic Degradation and Cometabolic Oxidation of Chlorinated Ethylenes. ER-201584. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201584/ER-201584 ER-201584]</ref>.

Under anaerobic conditions, the chlorinated solvents act as an electron acceptor. In such cases, electron donors may be in the form of naturally occurring, bioavailable organic carbon, or possibly from comingled plumes of petroleum hydrocarbons. The chlorinated solvents function in bacterial metabolism in the same fashion as oxygen functions in human metabolism. The chlorinated solvents are essentially something for the bacteria to breath in the absence of other electron acceptors such as oxygen, nitrate, or sulfate.

[[File:Wilson 3 Fig1.png|thumbnail|450 px|right|Figure 1. Degradation chlorinated alkenes to ethene.]]
In anaerobic groundwater, when conditions are favorable, chlorinated alkenes can undergo a sequential reductive dehalogenation where a chlorine atom is replaced with a hydrogen atom. Degradation proceeds from PCE to TCE, then to DCE, then to VC and finally to ethene (Fig. 1). The minimal geochemical conditions<ref>Wiedemeier, T.H., Swanson, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T., Wilson, B.H., Kampbell, D.H., Haas, P.E., Hansen, J.E., Chapelle, F.H., 1998. Technical protocol for evaluating natural attenuation of chlorinated solvents in ground water. EPA-600-R-98-128. [[Media:Wiedemeier-1998-Technical_Protocol_for_Evaluating_Natuaral_Attenuation.pdf|Report pdf]]</ref> that must be taken into account include pH, oxidation-reduction potential (ORP), dissolved oxygen (DO) concentration, total organic carbon (TOC) and competing electron acceptors including oxygen, nitrate, sulfate and ferric iron.

PCE and TCE can be used as an electron acceptor by a wide variety of bacteria<ref>Nyer, E.K., Payne, F., Sutherson, S., 2003. Discussion of environment vs. bacteria or let's play,‘name that bacteria’. Groundwater Monitoring & Remediation, 23(2), 32-48. [http://dx.doi.org/10.1111/j.1745-6592.2003.tb00665.x doi: 10.1111/j.1745-6592.2003.tb00665.x]</ref>. The bacteria can degrade PCE or TCE as far as DCE. The only organisms that can degrade DCE to VC and then degrade VC to the harmless end product ethene are stains of ''Dehalococcoides mccartyi''<ref>Löffler, F.E., Ritalahti, K.M., Zinder, S.H., 2013. Dehalococcoides and reductive dechlorination of chlorinated solvents. Bioaugmentation for groundwater remediation, ed. H.F. Stroo, Leeson, A., Ward, C.H. Springer, New York, NY. pgs. 39-88. ISBN: 978-1-4614-4114-4 ISBN 978-1-4614-4115-1. [http://dx.doi.org/10.1007/978-1-4614-4115-1 doi: 10.1007/978-1-4614-4115-1]</ref>.

The degradation of chlorinated alkanes in anaerobic groundwater is more complicated (Fig. 2). Chlorinated alkanes can undergo a sequential reductive dehalogenation. In addition, they can undergo the loss of a hydrogen and a chlorine atom to form an alkene (a dehydrochlorination) or the loss of two chlorine atoms to form an alkene (a dichloroelimination).

Three reactions have been demonstrated for 1,1,1-TCA in groundwater (Fig. 2)<ref>Scheutz, C., Durant, N.D., Hansen, M.H., Bjerg, P.L., 2011. Natural and enhanced anaerobic degradation of 1,1,1-trichloroethane and its degradation products in the subsurface–a critical review. Water Research, 45(9), 2701-2723. [http://dx.doi.org/10.1016/j.watres.2011.02.027 doi:10.1016/j.watres.2011.02.027]</ref>. It can undergo an abiotic hydrolysis reaction to produce acetate, an abiotic dehydrochlorination to produce 1,1-DCE, and a biological reductive dechlorination reaction to 1,1-DCA and then chloroethane. In addition to being reduced to chloroethane, 1,1-DCA can undergo a dichloroelimination reaction<ref>Lollar, B.S., Hirschorn, S., Mundle, S.O., Grostern, A., Edwards, E.A., Lacrampe-Couloume, G., 2010. Insights into enzyme kinetics of chloroethane biodegradation using compound specific stable isotopes. Environmental Science & Technology, 44(19), 7498-7503. [http://dx.doi.org/10.1021/es101330r doi: 10.1021/es101330r]</ref> to produce ethene.

[[File:Wilson 3 Fig2.PNG|thumbnail|400 px|left|Figure 2. Degradation of Chlorinated alkanes to ethane.]]

The degradation of 1,1,2-TCA follows a similar pattern (Fig. 2). One strain of ''Desulfitobacterium'' has been shown to dechlorinate 1,1,2-TCA to 1,2-DCA and chloroethane<ref>Zhao, S., Ding, C., He, J., 2015. Detoxification of 1,1,2-trichloroethane to ethene by desulfitobacterium and identification of its functional reductase gene. PloS One, 10(4), p.e0119507. [http://dx.doi.org/10.1371/journal.pone.0119507 doi:10.1371/journal.pone.0119507]</ref> through a sequential reductive dehalogenation. Certain strains of ''Dehalogenimonas'' go through a dichloroelimination reaction<ref>Bowman, K.S., Nobre, M.F., da Costa, M.S., Rainey, F.A. and Moe, W.M., 2013. Dehalogenimonas alkenigignens sp. nov., a chlorinated-alkane-dehalogenating bacterium isolated from groundwater. International Journal of Systematic and Evolutionary Microbiology, 63(4), 1492-1498. [http://dx.doi.org/10.1099/ijs.0.045054-0 doi: 10.1099/ijs.0.045054-0]</ref> to dechlorinate 1,1,2-TCA to VC and 1,2-DCA to ethene. A strain of ''Dehalobacter'' can also dechlorinate 1,2-DCA to ethene<ref>Grostern, A., Edwards, E.A., 2009. Characterization of a Dehalobacter coculture that dechlorinates 1,2-dichloroethane to ethene and identification of the putative reductive dehalogenase gene. Applied and Environmental Microbiology, 75(9), 2684-2693. [http://dx.doi.org/10.1128/aem.02037-08 doi: 10.1128/AEM.02037-08]</ref>.

A quantitative framework (BioPIC)<ref name="Lebron2015">Lebron, C. A., Wiedemeier, T. H., Wilson, J.T., Löffler, F.E., Hinchee, R.E., Singletary, M.A., 2015. Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites. ER-201129. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129/ER-201129 ER-201129]</ref> is now available that allows an evaluation of the rate constant for anaerobic biological degradation of cDCE and VC based on the abundance of gene markers for ''Dehalococcoides mccartyi''. The relationships between the rate constants for degradation of the chlorinated alkanes and abundance of gene copies of ''Dehalobacter'', ''Dehalogenimonas'' and other active bacteria are still being explored.

==Abiotic Degradation==
Chlorinated solvents can chemically react with a number of iron minerals in aquifers<ref name="HE2009"/>. The most important of these are magnetite, iron mono-sulfide, and pyrite.

Iron sulfide minerals form as a consequence of sulfate reduction in groundwater. The sulfide produced from sulfate reduction will react with Iron (III) minerals to form iron mono-sulfide. Over time the iron mono-sulfide will react with excess sulfide to produce pyrite.

The reactions of the chlorinated alkanes with the iron sulfide minerals is a sequential reductive dechlorination. However, the reaction of iron sulfide minerals with chlorinated alkenes is more complex (Fig. 3). Reductive dechlorination and dichloroelimination can proceed at the same time.

[[File:Wilson 3 Fig3.png|thumbnail|250 px|right|Figure 3. Degradation of chlorinated alkenes carried out by iron sulfide minerals.]]

The U.S. EPA regulates the maximum concentration of contaminants that are allowed in water that is supplied as drinking water. These U.S. EPA regulations are referred to as the Maximum Contaminant Level<ref>U.S. Environmental Protection Agency, 2016. Table of Regulated Drinking Water Contaminants. [http://www.epa.gov/your-drinking-water/table-regulated-drinking-water-contaminants Table of Regulated Drinking Water]</ref> or MCL. There are MCLs for the transformation products of reductive dechlorination (the DCEs and VC) and these products will be included in the target list of analytes in any conventional monitoring program. The products of dichloroelimination do not have MCLs and are not usually on the target list of analytes for conventional monitoring.

If the major pathway of abiotic degradation is dichloroelimination, then conventional monitoring will fail to recognize the contribution of abiotic degradation on iron sulfide minerals. However, the stable isotopes of carbon in chlorinated solvents are strongly fractionated during abiotic degradation on iron sulfide minerals. [[Compound Specific Isotope Analysis (CSIA) | Compound Specific Isotope Analysis (CSIA)]]<ref>Hunkeler, D., Meckenstock, R. U., Sherwood Lollar, B., Schmidt, T.C., Wilson, J.T., 2008. A Guide for Assessing Biodegradation and Source Identification of Organic Groundwater Contaminants Using Compound Specific Isotope Analysis (CSIA). U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-08/148. [[Media:Hunkeler-2008-A_Guide.pdf|Report pdf]]</ref> can be a useful tool to recognize abiotic degradation of chlorinated alkenes on iron sulfide minerals.

Magnetite is often present in unconsolidated glacial aquifers and aquifers that form in sediments that are shed by uplands composed of granite or other igneous rocks. Magnetite reacts readily with the chlorinated alkenes. The actual chemical interactions on magnetite are not well understood (Fig. 4). The ultimate degradation products are oxidized organic compounds and carbon dioxide<ref>Darlington, R., Rectanus, H., 2015. Biogeochemical Transformation Handbook. TR-NAVFAC EXWC-EV-1601, 41 pgs. [[Media:Darlington-2015-Biogeochem_Transformation_Handbook.pdf|Report pdf]]</ref>.

[[File:Wilson 3 Fig4.png|thumbnail|300 px|left|Figure 4. Degradation of chlorinated alkenes carried out by magnetite.]]

==Footprints==
Most plumes have some contribution of anaerobic sequential reductive dechlorination. As a result, the primary contaminant and the transformation products of reductive dechlorination are present in the groundwater. The highest concentrations of the primary contaminant will be near the source of contamination, and the flow of groundwater carries the transformation products further downgradient from the source (Figure 5).

Many plumes of chlorinated solvents also have a contribution of abiotic degradation. As a result, the intermediate degradation products (such as DCE) do not accumulate to stoichiometric concentrations. There is an appearance that degradation of the cDCE has stalled, when in fact it is actively degrading, but not to vinyl chloride (Fig. 5). A quantitative framework<ref name="Lebron2015"/> is now available that allows an evaluation of the contribution of abiotic degradation on magnetite based on the magnetic susceptibility of the sediment, and the contribution of abiotic degradation on pyrite based on the extent of sulfate reduction and the geochemistry of the groundwater.
[[File:Wilson 3 Fig5.png|thumbnail|400 px|center|Figure 5. Comparison of a chlorinated alkenes plume undergoing biodegradation alone vs. biodegradation with abiotic degradation.]]

==Tools and Databases for Chlorinated Solvent MNA==
The Scenarios Evaluation Tool for Chlorinated Solvent MNA<ref>Truex, M.J., Newell, C.J., Looney, B.B, Vangelas, K., 2006. Scenarios evaluation tool for chlorinated solvent MNA. Savannah River National Laboratory, Aiken, South Carolina. WSRC-STI-2006-0096. [[Media:Truex-2006-Scenarios_Evaluation_Tool_for_Chlorinated_Solvent_MNA.pdf|Report pdf]]</ref> was designed to provide a structure where the MNA methods and decision logic are linked together in one of 13 different “scenarios” or site types. Based on site data (e.g. Table 2), one selects which of the 13 scenarios best fits their site or portion of a site. Then one goes to the description of that scenario to learn which attenuation reactions are likely to be active, how to design a MNA monitoring program, whether MNA will work, and other relevant factors.
[[File:Wilson 3 Table2.png|thumbnail|600 px|center|Table 2. Key elements of the scenarios tool for chlorinated solvent MNA.]]

A data mining study of MNA at 45 chlorinated solvent sites<ref>McGuire, T.M., Newell, C.J., Looney, B.B., Vangelas, K.M., 2003. Historical and retrospective survey of monitored natural attenuation: A line of inquiry supporting monitored natural attenuation and enhanced passive remediation of chlorinated solvents. Westinghouse Savannah River Company, Aiken, SC. [[Media:McGuire-2003-Historical_and_Retrospective_Survey_of_MNA.pdf|Report pdf]]</ref> provides some interesting information about plume sources, strength, and size (Fig. 6).

[[File:Wilson 3 Fig6.png|thumbnail|500 px|center|Figure 6. Plume characteristics evaluation of 45 chlorinated solvent sites.]]

The performance of MNA was evaluated<ref>McGuire, T., 2016. Development of an Expanded, High-Reliability Cost and Performance Database for In-Situ Remediation Technologies. ESTCP Project No. ER-201120. [https://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201120/ER-201120 ER-201120]</ref> by comparing the change in concentrations of chlorinated organic compounds in wells in the source zone of plumes from the beginning to the end of an MNA monitoring period (Fig. 7).

[[File:Wilson 3 Fig7.png|thumbnail|500 px|center|Figure 7. Each dot represents an individual project, showing the geometric mean of the concentration at the beginning of the monitoring record (X-axis) and at the end of the monitoring record (Y-axis). The median duration of MNA monitoring for these 45 sites was 8.7 years and ranged from 4.1 to 15 years.]]

One study evaluated the change in source concentration over time at 23 chlorinated solvent sites by calculating concentration vs. time decay rates for source zone wells<ref>Newell, C.J., Cowie, I., McGuire, T.M., McNab Jr, W.W., 2006. Multiyear temporal changes in chlorinated solvent concentrations at 23 monitored natural attenuation sites. Journal of Environmental Engineering, American Society of Environmental Engineers, 132(6), 653-663. [http://dx.doi.org/10.1061/(asce)0733-9372(2006)132:6(653) doi: 10.1061/(asce)0733-9372(2006)132:6(653)]</ref>. The authors concluded, “If the median point decay rates from these sites are maintained over a 20 year period, the resulting reduction in concentration will be similar to the reported reduction in source zone concentrations achieved by active in situ source remediation technologies (typical project length: 1–2 years)".

As part of the development process for the chlorinated solvent natural attenuation model<ref>Aziz, C.E., Smith, A.P., Newell, C.J., Gonzales, J.R., 2000. BIOCHLOR Chlorinated solvent plume database report. Air Force Center for Environmental Excellence, Texas. [[Media:Aziz-2000-BIOCHLOR-plume-database.pdf|Report pdf]]</ref> BIOCHLOR, 24 chlorinated solvent plumes were studied in detail. Key findings included:

*TCE and c-DCE had median plume lengths of 1215 ft and 1205 ft, respectively.
*Chlorinated ethene plume lengths were moderately correlated with seepage velocity and source width (Fig. 8).
*First order decay rates ranged between 1 and 2 per year for the chlorinated ethane plumes.

[[File:Wilson 3 Fig8.png|thumbnail|900 px|center|Figure 8. Effect of estimated source size and groundwater seepage velocity on plume length.]]

==Summary==
MNA is an important remediation technology at some chlorinated solvent sites. There are numerous reactions, both biotic and abiotic, that can act on different chlorinated solvent compounds. Several tools and databases are available to help understand how chlorinated solvent plumes behave and to design and implement appropriate MNA programs.

==References==

<references/>

==See Also==
*[[Media:EPA-MNA-Chlorinated-Organics-Symposium.pdf|Proceedings of the Symposium on Natural Attenuation of Chlorinated Organics in Ground Water]]
*[[Media:AFCEE-Natural_Attenuation-Chlorinated_Solvents-1999.pdf|Natural Attenuation of Chlorinated Solvents Performance and Cost Results From Multiple Air Force Demonstration Sites]]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-1348 Using Advanced Analysis Approaches to Complete Long-Term Evaluations of Natural Attenuation Processes on the Remediation of Dissolved Chlorinated Solvent Contamination]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-1349/ER-1349 Integrated Protocol for Assessment of Long-Term Sustainability of Monitored Natural Attenuation of Chlorinated Solvent Plumes]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200019 Impact of Landfill Closure Designs on Long-Term Natural Attenuation of Chlorinated Hydrocarbons]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436 Estimating Cleanup Times Associated with Combining Source-Area Remediation with Monitored Natural Attenuation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200708/ER-200708 Use of Enzyme Probes for Estimation of Trichloroethene Degradation Rates and Acceptance of Monitored Natural Attenuation ]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200824/ER-200824 Verification of Methods for Assessing the Sustainability of Monitored Natural Attenuation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129 Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201211/ER-201211 Frequently Asked Questions about Monitored Natural Attenuation in the 21st Century]
*[https://www.coursera.org/learn/natural-attenuation-of-groundwater-contaminants/lecture/kBe2j/abiotic-degradation-principles Online Lecture Course - Abiotic Degradation]

Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions

2026-05-07T16:52:23Z

Admin:

The U.S. Department of Defense (DoD) faces many challenges in restoring aquifers at contaminated sites, often due to back-diffusion of tetrachloroethene (PCE) and trichloroethene (TCE) from low-permeability clay zones. The uptake, storage, and subsequent long-term release of these dissolved contaminants from clays are key processes in understanding the longevity, intensity, and risks associated with many persistent chlorinated ethene groundwater plumes. Although naturally occurring abiotic and biotic dechlorination processes in clays may reduce stored contaminant mass and significantly aid natural attenuation, no standardized field method currently exists to verify or quantify these reactions. It is critical to remediation design efforts to demonstrate and validate a cost-effective ''in situ'' approach for assessing these dechlorination processes using first-order rate constants. An approach was developed and applied across eight DoD sites to support Remedial Project Managers (RPMs) and regulators in evaluating natural attenuation potential in clay-rich environments.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Chlorinated Solvents]]
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| Monitored Natural Attenuation of Chlorinated Solvents]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
*[[Matrix Diffusion]]
*[[REMChlor - MD]]

'''Contributor(s):''' [[Dani Tran]], [[Dr. Charles Schaefer]], and [[Dr. Charles Werth]]

'''Key Resource(s):'''
*Schaefer, C.E, Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils<ref name="SchaeferEtAl2025"/>

==Introduction==
Cost-effective methods are needed to verify the occurrence of natural dechlorination processes and quantify their dechlorination rates in clays under ambient in situ conditions in order to reliably predict their long-term influence on plume longevity and mass discharge. However, accurately determining these rates is challenging due to slow reaction kinetics, the transient nature of transformation products, and the interplay of biotic and abiotic mechanisms within the clay matrix or at clay-sand interfaces. Tools capable of quantifying these reactions and assessing their role in mitigating plume persistence would be a significant aid for long-term site management.

For reductive abiotic dechlorination under anoxic conditions, a 1% hydrochloric acid (HCl) extraction of a sample of native clay coupled with X-ray diffraction (XRD) data can be used as a screening level tool to estimate reductive dechlorination rate constants. These rate constants can be inserted into fate and transport models such as [[REMChlor - MD]]<ref>Falta, R., and Wang, W., 2017. A semi-analytical method for simulating matrix diffusion in numerical transport models. Journal of Contaminant Hydrology, 197, pp. 39-49. [https://doi.org/10.1016/j.jconhyd.2016.12.007 doi: 10.1016/j.jconhyd.2016.12.007]  [[Media: FaltaWang2017.pdf | Open Access Manuscript]]</ref><ref>Kulkarni, P.R., Adamson, D.T., Popovic, J., Newell, C.J., 2022. Modeling a well-charactized perfluorooctane sulfate (PFOS) source and plume using the REMChlor-MD model to account for matrix diffusion. Journal of Contaminant Hydrology, 247, Article 103986. [https://doi.org/10.1016/j.jconhyd.2022.103986 doi: 10.1016/j.jconhyd.2022.103986]  [[Media: KulkarniEtAl2022.pdf | Open Access Manuscript]]</ref> to quantify abiotic dechlorination impacts within clay aquitards on chlorinated solvent plumes. Thus, determination of the abiotic reductive dechlorination rate constant for a particular clayey soil can be readily utilized to provide a more accurate assessment of aquifer cleanup timeframes for groundwater plumes that are being sustained by contaminant back-diffusion.

==Recommended Approach==
[[File: TranFig1.png | thumb | 500 px | Figure 1: First-order rate constants for abiotic reductive dechlorination of TCE under anaerobic conditions. Circles are data from Schaefer ''et al.'', 2021<ref>Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2021. Abiotic dechlorination in the presence of ferrous minerals. Journal of Contaminant Hydrology, 241, 103839. [https://doi.org/10.1016/j.jconhyd.2021.103839 doi: 10.1016/j.jconhyd.2021.103839]  [[Media: SchaeferEtAl2021.pdf | Open Access Manuscript]]</ref>, filled squares from Schaefer ''et al.'', 2018<ref name="SchaeferEtAl2018"/>, and Schaefer ''et al.'', 2017<ref>Schaefer, C.E., Ho., Gurr, C., Berns, E., Werth, C., 2017. Abiotic dechlorination of chlorinated ethenes in natural clayey soils: impacts of mineralogy and temperature. Journal of Contaminant Hydrology, 206, pp. 10-17. [https://doi.org/10.1016/j.jconhyd.2017.09.007 doi: 10.1016/j.jconhyd.2017.09.007]  [[Media: SchaeferEtAl2017.pdf | Open Access Manuscript]]</ref>, and open squares from Schaefer ''et al.'', 2025<ref name="SchaeferEtAl2025"/>. ]]
[[File: TranFig2.png | thumb | 600 px | Figure 2: Flowchart diagram of field screening procedures]]
The recommended approach builds upon the methodology and findings of a recent study<ref name="SchaeferEtAl2025">Schaefer, C.E., Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils. Groundwater Monitoring and Remediation, 45(2), pp. 31-39. [https://doi.org/10.1111/gwmr.12709 doi: 10.1111/gwmr.12709]</ref>, emphasizing field-based and analytical techniques to quantify abiotic first-order reductive dechlorination rate constants for PCE and TCE in clayey soils under anoxic conditions. Key components of this evaluation are listed below:
#Zone Identification: The focus of the investigation should be to delineate clayey zones adjacent to hydraulically conductive zones.
#Ferrous Mineral Quantification: Assess ferrous mineral context in clay via 1% HCl extraction at ambient temperature over a 10-minute interval.
#Mineralogical Characterization: Conduct XRD analysis with the specific intent of identifying the presence of pyrite and biotite.
#Reduced Gas Analysis: Measurement of reduced gases such as acetylene, ethene, and ethane concentrations in clay samples. Gas-tight sampling devices (e.g., En Core® soil samplers by En Novative Technologies, Inc.) should be used to ensure sample integrity during collection and transport.

Clay samples should be collected within a few centimeters of the high-permeability interface, with optional additional sampling further inward. For mineralogical analysis, a defined interval may be collected and subsequently subsampled. To preserve sample integrity, exposure to air should be minimized during collection, transport, and handling. Homogenization should occur within an anaerobic chamber, and if subsamples are required for external analysis, they must be shipped in gas-tight, anaerobic containers.

Estimation of the abiotic reductive first-order rate constant for PCE and TCE is based on the “reactive” ferrous content in the clay. Reactive ferrous content (Fe(II)r) is estimated as shown in Equation 1:

::'''Equation 1:'''       <big>''Fe(II)r = DA + XRDpyr - XRDbiotite''</big>

where ''DA'' is the ferrous content from the dilute acid (1% HCl) extraction, ''XRDpyr'' is the pyrite content from XRD analysis, and ''XRDbiotite'' is the biotite content from XRD analysis<ref name="SchaeferEtAl2025"/>.

Abiotic dechlorination is unlikely to contribute to mitigating contaminant back-diffusion when reactive ferrous iron (Fe(II)r) concentrations are below 100 mg/kg (Figure 1). For Fe(II)r above 100 mg/kg, the first-order rate constant for PCE and TCE reductive dechlorination can be estimated using the correlation shown in Figure 1<ref name="SchaeferEtAl2018">Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2018. Mechanisms for abiotic dechlorination of trichloroethene by ferrous minerals under oxic and anoxic conditions in natural sediments. Environmental Science and Technology, 52(23), pp.13747-13755. [https://doi.org/10.1021/acs.est.8b04108 doi: 10.1021/acs.est.8b04108]</ref><ref>Borden, R.C., Cha, K.Y., 2021. Evaluating the impact of back diffusion on groundwater cleanup time. Journal of Contaminant Hydrology, 243, Article 103889. [https://doi.org/10.1016/j.jconhyd.2021.103889 doi: 10.1016/j.jconhyd.2021]  [[Media: BordenCha2021.pdf | Open Access Manuscript]]</ref>. The rate constant exhibits a strong positive correlation with the logarithm of reactive Fe(II) content (Pearson’s ''r'' = 0.82), with a slope of 4.7 × 10⁻⁸ L g⁻¹ d⁻¹ (log mg kg⁻¹)⁻¹.

Figure 2 presents a decision flowchart designed to evaluate the significance and extent of abiotic reductive dechlorination. By applying Equation 1 to the dilute acid extractable Fe(II) plus measured mineral species data from clay samples, the reactive ferrous iron content (Fe(II)r) can be quantified, enabling a streamlined assessment of the extent to which abiotic processes are contributing to the mitigation of contaminant back-diffusion.

If Fe(II)r is ≥ 100 mg/kg, a first-order dechlorination rate constant can be estimated and subsequently used within a contaminant fate and transport model. However, if acetylene is detected in the clay, even with Fe(II)r less than 100 mg/kg, then bench-scale testing using methods similar to those described in a recent study<ref name="SchaeferEtAl2025"/> is recommended, as such results would likely be inconsistent with those shown in Figure 1, suggesting some other mechanism might be involved, or that the system mineralogy might be more complex than anticipated. Even if Fe(II)r ≥ 100 mg/kg, confirmatory bench-scale testing may be conducted for additional verification and to refine estimation of the abiotic dechlorination rate constant.

==Summary and Recommendations==
The approach outlined above is intended to serve as a generalized guide for practitioners and site managers to cost-effectively determine the extent to which beneficial abiotic reductive dechlorination reactions are likely occurring in low permeability (e.g., clayey) zones. This approach may be contraindicated if co-contaminants are present. It is currently unclear whether other classes of potentially reactive chemicals, such as trinitrotoluene (TNT) or chlorinated ethanes, could interact competitively with PCE and TCE.

In addition, it remains unclear how other classes of compounds such as per- and polyfluoroalkyl substances (PFAS) may interact or sorb with ferrous minerals and potentially inhibit abiotic dechlorination reactions. Coupling these recommended activities with conventional site investigation tasks would provide an opportunity to perform many of the up-front screening activities with minimal additional project costs. It is important to note that the guidance proposed herein pertains to particularly low permeability media. Sites with complex or varying lithology, where the mineralogy and/or redox conditions may vary, might require evaluation of multiple samples to provide appropriate site-wide information.
 

==References==
<references />

==See Also==
*[https://serdp-estcp.mil/projects/details/a7e3f7b5-ed82-4591-adaa-6196ff33dd60 ESTCP Project ER20-5031 – In Situ Verification and Quantification of Naturally Occurring Dechlorination Rates in Clays: Demonstrating Processes that Mitigate Back-Diffusion and Plume Persistence]

Chlorinated Solvents

2026-05-07T16:51:07Z

Admin:

Chlorinated solvents, including chlorinated volatile organic compounds (CVOC or CVOCs), are chemical compounds containing chlorine that have been widely used in various industries. They are divided in three groups (methanes, ethanes, ethenes) based on their structures, and include common groundwater contaminants such as carbon tetrachloride (CT), perchloroethene (PCE), trichloroethene (TCE), and vinyl chloride (VC). Chlorinated solvents tend to be colorless liquids at room temperatures, heavier than water, volatile, sparingly soluble, and moderately hydrophobic.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
'''Related Article(s):'''

*[[Biodegradation - Cometabolic]]
*[[Biodegradation - Reductive Processes]]
*[[Bioremediation - Anaerobic]]
*[[Bioremediation - Anaerobic Design Considerations]]
*[[Chemical Oxidation (In Situ - ISCO)]]
*[[Chemical Reduction (In Situ - ISCR)]]
*[[Design Tool - Base Addition for ERD]]
*[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation| Emulsified Vegetable Oil for Anaerobic Bioremediation]]
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Low pH Inhibition of Reductive Dechlorination]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| Monitored Natural Attenuation of Chlorinated Solvents]]
*[[pH Buffering in Aquifers]]
*[[Remediation Performance Assessment at Chlorinated Solvent Sites]]
*[[Soil & Groundwater Contaminants]]
*[[Thermal Remediation]]

'''Contributor(s): ''' [[Dr. Bilgen Yuncu, P.E.]] and [[M. Tony Lieberman]]

'''Key Resource(s):'''

*[http://dx.doi.org/10.1007/978-1-4419-1401-9_2 Chlorinated Solvent Chemistry: Structures, Nomenclature and Properties]<ref name="CS2010">Cwiertny, D.M., Scherer, M.M., 2010. Chlorinated solvent chemistry: structures, nomenclature and properties. In In situ remediation of chlorinated solvent plumes. Springer New York. pgs. 29-37. [http://dx.doi.org/10.1007/978-1-4419-1401-9_2 doi:10.1007/978-1-4419-1401-9_2]</ref>

==Introduction==

Chlorinated solvents are a large family of organic solvents that contain chlorine atoms in their molecular structure. They were first produced in Germany in the 1800s, and widespread use in the United States (U.S.) began after World War II. In the period of 1940-1980, the U.S. produced about 2 billion pounds of chlorinated solvents each year<ref name="PC 1996"> Pankow, J.F., Cherry, J.A., 1996. Dense Chlorinated Solvents and Other DNAPLs in Groundwater, Waterloo Press, Portland, OR. ISBN 0964801418</ref>. Chlorinated solvents, including [[wikipedia:Carbon_tetrachloride|carbon tetrachloride (CT)]], [[wikipedia:1,1,1-Trichloroethane|1,1,1-trichloroethane (TCA)]], [[wikipedia:Tetrachloroethylene|perchloroethene or tetrachloroethene (PCE)]] and [[wikipedia:Trichloroethylene|trichloroethene (TCE)]] have been among the most widely used cleaning and degreasing solvents in the U.S<ref>Doherty, R.E., 2000. A history of the production and use of carbon tetrachloride, tetrachloroethylene, trichloroethylene and 1, 1, 1-trichloroethane in the United States: Part 1--historical background; carbon tetrachloride and tetrachloroethylene. Environmental Forensics, 1(2), 69-81. [http://dx.doi.org/10.1006/enfo.2000.0010 doi:10.1006/enfo.2000.0010]</ref>. They also have been used in a wide variety of other purposes such as adhesives, chemical intermediates, clothes, pharmaceuticals, pesticides, and textile processing.

==Physical & Chemical Properties==

Chlorinated solvents are organic compounds generally constructed of a simple hydrocarbon chain (typically one to three carbon atoms in length). They can be divided into three categories based on their structural characteristics: chlorinated methanes, chlorinated ethanes and chlorinated ethenes.

Chlorinated methanes represent the most structurally simple solvent class and consist of a single carbon center (known as a methyl carbon) to which as many as four chlorine atoms are bonded. From the perspective of groundwater contamination, perhaps the most well-known chlorinated methanes are [[wikipedia:carbon tetrachloride|carbon tetrachloride (CT)]] or [[wikipedia:tetrachloromethane|tetrachloromethane]], [[wikipedia:trichloromethane|trichloromethane]] (commonly known as [[wikipedia:chloroform|chloroform (CF)]]), [[wikipedia:dichloromethane|dichloromethane (DCM)]], or [[wikipedia:methylene chloride|methylene chloride (MC)]] and [[wikipedia:chloromethane|chloromethane (CM)]], or [[wikipedia:methyl chloride|methyl chloride]].

Chlorinated ethanes consist of two carbon centers joined by a single [[wikipedia:Covalent_bond|covalent bond]]. The most frequently encountered groundwater pollutants of this class include [[wikipedia:1,1,1-trichloroethane|1,1,1-trichloroethane (1,1,1-TCA)]] and [[wikipedia:1,2-dichloroethane|1,2-dichloroethane]].

Chlorinated ethenes (also referred to as chlorinated ethylenes) also possess two carbon centers, but unlike chlorinated ethanes, these carbon atoms are joined by a carbon-carbon double bond. Chlorinated ethenes that are important groundwater contaminants include [[wikipedia:tetrachloroethene|tetrachloroethene]], or [[wikipedia:perchloroethene|perchloroethene (PCE)]], [[wikipedia:trichloroethene|trichloroethene (TCE)]], [[wikipedia:dichloroethene|dichloroethene (DCE)]]) (DCE, mainly two geometric isomers cis-1,2-dichloroethene and trans-1,2-dichloroethene), and [[wikipedia:vinyl chloride|vinyl chloride (VC)]].

Nomenclature and structure of selected compounds from each solvent class as well as some physical and chemical properties of most widely used chlorinated solvents are listed in Table 1.


{| class="mw-collapsible wikitable" style="margin: auto; color:black; background-color:white; width: 100%;"
|+Table 1. Nomenclature, Structure, Chemical and Physical Properties of Most Widely Used Chlorinated Solvents<ref name="CS2010" />.
|- style="color:white; background-color:#476b6b; text-align:center;"
|IUPAC Name
|Common Name
|Acronym
|Molecular Formula
|Chemical Structure
|Formula Weight
|Density (ρ)(g/mL)
|Aqueous Solubility (mg/L)
|Vapor Pressure (ρ0)(kPa)
|Henry's Law Constanta
|Log Kow
|MCLb (mg/L)
|-
| colspan="12" style="color:black; background-color:#94b8b8;" |Chlorinated Methanes
|-
| style="color:black; background-color:#d1e0e0;" |tetrachloromethane
| style="text-align:center;" |carbon tetrachloride
| style="text-align:center;" |CT
| style="text-align:center;" |CCl4
|
[[File:Tetrachloromethane.png|center|70 px|frameless]]
| style="text-align:center;" |153.8
| style="text-align:center;" |1.59
| style="text-align:center;" |800
| style="text-align:center;" |20.5
| style="text-align:center;" |28.9
| style="text-align:center;" |2.64
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |trichloromethane
| style="text-align:center;" |chloroform
| style="text-align:center;" |CF
| style="text-align:center;" |CHCl3
|
[[File:Trichloromethane.png|center|70px|frameless]]
| style="text-align:center;" |119.4
| style="text-align:center;" |1.49
| style="text-align:center;" |8,200
| style="text-align:center;" |26.2
| style="text-align:center;" |3.8
| style="text-align:center;" |1.97
| style="text-align:center;" |0.080c
|-
| style="color:black; background-color:#d1e0e0;" |dichloromethane
| style="text-align:center;" |methylene chloride
| style="text-align:center;" |DCM
| style="text-align:center;" |CH2Cl2
|
[[File:Dichloromethane.png|center|70px|frameless]]
| style="text-align:center;" |84.9
| style="text-align:center;" |1.33
| style="text-align:center;" |13,200
| style="text-align:center;" |55.3
| style="text-align:center;" |1.7
| style="text-align:center;" |1.25
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |chloromethane
| style="text-align:center;" |methyl chloride
| style="text-align:center;" |CM
| style="text-align:center;" |CH3Cl
|
[[File:Chloromethane.png|center|70px|frameless]]
| style="text-align:center;" |50.5
| style="text-align:center;" |0.92
| style="text-align:center;" |5,235
| style="text-align:center;" |570
| style="text-align:center;" |9.6
| style="text-align:center;" |0.91
| style="text-align:center;" |NRd
|-
| colspan="12" style="color:black; background-color:#94b8b8;" |Chlorinated Ethanes
|-
| style="color:black; background-color:#d1e0e0;" |hexachloroethane
| style="text-align:center;" |perchloroethane
| style="text-align:center;" |HCA
| style="text-align:center;" |C2Cl6
|
[[File:Hexachloroethane.png|center|70px|frameless]]
| style="text-align:center;" |236.7
| style="text-align:center;" |2.09
| style="text-align:center;" |50
| style="text-align:center;" |0.05e
| style="text-align:center;" | -
| style="text-align:center;" |3.93
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |pentachloroethane
| style="text-align:center;" | -
| style="text-align:center;" |PCA
| style="text-align:center;" |C2HCl5
|
[[File:Pentachloroethane.png|center|70px|frameless]]
| style="text-align:center;" |202.3
| style="text-align:center;" |1.68
| style="text-align:center;" |500
| style="text-align:center;" |0.6
| style="text-align:center;" |2.5
| style="text-align:center;" |2.89
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |1,1,1,2-tetrachloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1,1,2-TeCA
| style="text-align:center;" |C2H2Cl4
|
[[File:1,1,1,2-Tetrachloroethane.png|72px|frameless|center]]
| style="text-align:center;" |167.9
| style="text-align:center;" |1.54
| style="text-align:center;" |1,100
| style="text-align:center;" |1.6
| style="text-align:center;" |2.4
| style="text-align:center;" | -
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |1,1,2,2-tetrachloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1,2,2-TeCA
| style="text-align:center;" |C2H2Cl4
|
[[File:1,1,2,2-Tetrachloroethane.png|72px|frameless|center]]
| style="text-align:center;" |167.9
| style="text-align:center;" |1.60
| style="text-align:center;" |2,962
| style="text-align:center;" |0.8
| style="text-align:center;" |0.44
| style="text-align:center;" |2.39
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |1,1,2-trichloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1,2-TCA
| style="text-align:center;" |C2H3Cl3
|
[[File:1,1,2-Trichloroethane.svg.png|72px|frameless|center]]
| style="text-align:center;" |133.4
| style="text-align:center;" |1.44
| style="text-align:center;" |4,394
| style="text-align:center;" |3.22
| style="text-align:center;" |0.96
| style="text-align:center;" |2.38
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |1,1,1-trichloroethane
| style="text-align:center;" |methyl chloroform
| style="text-align:center;" |1,1,1-TCA
| style="text-align:center;" |C2H3Cl3
|
[[File:1,1,1-trichloroethane.png|center|70px|frameless]]
| style="text-align:center;" |133.4
| style="text-align:center;" |1.35
| style="text-align:center;" |1,495
| style="text-align:center;" |16.5
| style="text-align:center;" |14.5
| style="text-align:center;" |2.49
| style="text-align:center;" |0.20
|-
| style="color:black; background-color:#d1e0e0;" |1,2-dichloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,2-DCA
| style="text-align:center;" |C2H4Cl2
|
[[File:1,2-dichloroethane.png|center|70px|frameless]]
| style="text-align:center;" |99.0
| style="text-align:center;" |1.25
| style="text-align:center;" |8,606
| style="text-align:center;" |10.5
| style="text-align:center;" |1.2
| style="text-align:center;" |1.48
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |1,1-dichloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1-DCA
| style="text-align:center;" |C2H4Cl2
|
[[File:1,1-Dichloroethane 2.svg.png|72px|frameless|center]]
| style="text-align:center;" |99.0
| style="text-align:center;" |1.17
| style="text-align:center;" |4,676
| style="text-align:center;" |30.3
| style="text-align:center;" |6.2
| style="text-align:center;" |1.79
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |chloroethane
| style="text-align:center;" | -
| style="text-align:center;" |CA
| style="text-align:center;" |C2H5Cl
|
[[File:Chloroethane.png|center|70px|frameless]]
| style="text-align:center;" |64.5
| style="text-align:center;" |0.92
| style="text-align:center;" |5,700
| style="text-align:center;" |16.0
| style="text-align:center;" |1.8
| style="text-align:center;" |1.43
| style="text-align:center;" |NR
|-
| colspan="12" style="color:black; background-color:#94b8b8;" |Chlorinated Ethenes
|-
| style="color:black; background-color:#d1e0e0;" |tetrachloroethene
| style="text-align:center;" |perchloroethene
| style="text-align:center;" |PCE
| style="text-align:center;" |C2Cl4
|
[[File:Tetrachloroethene.png|center|70px|frameless]]
| style="text-align:center;" |165.8
| style="text-align:center;" |1.63
| style="text-align:center;" |150
| style="text-align:center;" |2.4
| style="text-align:center;" |26.3
| style="text-align:center;" |2.88
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |trichloroethene
| style="text-align:center;" | -
| style="text-align:center;" |TCE
| style="text-align:center;" |C2HCl3
|
[[File:Trichloroethene.png|72px|frameless|center]]
| style="text-align:center;" |131.4
| style="text-align:center;" |1.46
| style="text-align:center;" |1,100
| style="text-align:center;" |9.9
| style="text-align:center;" |11.7
| style="text-align:center;" |2.53
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |cis-1,2-dichloroethene
| style="text-align:center;" |cis-dichloroethene
| style="text-align:center;" |cis-DCE
| style="text-align:center;" |C2H2Cl2
|
[[File:Cis-1,2-dichloroethene.png|center|70px|frameless]]
| style="text-align:center;" |96.9
| style="text-align:center;" |1.28
| style="text-align:center;" |3,500
| style="text-align:center;" |27.1
| style="text-align:center;" |7.4
| style="text-align:center;" |1.86
| style="text-align:center;" |0.07
|-
| style="color:black; background-color:#d1e0e0;" |trans-1,2-dichloroethene
| style="text-align:center;" |trans-dichloroethene
| style="text-align:center;" |trans-DCE
| style="text-align:center;" |C2H2Cl2
|
[[File:Trans-1,2-dichloroethene.png|72px|frameless|center]]
| style="text-align:center;" |96.9
| style="text-align:center;" |1.26
| style="text-align:center;" |6,260
| style="text-align:center;" |44.4
| style="text-align:center;" |6.8
| style="text-align:center;" |1.93
| style="text-align:center;" |0.1
|-
| style="color:black; background-color:#d1e0e0;" |1,1-dichloroethene
| style="text-align:center;" |vinylidene chloride
| style="text-align:center;" |1,1-DCE
| style="text-align:center;" |C2H2Cl2
|
[[File:1,1-Dichloroethene.svg.png|72px|frameless|center]]
| style="text-align:center;" |96.9
| style="text-align:center;" |1.22
| style="text-align:center;" |3,344
| style="text-align:center;" |80.5
| style="text-align:center;" |23.0
| style="text-align:center;" |2.13
| style="text-align:center;" |0.007
|-
| style="color:black; background-color:#d1e0e0;" |chloroethene
| style="text-align:center;" |vinyl chloride
| style="text-align:center;" |VC
| style="text-align:center;" |C2H3Cl
|
[[File:Chloroethene.png|center|70px|frameless]]
| style="text-align:center;" |62.5
| style="text-align:center;" |0.91
| style="text-align:center;" |2,763
| style="text-align:center;" |355
| style="text-align:center;" |79.2
| style="text-align:center;" |1.38
| style="text-align:center;" |0.002
|-
| colspan="12" style="color:black; background-color:#d1e0e0;" |Notes:
atm = atmosphere; g = gram; Kow = octanol/water partitioning coefficient; Koc -- soil organic carbon/water partitioning coefficient; L = liter; MCL = maximum contaminant level; mg = milligram; mL = milliliter; mol = mole.

aHenry's Law Constant (KH)(x10-3 atm ・ m3/mol)

bSource: http://water.epa.gov/drink/contaminants/#List

cMCL for total trihalomethanes is defined as the summed concentration of chloroform, bromoform (CHBr3),bromodichloromethane (CHBrCl2), and dibromochloromethane (CHBr2Cl). http://water.epa.gov/drink/contaminants/basicinformation/disinfectionbyproducts.cfm

dNR : Not regulated.

eReported vapor pressure for solid-phase hexachloroethane.

|-
|}

Chlorinated solvents and many of their transformation products are colorless liquids at room temperature. They are heavier than water with densities greater than 1 gram per cubic centimeter (g/cm3) which means they can penetrate deeply into an aquifer. They are relatively volatile compounds with relatively high [[wikipedia:Henry’s Law|Henry’s Law]] constants(KH), a measure of the strength of partitioning from water into air). Generally, when KH for a compound exceeds 0.2 atmosphere/mole fraction (atm/M), they can readily be removed from water by air stripping it. Most chlorinated solvents can be classified as sparingly soluble in water, with aqueous solubilities generally on the order of 10s to 100s of mg/L. As the number of chlorine atoms on a compound increases, the solubility decreases. Because of their relatively low solubilities, chlorinated solvents dissolve slowly in groundwater. Another consequence of their limited solubility is their tendency to occur in the subsurface as a separate immiscible liquid phase which, because of its density compared to water, tends to sink in groundwater. Under these conditions, these are referred to as [[wikipedia:DNAPL|dense non-aqueous phase liquid (DNAPL)]]. Although chlorinated solvents are not very soluble in water, their solubility is typically orders of magnitude greater than their established [http://water.epa.gov/drink/contaminants/#Organic drinking water standards].

Chlorinated solvents can be considered moderately hydrophobic which can be determined by their [[wikipedia:Partition coefficient|octanol-water partition coefficient]]s (Kow, a measure of the tendency of a substance to prefer an organic or oily phase rather than an aqueous phase). Log Kow values less than 3 indicate that the compound does not sorb strongly to aquifer solids, but can be removed readily by activated carbon. On the other hand, compounds with log Kow less than 2, such as VC, generally are not removed well by activated carbon either<ref name="CS2010" />.

==References==

<references />

==See Also==

*[https://serdp-estcp.org/Tools-and-Training/Environmental-Restoration/DNAPL-Source-Zones/Frequently-Asked-Questions-Regarding-Management-of-Chlorinated-Solvents-in-Soils-and-Groundwater FAQ Regarding Management of Chlorinated Solvents in Soil and Groundwater]
*[//www.enviro.wiki/images/6/6b/AFCEE_Protocol_2007_chlorinated_solvents.pdf Protocol for In Situ Bioremediation of Chlorinated Solvents Using Edible Oil]

Monitored Natural Attenuation (MNA)

2026-05-07T16:49:39Z

Admin:

Monitored Natural Attenuation (MNA) is an important, common groundwater remediation technology used for treating some dissolved groundwater contaminants. MNA relies on natural attenuation processes to achieve site-specific remediation objectives within a reasonable time frame compared to more active approaches. While MNA has primarily focused on managing plumes with low residual contamination, there is an growing movement to also apply it to source zones via [[ Natural Source Zone Depletion (NSZD) | natural source zone depletion (NSZD)]]. [[Long-Term Monitoring (LTM) | Long-term monitoring]] is required to determine if the concentrations of target contaminants are behaving as predicted. 
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
'''Related Article(s):'''

*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| Monitored Natural Attenuation of Chlorinated Solvents]]
*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| Monitored Natural Attenuation of Metals and Metalloids]]
*[[Monitored Natural Attenuation (MNA) of Fuels|Monitored Natural Attenuation of Petroleum Hydrocarbons and Fuel Components]]
*[[Natural Source Zone Depletion (NSZD)]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]

'''Contributor(s):''' [[Dr. John Wilson]]

'''Key Resource(s):'''
*[[Media:EPA-1999-Use_of_MNA_at_Superfund%2C_RCRA_and_UST_sites.pdf|Use of monitored natural attenuation at superfund, RCRA corrective action, and underground storage tank sites]]<ref name="EPA1999"> U.S. Environmental Protection Agency, 1999. Use of monitored natural attenuation at superfund, RCRA corrective action, and underground storage tank sites. [[Media:EPA-1999-Use_of_MNA_at_Superfund%2C_RCRA_and_UST_sites.pdf|Report.pdf]]</ref>

==Introduction==
A number of natural processes can attenuate the concentrations of contaminants in groundwater including biological degradation, abiotic degradation, sorption, dispersion into groundwater adjacent to the contaminant plume, and volatilization to soil gas above the groundwater. As the concentration declines, it may reach a point where it is no longer considered hazardous. If the natural processes that attenuate the concentrations of a particular hazardous chemical can meet the cleanup goals for a site, the processes can provide the basis for a cleanup technology. The United States Environmental Protection Agency (U.S. EPA), defines Monitored Natural Attenuation (MNA) as ''“the reliance on natural attenuation processes (within the context of a carefully controlled and monitored site cleanup approach) to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods”''<ref name="EPA1999"/>.

The concentration at which a contaminant is no longer hazardous is defined by U.S. EPA and state regulations. The U.S. EPA regulates the maximum concentration of contaminants that are allowed in water that is supplied as drinking water. These U.S. EPA regulations are referred to as the Maximum Contaminant Level (MCL)<ref> U.S. Environmental Protection Agency (USEPA), 2016. Table of Regulated Drinking Water Contaminants.[http://www.epa.gov/your-drinking-water/table-regulated-drinking-water-contaminants Table of Regulated Drinking Water]</ref>. Often, the MCL is selected as the cleanup goal for MNA. However, other goals<ref> Deeb, R., Hawley, E., Kell, L. and O'Laskey, R., 2011. Assessing alternative endpoints for groundwater remediation at contaminated sites. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200832 ER-200832]</ref> are occasionally selected.

To accept MNA as remedial technology on the same basis as engineered remedial technologies, it is necessary to characterize the distribution of contamination at a site, characterize the [[ Advection and Groundwater Flow | flow of groundwater]], understand the processes that contribute to natural attenuation and use this information to build a conceptual model of the site. Sometimes the site conceptual model is used to organize an analytical model of the transport and fate<ref> VirginiaTech, United States Geological Survey (USGS), and Naval Facilites Engineering Command (NAVFAC). 2016. Natural Attenuation Software (NAS). [http://www.nas.cee.vt.edu/index.php Software]</ref> of the contaminants in groundwater. The forecasts of the transport and fate model are compared to the cleanup goals for the site to determine if natural attenuation is an appropriate remedy. If natural attenuation is selected as a remedy, the site is monitored over time to ensure that the attenuation of the contaminant proceeds as anticipated. The entire package of site characterization<ref> Pivetz, B.E., Abshire, D., Brandon, W., Mangion,S., Roberts, B., Stuart, B., Vanderpool, L., Wilson, B., Acree, S.D., 2012. Framework for Site Characterization for Monitored Natural Attenuation of Volatile Organic Compounds in Ground Water. EPA 600-R-12-712, 89 pgs. [[Media:Pivetz-2012-Framework_for_Site_Char_for_MNA.pdf|Report pdf]]</ref>, a site conceptual model, and monitoring<ref> Pope, D.F., Acree, S.D., Levine, H., Mangion, S., Van Ee, J., Hurt, K., Wilson, B. and Burden, D.S., 2004. Performance monitoring of MNA remedies for VOCs in ground water. US Environmental Protection Agency, National Risk Management Research Laboratory. [[Media:Pope-2012-Performance_Monitoring_of_MNA_Remedies.pdf|Report pdf]]</ref> are necessary components of MNA as a formal remedy for any site selected.

The U.S. EPA considers three lines of evidence<ref name= "EPA1999"/> before MNA can be accepted as the remedy for a site:
*Historical groundwater and/or soil chemistry data that demonstrate a clear and meaningful trend of decreasing contaminant mass and/or concentration over time at appropriate monitoring or sampling points.
*Hydrogeologic and geochemical data that can be used to demonstrate indirectly the type(s) of natural attenuation processes active at the site, and the rate at which such processes will reduce contaminant concentrations to required levels.
*Data from field or microcosm studies (conducted in or with actual contaminated site media) which directly demonstrate the occurrence of a particular natural attenuation process at the site and its ability to degrade the contaminants of concern (typically used to demonstrate biological degradation processes only).

At most sites, U.S. EPA requires the first two lines of evidence. The third line of evidence is reserved for contaminants that are not well understood.

MNA is often used as a remedy, or part of a remedy, where contaminants have been demonstrated to be degrading or sequestered in groundwater. A number of technical protocols have been developed to guide the application of MNA for particular contaminants, including [[Monitored Natural Attenuation (MNA) of Fuels|fuel hydrocarbons]]<ref>Wiedemeier, T.H., Wilson, J.T., Kampbell, D.H., Miller, R.N., Hansen, J.E., 1999. Technical Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural Attenuation of Fuel Contamination Dissolved in Groundwater. Volume I. [[Media:Wiedemeier-1999-technical_Protocol_for_implementing_Intrinsic_remediation.pdf|Report pdf]]</ref>, [[Monitored Natural Attenuation (MNA) of Chlorinated Solvents|chlorinated solvents]]<ref> Wiedemeier, T.H., Swanson, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T., Wilson, B.H., Kampbell, D.H., Haas, P.E., Hansen, J.E., Chapelle, F.H., 1998. Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water. EPA-600-R-98-128. [[Media:Wiedemeier-1998-Technical_Protocol_for_Evaluating_Natuaral_Attenuation.pdf|Report pdf]]</ref>, methyl ''tert''-butyl ether (MTBE<ref>Wilson, J.T., Kaiser, P.M., Adair, C., 2005. Monitored Natural Attenuation of MTBE as a Risk Management Option at Leaking Underground Storage Tank Sites EPA/600/R-04/1790. [[Media:Wilson-2005-MNA_of_MTBE.pdf|Report pdf]]</ref>), inorganics , metals , radionuclides<ref> Truex, M., Brady, P., Newell, C.J., Rysz, M., Denham, M., Vangelas, K. 2011. The Scenarios Approach to Attenuation-Based Remedies for Inorganic and Radionuclide Contaminants. Savannah-River National Laboratory U.S. Department of Energy. [[Media:TRUEX-2011-Scenarios_Approach_to_Attenuation-Based_Remedies.pdf|Report pdf]]</ref>, and explosives<ref> Pennington, J.C., Zakikhani, M., Harrelson, D., 1999. Monitored Natural Attenuation of Explosives in Groundwater. ESTCP Completion Report ER-199518. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-199518 ER-199518]</ref><ref> Borden, R.C., Knox, S.L., Lieberman, M.T., Ogles, D., 2014. Perchlorate natural attenuation in a riparian zone. Journal of Environmental Science and Health, Part A, Toxic/Hazardous Substances and Environmental Engineering, 49(10), 1100-1109. [http://dx.doi.org/10.1080/10934529.2014.897145 doi: 10.1080/10934529.2014.897145]</ref>. These protocols were developed from 1999 to 2010, in the same time period when U.S. EPA developed its policy guidance. Since that time, there have been significant advances<ref name="Adamson2014"> Adamson, D., Newell, C., 2014. Frequently Asked Questions about Monitored Natural Attenuation in the 21st Century. ER-201211. Environmental Security and Technology Certification Program, Arlington, Virginia. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201211 ER-201211]</ref> in our understanding of the processes that degrade contaminants in groundwater.

==Abiotic Process==
Abiotic processes<ref> Darlington, R., Rectanus, H. 2015. Biogeochemical Transformation Handbook. TR-NAVFAC EXWC-EV-1601, 41 pgs. [[Media:Darlington-2015-Biogeochem_Transformation_Handbook.pdf|Report pdf]]</ref> can contribute to natural attenuation of certain contaminants such as chlorinated solvents. For example, chlorinated alkenes can react with naturally occurring magnetite or other iron minerals in aquifer materials<ref>He, Y., Su, C., Wilson, J., Wilkin, R., Adair, C., Lee, T., Bradley, P., Ferrey, M., 2009. Identification and characterization methods for reactive minerals responsible for natural attenuation of chlorinated organic compounds in ground water. US Environmental Protection Agency. [[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Report pdf]]</ref>. The rate constants are generally slow, but abiotic degradation can be important if the travel time of the contamination to the point of compliance is long.

==Tools for Assessing Monitored Natural Attenuation==
# '''Statistical Tools to Evaluate Trends'''. Computer programs such as MAROS<ref>Aziz, J.J., Ling, M., Rifai, H.S., Newell, C.J., Gonzales, J.R., 2003. MAROS: A decision support system for optimizing monitoring plans. Ground Water, 41(3), 355-367. [http://dx.doi.org/10.1111/j.1745-6584.2003.tb02605.x doi: 10.1111/j.1745-6584.2003.tb02605.x]</ref><ref> Aziz, J.J., Newell, C.J., Rifai, H.S., Ling, M., Gonzales, J.R., 2000. Monitoring and Remediation Optimization System (MAROS): Software User’s Guide. [[Media:Aziz-2000-Monitoring_and_Remed._Opt._Syst._Guide.pdf|Report pdf]]</ref> and the Mann-Kendall Toolkit<ref>Connor, J., Farhat, S. K., Vanderford, M. V., Newell, C. J., 2012. GSI Mann-Kendall Toolkit. [http://www.gsi-net.com/en/software/free-software/gsi-mann-kendall-toolkit.html Mann Kendall Toolkit]</ref> can be used to help confirm trends in groundwater data used as a line of evidence for MNA. 
# '''[[Molecular Biological Tools - MBTs|Molecular Biological Tools (MBTs)]]'''. MBTs are used to identify and characterize the bacteria that carry out critical steps in the biodegradation of the contaminants in groundwater. In the case of chlorinated solvents tetrachloroethene (PCE) and trichloroethene (TCE), a key bacterium is ''Dehalococcoides mccartyi''<ref name ="Löffle2013">Löffler, F.E., Ritalahti, K.M., Zinder, S.H., 2013. Dehalococcoides and reductive dechlorination of chlorinated solvents. Bioaugmentation for groundwater remediation, ed. H.F. Stroo, A. Leeson, C.H. Ward, Springer, New York, NY. pgs. 39-88. ISBN: 978-1-4614-4114-4. [http://dx.doi.org/10.1007/978-1-4614-4115-1 doi: 10.1007/978-1-4614-4115-1]</ref>. In anaerobic groundwater, chlorinated alkenes can undergo a sequential reductive dehalogenation from PCE, to TCE, to dichloroethene (DCE) and then to vinyl chloride (VC) and finally to ethane. Anaerobic microbial communities that contain ''Dehalococcoides'' can degrade PCE and TCE all the way to harmless end products. The abundance of ''Dehalococcoides'' cells in groundwater can be determined by an assay based on the polymerase chain reaction<ref>Lebron, C.A., Petrovskis, E., Loffler, F., Henn, K., 2011. Application of Nucleic Acid-Based Tools for Monitoring Monitored Natural Attenuation (MNA), Biostimulation and Bioaugmentation at Chlorinated Solvent Sites (No. NFESC-CR-11-028-ENV). ER-200518. Naval Facilities Engineering Command Port Hueneme CA Engineering Service Center. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200518/ER-200518 ER-200518]</ref>. Other assays can determine the abundance of reductase genes<ref name ="Löffle2013"/> that code for enzymes that can carry out specific steps in the dechlorination pathway. Similar assays are available to determine the abundance of ''Dehalobacter, Dehalogenimonas, and Desulfitobacterium'' strains that degrade chlorinated alkanes, and MBT assays are available for several of their reductase genes. A great variety of bacteria degrade petroleum hydrocarbons. Bacteria that degrade hydrocarbons using oxygen initiate degradation with an oxygenase enzyme, and [[Quantitative Polymerase Chain Reaction (qPCR) | qPCR]] assays are available for a variety of oxygenase enzymes<ref>Baldwin, B.R., Nakatsu, C.H., Nies, L., 2008. Enumeration of aromatic oxygenase genes to evaluate monitored natural attenuation at gasoline-contaminated sites. Water Research, 42(3), 723-731. [http://dx.doi.org/10.1016/j.watres.2007.07.052 doi:10.1016/j.watres.2007.07.052]</ref>. The bacteria that degrade hydrocarbons under anaerobic conditions are particularly important for natural attenuation, and there are qPCR assays for the enzymes that initiate degradation under anaerobic conditions<ref> da Silva, M.L.B., Corseuil, H.X., 2012. Groundwater microbial analysis to assess enhanced BTEX biodegradation by nitrate injection at a gasohol-contaminated site. International Biodeterioration & Biodegradation, 67, 21-27. [http://dx.doi.org/10.1016/j.ibiod.2011.11.005 doi:10.1016/j.ibiod.2011.11.005]</ref>. See an entire article on MBTs here: [[Molecular Biological Tools - MBTs]] 
# '''[[Compound Specific Isotope Analysis (CSIA) | Compound Specific Isotope Analysis (CSIA)]]'''. CSIA can unequivocally demonstrate that a compound has degraded in groundwater. It is difficult to document the degradation of a compound in groundwater if the only information available is an apparent attenuation in concentrations along a flow path in the plume. There is always a possibility that a downgradient well is askew of the true flow path, and the attenuation is caused by dilution and not degradation. CSIA determines the ratio of stable isotopes in a compound. As a compound degrades, molecules with lighter isotopes degrade faster. As degradation progresses, the material that has not degraded becomes enriched in the heavier stable isotope. At many sites, degradation of the compound can be recognized and documented from a change in the ratio of isotopes<ref>Hunkeler, D., Meckenstock, R.U., Sherwood Lollar, B., Schmidt, T.C., Wilson, J.T., 2008. A Guide for Assessing Biodegradation and Source Identification of Organic Groundwater Contaminants Using Compound Specific Isotope Analysis (CSIA). U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-08/148, 2008. [[Media:Hunkeler-2008-A_Guide.pdf|Report pdf]]</ref>. At some sites, it is possible to use CSIA and reactive transport modeling<ref> Kuder, T., Philp, P., van Breukelen, B., Thouement, H., Vanderford, M., Newell, C. 2014. Integrated Stable Isotope-Reactive Transport Model Approach for Assessment of Chlorinated Solvent Degradation. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-201029/ER-201029 ER-201029]</ref> to evaluate the plausibility of alternate degradation pathways, and to estimate the extent of degradation. See an entire article on CSIA here: [[Compound Specific Isotope Analysis (CSIA) | Compound Specific Isotope Analysis (CSIA)]] 
# '''Computer Models'''. Groundwater fate and transport computer models are often used to evaluate how attenuation processes can control the migration of a plume. Public domain software is available that can incorporate terms for advective flow of groundwater, [[ Dispersion and Diffusion | dispersion]] (and more recently diffusion) of contaminations in groundwater, and biotic or abiotic reactions. Examples of commonly used models include analytical models REMChlor<ref name= "Falta2007">Falta, R.W., Stacy, M.B., Ahsanuzzaman, A.N.M., Wang, M., Earle, R., 2007. REMChlor remediation evaluation model for chlorinated solvents user’s manual Version 1.0. Cent. for subsurface model. support, US Environ. Prot. Agency, Ada, Okla.[https://www.epa.gov/water-research/remediation-evaluation-model-chlorinated-solvents-remchlor User's Manual v1.0]</ref> and REMFuel<ref name="Falta2007"/> , and the numerical models MODFLOW/RT3D<ref>2005. MODFLOW and Related Programs [http://water.usgs.gov/ogw/modflow Modflow]</ref>, MODFLOW/MT3DMS, and the Natural Attenuation Software (NAS<ref> Widdowson, M.A., Mendez III, E., Chapelle, F.H., Casey, C.C., 2005. Natural Attenuation Software (NAS) User’s Manual Version 2. [[Media:Widdowson2005-NAS_Users_Guide.pdf|Report pdf]]</ref>).
[[File:Wilson 1 Fig1a.JPG|375px|thumbnail|right|Figure 1a. Evolution of a plume when the plume and source do not attenuate.]]
[[File:Wilson 1 Fig1b.JPG|375px|thumbnail|right|Figure 1b. Evolution of a plume when the source and concentrations in groundwater both attenuate.]]
[[File:Wilson 1 Fig1c.JPG|375px|thumbnail|right|Figure 1c. Evolution of a plume when the source attenuates faster than the plume.]]

==Source Area Considerations==
In most plumes, the time frame that is required for natural attenuation to reach a cleanup goal across the entire plume is not controlled by the rate of attenuation in the groundwater. In many plumes, a source of contamination, such as residual oily phase material (non-aqueous phase liquid [NAPL]), contaminated soils, and matrix diffusion sources, provides a continuous supply of new contamination to the groundwater.

As a result, the lifecycle of the source<ref>Newell, C.J., Kueper, B.H., Wilson, J.T., Johnson, P.C., 2014. Natural Attenuation of Chlorinated Solvent Source Zones. Chlorinated Solvent Source Zone Remediation, Editors: Kueper, B.H., Stroo, H.F., Vogel, C.M., Ward, C. H. Springer New York. pgs. 459-508. [http://dx.doi.org/10.1007/978-1-4614-6922-3 doi: 10.1007/978-1-4614-6922-3]</ref> largely controls the lifecycle of contamination in groundwater. As a consequence, at many sites, some attempt is made to actively remediate the source of contamination. In almost every instance, active remediation is successful in reducing the concentration of the contamination, but fails to reduce the concentration to the cleanup goal. The final remedy is a pragmatic combination of active source remediation and MNA. Transport and fate models<ref> Widdowson, M., Chapelle, F., Casey, C., Kram, M., 2008. Estimating Cleanup Times Associated With Combining Source-Area Remediation With Monitored Natural Attenuation. ER-200436 [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436/ER-200436 ER-200436]</ref> can be used to evaluate the benefits from source remediation on the size and lifecycle of the plume of contaminated ground water. The models can estimate the reduction in concentration at the source that is necessary to pull a plume back behind a point of compliance and the time that is required for the plume to recede behind the point of compliance.

==Regulatory Considerations==
If a site is regulated under the Resource Conservation and Recovery Act (RCRA)<ref>[https://www.epa.gov/rcra US EPA RCRA Laws & Regulations]</ref>, the usual goal is for the contaminants to attenuate to acceptable concentrations before groundwater can migrate off-site and impact receptors. Under this MNA approach, the groundwater must reach a cleanup goal before it reaches a point of compliance. For this implementation, a quantitative framework (BioPIC)<ref>Lebron, C. A., Wiedemeier, T. H., Wilson, J.T., Löffler, F.E., Hinchee, R.E., Singletary, M.A., 2015. Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites. ER-201129. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129/ER-201129 ER-201129]</ref> is now available that integrates new discoveries on degradation processes into the U.S. EPA’s approach to evaluate MNA.

When a site is regulated under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA or Superfund)<ref>[https://www.epa.gov/laws-regulations/summary-comprehensive-environmental-response-compensation-and-liability-act US EPA CERCLA Act]</ref>, there is often an additional requirement that all the contamination must reach the cleanup goal by a specified date. The performance of a remedy at a Superfund site is reviewed on a five-year cycle. A framework<ref> Wilson, J.T., 2011. An Approach for Evaluating the Progress of Natural Attenuation in Groundwater. EPA 600-R-11-204. [[Media:Wilson-2011-An_Approach_for_Evaluating_Progress.pdf|Report pdf]]</ref> is available to review long-term monitoring data to determine whether the attenuation within the review cycle is adequate to meet the cleanup goal by the specified date.

In the USA, the individual states have provided regulations to supplement the U.S. EPA guidance. Examples include general guidance on MNA provided by California<ref>California Regional Water Quality Control Board, 2014. Workshop - Monitored Natural Attenuation. Barstow, California, September 10 & 11. [[Media:MNA_Workshop-2014_California_Water_Boards.pdf|Report pdf]]</ref>, Minnesota<ref>Minnesota Pollution Control Agency. Natural Attenuation of Groundwater. [https://www.pca.state.mn.us/water/natural-attenuation-groundwater Natural Attenuation of Groundwater]</ref>, New Jersey<ref> New Jersey Department of Environmental Protection - Site Remediation Program. 2012. Monitored Natural Attenuation Technical Guidance. [[Media:NJDEP-SRP-2012-MNA_Technical_Guidance_v_1_0.pdf|Report pdf]]</ref> , Ohio<ref>Ohio Environmental Protection Agency - Division of Environmental Response and Revitalization, 2001. Remedial Response Program Fact Sheet. Remediation Using Monitored Natural Attenuation.[[Media:OhioEPA-2001-Division_of_Envl_Response_and_Revitalization_fact_sheet.pdf|Report pdf]]</ref>, and Texas<ref> Texas Commission on Environmental Quality - Remediation Division, 2010. Monitored Natural Attenuation Demonstrations under TRRP. RG-366/TRRP-33. [[Media:TRRP-TCEQ-2010-Regulatory_Guidance-RG-366-TRRP-33.pdf|Report pdf]]</ref>. In addition, California<ref>California State Water Resources Control Board. 2012. Low-threat Underground Storage Tank Case Closure Policy. [[Media:CA-SWB-2012-Low-threat_UST_Case_Closure_Policy.pdf|Report pdf]]</ref>, Minnesota<ref> Minnesota Pollution Control Agency, 2005. Assessment of Natural Biogradation at Petroleum Release Sites. Guidance Document 4-03. [[Media:MINN-PCA-2005-Assessment_of_Natural_Biogradation_at_Petroleum_Rel_Sites.pdf|Report pdf]]</ref>, Washington State<ref> Washington State Department of Ecology, 2005. Guidance on Remediation of Petroleum-Contaminated Ground Water by Natural Attenuation. Publication Number 05-09-091 (Version 1.0). [[Media:WASH-ECOL-2005-Guidance_on_Remediation_of_Petroleum_Contaminated_GW.pdf|Report pdf]]</ref>, and Wisconsin<ref>Wisconsin Department of Natural Resources, 2014. Guidance on Natural Attenuation For Petroleum Releases. Remediation and Redevelopment Program. RR-614. [[Media:WIS-DNR-2014-Guidance_on_Natural_Attenuation_for_Petroleum_Releases.pdf|Report pdf]]</ref> provide guidance on petroleum releases. Minnesota<ref> Minnesota Pollution Control Agency Site Remediation Section. 2006. Guidelines Natural Attenuation of Chlorinated Solvents in Ground Water. [[Media:MINN-PCA-2006-Guidelines_Natural_Attenuation_of_Chlorinated_Solvents_in_GW.pdf|Report pdf]]</ref> and Wisconsin<ref> Wisconsin Department of Natural Resources, 2014. Understanding Chlorinated Hydrocarbon Behavior in Groundwater: Guidance on the Investigation, Assessment and Limitations of Monitored Natural Attenuation. RR-699. [[Media:WIS-DNR-2014-Understanding_Chlorinated_Hydrocarbon_Behavior_In_GW.pdf|Report pdf]]</ref> provide guidance on chlorinated solvents.

==Additional Information==
Additional information on MNA is available on web pages that are maintained by the United State Environmental Protection Agency<ref>U.S. Environmental Protection Agency, 2016. Natural Attenuation Overview. Technology Innovation and Field Services Division. [https://clu-in.org/techfocus/default.focus/sec/Natural_Attenuation/cat/Overview Natural Attenuation Overview]</ref>, the United States Geological Survey<ref> Natural Attenuation Definitions. 2015. United States Geological Survey. </ref>, Department of Energy, and the Interstate Technology Regulatory Council<ref>ITRC, 2008. Enhanced attenuation of chlorinated organics (EACO): A decision framework for site transition. [[Media:ITRC-2008-EACO_Framework_General.pdf|Report pdf]]</ref>. In addition, ESTCP has published “Frequently Asked Questions Regarding MNA in Groundwater” which provides a recent summary overview of key approaches, technologies, and best practices for applying MNA<ref name="Adamson2014"/>.

==References==

<references/>

==See Also==
*[[Long-Term Monitoring (LTM)]]
*[[Media:AFCEE_Long_Term_Monitoring_Protocol_2000.pdf|Designing Monitoring Programs to Effectively Evaluate the Performance of Natural Attenuation]]
*[[Media:ER-201032_Final_Report.pdf|Determining Source Attenuation History to Support Closure by Natural Attenuation]]
*[[Media:Role-of-DHC-Organism-Natural-Attenuation-Chlorinated-Ethylenes.pdf|Evaluation of the Role of Dehalococcoides Organisms in the Natural Attenuation of Chlorinated Ethylenes in Ground Water]]
*[[Media:Natatt_Cr.pdf|EPA Ground Water Issue: Natural Attenuation of Hexavalent Chromium in Groundwater and Soils]]
*[[Media:Parsons_MNA-Altus.pdf|Remediation by Natural Attenuation Treatability Study at Altus Air Force Base]]
*[[Media:Mnatoolbox.pdf|Site Screening and Technical Guidance for Monitored Natural Attenuation at DOE Sites]]
*[https://www.enviro.wiki/images/3/33/mna1198.pdf Technical Guidelines for Evaluating Monitored Natural Attenuation of Petroleum Hydrocarbons and Chlorinated Solvents in Groundwater at Naval and Marine Corps Facilities]
*[[Media:MNA-Guidance-2015.pdf|Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites]]
*[https://www.serdp-estcp.org/index.php/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-199518/ER-199518/(language)/eng-US Monitored Natural Attenuation of Explosives in Groundwater]

Chlorinated Solvents

2026-05-07T16:48:32Z

Admin:

Chlorinated solvents, including chlorinated volatile organic compounds (CVOC or CVOCs), are chemical compounds containing chlorine that have been widely used in various industries. They are divided in three groups (methanes, ethanes, ethenes) based on their structures, and include common groundwater contaminants such as carbon tetrachloride (CT), perchloroethene (PCE), trichloroethene (TCE), and vinyl chloride (VC). Chlorinated solvents tend to be colorless liquids at room temperatures, heavier than water, volatile, sparingly soluble, and moderately hydrophobic.
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'''Related Article(s):'''

*[[Biodegradation - Cometabolic]]
*[[Biodegradation - Reductive Processes]]
*[[Bioremediation - Anaerobic]]
*[[Bioremediation - Anaerobic Design Considerations]]
*[[Chemical Oxidation (In Situ - ISCO)]]
*[[Chemical Reduction (In Situ - ISCR)]]
*[[Design Tool - Base Addition for ERD]]
*[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation]]
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Low pH Inhibition of Reductive Dechlorination]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| Monitored Natural Attenuation of Chlorinated Solvents]]
*[[pH Buffering in Aquifers]]
*[[Remediation Performance Assessment at Chlorinated Solvent Sites]]
*[[Soil & Groundwater Contaminants]]
*[[Thermal Remediation]]

'''Contributor(s): ''' [[Dr. Bilgen Yuncu, P.E.]] and [[M. Tony Lieberman]]

'''Key Resource(s):'''

*[http://dx.doi.org/10.1007/978-1-4419-1401-9_2 Chlorinated Solvent Chemistry: Structures, Nomenclature and Properties]<ref name="CS2010">Cwiertny, D.M., Scherer, M.M., 2010. Chlorinated solvent chemistry: structures, nomenclature and properties. In In situ remediation of chlorinated solvent plumes. Springer New York. pgs. 29-37. [http://dx.doi.org/10.1007/978-1-4419-1401-9_2 doi:10.1007/978-1-4419-1401-9_2]</ref>

==Introduction==

Chlorinated solvents are a large family of organic solvents that contain chlorine atoms in their molecular structure. They were first produced in Germany in the 1800s, and widespread use in the United States (U.S.) began after World War II. In the period of 1940-1980, the U.S. produced about 2 billion pounds of chlorinated solvents each year<ref name="PC 1996"> Pankow, J.F., Cherry, J.A., 1996. Dense Chlorinated Solvents and Other DNAPLs in Groundwater, Waterloo Press, Portland, OR. ISBN 0964801418</ref>. Chlorinated solvents, including [[wikipedia:Carbon_tetrachloride|carbon tetrachloride (CT)]], [[wikipedia:1,1,1-Trichloroethane|1,1,1-trichloroethane (TCA)]], [[wikipedia:Tetrachloroethylene|perchloroethene or tetrachloroethene (PCE)]] and [[wikipedia:Trichloroethylene|trichloroethene (TCE)]] have been among the most widely used cleaning and degreasing solvents in the U.S<ref>Doherty, R.E., 2000. A history of the production and use of carbon tetrachloride, tetrachloroethylene, trichloroethylene and 1, 1, 1-trichloroethane in the United States: Part 1--historical background; carbon tetrachloride and tetrachloroethylene. Environmental Forensics, 1(2), 69-81. [http://dx.doi.org/10.1006/enfo.2000.0010 doi:10.1006/enfo.2000.0010]</ref>. They also have been used in a wide variety of other purposes such as adhesives, chemical intermediates, clothes, pharmaceuticals, pesticides, and textile processing.

==Physical & Chemical Properties==

Chlorinated solvents are organic compounds generally constructed of a simple hydrocarbon chain (typically one to three carbon atoms in length). They can be divided into three categories based on their structural characteristics: chlorinated methanes, chlorinated ethanes and chlorinated ethenes.

Chlorinated methanes represent the most structurally simple solvent class and consist of a single carbon center (known as a methyl carbon) to which as many as four chlorine atoms are bonded. From the perspective of groundwater contamination, perhaps the most well-known chlorinated methanes are [[wikipedia:carbon tetrachloride|carbon tetrachloride (CT)]] or [[wikipedia:tetrachloromethane|tetrachloromethane]], [[wikipedia:trichloromethane|trichloromethane]] (commonly known as [[wikipedia:chloroform|chloroform (CF)]]), [[wikipedia:dichloromethane|dichloromethane (DCM)]], or [[wikipedia:methylene chloride|methylene chloride (MC)]] and [[wikipedia:chloromethane|chloromethane (CM)]], or [[wikipedia:methyl chloride|methyl chloride]].

Chlorinated ethanes consist of two carbon centers joined by a single [[wikipedia:Covalent_bond|covalent bond]]. The most frequently encountered groundwater pollutants of this class include [[wikipedia:1,1,1-trichloroethane|1,1,1-trichloroethane (1,1,1-TCA)]] and [[wikipedia:1,2-dichloroethane|1,2-dichloroethane]].

Chlorinated ethenes (also referred to as chlorinated ethylenes) also possess two carbon centers, but unlike chlorinated ethanes, these carbon atoms are joined by a carbon-carbon double bond. Chlorinated ethenes that are important groundwater contaminants include [[wikipedia:tetrachloroethene|tetrachloroethene]], or [[wikipedia:perchloroethene|perchloroethene (PCE)]], [[wikipedia:trichloroethene|trichloroethene (TCE)]], [[wikipedia:dichloroethene|dichloroethene (DCE)]]) (DCE, mainly two geometric isomers cis-1,2-dichloroethene and trans-1,2-dichloroethene), and [[wikipedia:vinyl chloride|vinyl chloride (VC)]].

Nomenclature and structure of selected compounds from each solvent class as well as some physical and chemical properties of most widely used chlorinated solvents are listed in Table 1.


{| class="mw-collapsible wikitable" style="margin: auto; color:black; background-color:white; width: 100%;"
|+Table 1. Nomenclature, Structure, Chemical and Physical Properties of Most Widely Used Chlorinated Solvents<ref name="CS2010" />.
|- style="color:white; background-color:#476b6b; text-align:center;"
|IUPAC Name
|Common Name
|Acronym
|Molecular Formula
|Chemical Structure
|Formula Weight
|Density (ρ)(g/mL)
|Aqueous Solubility (mg/L)
|Vapor Pressure (ρ0)(kPa)
|Henry's Law Constanta
|Log Kow
|MCLb (mg/L)
|-
| colspan="12" style="color:black; background-color:#94b8b8;" |Chlorinated Methanes
|-
| style="color:black; background-color:#d1e0e0;" |tetrachloromethane
| style="text-align:center;" |carbon tetrachloride
| style="text-align:center;" |CT
| style="text-align:center;" |CCl4
|
[[File:Tetrachloromethane.png|center|70 px|frameless]]
| style="text-align:center;" |153.8
| style="text-align:center;" |1.59
| style="text-align:center;" |800
| style="text-align:center;" |20.5
| style="text-align:center;" |28.9
| style="text-align:center;" |2.64
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |trichloromethane
| style="text-align:center;" |chloroform
| style="text-align:center;" |CF
| style="text-align:center;" |CHCl3
|
[[File:Trichloromethane.png|center|70px|frameless]]
| style="text-align:center;" |119.4
| style="text-align:center;" |1.49
| style="text-align:center;" |8,200
| style="text-align:center;" |26.2
| style="text-align:center;" |3.8
| style="text-align:center;" |1.97
| style="text-align:center;" |0.080c
|-
| style="color:black; background-color:#d1e0e0;" |dichloromethane
| style="text-align:center;" |methylene chloride
| style="text-align:center;" |DCM
| style="text-align:center;" |CH2Cl2
|
[[File:Dichloromethane.png|center|70px|frameless]]
| style="text-align:center;" |84.9
| style="text-align:center;" |1.33
| style="text-align:center;" |13,200
| style="text-align:center;" |55.3
| style="text-align:center;" |1.7
| style="text-align:center;" |1.25
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |chloromethane
| style="text-align:center;" |methyl chloride
| style="text-align:center;" |CM
| style="text-align:center;" |CH3Cl
|
[[File:Chloromethane.png|center|70px|frameless]]
| style="text-align:center;" |50.5
| style="text-align:center;" |0.92
| style="text-align:center;" |5,235
| style="text-align:center;" |570
| style="text-align:center;" |9.6
| style="text-align:center;" |0.91
| style="text-align:center;" |NRd
|-
| colspan="12" style="color:black; background-color:#94b8b8;" |Chlorinated Ethanes
|-
| style="color:black; background-color:#d1e0e0;" |hexachloroethane
| style="text-align:center;" |perchloroethane
| style="text-align:center;" |HCA
| style="text-align:center;" |C2Cl6
|
[[File:Hexachloroethane.png|center|70px|frameless]]
| style="text-align:center;" |236.7
| style="text-align:center;" |2.09
| style="text-align:center;" |50
| style="text-align:center;" |0.05e
| style="text-align:center;" | -
| style="text-align:center;" |3.93
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |pentachloroethane
| style="text-align:center;" | -
| style="text-align:center;" |PCA
| style="text-align:center;" |C2HCl5
|
[[File:Pentachloroethane.png|center|70px|frameless]]
| style="text-align:center;" |202.3
| style="text-align:center;" |1.68
| style="text-align:center;" |500
| style="text-align:center;" |0.6
| style="text-align:center;" |2.5
| style="text-align:center;" |2.89
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |1,1,1,2-tetrachloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1,1,2-TeCA
| style="text-align:center;" |C2H2Cl4
|
[[File:1,1,1,2-Tetrachloroethane.png|72px|frameless|center]]
| style="text-align:center;" |167.9
| style="text-align:center;" |1.54
| style="text-align:center;" |1,100
| style="text-align:center;" |1.6
| style="text-align:center;" |2.4
| style="text-align:center;" | -
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |1,1,2,2-tetrachloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1,2,2-TeCA
| style="text-align:center;" |C2H2Cl4
|
[[File:1,1,2,2-Tetrachloroethane.png|72px|frameless|center]]
| style="text-align:center;" |167.9
| style="text-align:center;" |1.60
| style="text-align:center;" |2,962
| style="text-align:center;" |0.8
| style="text-align:center;" |0.44
| style="text-align:center;" |2.39
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |1,1,2-trichloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1,2-TCA
| style="text-align:center;" |C2H3Cl3
|
[[File:1,1,2-Trichloroethane.svg.png|72px|frameless|center]]
| style="text-align:center;" |133.4
| style="text-align:center;" |1.44
| style="text-align:center;" |4,394
| style="text-align:center;" |3.22
| style="text-align:center;" |0.96
| style="text-align:center;" |2.38
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |1,1,1-trichloroethane
| style="text-align:center;" |methyl chloroform
| style="text-align:center;" |1,1,1-TCA
| style="text-align:center;" |C2H3Cl3
|
[[File:1,1,1-trichloroethane.png|center|70px|frameless]]
| style="text-align:center;" |133.4
| style="text-align:center;" |1.35
| style="text-align:center;" |1,495
| style="text-align:center;" |16.5
| style="text-align:center;" |14.5
| style="text-align:center;" |2.49
| style="text-align:center;" |0.20
|-
| style="color:black; background-color:#d1e0e0;" |1,2-dichloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,2-DCA
| style="text-align:center;" |C2H4Cl2
|
[[File:1,2-dichloroethane.png|center|70px|frameless]]
| style="text-align:center;" |99.0
| style="text-align:center;" |1.25
| style="text-align:center;" |8,606
| style="text-align:center;" |10.5
| style="text-align:center;" |1.2
| style="text-align:center;" |1.48
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |1,1-dichloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1-DCA
| style="text-align:center;" |C2H4Cl2
|
[[File:1,1-Dichloroethane 2.svg.png|72px|frameless|center]]
| style="text-align:center;" |99.0
| style="text-align:center;" |1.17
| style="text-align:center;" |4,676
| style="text-align:center;" |30.3
| style="text-align:center;" |6.2
| style="text-align:center;" |1.79
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |chloroethane
| style="text-align:center;" | -
| style="text-align:center;" |CA
| style="text-align:center;" |C2H5Cl
|
[[File:Chloroethane.png|center|70px|frameless]]
| style="text-align:center;" |64.5
| style="text-align:center;" |0.92
| style="text-align:center;" |5,700
| style="text-align:center;" |16.0
| style="text-align:center;" |1.8
| style="text-align:center;" |1.43
| style="text-align:center;" |NR
|-
| colspan="12" style="color:black; background-color:#94b8b8;" |Chlorinated Ethenes
|-
| style="color:black; background-color:#d1e0e0;" |tetrachloroethene
| style="text-align:center;" |perchloroethene
| style="text-align:center;" |PCE
| style="text-align:center;" |C2Cl4
|
[[File:Tetrachloroethene.png|center|70px|frameless]]
| style="text-align:center;" |165.8
| style="text-align:center;" |1.63
| style="text-align:center;" |150
| style="text-align:center;" |2.4
| style="text-align:center;" |26.3
| style="text-align:center;" |2.88
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |trichloroethene
| style="text-align:center;" | -
| style="text-align:center;" |TCE
| style="text-align:center;" |C2HCl3
|
[[File:Trichloroethene.png|72px|frameless|center]]
| style="text-align:center;" |131.4
| style="text-align:center;" |1.46
| style="text-align:center;" |1,100
| style="text-align:center;" |9.9
| style="text-align:center;" |11.7
| style="text-align:center;" |2.53
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |cis-1,2-dichloroethene
| style="text-align:center;" |cis-dichloroethene
| style="text-align:center;" |cis-DCE
| style="text-align:center;" |C2H2Cl2
|
[[File:Cis-1,2-dichloroethene.png|center|70px|frameless]]
| style="text-align:center;" |96.9
| style="text-align:center;" |1.28
| style="text-align:center;" |3,500
| style="text-align:center;" |27.1
| style="text-align:center;" |7.4
| style="text-align:center;" |1.86
| style="text-align:center;" |0.07
|-
| style="color:black; background-color:#d1e0e0;" |trans-1,2-dichloroethene
| style="text-align:center;" |trans-dichloroethene
| style="text-align:center;" |trans-DCE
| style="text-align:center;" |C2H2Cl2
|
[[File:Trans-1,2-dichloroethene.png|72px|frameless|center]]
| style="text-align:center;" |96.9
| style="text-align:center;" |1.26
| style="text-align:center;" |6,260
| style="text-align:center;" |44.4
| style="text-align:center;" |6.8
| style="text-align:center;" |1.93
| style="text-align:center;" |0.1
|-
| style="color:black; background-color:#d1e0e0;" |1,1-dichloroethene
| style="text-align:center;" |vinylidene chloride
| style="text-align:center;" |1,1-DCE
| style="text-align:center;" |C2H2Cl2
|
[[File:1,1-Dichloroethene.svg.png|72px|frameless|center]]
| style="text-align:center;" |96.9
| style="text-align:center;" |1.22
| style="text-align:center;" |3,344
| style="text-align:center;" |80.5
| style="text-align:center;" |23.0
| style="text-align:center;" |2.13
| style="text-align:center;" |0.007
|-
| style="color:black; background-color:#d1e0e0;" |chloroethene
| style="text-align:center;" |vinyl chloride
| style="text-align:center;" |VC
| style="text-align:center;" |C2H3Cl
|
[[File:Chloroethene.png|center|70px|frameless]]
| style="text-align:center;" |62.5
| style="text-align:center;" |0.91
| style="text-align:center;" |2,763
| style="text-align:center;" |355
| style="text-align:center;" |79.2
| style="text-align:center;" |1.38
| style="text-align:center;" |0.002
|-
| colspan="12" style="color:black; background-color:#d1e0e0;" |Notes:
atm = atmosphere; g = gram; Kow = octanol/water partitioning coefficient; Koc -- soil organic carbon/water partitioning coefficient; L = liter; MCL = maximum contaminant level; mg = milligram; mL = milliliter; mol = mole.

aHenry's Law Constant (KH)(x10-3 atm ・ m3/mol)

bSource: http://water.epa.gov/drink/contaminants/#List

cMCL for total trihalomethanes is defined as the summed concentration of chloroform, bromoform (CHBr3),bromodichloromethane (CHBrCl2), and dibromochloromethane (CHBr2Cl). http://water.epa.gov/drink/contaminants/basicinformation/disinfectionbyproducts.cfm

dNR : Not regulated.

eReported vapor pressure for solid-phase hexachloroethane.

|-
|}

Chlorinated solvents and many of their transformation products are colorless liquids at room temperature. They are heavier than water with densities greater than 1 gram per cubic centimeter (g/cm3) which means they can penetrate deeply into an aquifer. They are relatively volatile compounds with relatively high [[wikipedia:Henry’s Law|Henry’s Law]] constants(KH), a measure of the strength of partitioning from water into air). Generally, when KH for a compound exceeds 0.2 atmosphere/mole fraction (atm/M), they can readily be removed from water by air stripping it. Most chlorinated solvents can be classified as sparingly soluble in water, with aqueous solubilities generally on the order of 10s to 100s of mg/L. As the number of chlorine atoms on a compound increases, the solubility decreases. Because of their relatively low solubilities, chlorinated solvents dissolve slowly in groundwater. Another consequence of their limited solubility is their tendency to occur in the subsurface as a separate immiscible liquid phase which, because of its density compared to water, tends to sink in groundwater. Under these conditions, these are referred to as [[wikipedia:DNAPL|dense non-aqueous phase liquid (DNAPL)]]. Although chlorinated solvents are not very soluble in water, their solubility is typically orders of magnitude greater than their established [http://water.epa.gov/drink/contaminants/#Organic drinking water standards].

Chlorinated solvents can be considered moderately hydrophobic which can be determined by their [[wikipedia:Partition coefficient|octanol-water partition coefficient]]s (Kow, a measure of the tendency of a substance to prefer an organic or oily phase rather than an aqueous phase). Log Kow values less than 3 indicate that the compound does not sorb strongly to aquifer solids, but can be removed readily by activated carbon. On the other hand, compounds with log Kow less than 2, such as VC, generally are not removed well by activated carbon either<ref name="CS2010" />.

==References==

<references />

==See Also==

*[https://serdp-estcp.org/Tools-and-Training/Environmental-Restoration/DNAPL-Source-Zones/Frequently-Asked-Questions-Regarding-Management-of-Chlorinated-Solvents-in-Soils-and-Groundwater FAQ Regarding Management of Chlorinated Solvents in Soil and Groundwater]
*[//www.enviro.wiki/images/6/6b/AFCEE_Protocol_2007_chlorinated_solvents.pdf Protocol for In Situ Bioremediation of Chlorinated Solvents Using Edible Oil]

Monitored Natural Attenuation (MNA)

2026-05-07T16:42:20Z

Admin:

Monitored Natural Attenuation (MNA) is an important, common groundwater remediation technology used for treating some dissolved groundwater contaminants. MNA relies on natural attenuation processes to achieve site-specific remediation objectives within a reasonable time frame compared to more active approaches. While MNA has primarily focused on managing plumes with low residual contamination, there is an growing movement to also apply it to source zones via [[ Natural Source Zone Depletion (NSZD) | natural source zone depletion (NSZD)]]. [[Long-Term Monitoring (LTM) | Long-term monitoring]] is required to determine if the concentrations of target contaminants are behaving as predicted. 
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
'''Related Article(s):'''

*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| MNA of Chlorinated Solvents]]
*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| MNA of Metals and Metalloids]]
*[[Monitored Natural Attenuation (MNA) of Fuels|MNA of Petroleum Hydrocarbons and Fuel Components]]
*[[Natural Source Zone Depletion (NSZD)]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]

'''Contributor(s):''' [[Dr. John Wilson]]

'''Key Resource(s):'''
*[[Media:EPA-1999-Use_of_MNA_at_Superfund%2C_RCRA_and_UST_sites.pdf|Use of monitored natural attenuation at superfund, RCRA corrective action, and underground storage tank sites]]<ref name="EPA1999"> U.S. Environmental Protection Agency, 1999. Use of monitored natural attenuation at superfund, RCRA corrective action, and underground storage tank sites. [[Media:EPA-1999-Use_of_MNA_at_Superfund%2C_RCRA_and_UST_sites.pdf|Report.pdf]]</ref>

==Introduction==
A number of natural processes can attenuate the concentrations of contaminants in groundwater including biological degradation, abiotic degradation, sorption, dispersion into groundwater adjacent to the contaminant plume, and volatilization to soil gas above the groundwater. As the concentration declines, it may reach a point where it is no longer considered hazardous. If the natural processes that attenuate the concentrations of a particular hazardous chemical can meet the cleanup goals for a site, the processes can provide the basis for a cleanup technology. The United States Environmental Protection Agency (U.S. EPA), defines Monitored Natural Attenuation (MNA) as ''“the reliance on natural attenuation processes (within the context of a carefully controlled and monitored site cleanup approach) to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods”''<ref name="EPA1999"/>.

The concentration at which a contaminant is no longer hazardous is defined by U.S. EPA and state regulations. The U.S. EPA regulates the maximum concentration of contaminants that are allowed in water that is supplied as drinking water. These U.S. EPA regulations are referred to as the Maximum Contaminant Level (MCL)<ref> U.S. Environmental Protection Agency (USEPA), 2016. Table of Regulated Drinking Water Contaminants.[http://www.epa.gov/your-drinking-water/table-regulated-drinking-water-contaminants Table of Regulated Drinking Water]</ref>. Often, the MCL is selected as the cleanup goal for MNA. However, other goals<ref> Deeb, R., Hawley, E., Kell, L. and O'Laskey, R., 2011. Assessing alternative endpoints for groundwater remediation at contaminated sites. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200832 ER-200832]</ref> are occasionally selected.

To accept MNA as remedial technology on the same basis as engineered remedial technologies, it is necessary to characterize the distribution of contamination at a site, characterize the [[ Advection and Groundwater Flow | flow of groundwater]], understand the processes that contribute to natural attenuation and use this information to build a conceptual model of the site. Sometimes the site conceptual model is used to organize an analytical model of the transport and fate<ref> VirginiaTech, United States Geological Survey (USGS), and Naval Facilites Engineering Command (NAVFAC). 2016. Natural Attenuation Software (NAS). [http://www.nas.cee.vt.edu/index.php Software]</ref> of the contaminants in groundwater. The forecasts of the transport and fate model are compared to the cleanup goals for the site to determine if natural attenuation is an appropriate remedy. If natural attenuation is selected as a remedy, the site is monitored over time to ensure that the attenuation of the contaminant proceeds as anticipated. The entire package of site characterization<ref> Pivetz, B.E., Abshire, D., Brandon, W., Mangion,S., Roberts, B., Stuart, B., Vanderpool, L., Wilson, B., Acree, S.D., 2012. Framework for Site Characterization for Monitored Natural Attenuation of Volatile Organic Compounds in Ground Water. EPA 600-R-12-712, 89 pgs. [[Media:Pivetz-2012-Framework_for_Site_Char_for_MNA.pdf|Report pdf]]</ref>, a site conceptual model, and monitoring<ref> Pope, D.F., Acree, S.D., Levine, H., Mangion, S., Van Ee, J., Hurt, K., Wilson, B. and Burden, D.S., 2004. Performance monitoring of MNA remedies for VOCs in ground water. US Environmental Protection Agency, National Risk Management Research Laboratory. [[Media:Pope-2012-Performance_Monitoring_of_MNA_Remedies.pdf|Report pdf]]</ref> are necessary components of MNA as a formal remedy for any site selected.

The U.S. EPA considers three lines of evidence<ref name= "EPA1999"/> before MNA can be accepted as the remedy for a site:
*Historical groundwater and/or soil chemistry data that demonstrate a clear and meaningful trend of decreasing contaminant mass and/or concentration over time at appropriate monitoring or sampling points.
*Hydrogeologic and geochemical data that can be used to demonstrate indirectly the type(s) of natural attenuation processes active at the site, and the rate at which such processes will reduce contaminant concentrations to required levels.
*Data from field or microcosm studies (conducted in or with actual contaminated site media) which directly demonstrate the occurrence of a particular natural attenuation process at the site and its ability to degrade the contaminants of concern (typically used to demonstrate biological degradation processes only).

At most sites, U.S. EPA requires the first two lines of evidence. The third line of evidence is reserved for contaminants that are not well understood.

MNA is often used as a remedy, or part of a remedy, where contaminants have been demonstrated to be degrading or sequestered in groundwater. A number of technical protocols have been developed to guide the application of MNA for particular contaminants, including [[Monitored Natural Attenuation (MNA) of Fuels|fuel hydrocarbons]]<ref>Wiedemeier, T.H., Wilson, J.T., Kampbell, D.H., Miller, R.N., Hansen, J.E., 1999. Technical Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural Attenuation of Fuel Contamination Dissolved in Groundwater. Volume I. [[Media:Wiedemeier-1999-technical_Protocol_for_implementing_Intrinsic_remediation.pdf|Report pdf]]</ref>, [[Monitored Natural Attenuation (MNA) of Chlorinated Solvents|chlorinated solvents]]<ref> Wiedemeier, T.H., Swanson, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T., Wilson, B.H., Kampbell, D.H., Haas, P.E., Hansen, J.E., Chapelle, F.H., 1998. Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water. EPA-600-R-98-128. [[Media:Wiedemeier-1998-Technical_Protocol_for_Evaluating_Natuaral_Attenuation.pdf|Report pdf]]</ref>, methyl ''tert''-butyl ether (MTBE<ref>Wilson, J.T., Kaiser, P.M., Adair, C., 2005. Monitored Natural Attenuation of MTBE as a Risk Management Option at Leaking Underground Storage Tank Sites EPA/600/R-04/1790. [[Media:Wilson-2005-MNA_of_MTBE.pdf|Report pdf]]</ref>), inorganics , metals , radionuclides<ref> Truex, M., Brady, P., Newell, C.J., Rysz, M., Denham, M., Vangelas, K. 2011. The Scenarios Approach to Attenuation-Based Remedies for Inorganic and Radionuclide Contaminants. Savannah-River National Laboratory U.S. Department of Energy. [[Media:TRUEX-2011-Scenarios_Approach_to_Attenuation-Based_Remedies.pdf|Report pdf]]</ref>, and explosives<ref> Pennington, J.C., Zakikhani, M., Harrelson, D., 1999. Monitored Natural Attenuation of Explosives in Groundwater. ESTCP Completion Report ER-199518. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-199518 ER-199518]</ref><ref> Borden, R.C., Knox, S.L., Lieberman, M.T., Ogles, D., 2014. Perchlorate natural attenuation in a riparian zone. Journal of Environmental Science and Health, Part A, Toxic/Hazardous Substances and Environmental Engineering, 49(10), 1100-1109. [http://dx.doi.org/10.1080/10934529.2014.897145 doi: 10.1080/10934529.2014.897145]</ref>. These protocols were developed from 1999 to 2010, in the same time period when U.S. EPA developed its policy guidance. Since that time, there have been significant advances<ref name="Adamson2014"> Adamson, D., Newell, C., 2014. Frequently Asked Questions about Monitored Natural Attenuation in the 21st Century. ER-201211. Environmental Security and Technology Certification Program, Arlington, Virginia. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201211 ER-201211]</ref> in our understanding of the processes that degrade contaminants in groundwater.

==Abiotic Process==
Abiotic processes<ref> Darlington, R., Rectanus, H. 2015. Biogeochemical Transformation Handbook. TR-NAVFAC EXWC-EV-1601, 41 pgs. [[Media:Darlington-2015-Biogeochem_Transformation_Handbook.pdf|Report pdf]]</ref> can contribute to natural attenuation of certain contaminants such as chlorinated solvents. For example, chlorinated alkenes can react with naturally occurring magnetite or other iron minerals in aquifer materials<ref>He, Y., Su, C., Wilson, J., Wilkin, R., Adair, C., Lee, T., Bradley, P., Ferrey, M., 2009. Identification and characterization methods for reactive minerals responsible for natural attenuation of chlorinated organic compounds in ground water. US Environmental Protection Agency. [[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Report pdf]]</ref>. The rate constants are generally slow, but abiotic degradation can be important if the travel time of the contamination to the point of compliance is long.

==Tools for Assessing Monitored Natural Attenuation==
# '''Statistical Tools to Evaluate Trends'''. Computer programs such as MAROS<ref>Aziz, J.J., Ling, M., Rifai, H.S., Newell, C.J., Gonzales, J.R., 2003. MAROS: A decision support system for optimizing monitoring plans. Ground Water, 41(3), 355-367. [http://dx.doi.org/10.1111/j.1745-6584.2003.tb02605.x doi: 10.1111/j.1745-6584.2003.tb02605.x]</ref><ref> Aziz, J.J., Newell, C.J., Rifai, H.S., Ling, M., Gonzales, J.R., 2000. Monitoring and Remediation Optimization System (MAROS): Software User’s Guide. [[Media:Aziz-2000-Monitoring_and_Remed._Opt._Syst._Guide.pdf|Report pdf]]</ref> and the Mann-Kendall Toolkit<ref>Connor, J., Farhat, S. K., Vanderford, M. V., Newell, C. J., 2012. GSI Mann-Kendall Toolkit. [http://www.gsi-net.com/en/software/free-software/gsi-mann-kendall-toolkit.html Mann Kendall Toolkit]</ref> can be used to help confirm trends in groundwater data used as a line of evidence for MNA. 
# '''[[Molecular Biological Tools - MBTs|Molecular Biological Tools (MBTs)]]'''. MBTs are used to identify and characterize the bacteria that carry out critical steps in the biodegradation of the contaminants in groundwater. In the case of chlorinated solvents tetrachloroethene (PCE) and trichloroethene (TCE), a key bacterium is ''Dehalococcoides mccartyi''<ref name ="Löffle2013">Löffler, F.E., Ritalahti, K.M., Zinder, S.H., 2013. Dehalococcoides and reductive dechlorination of chlorinated solvents. Bioaugmentation for groundwater remediation, ed. H.F. Stroo, A. Leeson, C.H. Ward, Springer, New York, NY. pgs. 39-88. ISBN: 978-1-4614-4114-4. [http://dx.doi.org/10.1007/978-1-4614-4115-1 doi: 10.1007/978-1-4614-4115-1]</ref>. In anaerobic groundwater, chlorinated alkenes can undergo a sequential reductive dehalogenation from PCE, to TCE, to dichloroethene (DCE) and then to vinyl chloride (VC) and finally to ethane. Anaerobic microbial communities that contain ''Dehalococcoides'' can degrade PCE and TCE all the way to harmless end products. The abundance of ''Dehalococcoides'' cells in groundwater can be determined by an assay based on the polymerase chain reaction<ref>Lebron, C.A., Petrovskis, E., Loffler, F., Henn, K., 2011. Application of Nucleic Acid-Based Tools for Monitoring Monitored Natural Attenuation (MNA), Biostimulation and Bioaugmentation at Chlorinated Solvent Sites (No. NFESC-CR-11-028-ENV). ER-200518. Naval Facilities Engineering Command Port Hueneme CA Engineering Service Center. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200518/ER-200518 ER-200518]</ref>. Other assays can determine the abundance of reductase genes<ref name ="Löffle2013"/> that code for enzymes that can carry out specific steps in the dechlorination pathway. Similar assays are available to determine the abundance of ''Dehalobacter, Dehalogenimonas, and Desulfitobacterium'' strains that degrade chlorinated alkanes, and MBT assays are available for several of their reductase genes. A great variety of bacteria degrade petroleum hydrocarbons. Bacteria that degrade hydrocarbons using oxygen initiate degradation with an oxygenase enzyme, and [[Quantitative Polymerase Chain Reaction (qPCR) | qPCR]] assays are available for a variety of oxygenase enzymes<ref>Baldwin, B.R., Nakatsu, C.H., Nies, L., 2008. Enumeration of aromatic oxygenase genes to evaluate monitored natural attenuation at gasoline-contaminated sites. Water Research, 42(3), 723-731. [http://dx.doi.org/10.1016/j.watres.2007.07.052 doi:10.1016/j.watres.2007.07.052]</ref>. The bacteria that degrade hydrocarbons under anaerobic conditions are particularly important for natural attenuation, and there are qPCR assays for the enzymes that initiate degradation under anaerobic conditions<ref> da Silva, M.L.B., Corseuil, H.X., 2012. Groundwater microbial analysis to assess enhanced BTEX biodegradation by nitrate injection at a gasohol-contaminated site. International Biodeterioration & Biodegradation, 67, 21-27. [http://dx.doi.org/10.1016/j.ibiod.2011.11.005 doi:10.1016/j.ibiod.2011.11.005]</ref>. See an entire article on MBTs here: [[Molecular Biological Tools - MBTs]] 
# '''[[Compound Specific Isotope Analysis (CSIA) | Compound Specific Isotope Analysis (CSIA)]]'''. CSIA can unequivocally demonstrate that a compound has degraded in groundwater. It is difficult to document the degradation of a compound in groundwater if the only information available is an apparent attenuation in concentrations along a flow path in the plume. There is always a possibility that a downgradient well is askew of the true flow path, and the attenuation is caused by dilution and not degradation. CSIA determines the ratio of stable isotopes in a compound. As a compound degrades, molecules with lighter isotopes degrade faster. As degradation progresses, the material that has not degraded becomes enriched in the heavier stable isotope. At many sites, degradation of the compound can be recognized and documented from a change in the ratio of isotopes<ref>Hunkeler, D., Meckenstock, R.U., Sherwood Lollar, B., Schmidt, T.C., Wilson, J.T., 2008. A Guide for Assessing Biodegradation and Source Identification of Organic Groundwater Contaminants Using Compound Specific Isotope Analysis (CSIA). U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-08/148, 2008. [[Media:Hunkeler-2008-A_Guide.pdf|Report pdf]]</ref>. At some sites, it is possible to use CSIA and reactive transport modeling<ref> Kuder, T., Philp, P., van Breukelen, B., Thouement, H., Vanderford, M., Newell, C. 2014. Integrated Stable Isotope-Reactive Transport Model Approach for Assessment of Chlorinated Solvent Degradation. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-201029/ER-201029 ER-201029]</ref> to evaluate the plausibility of alternate degradation pathways, and to estimate the extent of degradation. See an entire article on CSIA here: [[Compound Specific Isotope Analysis (CSIA) | Compound Specific Isotope Analysis (CSIA)]] 
# '''Computer Models'''. Groundwater fate and transport computer models are often used to evaluate how attenuation processes can control the migration of a plume. Public domain software is available that can incorporate terms for advective flow of groundwater, [[ Dispersion and Diffusion | dispersion]] (and more recently diffusion) of contaminations in groundwater, and biotic or abiotic reactions. Examples of commonly used models include analytical models REMChlor<ref name= "Falta2007">Falta, R.W., Stacy, M.B., Ahsanuzzaman, A.N.M., Wang, M., Earle, R., 2007. REMChlor remediation evaluation model for chlorinated solvents user’s manual Version 1.0. Cent. for subsurface model. support, US Environ. Prot. Agency, Ada, Okla.[https://www.epa.gov/water-research/remediation-evaluation-model-chlorinated-solvents-remchlor User's Manual v1.0]</ref> and REMFuel<ref name="Falta2007"/> , and the numerical models MODFLOW/RT3D<ref>2005. MODFLOW and Related Programs [http://water.usgs.gov/ogw/modflow Modflow]</ref>, MODFLOW/MT3DMS, and the Natural Attenuation Software (NAS<ref> Widdowson, M.A., Mendez III, E., Chapelle, F.H., Casey, C.C., 2005. Natural Attenuation Software (NAS) User’s Manual Version 2. [[Media:Widdowson2005-NAS_Users_Guide.pdf|Report pdf]]</ref>).
[[File:Wilson 1 Fig1a.JPG|375px|thumbnail|right|Figure 1a. Evolution of a plume when the plume and source do not attenuate.]]
[[File:Wilson 1 Fig1b.JPG|375px|thumbnail|right|Figure 1b. Evolution of a plume when the source and concentrations in groundwater both attenuate.]]
[[File:Wilson 1 Fig1c.JPG|375px|thumbnail|right|Figure 1c. Evolution of a plume when the source attenuates faster than the plume.]]

==Source Area Considerations==
In most plumes, the time frame that is required for natural attenuation to reach a cleanup goal across the entire plume is not controlled by the rate of attenuation in the groundwater. In many plumes, a source of contamination, such as residual oily phase material (non-aqueous phase liquid [NAPL]), contaminated soils, and matrix diffusion sources, provides a continuous supply of new contamination to the groundwater.

As a result, the lifecycle of the source<ref>Newell, C.J., Kueper, B.H., Wilson, J.T., Johnson, P.C., 2014. Natural Attenuation of Chlorinated Solvent Source Zones. Chlorinated Solvent Source Zone Remediation, Editors: Kueper, B.H., Stroo, H.F., Vogel, C.M., Ward, C. H. Springer New York. pgs. 459-508. [http://dx.doi.org/10.1007/978-1-4614-6922-3 doi: 10.1007/978-1-4614-6922-3]</ref> largely controls the lifecycle of contamination in groundwater. As a consequence, at many sites, some attempt is made to actively remediate the source of contamination. In almost every instance, active remediation is successful in reducing the concentration of the contamination, but fails to reduce the concentration to the cleanup goal. The final remedy is a pragmatic combination of active source remediation and MNA. Transport and fate models<ref> Widdowson, M., Chapelle, F., Casey, C., Kram, M., 2008. Estimating Cleanup Times Associated With Combining Source-Area Remediation With Monitored Natural Attenuation. ER-200436 [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436/ER-200436 ER-200436]</ref> can be used to evaluate the benefits from source remediation on the size and lifecycle of the plume of contaminated ground water. The models can estimate the reduction in concentration at the source that is necessary to pull a plume back behind a point of compliance and the time that is required for the plume to recede behind the point of compliance.

==Regulatory Considerations==
If a site is regulated under the Resource Conservation and Recovery Act (RCRA)<ref>[https://www.epa.gov/rcra US EPA RCRA Laws & Regulations]</ref>, the usual goal is for the contaminants to attenuate to acceptable concentrations before groundwater can migrate off-site and impact receptors. Under this MNA approach, the groundwater must reach a cleanup goal before it reaches a point of compliance. For this implementation, a quantitative framework (BioPIC)<ref>Lebron, C. A., Wiedemeier, T. H., Wilson, J.T., Löffler, F.E., Hinchee, R.E., Singletary, M.A., 2015. Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites. ER-201129. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129/ER-201129 ER-201129]</ref> is now available that integrates new discoveries on degradation processes into the U.S. EPA’s approach to evaluate MNA.

When a site is regulated under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA or Superfund)<ref>[https://www.epa.gov/laws-regulations/summary-comprehensive-environmental-response-compensation-and-liability-act US EPA CERCLA Act]</ref>, there is often an additional requirement that all the contamination must reach the cleanup goal by a specified date. The performance of a remedy at a Superfund site is reviewed on a five-year cycle. A framework<ref> Wilson, J.T., 2011. An Approach for Evaluating the Progress of Natural Attenuation in Groundwater. EPA 600-R-11-204. [[Media:Wilson-2011-An_Approach_for_Evaluating_Progress.pdf|Report pdf]]</ref> is available to review long-term monitoring data to determine whether the attenuation within the review cycle is adequate to meet the cleanup goal by the specified date.

In the USA, the individual states have provided regulations to supplement the U.S. EPA guidance. Examples include general guidance on MNA provided by California<ref>California Regional Water Quality Control Board, 2014. Workshop - Monitored Natural Attenuation. Barstow, California, September 10 & 11. [[Media:MNA_Workshop-2014_California_Water_Boards.pdf|Report pdf]]</ref>, Minnesota<ref>Minnesota Pollution Control Agency. Natural Attenuation of Groundwater. [https://www.pca.state.mn.us/water/natural-attenuation-groundwater Natural Attenuation of Groundwater]</ref>, New Jersey<ref> New Jersey Department of Environmental Protection - Site Remediation Program. 2012. Monitored Natural Attenuation Technical Guidance. [[Media:NJDEP-SRP-2012-MNA_Technical_Guidance_v_1_0.pdf|Report pdf]]</ref> , Ohio<ref>Ohio Environmental Protection Agency - Division of Environmental Response and Revitalization, 2001. Remedial Response Program Fact Sheet. Remediation Using Monitored Natural Attenuation.[[Media:OhioEPA-2001-Division_of_Envl_Response_and_Revitalization_fact_sheet.pdf|Report pdf]]</ref>, and Texas<ref> Texas Commission on Environmental Quality - Remediation Division, 2010. Monitored Natural Attenuation Demonstrations under TRRP. RG-366/TRRP-33. [[Media:TRRP-TCEQ-2010-Regulatory_Guidance-RG-366-TRRP-33.pdf|Report pdf]]</ref>. In addition, California<ref>California State Water Resources Control Board. 2012. Low-threat Underground Storage Tank Case Closure Policy. [[Media:CA-SWB-2012-Low-threat_UST_Case_Closure_Policy.pdf|Report pdf]]</ref>, Minnesota<ref> Minnesota Pollution Control Agency, 2005. Assessment of Natural Biogradation at Petroleum Release Sites. Guidance Document 4-03. [[Media:MINN-PCA-2005-Assessment_of_Natural_Biogradation_at_Petroleum_Rel_Sites.pdf|Report pdf]]</ref>, Washington State<ref> Washington State Department of Ecology, 2005. Guidance on Remediation of Petroleum-Contaminated Ground Water by Natural Attenuation. Publication Number 05-09-091 (Version 1.0). [[Media:WASH-ECOL-2005-Guidance_on_Remediation_of_Petroleum_Contaminated_GW.pdf|Report pdf]]</ref>, and Wisconsin<ref>Wisconsin Department of Natural Resources, 2014. Guidance on Natural Attenuation For Petroleum Releases. Remediation and Redevelopment Program. RR-614. [[Media:WIS-DNR-2014-Guidance_on_Natural_Attenuation_for_Petroleum_Releases.pdf|Report pdf]]</ref> provide guidance on petroleum releases. Minnesota<ref> Minnesota Pollution Control Agency Site Remediation Section. 2006. Guidelines Natural Attenuation of Chlorinated Solvents in Ground Water. [[Media:MINN-PCA-2006-Guidelines_Natural_Attenuation_of_Chlorinated_Solvents_in_GW.pdf|Report pdf]]</ref> and Wisconsin<ref> Wisconsin Department of Natural Resources, 2014. Understanding Chlorinated Hydrocarbon Behavior in Groundwater: Guidance on the Investigation, Assessment and Limitations of Monitored Natural Attenuation. RR-699. [[Media:WIS-DNR-2014-Understanding_Chlorinated_Hydrocarbon_Behavior_In_GW.pdf|Report pdf]]</ref> provide guidance on chlorinated solvents.

==Additional Information==
Additional information on MNA is available on web pages that are maintained by the United State Environmental Protection Agency<ref>U.S. Environmental Protection Agency, 2016. Natural Attenuation Overview. Technology Innovation and Field Services Division. [https://clu-in.org/techfocus/default.focus/sec/Natural_Attenuation/cat/Overview Natural Attenuation Overview]</ref>, the United States Geological Survey<ref> Natural Attenuation Definitions. 2015. United States Geological Survey. </ref>, Department of Energy, and the Interstate Technology Regulatory Council<ref>ITRC, 2008. Enhanced attenuation of chlorinated organics (EACO): A decision framework for site transition. [[Media:ITRC-2008-EACO_Framework_General.pdf|Report pdf]]</ref>. In addition, ESTCP has published “Frequently Asked Questions Regarding MNA in Groundwater” which provides a recent summary overview of key approaches, technologies, and best practices for applying MNA<ref name="Adamson2014"/>.

==References==

<references/>

==See Also==
*[[Long-Term Monitoring (LTM)]]
*[[Media:AFCEE_Long_Term_Monitoring_Protocol_2000.pdf|Designing Monitoring Programs to Effectively Evaluate the Performance of Natural Attenuation]]
*[[Media:ER-201032_Final_Report.pdf|Determining Source Attenuation History to Support Closure by Natural Attenuation]]
*[[Media:Role-of-DHC-Organism-Natural-Attenuation-Chlorinated-Ethylenes.pdf|Evaluation of the Role of Dehalococcoides Organisms in the Natural Attenuation of Chlorinated Ethylenes in Ground Water]]
*[[Media:Natatt_Cr.pdf|EPA Ground Water Issue: Natural Attenuation of Hexavalent Chromium in Groundwater and Soils]]
*[[Media:Parsons_MNA-Altus.pdf|Remediation by Natural Attenuation Treatability Study at Altus Air Force Base]]
*[[Media:Mnatoolbox.pdf|Site Screening and Technical Guidance for Monitored Natural Attenuation at DOE Sites]]
*[https://www.enviro.wiki/images/3/33/mna1198.pdf Technical Guidelines for Evaluating Monitored Natural Attenuation of Petroleum Hydrocarbons and Chlorinated Solvents in Groundwater at Naval and Marine Corps Facilities]
*[[Media:MNA-Guidance-2015.pdf|Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites]]
*[https://www.serdp-estcp.org/index.php/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-199518/ER-199518/(language)/eng-US Monitored Natural Attenuation of Explosives in Groundwater]

Monitored Natural Attenuation (MNA)

2026-05-07T16:41:24Z

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Monitored Natural Attenuation (MNA) is an important, common groundwater remediation technology used for treating some dissolved groundwater contaminants. MNA relies on natural attenuation processes to achieve site-specific remediation objectives within a reasonable time frame compared to more active approaches. While MNA has primarily focused on managing plumes with low residual contamination, there is an growing movement to also apply it to source zones via [[ Natural Source Zone Depletion (NSZD) | natural source zone depletion (NSZD)]]. [[Long-Term Monitoring (LTM) | Long-term monitoring]] is required to determine if the concentrations of target contaminants are behaving as predicted. 
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'''Related Article(s):'''

*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| MNA of Chlorinated Solvents]]
**[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| MNA of Metals and Metalloids]]
*[[Monitored Natural Attenuation (MNA) of Fuels|MNA of Petroleum Hydrocarbons and Fuel Components]]
*[[Natural Source Zone Depletion (NSZD)]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]

'''Contributor(s):''' [[Dr. John Wilson]]

'''Key Resource(s):'''
*[[Media:EPA-1999-Use_of_MNA_at_Superfund%2C_RCRA_and_UST_sites.pdf|Use of monitored natural attenuation at superfund, RCRA corrective action, and underground storage tank sites]]<ref name="EPA1999"> U.S. Environmental Protection Agency, 1999. Use of monitored natural attenuation at superfund, RCRA corrective action, and underground storage tank sites. [[Media:EPA-1999-Use_of_MNA_at_Superfund%2C_RCRA_and_UST_sites.pdf|Report.pdf]]</ref>

==Introduction==
A number of natural processes can attenuate the concentrations of contaminants in groundwater including biological degradation, abiotic degradation, sorption, dispersion into groundwater adjacent to the contaminant plume, and volatilization to soil gas above the groundwater. As the concentration declines, it may reach a point where it is no longer considered hazardous. If the natural processes that attenuate the concentrations of a particular hazardous chemical can meet the cleanup goals for a site, the processes can provide the basis for a cleanup technology. The United States Environmental Protection Agency (U.S. EPA), defines Monitored Natural Attenuation (MNA) as ''“the reliance on natural attenuation processes (within the context of a carefully controlled and monitored site cleanup approach) to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods”''<ref name="EPA1999"/>.

The concentration at which a contaminant is no longer hazardous is defined by U.S. EPA and state regulations. The U.S. EPA regulates the maximum concentration of contaminants that are allowed in water that is supplied as drinking water. These U.S. EPA regulations are referred to as the Maximum Contaminant Level (MCL)<ref> U.S. Environmental Protection Agency (USEPA), 2016. Table of Regulated Drinking Water Contaminants.[http://www.epa.gov/your-drinking-water/table-regulated-drinking-water-contaminants Table of Regulated Drinking Water]</ref>. Often, the MCL is selected as the cleanup goal for MNA. However, other goals<ref> Deeb, R., Hawley, E., Kell, L. and O'Laskey, R., 2011. Assessing alternative endpoints for groundwater remediation at contaminated sites. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200832 ER-200832]</ref> are occasionally selected.

To accept MNA as remedial technology on the same basis as engineered remedial technologies, it is necessary to characterize the distribution of contamination at a site, characterize the [[ Advection and Groundwater Flow | flow of groundwater]], understand the processes that contribute to natural attenuation and use this information to build a conceptual model of the site. Sometimes the site conceptual model is used to organize an analytical model of the transport and fate<ref> VirginiaTech, United States Geological Survey (USGS), and Naval Facilites Engineering Command (NAVFAC). 2016. Natural Attenuation Software (NAS). [http://www.nas.cee.vt.edu/index.php Software]</ref> of the contaminants in groundwater. The forecasts of the transport and fate model are compared to the cleanup goals for the site to determine if natural attenuation is an appropriate remedy. If natural attenuation is selected as a remedy, the site is monitored over time to ensure that the attenuation of the contaminant proceeds as anticipated. The entire package of site characterization<ref> Pivetz, B.E., Abshire, D., Brandon, W., Mangion,S., Roberts, B., Stuart, B., Vanderpool, L., Wilson, B., Acree, S.D., 2012. Framework for Site Characterization for Monitored Natural Attenuation of Volatile Organic Compounds in Ground Water. EPA 600-R-12-712, 89 pgs. [[Media:Pivetz-2012-Framework_for_Site_Char_for_MNA.pdf|Report pdf]]</ref>, a site conceptual model, and monitoring<ref> Pope, D.F., Acree, S.D., Levine, H., Mangion, S., Van Ee, J., Hurt, K., Wilson, B. and Burden, D.S., 2004. Performance monitoring of MNA remedies for VOCs in ground water. US Environmental Protection Agency, National Risk Management Research Laboratory. [[Media:Pope-2012-Performance_Monitoring_of_MNA_Remedies.pdf|Report pdf]]</ref> are necessary components of MNA as a formal remedy for any site selected.

The U.S. EPA considers three lines of evidence<ref name= "EPA1999"/> before MNA can be accepted as the remedy for a site:
*Historical groundwater and/or soil chemistry data that demonstrate a clear and meaningful trend of decreasing contaminant mass and/or concentration over time at appropriate monitoring or sampling points.
*Hydrogeologic and geochemical data that can be used to demonstrate indirectly the type(s) of natural attenuation processes active at the site, and the rate at which such processes will reduce contaminant concentrations to required levels.
*Data from field or microcosm studies (conducted in or with actual contaminated site media) which directly demonstrate the occurrence of a particular natural attenuation process at the site and its ability to degrade the contaminants of concern (typically used to demonstrate biological degradation processes only).

At most sites, U.S. EPA requires the first two lines of evidence. The third line of evidence is reserved for contaminants that are not well understood.

MNA is often used as a remedy, or part of a remedy, where contaminants have been demonstrated to be degrading or sequestered in groundwater. A number of technical protocols have been developed to guide the application of MNA for particular contaminants, including [[Monitored Natural Attenuation (MNA) of Fuels|fuel hydrocarbons]]<ref>Wiedemeier, T.H., Wilson, J.T., Kampbell, D.H., Miller, R.N., Hansen, J.E., 1999. Technical Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural Attenuation of Fuel Contamination Dissolved in Groundwater. Volume I. [[Media:Wiedemeier-1999-technical_Protocol_for_implementing_Intrinsic_remediation.pdf|Report pdf]]</ref>, [[Monitored Natural Attenuation (MNA) of Chlorinated Solvents|chlorinated solvents]]<ref> Wiedemeier, T.H., Swanson, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T., Wilson, B.H., Kampbell, D.H., Haas, P.E., Hansen, J.E., Chapelle, F.H., 1998. Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water. EPA-600-R-98-128. [[Media:Wiedemeier-1998-Technical_Protocol_for_Evaluating_Natuaral_Attenuation.pdf|Report pdf]]</ref>, methyl ''tert''-butyl ether (MTBE<ref>Wilson, J.T., Kaiser, P.M., Adair, C., 2005. Monitored Natural Attenuation of MTBE as a Risk Management Option at Leaking Underground Storage Tank Sites EPA/600/R-04/1790. [[Media:Wilson-2005-MNA_of_MTBE.pdf|Report pdf]]</ref>), inorganics , metals , radionuclides<ref> Truex, M., Brady, P., Newell, C.J., Rysz, M., Denham, M., Vangelas, K. 2011. The Scenarios Approach to Attenuation-Based Remedies for Inorganic and Radionuclide Contaminants. Savannah-River National Laboratory U.S. Department of Energy. [[Media:TRUEX-2011-Scenarios_Approach_to_Attenuation-Based_Remedies.pdf|Report pdf]]</ref>, and explosives<ref> Pennington, J.C., Zakikhani, M., Harrelson, D., 1999. Monitored Natural Attenuation of Explosives in Groundwater. ESTCP Completion Report ER-199518. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-199518 ER-199518]</ref><ref> Borden, R.C., Knox, S.L., Lieberman, M.T., Ogles, D., 2014. Perchlorate natural attenuation in a riparian zone. Journal of Environmental Science and Health, Part A, Toxic/Hazardous Substances and Environmental Engineering, 49(10), 1100-1109. [http://dx.doi.org/10.1080/10934529.2014.897145 doi: 10.1080/10934529.2014.897145]</ref>. These protocols were developed from 1999 to 2010, in the same time period when U.S. EPA developed its policy guidance. Since that time, there have been significant advances<ref name="Adamson2014"> Adamson, D., Newell, C., 2014. Frequently Asked Questions about Monitored Natural Attenuation in the 21st Century. ER-201211. Environmental Security and Technology Certification Program, Arlington, Virginia. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201211 ER-201211]</ref> in our understanding of the processes that degrade contaminants in groundwater.

==Abiotic Process==
Abiotic processes<ref> Darlington, R., Rectanus, H. 2015. Biogeochemical Transformation Handbook. TR-NAVFAC EXWC-EV-1601, 41 pgs. [[Media:Darlington-2015-Biogeochem_Transformation_Handbook.pdf|Report pdf]]</ref> can contribute to natural attenuation of certain contaminants such as chlorinated solvents. For example, chlorinated alkenes can react with naturally occurring magnetite or other iron minerals in aquifer materials<ref>He, Y., Su, C., Wilson, J., Wilkin, R., Adair, C., Lee, T., Bradley, P., Ferrey, M., 2009. Identification and characterization methods for reactive minerals responsible for natural attenuation of chlorinated organic compounds in ground water. US Environmental Protection Agency. [[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Report pdf]]</ref>. The rate constants are generally slow, but abiotic degradation can be important if the travel time of the contamination to the point of compliance is long.

==Tools for Assessing Monitored Natural Attenuation==
# '''Statistical Tools to Evaluate Trends'''. Computer programs such as MAROS<ref>Aziz, J.J., Ling, M., Rifai, H.S., Newell, C.J., Gonzales, J.R., 2003. MAROS: A decision support system for optimizing monitoring plans. Ground Water, 41(3), 355-367. [http://dx.doi.org/10.1111/j.1745-6584.2003.tb02605.x doi: 10.1111/j.1745-6584.2003.tb02605.x]</ref><ref> Aziz, J.J., Newell, C.J., Rifai, H.S., Ling, M., Gonzales, J.R., 2000. Monitoring and Remediation Optimization System (MAROS): Software User’s Guide. [[Media:Aziz-2000-Monitoring_and_Remed._Opt._Syst._Guide.pdf|Report pdf]]</ref> and the Mann-Kendall Toolkit<ref>Connor, J., Farhat, S. K., Vanderford, M. V., Newell, C. J., 2012. GSI Mann-Kendall Toolkit. [http://www.gsi-net.com/en/software/free-software/gsi-mann-kendall-toolkit.html Mann Kendall Toolkit]</ref> can be used to help confirm trends in groundwater data used as a line of evidence for MNA. 
# '''[[Molecular Biological Tools - MBTs|Molecular Biological Tools (MBTs)]]'''. MBTs are used to identify and characterize the bacteria that carry out critical steps in the biodegradation of the contaminants in groundwater. In the case of chlorinated solvents tetrachloroethene (PCE) and trichloroethene (TCE), a key bacterium is ''Dehalococcoides mccartyi''<ref name ="Löffle2013">Löffler, F.E., Ritalahti, K.M., Zinder, S.H., 2013. Dehalococcoides and reductive dechlorination of chlorinated solvents. Bioaugmentation for groundwater remediation, ed. H.F. Stroo, A. Leeson, C.H. Ward, Springer, New York, NY. pgs. 39-88. ISBN: 978-1-4614-4114-4. [http://dx.doi.org/10.1007/978-1-4614-4115-1 doi: 10.1007/978-1-4614-4115-1]</ref>. In anaerobic groundwater, chlorinated alkenes can undergo a sequential reductive dehalogenation from PCE, to TCE, to dichloroethene (DCE) and then to vinyl chloride (VC) and finally to ethane. Anaerobic microbial communities that contain ''Dehalococcoides'' can degrade PCE and TCE all the way to harmless end products. The abundance of ''Dehalococcoides'' cells in groundwater can be determined by an assay based on the polymerase chain reaction<ref>Lebron, C.A., Petrovskis, E., Loffler, F., Henn, K., 2011. Application of Nucleic Acid-Based Tools for Monitoring Monitored Natural Attenuation (MNA), Biostimulation and Bioaugmentation at Chlorinated Solvent Sites (No. NFESC-CR-11-028-ENV). ER-200518. Naval Facilities Engineering Command Port Hueneme CA Engineering Service Center. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200518/ER-200518 ER-200518]</ref>. Other assays can determine the abundance of reductase genes<ref name ="Löffle2013"/> that code for enzymes that can carry out specific steps in the dechlorination pathway. Similar assays are available to determine the abundance of ''Dehalobacter, Dehalogenimonas, and Desulfitobacterium'' strains that degrade chlorinated alkanes, and MBT assays are available for several of their reductase genes. A great variety of bacteria degrade petroleum hydrocarbons. Bacteria that degrade hydrocarbons using oxygen initiate degradation with an oxygenase enzyme, and [[Quantitative Polymerase Chain Reaction (qPCR) | qPCR]] assays are available for a variety of oxygenase enzymes<ref>Baldwin, B.R., Nakatsu, C.H., Nies, L., 2008. Enumeration of aromatic oxygenase genes to evaluate monitored natural attenuation at gasoline-contaminated sites. Water Research, 42(3), 723-731. [http://dx.doi.org/10.1016/j.watres.2007.07.052 doi:10.1016/j.watres.2007.07.052]</ref>. The bacteria that degrade hydrocarbons under anaerobic conditions are particularly important for natural attenuation, and there are qPCR assays for the enzymes that initiate degradation under anaerobic conditions<ref> da Silva, M.L.B., Corseuil, H.X., 2012. Groundwater microbial analysis to assess enhanced BTEX biodegradation by nitrate injection at a gasohol-contaminated site. International Biodeterioration & Biodegradation, 67, 21-27. [http://dx.doi.org/10.1016/j.ibiod.2011.11.005 doi:10.1016/j.ibiod.2011.11.005]</ref>. See an entire article on MBTs here: [[Molecular Biological Tools - MBTs]] 
# '''[[Compound Specific Isotope Analysis (CSIA) | Compound Specific Isotope Analysis (CSIA)]]'''. CSIA can unequivocally demonstrate that a compound has degraded in groundwater. It is difficult to document the degradation of a compound in groundwater if the only information available is an apparent attenuation in concentrations along a flow path in the plume. There is always a possibility that a downgradient well is askew of the true flow path, and the attenuation is caused by dilution and not degradation. CSIA determines the ratio of stable isotopes in a compound. As a compound degrades, molecules with lighter isotopes degrade faster. As degradation progresses, the material that has not degraded becomes enriched in the heavier stable isotope. At many sites, degradation of the compound can be recognized and documented from a change in the ratio of isotopes<ref>Hunkeler, D., Meckenstock, R.U., Sherwood Lollar, B., Schmidt, T.C., Wilson, J.T., 2008. A Guide for Assessing Biodegradation and Source Identification of Organic Groundwater Contaminants Using Compound Specific Isotope Analysis (CSIA). U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-08/148, 2008. [[Media:Hunkeler-2008-A_Guide.pdf|Report pdf]]</ref>. At some sites, it is possible to use CSIA and reactive transport modeling<ref> Kuder, T., Philp, P., van Breukelen, B., Thouement, H., Vanderford, M., Newell, C. 2014. Integrated Stable Isotope-Reactive Transport Model Approach for Assessment of Chlorinated Solvent Degradation. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-201029/ER-201029 ER-201029]</ref> to evaluate the plausibility of alternate degradation pathways, and to estimate the extent of degradation. See an entire article on CSIA here: [[Compound Specific Isotope Analysis (CSIA) | Compound Specific Isotope Analysis (CSIA)]] 
# '''Computer Models'''. Groundwater fate and transport computer models are often used to evaluate how attenuation processes can control the migration of a plume. Public domain software is available that can incorporate terms for advective flow of groundwater, [[ Dispersion and Diffusion | dispersion]] (and more recently diffusion) of contaminations in groundwater, and biotic or abiotic reactions. Examples of commonly used models include analytical models REMChlor<ref name= "Falta2007">Falta, R.W., Stacy, M.B., Ahsanuzzaman, A.N.M., Wang, M., Earle, R., 2007. REMChlor remediation evaluation model for chlorinated solvents user’s manual Version 1.0. Cent. for subsurface model. support, US Environ. Prot. Agency, Ada, Okla.[https://www.epa.gov/water-research/remediation-evaluation-model-chlorinated-solvents-remchlor User's Manual v1.0]</ref> and REMFuel<ref name="Falta2007"/> , and the numerical models MODFLOW/RT3D<ref>2005. MODFLOW and Related Programs [http://water.usgs.gov/ogw/modflow Modflow]</ref>, MODFLOW/MT3DMS, and the Natural Attenuation Software (NAS<ref> Widdowson, M.A., Mendez III, E., Chapelle, F.H., Casey, C.C., 2005. Natural Attenuation Software (NAS) User’s Manual Version 2. [[Media:Widdowson2005-NAS_Users_Guide.pdf|Report pdf]]</ref>).
[[File:Wilson 1 Fig1a.JPG|375px|thumbnail|right|Figure 1a. Evolution of a plume when the plume and source do not attenuate.]]
[[File:Wilson 1 Fig1b.JPG|375px|thumbnail|right|Figure 1b. Evolution of a plume when the source and concentrations in groundwater both attenuate.]]
[[File:Wilson 1 Fig1c.JPG|375px|thumbnail|right|Figure 1c. Evolution of a plume when the source attenuates faster than the plume.]]

==Source Area Considerations==
In most plumes, the time frame that is required for natural attenuation to reach a cleanup goal across the entire plume is not controlled by the rate of attenuation in the groundwater. In many plumes, a source of contamination, such as residual oily phase material (non-aqueous phase liquid [NAPL]), contaminated soils, and matrix diffusion sources, provides a continuous supply of new contamination to the groundwater.

As a result, the lifecycle of the source<ref>Newell, C.J., Kueper, B.H., Wilson, J.T., Johnson, P.C., 2014. Natural Attenuation of Chlorinated Solvent Source Zones. Chlorinated Solvent Source Zone Remediation, Editors: Kueper, B.H., Stroo, H.F., Vogel, C.M., Ward, C. H. Springer New York. pgs. 459-508. [http://dx.doi.org/10.1007/978-1-4614-6922-3 doi: 10.1007/978-1-4614-6922-3]</ref> largely controls the lifecycle of contamination in groundwater. As a consequence, at many sites, some attempt is made to actively remediate the source of contamination. In almost every instance, active remediation is successful in reducing the concentration of the contamination, but fails to reduce the concentration to the cleanup goal. The final remedy is a pragmatic combination of active source remediation and MNA. Transport and fate models<ref> Widdowson, M., Chapelle, F., Casey, C., Kram, M., 2008. Estimating Cleanup Times Associated With Combining Source-Area Remediation With Monitored Natural Attenuation. ER-200436 [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436/ER-200436 ER-200436]</ref> can be used to evaluate the benefits from source remediation on the size and lifecycle of the plume of contaminated ground water. The models can estimate the reduction in concentration at the source that is necessary to pull a plume back behind a point of compliance and the time that is required for the plume to recede behind the point of compliance.

==Regulatory Considerations==
If a site is regulated under the Resource Conservation and Recovery Act (RCRA)<ref>[https://www.epa.gov/rcra US EPA RCRA Laws & Regulations]</ref>, the usual goal is for the contaminants to attenuate to acceptable concentrations before groundwater can migrate off-site and impact receptors. Under this MNA approach, the groundwater must reach a cleanup goal before it reaches a point of compliance. For this implementation, a quantitative framework (BioPIC)<ref>Lebron, C. A., Wiedemeier, T. H., Wilson, J.T., Löffler, F.E., Hinchee, R.E., Singletary, M.A., 2015. Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites. ER-201129. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129/ER-201129 ER-201129]</ref> is now available that integrates new discoveries on degradation processes into the U.S. EPA’s approach to evaluate MNA.

When a site is regulated under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA or Superfund)<ref>[https://www.epa.gov/laws-regulations/summary-comprehensive-environmental-response-compensation-and-liability-act US EPA CERCLA Act]</ref>, there is often an additional requirement that all the contamination must reach the cleanup goal by a specified date. The performance of a remedy at a Superfund site is reviewed on a five-year cycle. A framework<ref> Wilson, J.T., 2011. An Approach for Evaluating the Progress of Natural Attenuation in Groundwater. EPA 600-R-11-204. [[Media:Wilson-2011-An_Approach_for_Evaluating_Progress.pdf|Report pdf]]</ref> is available to review long-term monitoring data to determine whether the attenuation within the review cycle is adequate to meet the cleanup goal by the specified date.

In the USA, the individual states have provided regulations to supplement the U.S. EPA guidance. Examples include general guidance on MNA provided by California<ref>California Regional Water Quality Control Board, 2014. Workshop - Monitored Natural Attenuation. Barstow, California, September 10 & 11. [[Media:MNA_Workshop-2014_California_Water_Boards.pdf|Report pdf]]</ref>, Minnesota<ref>Minnesota Pollution Control Agency. Natural Attenuation of Groundwater. [https://www.pca.state.mn.us/water/natural-attenuation-groundwater Natural Attenuation of Groundwater]</ref>, New Jersey<ref> New Jersey Department of Environmental Protection - Site Remediation Program. 2012. Monitored Natural Attenuation Technical Guidance. [[Media:NJDEP-SRP-2012-MNA_Technical_Guidance_v_1_0.pdf|Report pdf]]</ref> , Ohio<ref>Ohio Environmental Protection Agency - Division of Environmental Response and Revitalization, 2001. Remedial Response Program Fact Sheet. Remediation Using Monitored Natural Attenuation.[[Media:OhioEPA-2001-Division_of_Envl_Response_and_Revitalization_fact_sheet.pdf|Report pdf]]</ref>, and Texas<ref> Texas Commission on Environmental Quality - Remediation Division, 2010. Monitored Natural Attenuation Demonstrations under TRRP. RG-366/TRRP-33. [[Media:TRRP-TCEQ-2010-Regulatory_Guidance-RG-366-TRRP-33.pdf|Report pdf]]</ref>. In addition, California<ref>California State Water Resources Control Board. 2012. Low-threat Underground Storage Tank Case Closure Policy. [[Media:CA-SWB-2012-Low-threat_UST_Case_Closure_Policy.pdf|Report pdf]]</ref>, Minnesota<ref> Minnesota Pollution Control Agency, 2005. Assessment of Natural Biogradation at Petroleum Release Sites. Guidance Document 4-03. [[Media:MINN-PCA-2005-Assessment_of_Natural_Biogradation_at_Petroleum_Rel_Sites.pdf|Report pdf]]</ref>, Washington State<ref> Washington State Department of Ecology, 2005. Guidance on Remediation of Petroleum-Contaminated Ground Water by Natural Attenuation. Publication Number 05-09-091 (Version 1.0). [[Media:WASH-ECOL-2005-Guidance_on_Remediation_of_Petroleum_Contaminated_GW.pdf|Report pdf]]</ref>, and Wisconsin<ref>Wisconsin Department of Natural Resources, 2014. Guidance on Natural Attenuation For Petroleum Releases. Remediation and Redevelopment Program. RR-614. [[Media:WIS-DNR-2014-Guidance_on_Natural_Attenuation_for_Petroleum_Releases.pdf|Report pdf]]</ref> provide guidance on petroleum releases. Minnesota<ref> Minnesota Pollution Control Agency Site Remediation Section. 2006. Guidelines Natural Attenuation of Chlorinated Solvents in Ground Water. [[Media:MINN-PCA-2006-Guidelines_Natural_Attenuation_of_Chlorinated_Solvents_in_GW.pdf|Report pdf]]</ref> and Wisconsin<ref> Wisconsin Department of Natural Resources, 2014. Understanding Chlorinated Hydrocarbon Behavior in Groundwater: Guidance on the Investigation, Assessment and Limitations of Monitored Natural Attenuation. RR-699. [[Media:WIS-DNR-2014-Understanding_Chlorinated_Hydrocarbon_Behavior_In_GW.pdf|Report pdf]]</ref> provide guidance on chlorinated solvents.

==Additional Information==
Additional information on MNA is available on web pages that are maintained by the United State Environmental Protection Agency<ref>U.S. Environmental Protection Agency, 2016. Natural Attenuation Overview. Technology Innovation and Field Services Division. [https://clu-in.org/techfocus/default.focus/sec/Natural_Attenuation/cat/Overview Natural Attenuation Overview]</ref>, the United States Geological Survey<ref> Natural Attenuation Definitions. 2015. United States Geological Survey. </ref>, Department of Energy, and the Interstate Technology Regulatory Council<ref>ITRC, 2008. Enhanced attenuation of chlorinated organics (EACO): A decision framework for site transition. [[Media:ITRC-2008-EACO_Framework_General.pdf|Report pdf]]</ref>. In addition, ESTCP has published “Frequently Asked Questions Regarding MNA in Groundwater” which provides a recent summary overview of key approaches, technologies, and best practices for applying MNA<ref name="Adamson2014"/>.

==References==

<references/>

==See Also==
*[[Long-Term Monitoring (LTM)]]
*[[Media:AFCEE_Long_Term_Monitoring_Protocol_2000.pdf|Designing Monitoring Programs to Effectively Evaluate the Performance of Natural Attenuation]]
*[[Media:ER-201032_Final_Report.pdf|Determining Source Attenuation History to Support Closure by Natural Attenuation]]
*[[Media:Role-of-DHC-Organism-Natural-Attenuation-Chlorinated-Ethylenes.pdf|Evaluation of the Role of Dehalococcoides Organisms in the Natural Attenuation of Chlorinated Ethylenes in Ground Water]]
*[[Media:Natatt_Cr.pdf|EPA Ground Water Issue: Natural Attenuation of Hexavalent Chromium in Groundwater and Soils]]
*[[Media:Parsons_MNA-Altus.pdf|Remediation by Natural Attenuation Treatability Study at Altus Air Force Base]]
*[[Media:Mnatoolbox.pdf|Site Screening and Technical Guidance for Monitored Natural Attenuation at DOE Sites]]
*[https://www.enviro.wiki/images/3/33/mna1198.pdf Technical Guidelines for Evaluating Monitored Natural Attenuation of Petroleum Hydrocarbons and Chlorinated Solvents in Groundwater at Naval and Marine Corps Facilities]
*[[Media:MNA-Guidance-2015.pdf|Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites]]
*[https://www.serdp-estcp.org/index.php/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-199518/ER-199518/(language)/eng-US Monitored Natural Attenuation of Explosives in Groundwater]

Chlorinated Solvents

2026-05-07T16:39:52Z

Admin:

Chlorinated solvents, including chlorinated volatile organic compounds (CVOC or CVOCs), are chemical compounds containing chlorine that have been widely used in various industries. They are divided in three groups (methanes, ethanes, ethenes) based on their structures, and include common groundwater contaminants such as carbon tetrachloride (CT), perchloroethene (PCE), trichloroethene (TCE), and vinyl chloride (VC). Chlorinated solvents tend to be colorless liquids at room temperatures, heavier than water, volatile, sparingly soluble, and moderately hydrophobic.
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'''Related Article(s):'''

*[[Biodegradation - Cometabolic]]
*[[Biodegradation - Reductive Processes]]
*[[Bioremediation - Anaerobic]]
*[[Bioremediation - Anaerobic Design Considerations]]
*[[Chemical Oxidation (In Situ - ISCO)]]
*[[Chemical Reduction (In Situ - ISCR)]]
*[[Design Tool - Base Addition for ERD]]
*[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation]]
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Low pH Inhibition of Reductive Dechlorination]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents]]
*[[pH Buffering in Aquifers]]
*[[Remediation Performance Assessment at Chlorinated Solvent Sites]]
*[[Soil & Groundwater Contaminants]]
*[[Thermal Remediation]]

'''Contributor(s): ''' [[Dr. Bilgen Yuncu, P.E.]] and [[M. Tony Lieberman]]

'''Key Resource(s):'''

*[http://dx.doi.org/10.1007/978-1-4419-1401-9_2 Chlorinated Solvent Chemistry: Structures, Nomenclature and Properties]<ref name="CS2010">Cwiertny, D.M., Scherer, M.M., 2010. Chlorinated solvent chemistry: structures, nomenclature and properties. In In situ remediation of chlorinated solvent plumes. Springer New York. pgs. 29-37. [http://dx.doi.org/10.1007/978-1-4419-1401-9_2 doi:10.1007/978-1-4419-1401-9_2]</ref>

==Introduction==

Chlorinated solvents are a large family of organic solvents that contain chlorine atoms in their molecular structure. They were first produced in Germany in the 1800s, and widespread use in the United States (U.S.) began after World War II. In the period of 1940-1980, the U.S. produced about 2 billion pounds of chlorinated solvents each year<ref name="PC 1996"> Pankow, J.F., Cherry, J.A., 1996. Dense Chlorinated Solvents and Other DNAPLs in Groundwater, Waterloo Press, Portland, OR. ISBN 0964801418</ref>. Chlorinated solvents, including [[wikipedia:Carbon_tetrachloride|carbon tetrachloride (CT)]], [[wikipedia:1,1,1-Trichloroethane|1,1,1-trichloroethane (TCA)]], [[wikipedia:Tetrachloroethylene|perchloroethene or tetrachloroethene (PCE)]] and [[wikipedia:Trichloroethylene|trichloroethene (TCE)]] have been among the most widely used cleaning and degreasing solvents in the U.S<ref>Doherty, R.E., 2000. A history of the production and use of carbon tetrachloride, tetrachloroethylene, trichloroethylene and 1, 1, 1-trichloroethane in the United States: Part 1--historical background; carbon tetrachloride and tetrachloroethylene. Environmental Forensics, 1(2), 69-81. [http://dx.doi.org/10.1006/enfo.2000.0010 doi:10.1006/enfo.2000.0010]</ref>. They also have been used in a wide variety of other purposes such as adhesives, chemical intermediates, clothes, pharmaceuticals, pesticides, and textile processing.

==Physical & Chemical Properties==

Chlorinated solvents are organic compounds generally constructed of a simple hydrocarbon chain (typically one to three carbon atoms in length). They can be divided into three categories based on their structural characteristics: chlorinated methanes, chlorinated ethanes and chlorinated ethenes.

Chlorinated methanes represent the most structurally simple solvent class and consist of a single carbon center (known as a methyl carbon) to which as many as four chlorine atoms are bonded. From the perspective of groundwater contamination, perhaps the most well-known chlorinated methanes are [[wikipedia:carbon tetrachloride|carbon tetrachloride (CT)]] or [[wikipedia:tetrachloromethane|tetrachloromethane]], [[wikipedia:trichloromethane|trichloromethane]] (commonly known as [[wikipedia:chloroform|chloroform (CF)]]), [[wikipedia:dichloromethane|dichloromethane (DCM)]], or [[wikipedia:methylene chloride|methylene chloride (MC)]] and [[wikipedia:chloromethane|chloromethane (CM)]], or [[wikipedia:methyl chloride|methyl chloride]].

Chlorinated ethanes consist of two carbon centers joined by a single [[wikipedia:Covalent_bond|covalent bond]]. The most frequently encountered groundwater pollutants of this class include [[wikipedia:1,1,1-trichloroethane|1,1,1-trichloroethane (1,1,1-TCA)]] and [[wikipedia:1,2-dichloroethane|1,2-dichloroethane]].

Chlorinated ethenes (also referred to as chlorinated ethylenes) also possess two carbon centers, but unlike chlorinated ethanes, these carbon atoms are joined by a carbon-carbon double bond. Chlorinated ethenes that are important groundwater contaminants include [[wikipedia:tetrachloroethene|tetrachloroethene]], or [[wikipedia:perchloroethene|perchloroethene (PCE)]], [[wikipedia:trichloroethene|trichloroethene (TCE)]], [[wikipedia:dichloroethene|dichloroethene (DCE)]]) (DCE, mainly two geometric isomers cis-1,2-dichloroethene and trans-1,2-dichloroethene), and [[wikipedia:vinyl chloride|vinyl chloride (VC)]].

Nomenclature and structure of selected compounds from each solvent class as well as some physical and chemical properties of most widely used chlorinated solvents are listed in Table 1.


{| class="mw-collapsible wikitable" style="margin: auto; color:black; background-color:white; width: 100%;"
|+Table 1. Nomenclature, Structure, Chemical and Physical Properties of Most Widely Used Chlorinated Solvents<ref name="CS2010" />.
|- style="color:white; background-color:#476b6b; text-align:center;"
|IUPAC Name
|Common Name
|Acronym
|Molecular Formula
|Chemical Structure
|Formula Weight
|Density (ρ)(g/mL)
|Aqueous Solubility (mg/L)
|Vapor Pressure (ρ0)(kPa)
|Henry's Law Constanta
|Log Kow
|MCLb (mg/L)
|-
| colspan="12" style="color:black; background-color:#94b8b8;" |Chlorinated Methanes
|-
| style="color:black; background-color:#d1e0e0;" |tetrachloromethane
| style="text-align:center;" |carbon tetrachloride
| style="text-align:center;" |CT
| style="text-align:center;" |CCl4
|
[[File:Tetrachloromethane.png|center|70 px|frameless]]
| style="text-align:center;" |153.8
| style="text-align:center;" |1.59
| style="text-align:center;" |800
| style="text-align:center;" |20.5
| style="text-align:center;" |28.9
| style="text-align:center;" |2.64
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |trichloromethane
| style="text-align:center;" |chloroform
| style="text-align:center;" |CF
| style="text-align:center;" |CHCl3
|
[[File:Trichloromethane.png|center|70px|frameless]]
| style="text-align:center;" |119.4
| style="text-align:center;" |1.49
| style="text-align:center;" |8,200
| style="text-align:center;" |26.2
| style="text-align:center;" |3.8
| style="text-align:center;" |1.97
| style="text-align:center;" |0.080c
|-
| style="color:black; background-color:#d1e0e0;" |dichloromethane
| style="text-align:center;" |methylene chloride
| style="text-align:center;" |DCM
| style="text-align:center;" |CH2Cl2
|
[[File:Dichloromethane.png|center|70px|frameless]]
| style="text-align:center;" |84.9
| style="text-align:center;" |1.33
| style="text-align:center;" |13,200
| style="text-align:center;" |55.3
| style="text-align:center;" |1.7
| style="text-align:center;" |1.25
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |chloromethane
| style="text-align:center;" |methyl chloride
| style="text-align:center;" |CM
| style="text-align:center;" |CH3Cl
|
[[File:Chloromethane.png|center|70px|frameless]]
| style="text-align:center;" |50.5
| style="text-align:center;" |0.92
| style="text-align:center;" |5,235
| style="text-align:center;" |570
| style="text-align:center;" |9.6
| style="text-align:center;" |0.91
| style="text-align:center;" |NRd
|-
| colspan="12" style="color:black; background-color:#94b8b8;" |Chlorinated Ethanes
|-
| style="color:black; background-color:#d1e0e0;" |hexachloroethane
| style="text-align:center;" |perchloroethane
| style="text-align:center;" |HCA
| style="text-align:center;" |C2Cl6
|
[[File:Hexachloroethane.png|center|70px|frameless]]
| style="text-align:center;" |236.7
| style="text-align:center;" |2.09
| style="text-align:center;" |50
| style="text-align:center;" |0.05e
| style="text-align:center;" | -
| style="text-align:center;" |3.93
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |pentachloroethane
| style="text-align:center;" | -
| style="text-align:center;" |PCA
| style="text-align:center;" |C2HCl5
|
[[File:Pentachloroethane.png|center|70px|frameless]]
| style="text-align:center;" |202.3
| style="text-align:center;" |1.68
| style="text-align:center;" |500
| style="text-align:center;" |0.6
| style="text-align:center;" |2.5
| style="text-align:center;" |2.89
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |1,1,1,2-tetrachloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1,1,2-TeCA
| style="text-align:center;" |C2H2Cl4
|
[[File:1,1,1,2-Tetrachloroethane.png|72px|frameless|center]]
| style="text-align:center;" |167.9
| style="text-align:center;" |1.54
| style="text-align:center;" |1,100
| style="text-align:center;" |1.6
| style="text-align:center;" |2.4
| style="text-align:center;" | -
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |1,1,2,2-tetrachloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1,2,2-TeCA
| style="text-align:center;" |C2H2Cl4
|
[[File:1,1,2,2-Tetrachloroethane.png|72px|frameless|center]]
| style="text-align:center;" |167.9
| style="text-align:center;" |1.60
| style="text-align:center;" |2,962
| style="text-align:center;" |0.8
| style="text-align:center;" |0.44
| style="text-align:center;" |2.39
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |1,1,2-trichloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1,2-TCA
| style="text-align:center;" |C2H3Cl3
|
[[File:1,1,2-Trichloroethane.svg.png|72px|frameless|center]]
| style="text-align:center;" |133.4
| style="text-align:center;" |1.44
| style="text-align:center;" |4,394
| style="text-align:center;" |3.22
| style="text-align:center;" |0.96
| style="text-align:center;" |2.38
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |1,1,1-trichloroethane
| style="text-align:center;" |methyl chloroform
| style="text-align:center;" |1,1,1-TCA
| style="text-align:center;" |C2H3Cl3
|
[[File:1,1,1-trichloroethane.png|center|70px|frameless]]
| style="text-align:center;" |133.4
| style="text-align:center;" |1.35
| style="text-align:center;" |1,495
| style="text-align:center;" |16.5
| style="text-align:center;" |14.5
| style="text-align:center;" |2.49
| style="text-align:center;" |0.20
|-
| style="color:black; background-color:#d1e0e0;" |1,2-dichloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,2-DCA
| style="text-align:center;" |C2H4Cl2
|
[[File:1,2-dichloroethane.png|center|70px|frameless]]
| style="text-align:center;" |99.0
| style="text-align:center;" |1.25
| style="text-align:center;" |8,606
| style="text-align:center;" |10.5
| style="text-align:center;" |1.2
| style="text-align:center;" |1.48
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |1,1-dichloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1-DCA
| style="text-align:center;" |C2H4Cl2
|
[[File:1,1-Dichloroethane 2.svg.png|72px|frameless|center]]
| style="text-align:center;" |99.0
| style="text-align:center;" |1.17
| style="text-align:center;" |4,676
| style="text-align:center;" |30.3
| style="text-align:center;" |6.2
| style="text-align:center;" |1.79
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |chloroethane
| style="text-align:center;" | -
| style="text-align:center;" |CA
| style="text-align:center;" |C2H5Cl
|
[[File:Chloroethane.png|center|70px|frameless]]
| style="text-align:center;" |64.5
| style="text-align:center;" |0.92
| style="text-align:center;" |5,700
| style="text-align:center;" |16.0
| style="text-align:center;" |1.8
| style="text-align:center;" |1.43
| style="text-align:center;" |NR
|-
| colspan="12" style="color:black; background-color:#94b8b8;" |Chlorinated Ethenes
|-
| style="color:black; background-color:#d1e0e0;" |tetrachloroethene
| style="text-align:center;" |perchloroethene
| style="text-align:center;" |PCE
| style="text-align:center;" |C2Cl4
|
[[File:Tetrachloroethene.png|center|70px|frameless]]
| style="text-align:center;" |165.8
| style="text-align:center;" |1.63
| style="text-align:center;" |150
| style="text-align:center;" |2.4
| style="text-align:center;" |26.3
| style="text-align:center;" |2.88
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |trichloroethene
| style="text-align:center;" | -
| style="text-align:center;" |TCE
| style="text-align:center;" |C2HCl3
|
[[File:Trichloroethene.png|72px|frameless|center]]
| style="text-align:center;" |131.4
| style="text-align:center;" |1.46
| style="text-align:center;" |1,100
| style="text-align:center;" |9.9
| style="text-align:center;" |11.7
| style="text-align:center;" |2.53
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |cis-1,2-dichloroethene
| style="text-align:center;" |cis-dichloroethene
| style="text-align:center;" |cis-DCE
| style="text-align:center;" |C2H2Cl2
|
[[File:Cis-1,2-dichloroethene.png|center|70px|frameless]]
| style="text-align:center;" |96.9
| style="text-align:center;" |1.28
| style="text-align:center;" |3,500
| style="text-align:center;" |27.1
| style="text-align:center;" |7.4
| style="text-align:center;" |1.86
| style="text-align:center;" |0.07
|-
| style="color:black; background-color:#d1e0e0;" |trans-1,2-dichloroethene
| style="text-align:center;" |trans-dichloroethene
| style="text-align:center;" |trans-DCE
| style="text-align:center;" |C2H2Cl2
|
[[File:Trans-1,2-dichloroethene.png|72px|frameless|center]]
| style="text-align:center;" |96.9
| style="text-align:center;" |1.26
| style="text-align:center;" |6,260
| style="text-align:center;" |44.4
| style="text-align:center;" |6.8
| style="text-align:center;" |1.93
| style="text-align:center;" |0.1
|-
| style="color:black; background-color:#d1e0e0;" |1,1-dichloroethene
| style="text-align:center;" |vinylidene chloride
| style="text-align:center;" |1,1-DCE
| style="text-align:center;" |C2H2Cl2
|
[[File:1,1-Dichloroethene.svg.png|72px|frameless|center]]
| style="text-align:center;" |96.9
| style="text-align:center;" |1.22
| style="text-align:center;" |3,344
| style="text-align:center;" |80.5
| style="text-align:center;" |23.0
| style="text-align:center;" |2.13
| style="text-align:center;" |0.007
|-
| style="color:black; background-color:#d1e0e0;" |chloroethene
| style="text-align:center;" |vinyl chloride
| style="text-align:center;" |VC
| style="text-align:center;" |C2H3Cl
|
[[File:Chloroethene.png|center|70px|frameless]]
| style="text-align:center;" |62.5
| style="text-align:center;" |0.91
| style="text-align:center;" |2,763
| style="text-align:center;" |355
| style="text-align:center;" |79.2
| style="text-align:center;" |1.38
| style="text-align:center;" |0.002
|-
| colspan="12" style="color:black; background-color:#d1e0e0;" |Notes:
atm = atmosphere; g = gram; Kow = octanol/water partitioning coefficient; Koc -- soil organic carbon/water partitioning coefficient; L = liter; MCL = maximum contaminant level; mg = milligram; mL = milliliter; mol = mole.

aHenry's Law Constant (KH)(x10-3 atm ・ m3/mol)

bSource: http://water.epa.gov/drink/contaminants/#List

cMCL for total trihalomethanes is defined as the summed concentration of chloroform, bromoform (CHBr3),bromodichloromethane (CHBrCl2), and dibromochloromethane (CHBr2Cl). http://water.epa.gov/drink/contaminants/basicinformation/disinfectionbyproducts.cfm

dNR : Not regulated.

eReported vapor pressure for solid-phase hexachloroethane.

|-
|}

Chlorinated solvents and many of their transformation products are colorless liquids at room temperature. They are heavier than water with densities greater than 1 gram per cubic centimeter (g/cm3) which means they can penetrate deeply into an aquifer. They are relatively volatile compounds with relatively high [[wikipedia:Henry’s Law|Henry’s Law]] constants(KH), a measure of the strength of partitioning from water into air). Generally, when KH for a compound exceeds 0.2 atmosphere/mole fraction (atm/M), they can readily be removed from water by air stripping it. Most chlorinated solvents can be classified as sparingly soluble in water, with aqueous solubilities generally on the order of 10s to 100s of mg/L. As the number of chlorine atoms on a compound increases, the solubility decreases. Because of their relatively low solubilities, chlorinated solvents dissolve slowly in groundwater. Another consequence of their limited solubility is their tendency to occur in the subsurface as a separate immiscible liquid phase which, because of its density compared to water, tends to sink in groundwater. Under these conditions, these are referred to as [[wikipedia:DNAPL|dense non-aqueous phase liquid (DNAPL)]]. Although chlorinated solvents are not very soluble in water, their solubility is typically orders of magnitude greater than their established [http://water.epa.gov/drink/contaminants/#Organic drinking water standards].

Chlorinated solvents can be considered moderately hydrophobic which can be determined by their [[wikipedia:Partition coefficient|octanol-water partition coefficient]]s (Kow, a measure of the tendency of a substance to prefer an organic or oily phase rather than an aqueous phase). Log Kow values less than 3 indicate that the compound does not sorb strongly to aquifer solids, but can be removed readily by activated carbon. On the other hand, compounds with log Kow less than 2, such as VC, generally are not removed well by activated carbon either<ref name="CS2010" />.

==References==

<references />

==See Also==

*[https://serdp-estcp.org/Tools-and-Training/Environmental-Restoration/DNAPL-Source-Zones/Frequently-Asked-Questions-Regarding-Management-of-Chlorinated-Solvents-in-Soils-and-Groundwater FAQ Regarding Management of Chlorinated Solvents in Soil and Groundwater]
*[//www.enviro.wiki/images/6/6b/AFCEE_Protocol_2007_chlorinated_solvents.pdf Protocol for In Situ Bioremediation of Chlorinated Solvents Using Edible Oil]

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2026-05-07T16:37:56Z

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[[File:WH Picture1.JPG|thumb|center|x350px|link=Matrix Diffusion|Molecular diffusion slowly transports solutes into clay-rich, lower permeability zones]]
[[File:WH Picture2.JPG|thumb|center|x350px|link=Subgrade Biogeochemical Reactor (SBGR)|Typical subgrade biogeochemical reactor (SBGR) layout. The SBGR is an in situ remediation technology for treatment of contaminated source areas and groundwater plume hot spots ]]
[[File:WH Picture3.JPG|thumb|center|x350px|link=Direct Push Logging|An Hydraulic Profiling Tool (HPT) log with electrical conductivity (EC) on left, injection pressure in middle, and flow rate on the right]]
[[File:WH Picture4.JPG|thumb|center|x350px|link=PH Buffering in Aquifers|Diagram of mineral surface exchanging hydrogen ions with varying pH. The surface of most aquifer minerals carries an electrical charge that varies with pH]]
[[File:WH Picture5.JPG|thumb|center|x350px|link=Biodegradation - Hydrocarbons|Comparison of the longitudinal redox zonation concept (A) and the plume fringe concept (B). Both concepts describe the spatial distribution of electron acceptors and respiration processes in a hydrocarbon contaminant plume]]
[[File:WH Picture6.JPG|thumb|center|x350px|link=Direct Push Logging|Schematic of an Hydraulic Profiling Tool (HPT) probe. HPT were developed to better understand formation permeability and the distribution of permeable and low permeability zones in unconsolidated formations]]
[[File:WH Picture7.JPG|thumb|center|x350px|link=Chemical Oxidation Design Considerations(In Situ - ISCO)|In situ chemical oxidation using (a) direct-push injection probes or (b) well-to-well flushing to delivery oxidants (shown in blue) into a target treatment zone of groundwater contaminated by dense nonaqueous phase liquid compounds (shown in red)]]
[[File:WH Picture8.JPG|thumb|center|x350px|link=Geophysical Methods - Case_Studies|High-resolution 3D cross-borehole electrical imaging of contaminated fractured rock at the former Naval Air Warfare Center in New Jersey. Cross-borehole resistivity tomography imaging is a geophysical technique that can be used for site characterization and monitoring by observing variations in the electrical properties of subsurface materials]]
[[File:WH Picture9.JPG|thumb|center|x350px|link=Stable_Isotope_Probing_(SIP)|Stable isotope probing (SIP) in use: Loading, deployment and recovery of Bio-Trap® passive sampler with 13C-labeled benzene. Stable isotope probing (SIP) is used to conclusively determine whether in situ biodegradation of a contaminant is occurring]]
[[File:WH Picture10.JPG|thumb|center|x350px|link=1,2,3-Trichloropropane|Summary of anticipated, primary reaction pathways for degradation of 1,2,3-Trichloropropane (TCP). TCP is a man-made chemical that was used in the past primarily as a solvent and extractive agent, a paint and varnish remover, and as a cleaning and degreasing agent]]
[[File:WH Picture11.JPG|thumb|center|x350px|link=Monitored Natural Attenuation (MNA) of Fuels|Distribution of BTEX plume lengths from 604 hydrocarbon sites. Monitored Natural Attenuation (MNA) is one of the most commonly used remediation approaches for groundwater contaminated with petroleum hydrocarbons (PHCs) and certain fuel additives such as fuel oxygenates or lead scavengers]]
[[File:WH Picture12.JPG|thumb|center|x350px|link=Groundwater Sampling - No-Purge/Passive|No-purge and passive sampling methods eliminate the pre-purging step for groundwater sample collection and represent alternatives to conventional sampling methods that rely on low-flow purging of a well prior to collection. The Snap SamplerTM is an example of a passive grab sampler]]
[[File:WH Picture13.JPG|thumb|center|x350px|link=Natural Source Zone Depletion (NSZD)|Conceptualization of Vapor Transport-related Natural Source Zone Depletion (NSZD) processes at a Petroleum Release Site]]
[[File:WH Picture14.JPG|thumb|center|x350px|link=Soil Vapor Extraction (SVE)|Conceptual diagram of basic Soil Vapor Extraction (SVE) system for vadose zone remediation. (SVE) is a common and typically effective physical treatment process for remediation of volatile contaminants in vadose zone (unsaturated) soils]]
[[File:WH Picture15.JPG|thumb|center|x350px|link=Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation|Emulsified Vegetable Oil (EVO) mixed in field during early pilot test. EVO is commonly added as a slowly fermentable substrate to stimulate the in situ anaerobic bioremediation of chlorinated solvents, explosives, perchlorate, chromate, and other contaminants]]
[[File:WH Picture16.JPG|thumb|center|x350px|link=Vapor_Intrusion_(VI)|Key elements of vapor intrusion pathways]]
[[File:WH Picture17.JPG|thumb|center|x350px|link=Sorption_of_Organic_Contaminants|Batch reactor experiments to generate points on a sorption isotherm]]
[[File:WH Picture18.JPG|thumb|center|x350px|link=Metagenomics|Results for metagenomic analysis of a groundwater sample obtained from a site impacted with petroleum hydrocarbons]]
[[File:WH Picture19.JPG|thumb|center|x350px|link=Perchlorate|Perchlorate releases and drinking water detections]]
[[File:WH Picture20.JPG|thumb|center|x350px|link=Mass_Flux_and_Mass_Discharge|Data input screen for ESTCP Mass Flux Toolkit]]
[[File:WH Picture21.JPG|thumb|center|x350px|link=Bioremediation_-_Anaerobic_Design_Considerations|Amendment addition for biobarrier]]
[[File:WH Picture22.JPG|thumb|center|x350px|link=Thermal Conduction Heating (TCH)|Thermal Remediation - Desorption schematic]]
[[File:WH_Picture23.jpg|thumb|center|x350px|link=Contaminated_Sediments_-_Introduction |Key exposure pathways for human health risk from contaminated sediments]]
[[File:WH_Picture24.jpg|thumb|center|x350px|link=Perfluoroalkyl_and_Polyfluoroalkyl_Substances_(PFAS)| The PFAS family of compounds]]

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{| style="width:100%; vertical-align:top;"
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'''[[Transport & Attenuation Processes | Attenuation & Transport Processes]]'''

*[[Biodegradation - 1,4-Dioxane]]
*[[Biodegradation - Cometabolic]]
*[[Biodegradation - Hydrocarbons]]
*[[Biodegradation - Reductive Processes]]
*[[Groundwater Flow and Solute Transport]]
*[[Matrix Diffusion]]
*[[Metals and Metalloids - Mobility in Groundwater | Mobility of Metals and Metalloids]]
*[[pH Buffering in Aquifers]]
*[[Sorption of Organic Contaminants]]
*[[Vapor Intrusion (VI)]]
**[[Vapor Intrusion - Separation Distances from Petroleum Sources]]
**[[Vapor Intrusion – Sewers and Utility Tunnels as Preferential Pathways|Vapor Intrusion - Sewers and Utility Tunnels as Preferential Pathways]]
**[[Assessing Vapor Intrusion (VI) Impacts in Neighborhoods with Groundwater Contaminated by Chlorinated Volatile Organic Chemicals (CVOCs)|Vapor Intrusion - Assessing VI Impacts in Neighborhoods with Groundwater Contaminated CVOCs]]

'''[[Characterization, Assessment & Monitoring]]'''

*[[Characterization Methods – Hydraulic Conductivity]]
*[[Compound Specific Isotope Analysis (CSIA)|Compound Specific Isotope Analysis (CSIA)]]
*[[Direct Push (DP) Technology]]
**[[Direct Push Logging |Direct Push Logging]]
**[[Direct Push Sampling |Direct Push Sampling]]
*[[Geophysical Methods | Geophysical Methods]]
**[[Geophysical Methods - Case Studies |Case Studies]]
**[[Hydrogeophysical Methods for Characterization and Monitoring of Groundwater-Surface Water Exchanges]]
*[[Groundwater Sampling - No-Purge/Passive]]
*[[Long-Term Monitoring (LTM)|Long-Term Monitoring (LTM)]]
**[[Long-Term Monitoring (LTM) - Data Analysis |LTM Data Analysis]]
**[[Long-Term Monitoring (LTM) - Data Variability |LTM Data Variability]]
*[[Molecular Biological Tools - MBTs |Molecular Biological Tools (MBTs)]]
**[[Metagenomics]]
**[[Proteomics and Proteogenomics]]
**[[Quantitative Polymerase Chain Reaction (qPCR)]]
**[[Stable Isotope Probing (SIP)]]
*[[Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill |Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill]]
*[[OPTically-based In-situ Characterization System (OPTICS)]]

'''[[Coastal and Estuarine Ecology]]'''

*[[Phytoplankton (Algae) Blooms]]

'''[[Contaminated Sediments - Introduction | Contaminated Sediments]]'''

*[[Contaminated Sediment Risk Assessment]]
*[[In Situ Toxicity Identification Evaluation (iTIE) | In Situ Toxicity Identification Evaluation]]
*[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
*[[Mercury in Sediments]]
*[[Passive Sampling of Sediments]]
**[[Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)]]
*[[Sediment Capping]]

| style="width:33%; vertical-align:top; " |

'''[[Light Non-Aqueous Phase Liquids (LNAPLs)]]'''

*[[LNAPL Conceptual Site Models]]
*[[LNAPL Remediation Technologies]]
*[[NAPL Mobility]]

'''[[Munitions Constituents]]'''

*[[Munitions Constituents - Abiotic Reduction|Abiotic Reduction]]
*[[Munitions Constituents - Alkaline Degradation|Alkaline Degradation]]
**[[Pyrogenic Carbonaceous Matter Enhanced Alkaline Hydrolysis]]
*[[Munitions Constituents - Composting|Composting]]
*[[Munitions Constituents - Deposition |Deposition]]
*[[Munitions Constituents - Dissolution |Dissolution]]
*[[Munitions Constituents - Electrochemical Treatment|Electrochemical Treatment]]
*[[Metal(loid)s - Small Arms Ranges]]
*[[Passive Sampling of Munitions Constituents|Passive Sampling]]
*[[Munitions Constituents – Photolysis |Photolysis]]
*[[Remediation of Stormwater Runoff Contaminated by Munition Constituents |Remediation of Stormwater Runoff ]]
*[[Munitions Constituents – Sample Extraction and Analytical Techniques|Sample Extraction and Analytical Techniques]]
*[[Munitions Constituents - Soil Sampling |Soil Sampling]]
*[[Munitions Constituents - Sorption |Sorption]]
*[[Munitions Constituents - IM Toxicology |Toxicology]]
*[[Munitions Constituents- TREECS™ Fate and Risk Modeling|TREECS™]]

'''[[Monitored Natural Attenuation (MNA)]]'''

*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| MNA of Chlorinated Solvents]]
**[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Monitored Natural Attenuation (MNA) of Fuels| MNA of Fuels]]
*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| MNA of Metals and Metalloids]]
*[[Natural Source Zone Depletion (NSZD)]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies| Transitioning from Active Remedies]]

'''[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]'''

*[[Hydrothermal Alkaline Treatment (HALT)]]
*[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]
*[[PFAS Destruction by Ultraviolet/Sulfite Treatment]]
*[[PFAS Ex Situ Water Treatment]]
**[[PFAS Treatment by Anion Exchange]]
*[[PFAS Monitored Retention (PMR) and PFAS Enhanced Retention (PER)]]
*[[PFAS Soil Remediation Technologies]]
*[[PFAS Sources]]
*[[PFAS Toxicology and Risk Assessment]]
*[[PFAS Transport and Fate]]
*[[PFAS Treatment by Electrical Discharge Plasma]]
*[[Photoactivated Reductive Defluorination - PFAS Destruction | Photoactivated Reductive Defluorination]]
*[[Reverse Osmosis and Nanofiltration Membrane Filtration Systems for PFAS Removal]]
*[[Thermal Conduction Heating for Treatment of PFAS-Impacted Soil]]
*[[Transition of Aqueous Film Forming Foam (AFFF) Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances (PFAS)| Transition of Aqueous Film Forming Foam Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances]]

| style="width:33%; vertical-align:top; " |
'''[[Regulatory Issues and Site Management]]'''

*[[Alternative Endpoints]]
*[[Mass Flux and Mass Discharge]]
*[[Plume Response Modeling]]
*[[REMChlor - MD | REMChlor-MD]]
*[[Source Zone Modeling]]
*[[Sustainable Remediation]]

'''[[Remediation Technologies]]'''
*[[Amendment Distribution in Low Conductivity Materials]]
*[[Bioremediation - Anaerobic|Anaerobic Bioremediation]]
**[[Bioremediation - Anaerobic Design Considerations | Design Considerations]]
**[[Design Tool - Base Addition for ERD]]
**[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation]]
**[[Low pH Inhibition of Reductive Dechlorination]]
**[[Bioremediation - Anaerobic Secondary Water Quality Impacts | Secondary Water Quality Impacts]]
*[[Chemical Oxidation (In Situ - ISCO) | In Situ Chemical Oxidation (ISCO)]]
**[[Chemical Oxidation Design Considerations(In Situ - ISCO) | Design Considerations]]
**[[Chemical Oxidation Oxidant Selection (In Situ - ISCO) | Oxidant Selection]]
*[[Chemical Reduction (In Situ - ISCR) | In Situ Chemical Reduction (ISCR)]]
**[[Zerovalent Iron (ZVI) (Chemical Reduction - ISCR) | Zero-Valent Iron (ZVI)]]
**[[Zerovalent Iron Permeable Reactive Barriers]]
*[[In Situ Groundwater Treatment with Activated Carbon]]
*[[Injection Techniques for Liquid Amendments]]
*[[Injection Techniques - Viscosity Modification]]
*[[Landfarming]]
*[[Metal and Metalloids - Remediation | Remediation of Metals and Metalloids]]
*[[Remediation Performance Assessment at Chlorinated Solvent Sites]]
*[[Soil Vapor Extraction (SVE)]]
*[[Stream Restoration]]
*[[Subgrade Biogeochemical Reactor (SBGR)]]
*[[Supercritical Water Oxidation (SCWO)]]
*[[Thermal Remediation]]
**[[Thermal Remediation - Combined Remedies | Combined Remedies]]
**[[Thermal Remediation - Electrical Resistance Heating | Electrical Resistance Heating (ERH)]]
**[[Thermal Remediation - Smoldering | Smoldering]]
**[[Thermal Remediation - Steam | Steam Enhanced Extraction (SEE)]]
**[[Thermal Conduction Heating (TCH)]]

'''[[Soil & Groundwater Contaminants]]'''

*[[1,2,3-Trichloropropane]]
*[[1,4-Dioxane]]
*[[Chlorinated Solvents]]
*[[Metal and Metalloid Contaminants|Metals and Metalloids]]
*[[N-nitrosodimethylamine (NDMA)]]
*[[Perchlorate|Perchlorate]]
*[[Petroleum Hydrocarbons (PHCs)]]
*[[Polycyclic Aromatic Hydrocarbons (PAHs)]]
|}
|}
|}

Polycyclic Aromatic Hydrocarbons (PAHs)

2026-04-28T20:19:40Z

Admin:

Polycyclic aromatic hydrocarbons (PAHs) are a class of organic compounds that consist solely of carbon and hydrogen atoms in aromatic ring structures. Sixteen PAHs are regulated by the U.S. Environmental Protection Agency (USEPA) based on their potential human and ecological health effects. These compounds can be naturally occurring (e.g., forest fires) or anthropogenic (e.g., coal gasification, automobile exhaust). [[Remediation Technologies | Remedial techniques]] are available for addressing PAH-contaminated soil, groundwater, and surface waters. However, such efforts must carefully consider the hydrophobic nature of PAHs, effects of PAH weathering in soil/sediment, and the poor biodegradability of high-molecular weight PAHs. Bioavailability of PAHs is also a key consideration for health and ecological risk assessment and selection of remedial techniques. Here, we review the physical and chemical properties of PAHs, their toxicity and rationale as priority pollutants, remedial options, and risk assessment considerations.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
'''Related Article(s):'''

*[[Biodegradation - Hydrocarbons]]
*[[Monitored Natural Attenuation (MNA) of Fuels]]
*[[Petroleum Hydrocarbons (PHCs)]]
*[[Remediation Technologies]]

'''Contributor(s):''' [[Dr. Stephen Richardson]]

'''Key Resource(s):'''

*[http://www.cambridge.org/us/academic/subjects/medicine/oncology/polycyclic-aromatic-hydrocarbons-chemistry-and-carcinogenicity Polycyclic aromatic hydrocarbons chemistry and carcinogenicity]<ref name="Harvey1991">Harvey, R.G., 1991. Polycyclic aromatic hydrocarbons: chemistry and carcinogenicity. Cambridge University Press: Cambridge, 396 pgs. ISBN 978-0521292047</ref>
*[//www.enviro.wiki/images/2/25/USEPA-1993-Provisional_Guidance_for_quantitative_Risk_Assessment....pdf Provisional guidance for quantitative risk assessment of polycyclic aromatic hydrocarbons]<ref name="USEPA1993">U.S. Environmental Protection Agency, 1993. Provisional guidance for quantitative risk assessment of polycyclic aromatic hydrocarbons. EPA 600-R-93-089. [//www.enviro.wiki/images/2/25/USEPA-1993-Provisional_Guidance_for_quantitative_Risk_Assessment....pdf Report pdf]</ref>

==Introduction==
[[wikipedia: Polycyclic aromatic hydrocarbon | Polycyclic aromatic hydrocarbons (PAHs)]], also known as polyaromatic hydrocarbons, are a class of hundreds of organic compounds that consist of two or more aromatic rings fused in linear, angular, or clustered arrangements<ref name="Harvey1991" />. PAHs are ubiquitous in the environment, predominantly formed by the incomplete combustion of organic materials from both natural sources (e.g., forest fires, volcanic events), and anthropogenic activities (e.g., coal gasification, automobile exhaust, incinerators, coke production, cooking, tobacco smoke)<ref name="Harvey1991" />. The USEPA has listed 16 PAHs as [https://www.epa.gov/eg/toxic-and-priority-pollutants-under-clean-water-act 'priority pollutants'] in aquatic and terrestrial ecosystems<ref>Keith, L. and Telliard, W., 1979. ES&T special report: priority pollutants: I-a perspective view. Environmental Science & Technology, 13(4), 416-423. [http://dx.doi.org/10.1021/es60152a601 doi: 10.1021/es60152a601]</ref>. Seven of these PAHs may cause cancer in humans<ref name="USEPA1993" />, and benzo[a]pyrene is considered the highest cancer risk amongst the 16 PAHs<ref name="USEPA1993" /><ref name="LaGoy1994">LaGoy, P.K., Quirk, T.C., 1994. Establishing generic remediation goals for the polycyclic aromatic hydrocarbons: critical issues. Environmental Health Perspectives, 102(4), 348-352. [//www.enviro.wiki/images/c/c0/LaGoy-1994-Establishing_generic_remediation_goals_for_the_plycyclic_aromatic_hydrocarbons.pdf Report pdf]</ref>.

==Physical and Chemical Properties==
PAHs are [[wikipedia: Hydrophobe | hydrophobic]] and do not readily dissolve in water or volatilize to the atmosphere (with the exception of [[wikipedia: Naphthalene | naphthalene]], which was once used in 'moth balls'). The chemical stability, low water solubility, and high sorption capacity of PAHs contribute greatly to their persistence in the environment<ref>Kanaly, R.A., Harayama, S., 2000. Biodegradation of high-molecular-weight polycyclic aromatic hydrocarbons by bacteria. Journal of Bacteriology, 182(8), 2059-2067. [http://dx.doi.org/10.1128/jb.182.8.2059-2067.2000 doi: 10.1128/JB.182.8.2059-2067.2000]</ref><ref>Van Hamme, J.D., Singh, A., Ward, O.P., 2003. Recent advances in petroleum microbiology. Microbiology and Molecular Biology Reviews, 67(4), 503-549. [http://dx.doi.org/10.1128/mmbr.67.4.503-549.2003 doi: 10.1128/MMBR.67.4.503-549.2003]</ref>. PAHs can be divided into two categories: '''(1) low molecular weight PAHs''' composed of less than four aromatic rings (e.g., naphthalene, acenaphthene, fluorene, phenanthrene), and '''(2) high molecular weight PAHs''' composed of four or more rings (e.g., pyrene, chrysene, benzo[a]pyrene, dibenz[a,h]anthracene). High molecular weight PAHs are generally less water soluble, have lower vapor pressures and Henry’s constants, and partition more readily into organic matter than low molecular weight PAHs. Selected physical and chemical properties of the 16 USEPA-regulated PAHs are presented in Table 1.

[[File:Richardson-Article 1-Table 1.JPG|thumbnail|600px|center|Table 1. Chemical structures and selected properties of the 16 USEPA priority pollutant PAHsab. aabbreviations: MW = molecular weight (g/mol); Cwsat = aqueous solubility (mg/L); p* = vapor pressure (mm Hg); Kow = octanol-water partitioning coefficient; Koc = organic carbon partitioning coefficient; TEF = toxic equivalency factor; PLHS = Priority List of Hazardous Substances. bAll data are from<ref>LaGrega, M.D., Buckingham, P.L., Evans, J.C., 2001. Hazardous waste management: 2nd edition. McGraw-Hill, Boston. ISBN 1577666933.</ref> unless otherwise noted; cdata from<ref name="NRC2003" />; ddata from<ref>Mackay, D., Shiu, W.Y., Ma, K.C., 1997. Illustrated handbook of physical-chemical properties of environmental fate for organic chemicals.</ref>.]]

==Toxicity==
The most important property driving PAH remediation is their toxicity (or carcinogenicity). PAHs ranked 9th on the 2015 Agency for Toxic Substances and Disease Registry (ATSDR) Priority List of Hazardous Substances (PLHS) based on their toxicity, frequency of occurrence at USEPA National Priorities List (i.e. Superfund) sites, and potential for human exposure<ref>ATSDR, 2015. Comprehensive environmental response, compensation, and liability act (CERCLA) priority list of hazardous substances. [http://www.atsdr.cdc.gov/spl List]</ref>. Individually, all 16 regulated PAHs are included on the ATSDR PLHS, with six ranked in the top 100; most notably, benzo[a]pyrene at 8th (Table 1).

Regulatory guidelines or site-specific cleanup goals for soil commonly account for PAH toxicity by assigning toxic equivalency factors (TEFs) to individual PAHs (Table 1), normalized to benzo[a]pyrene toxicity. Multiplying the measured concentration of each PAH by its respective TEF yields an equivalent concentration of benzo[a]pyrene for the PAH mixture, called a benzo[a]pyrene equivalent<ref name="LaGoy1994" />. These adjusted values often serve as soil remediation goals at PAH-contaminated sites. For drinking water, the USEPA has established a maximum contaminant level for benzo[a]pyrene of 0.2 µg/L.

==PAH Bioavailability==
Bioavailability is an important concept for PAH remediation and risk assessment in soil and sediments. With respect to bioremediation, bioavailability refers to the contaminant fraction that can be effectively accessed by contaminant-degrading microbial communities<ref name="NRC2003">National Research Council (US). Committee on Bioavailability of Contaminants in Soils and Sediments, 2003. Bioavailability of contaminants in soils and sediments: Processes, tools, and applications. Washington, DC: National Academies Press., 432 pgs. [http://dx.doi.org/10.17226/10523 doi: 10.17226/10523]</ref><ref>Ortega-Calvo, J.J., Harmsen, J., Parsons, J.R., Semple, K.T., Aitken, M.D., Ajao, C., Eadsforth, C., Galay-Burgos, M., Naidu, R., Oliver, R., Peijnenburg, W.J., Römbke, J., Streck, G., Versonnen, B. 2015. From bioavailability science to regulation of organic chemicals. Environmental Science & Technology, 49(17), 10255-10264. [http://dx.doi.org/10.1021/acs.est.5b02412 doi: 10.1021/acs.est.5b02412]</ref>. Bioavailability and degradation of PAHs in natural soils and sediment are generally two-phased, with an initial phase of rapid PAH removal followed by a longer period of limited PAH reduction. During the initial phase, bioavailability of PAHs is high and degradation rates may be limited by reaction rate (e.g., microbial uptake rather than mass transfer from soil particles). As bioavailable PAHs are removed, mass transfer mechanisms (desorption and diffusion) become controlling factors for the rate of PAH degradation<ref>Bosma, T.N., Middeldorp, P.J., Schraa, G., Zehnder, A.J., 1997. Mass transfer limitation of biotransformation: quantifying bioavailability. Environmental Science & Technology, 31(1), 248-252. [http://dx.doi.org/10.1021/es960383u doi: 10.1021/es960383u]</ref>.
Physical, chemical, and biological remedial methods have been developed to either increase PAH bioavailability (e.g., mixing, surfactants, cosolvents), or decrease PAH bioavailability (e.g., biostabilization, sediment capping, solidification), depending on the treatment design and site cleanup goals<ref>Ehlers, L.J., Luthy, R.G., 2003. Contaminant bioavailability in soil and sediment. Environmental Science & Technology, 37, 295A-302A. [http://dx.doi.org/10.1021/es032524f doi: 10.1021/es032524f]</ref>. The former generally incorporate a mass removal step (e.g., enhanced biodegradation, aqueous phase extraction) to minimize risk. The latter strategies reduce risk by creating a barrier between the contaminations and surrounding receptors.

==Remediation of PAHs==
Anthropogenic sources of PAHs vary widely, including former manufactured gas plants, petroleum fuel spills, coal- and gas-fired power plants, and industrial incinerators, as well as are present in a range of environments (e.g., harbor sediments, marine waters), often proximate to past or current industrial operations. The fate and transport of PAHs in these environments is controlled by PAH hydrophobicity, rates of dissolution, physicochemical properties of the soil, and the source phase. In sediments, for example, PAHs sorb to natural organic matter and are present in oils, tars, residues, and other nonaqueous phase liquids (NAPLs) deposited from industrial activities. These phases act as long-term sources of PAHs to the water column through dissolution processes and can greatly influence overall PAH transport, degradation, and bioavailability<ref>Wick, A.F., Haus, N.W., Sukkariyah, B.F., Haering, K.C., Daniels, W.L., 2011. Remediation of PAH-contaminated soils and sediments: a literature review. Virginia Polytechnic Institute and State University, USA. [//www.enviro.wiki/images/7/7f/Wick-2011-Virginia_Tech_PAH_Remediation_Lit_Review.pdf Report pdf]</ref>.
A variety of ''ex situ'' and ''in situ'' remediation methods have been used to address PAH contaminated soils, groundwater, and surface waters. The most common ''ex situ'' practices include landfill disposal, incineration, [[Thermal Remediation | thermal desorption]], and soil washing<ref>U.S. Environmental Protection Agency, 2004. Cleaning up the nation's waste sites: markets and technology trends. EPA 542-R-04-015. [//www.enviro.wiki/images/8/88/USEPA-2004-Cleaning_Up_the_Nations_Waste_Sites.pdf Report pdf]</ref>. Generally, these methods are expensive and can be cost-prohibitive for sites with large footprints, significant depth of contamination, and existing infrastructure. Alternatively, ''in situ'' treatments such as chemical oxidation, solvent and surfactant flushing, and bioremediation are available, although they are used to a lesser extent than ''ex situ'' methods. These remediation methods are generally less expensive, but require longer treatment times to meet regulatory criteria.

Physical treatments such as in situ stabilization and solidification prevent or minimize the release (or leaching) of PAHs from contaminated soils and sediments by using binding agents (e.g., cement, asphalt, fly ash, and clay) to limit water infiltration and bind PAHs into less mobile forms. The addition of granular activated carbon to contaminated sediments has also been tested as a means to strongly bind available PAHs and reduce the ecological risks to overlying surface waters<ref>Luthy, R.G., Zimmerman, J.R., McLeod, P.B., Zare, R.N., Mahajan, T., Ghosh, U., Bridges, T.S., Millward, R.N., Talley, J.W., 2004. In situ stabilization of persistent organic contaminants in Marine Sediments. Strategic Environmental Research and Development Program, Arlington, Virginia. SERDP Project ER-1207. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Sediments/ER-1207 ER-1207]</ref><ref>Luthy, R.G., Zimmerman, J.R., McLeod, P.B., Zare, R.N., Mahajan, T., Ghosh, U., Bridges, T.S., Millward, R.N., Talley, J.W., 2014. Demonstration of in situ treatment with reactive amendments for contaminated sediments in active DoD harbors. Project ER-201131. Strategic Environmental Research and Development Program, Arlington, Virginia. [https://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Sediments/ER-201131/ER-201131 ER-201131]</ref>.

Chemical treatments for PAHs include Fenton’s reagent, hydrogen peroxide, activated persulfate, and ozone<ref name="Huling2006">Huling, S. G., Pivetz, B. E., 2006. In-situ chemical oxidation (No. EPA/600/R-06/072). Environmental Protection Agency Washington DC Office of Water. [//www.enviro.wiki/images/7/79/Huling-EPA-ISCO.pdf Report pdf]</ref>. These methods generate very reactive free radicals (e.g., hydroxyl radical, sulfate radical, ozone radical) and other reactive species (e.g., persulfate anion, peroxides), capable of attacking the aromatic structure of PAHs. Advantages of in situ chemical oxidation over conventional remediation methods include reasonable treatment times, reactivity with a broad range of PAHs, and destruction of contaminants in situ. However, the use of chemical oxidants is complicated by oxidation of non-target species such as soil organic matter and limited control of oxidant delivery in heterogeneous media<ref name="Huling2006" />.
Biodegradation of PAHs can occur both aerobically and anaerobically in the subsurface. Numerous aerobic bacterial species and fungi are capable of transforming two-, three-, and four-ring PAHs to non-toxic end products such as water and carbon dioxide (i.e., PAH mineralization) and partially degrading five- and six-ring PAHs to intermediate compounds<ref>Cerniglia, C.E., 1992. Biodegradation of polycyclic aromatic hydrocarbons. Biodegradation, 3(2-3), 351-368. [http://dx.doi.org/10.1007/bf00129093 doi: 10.1007/BF00129093]</ref>. Under anaerobic conditions, degradation of two- and three-ring PAHs has been documented under nitrate<ref>Mihelcic, J.R., Luthy, R.G., 1988. Degradation of polycyclic aromatic hydrocarbon compounds under various redox conditions in soil-water systems. Applied and Environmental Microbiology, 54(5), 1182-1187. [http://aem.asm.org/content/54/5/1182.short Journal Article Page]</ref><ref>McNally, D.L., Mihelcic, J.R., Lueking, D.R., 1998. Biodegradation of three-and four-ring polycyclic aromatic hydrocarbons under aerobic and denitrifying conditions. Environmental Science & Technology, 32(17), 2633-2639. [http://dx.doi.org/10.1021/es980006c doi: 10.1021/es980006c]</ref>, iron<ref>Anderson, R.T., Lovley, D.R., 1999. Naphthalene and benzene degradation under Fe(III)-reducing conditions in petroleum-contaminated aquifers. Bioremediation Journal, 3(2), 121-135. [http://dx.doi.org/10.1080/10889869991219271 doi: 10.1080/10889869991219271]</ref>, and sulfate-reducing conditions<ref>Coates, J.D., Anderson, R.T., Woodward, J.C., Phillips, E.J., Lovley, D.R., 1996. Anaerobic hydrocarbon degradation in petroleum-contaminated harbor sediments under sulfate-reducing and artificially imposed iron-reducing conditions. Environmental Science & Technology, 30(9), 2784-2789. [http://dx.doi.org/10.1021/es9600441 doi: 10.1021/es9600441]</ref><ref>Coates, J.D., Anderson, R.T., Lovley, D.R., 1996. Oxidation of polycyclic aromatic hydrocarbons under sulfate-reducing conditions. Applied and Environmental Microbiology, 62(3), 1099-1101. [http://aem.asm.org/content/62/3/1099.short Journal Article]</ref>. However, rates of anaerobic PAH biodegradation are generally much lower (several orders of magnitude) than aerobic metabolism. Since many contaminated sites are oxygen- and nutrient-limited, a variety of biostimulation methods (e.g., can be composting, landfarming, biosparging, peroxide injection) can be used to deliver oxygen/nutrients into groundwater and soil/sediments to stimulate aerobic degradation of PAHs<ref>Mueller, J.G., Chapman, P.J., Pritchard, P.H., 1989. Creosote-contaminated sites. Their potential for bioremediation. Environmental Science & Technology, 23(10), 1197-1201. [http://dx.doi.org/10.1021/es00068a003 doi: 10.1021/es00068a003]</ref><ref>Johnson, C.R., Scow, K.M., 1999. Effect of nitrogen and phosphorus addition on phenanthrene biodegradation in four soils. Biodegradation, 10(1), 43-50. [http://dx.doi.org/10.1023/a:1008359606545 doi: 10.1023/A:1008359606545]</ref><ref>Breedveld, G.D., Sparrevik, M., 2000. Nutrient-limited biodegradation of PAH in various soil strata at a creosote contaminated site. Biodegradation, 11(6), pp.391-399. [http://dx.doi.org/10.1023/a:1011695023196 doi: 10.1023/A:1011695023196]</ref><ref>Carmichael, L.M., Pfaender, F.K., 1997. The effect of inorganic and organic supplements on the microbial degradation of phenanthrene and pyrene in soils. Biodegradation, 8(1), 1-13. [http://dx.doi.org/10.1023/a:1008258720649 doi: 10.1023/A:1008258720649]</ref>.

==References==

<references />

==See Also==

Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions

2026-04-28T20:19:15Z

Admin:

The U.S. Department of Defense (DoD) faces many challenges in restoring aquifers at contaminated sites, often due to back-diffusion of tetrachloroethene (PCE) and trichloroethene (TCE) from low-permeability clay zones. The uptake, storage, and subsequent long-term release of these dissolved contaminants from clays are key processes in understanding the longevity, intensity, and risks associated with many persistent chlorinated ethene groundwater plumes. Although naturally occurring abiotic and biotic dechlorination processes in clays may reduce stored contaminant mass and significantly aid natural attenuation, no standardized field method currently exists to verify or quantify these reactions. It is critical to remediation design efforts to demonstrate and validate a cost-effective ''in situ'' approach for assessing these dechlorination processes using first-order rate constants. An approach was developed and applied across eight DoD sites to support Remedial Project Managers (RPMs) and regulators in evaluating natural attenuation potential in clay-rich environments.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Chlorinated Solvents]]
*[[Monitored Natural Attenuation (MNA)]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
*[[Matrix Diffusion]]
*[[REMChlor - MD]]

'''Contributor(s):''' Dani Tran, [[Dr. Charles Schaefer]], Dr. Charles Werth

'''Key Resource(s):'''
*Schaefer, C.E, Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils<ref name="SchaeferEtAl2025"/>

==Introduction==
Cost-effective methods are needed to verify the occurrence of natural dechlorination processes and quantify their dechlorination rates in clays under ambient in situ conditions in order to reliably predict their long-term influence on plume longevity and mass discharge. However, accurately determining these rates is challenging due to slow reaction kinetics, the transient nature of transformation products, and the interplay of biotic and abiotic mechanisms within the clay matrix or at clay-sand interfaces. Tools capable of quantifying these reactions and assessing their role in mitigating plume persistence would be a significant aid for long-term site management.

For reductive abiotic dechlorination under anoxic conditions, a 1% hydrochloric acid (HCl) extraction of a sample of native clay coupled with X-ray diffraction (XRD) data can be used as a screening level tool to estimate reductive dechlorination rate constants. These rate constants can be inserted into fate and transport models such as [[REMChlor - MD]]<ref>Falta, R., and Wang, W., 2017. A semi-analytical method for simulating matrix diffusion in numerical transport models. Journal of Contaminant Hydrology, 197, pp. 39-49. [https://doi.org/10.1016/j.jconhyd.2016.12.007 doi: 10.1016/j.jconhyd.2016.12.007]  [[Media: FaltaWang2017.pdf | Open Access Manuscript]]</ref><ref>Kulkarni, P.R., Adamson, D.T., Popovic, J., Newell, C.J., 2022. Modeling a well-charactized perfluorooctane sulfate (PFOS) source and plume using the REMChlor-MD model to account for matrix diffusion. Journal of Contaminant Hydrology, 247, Article 103986. [https://doi.org/10.1016/j.jconhyd.2022.103986 doi: 10.1016/j.jconhyd.2022.103986]  [[Media: KulkarniEtAl2022.pdf | Open Access Manuscript]]</ref> to quantify abiotic dechlorination impacts within clay aquitards on chlorinated solvent plumes. Thus, determination of the abiotic reductive dechlorination rate constant for a particular clayey soil can be readily utilized to provide a more accurate assessment of aquifer cleanup timeframes for groundwater plumes that are being sustained by contaminant back-diffusion.

==Recommended Approach==
[[File: TranFig1.png | thumb | 500 px | Figure 1: First-order rate constants for abiotic reductive dechlorination of TCE under anaerobic conditions. Circles are data from Schaefer ''et al.'', 2021<ref>Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2021. Abiotic dechlorination in the presence of ferrous minerals. Journal of Contaminant Hydrology, 241, 103839. [https://doi.org/10.1016/j.jconhyd.2021.103839 doi: 10.1016/j.jconhyd.2021.103839]  [[Media: SchaeferEtAl2021.pdf | Open Access Manuscript]]</ref>, filled squares from Schaefer ''et al.'', 2018<ref name="SchaeferEtAl2018"/>, and Schaefer ''et al.'', 2017<ref>Schaefer, C.E., Ho., Gurr, C., Berns, E., Werth, C., 2017. Abiotic dechlorination of chlorinated ethenes in natural clayey soils: impacts of mineralogy and temperature. Journal of Contaminant Hydrology, 206, pp. 10-17. [https://doi.org/10.1016/j.jconhyd.2017.09.007 doi: 10.1016/j.jconhyd.2017.09.007]  [[Media: SchaeferEtAl2017.pdf | Open Access Manuscript]]</ref>, and open squares from Schaefer ''et al.'', 2025<ref name="SchaeferEtAl2025"/>. ]]
[[File: TranFig2.png | thumb | 600 px | Figure 2: Flowchart diagram of field screening procedures]]
The recommended approach builds upon the methodology and findings of a recent study<ref name="SchaeferEtAl2025">Schaefer, C.E., Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils. Groundwater Monitoring and Remediation, 45(2), pp. 31-39. [https://doi.org/10.1111/gwmr.12709 doi: 10.1111/gwmr.12709]</ref>, emphasizing field-based and analytical techniques to quantify abiotic first-order reductive dechlorination rate constants for PCE and TCE in clayey soils under anoxic conditions. Key components of this evaluation are listed below:
#Zone Identification: The focus of the investigation should be to delineate clayey zones adjacent to hydraulically conductive zones.
#Ferrous Mineral Quantification: Assess ferrous mineral context in clay via 1% HCl extraction at ambient temperature over a 10-minute interval.
#Mineralogical Characterization: Conduct XRD analysis with the specific intent of identifying the presence of pyrite and biotite.
#Reduced Gas Analysis: Measurement of reduced gases such as acetylene, ethene, and ethane concentrations in clay samples. Gas-tight sampling devices (e.g., En Core® soil samplers by En Novative Technologies, Inc.) should be used to ensure sample integrity during collection and transport.

Clay samples should be collected within a few centimeters of the high-permeability interface, with optional additional sampling further inward. For mineralogical analysis, a defined interval may be collected and subsequently subsampled. To preserve sample integrity, exposure to air should be minimized during collection, transport, and handling. Homogenization should occur within an anaerobic chamber, and if subsamples are required for external analysis, they must be shipped in gas-tight, anaerobic containers.

Estimation of the abiotic reductive first-order rate constant for PCE and TCE is based on the “reactive” ferrous content in the clay. Reactive ferrous content (Fe(II)r) is estimated as shown in Equation 1:

::'''Equation 1:'''       <big>''Fe(II)r = DA + XRDpyr - XRDbiotite''</big>

where ''DA'' is the ferrous content from the dilute acid (1% HCl) extraction, ''XRDpyr'' is the pyrite content from XRD analysis, and ''XRDbiotite'' is the biotite content from XRD analysis<ref name="SchaeferEtAl2025"/>.

Abiotic dechlorination is unlikely to contribute to mitigating contaminant back-diffusion when reactive ferrous iron (Fe(II)r) concentrations are below 100 mg/kg (Figure 1). For Fe(II)r above 100 mg/kg, the first-order rate constant for PCE and TCE reductive dechlorination can be estimated using the correlation shown in Figure 1<ref name="SchaeferEtAl2018">Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2018. Mechanisms for abiotic dechlorination of trichloroethene by ferrous minerals under oxic and anoxic conditions in natural sediments. Environmental Science and Technology, 52(23), pp.13747-13755. [https://doi.org/10.1021/acs.est.8b04108 doi: 10.1021/acs.est.8b04108]</ref><ref>Borden, R.C., Cha, K.Y., 2021. Evaluating the impact of back diffusion on groundwater cleanup time. Journal of Contaminant Hydrology, 243, Article 103889. [https://doi.org/10.1016/j.jconhyd.2021.103889 doi: 10.1016/j.jconhyd.2021]  [[Media: BordenCha2021.pdf | Open Access Manuscript]]</ref>. The rate constant exhibits a strong positive correlation with the logarithm of reactive Fe(II) content (Pearson’s ''r'' = 0.82), with a slope of 4.7 × 10⁻⁸ L g⁻¹ d⁻¹ (log mg kg⁻¹)⁻¹.

Figure 2 presents a decision flowchart designed to evaluate the significance and extent of abiotic reductive dechlorination. By applying Equation 1 to the dilute acid extractable Fe(II) plus measured mineral species data from clay samples, the reactive ferrous iron content (Fe(II)r) can be quantified, enabling a streamlined assessment of the extent to which abiotic processes are contributing to the mitigation of contaminant back-diffusion.

If Fe(II)r is ≥ 100 mg/kg, a first-order dechlorination rate constant can be estimated and subsequently used within a contaminant fate and transport model. However, if acetylene is detected in the clay, even with Fe(II)r less than 100 mg/kg, then bench-scale testing using methods similar to those described in a recent study<ref name="SchaeferEtAl2025"/> is recommended, as such results would likely be inconsistent with those shown in Figure 1, suggesting some other mechanism might be involved, or that the system mineralogy might be more complex than anticipated. Even if Fe(II)r ≥ 100 mg/kg, confirmatory bench-scale testing may be conducted for additional verification and to refine estimation of the abiotic dechlorination rate constant.

==Summary and Recommendations==
The approach outlined above is intended to serve as a generalized guide for practitioners and site managers to cost-effectively determine the extent to which beneficial abiotic reductive dechlorination reactions are likely occurring in low permeability (e.g., clayey) zones. This approach may be contraindicated if co-contaminants are present. It is currently unclear whether other classes of potentially reactive chemicals, such as trinitrotoluene (TNT) or chlorinated ethanes, could interact competitively with PCE and TCE.

In addition, it remains unclear how other classes of compounds such as per- and polyfluoroalkyl substances (PFAS) may interact or sorb with ferrous minerals and potentially inhibit abiotic dechlorination reactions. Coupling these recommended activities with conventional site investigation tasks would provide an opportunity to perform many of the up-front screening activities with minimal additional project costs. It is important to note that the guidance proposed herein pertains to particularly low permeability media. Sites with complex or varying lithology, where the mineralogy and/or redox conditions may vary, might require evaluation of multiple samples to provide appropriate site-wide information.
 

==References==
<references />

==See Also==
*[https://serdp-estcp.mil/projects/details/a7e3f7b5-ed82-4591-adaa-6196ff33dd60 ESTCP Project ER20-5031 – In Situ Verification and Quantification of Naturally Occurring Dechlorination Rates in Clays: Demonstrating Processes that Mitigate Back-Diffusion and Plume Persistence]

Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions

2026-04-28T20:18:23Z

Admin:

The U.S. Department of Defense (DoD) faces many challenges in restoring aquifers at contaminated sites, often due to back-diffusion of tetrachloroethene (PCE) and trichloroethene (TCE) from low-permeability clay zones. The uptake, storage, and subsequent long-term release of these dissolved contaminants from clays are key processes in understanding the longevity, intensity, and risks associated with many persistent chlorinated ethene groundwater plumes. Although naturally occurring abiotic and biotic dechlorination processes in clays may reduce stored contaminant mass and significantly aid natural attenuation, no standardized field method currently exists to verify or quantify these reactions. It is critical to remediation design efforts to demonstrate and validate a cost-effective ''in situ'' approach for assessing these dechlorination processes using first-order rate constants. An approach was developed and applied across eight DoD sites to support Remedial Project Managers (RPMs) and regulators in evaluating natural attenuation potential in clay-rich environments.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Chlorinated Solvents]]
*[[Monitored Natural Attenuation (MNA)]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
*[[Matrix Diffusion]]
*[[REMChlor - MD]]

'''Contributor(s):''' Dani Tran, [[Dr. Charles Schaefer]], Dr. Charles Werth

'''Key Resource (s):'''
*Schaefer, C.E, Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils<ref name="SchaeferEtAl2025"/>

==Introduction==
Cost-effective methods are needed to verify the occurrence of natural dechlorination processes and quantify their dechlorination rates in clays under ambient in situ conditions in order to reliably predict their long-term influence on plume longevity and mass discharge. However, accurately determining these rates is challenging due to slow reaction kinetics, the transient nature of transformation products, and the interplay of biotic and abiotic mechanisms within the clay matrix or at clay-sand interfaces. Tools capable of quantifying these reactions and assessing their role in mitigating plume persistence would be a significant aid for long-term site management.

For reductive abiotic dechlorination under anoxic conditions, a 1% hydrochloric acid (HCl) extraction of a sample of native clay coupled with X-ray diffraction (XRD) data can be used as a screening level tool to estimate reductive dechlorination rate constants. These rate constants can be inserted into fate and transport models such as [[REMChlor - MD]]<ref>Falta, R., and Wang, W., 2017. A semi-analytical method for simulating matrix diffusion in numerical transport models. Journal of Contaminant Hydrology, 197, pp. 39-49. [https://doi.org/10.1016/j.jconhyd.2016.12.007 doi: 10.1016/j.jconhyd.2016.12.007]  [[Media: FaltaWang2017.pdf | Open Access Manuscript]]</ref><ref>Kulkarni, P.R., Adamson, D.T., Popovic, J., Newell, C.J., 2022. Modeling a well-charactized perfluorooctane sulfate (PFOS) source and plume using the REMChlor-MD model to account for matrix diffusion. Journal of Contaminant Hydrology, 247, Article 103986. [https://doi.org/10.1016/j.jconhyd.2022.103986 doi: 10.1016/j.jconhyd.2022.103986]  [[Media: KulkarniEtAl2022.pdf | Open Access Manuscript]]</ref> to quantify abiotic dechlorination impacts within clay aquitards on chlorinated solvent plumes. Thus, determination of the abiotic reductive dechlorination rate constant for a particular clayey soil can be readily utilized to provide a more accurate assessment of aquifer cleanup timeframes for groundwater plumes that are being sustained by contaminant back-diffusion.

==Recommended Approach==
[[File: TranFig1.png | thumb | 500 px | Figure 1: First-order rate constants for abiotic reductive dechlorination of TCE under anaerobic conditions. Circles are data from Schaefer ''et al.'', 2021<ref>Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2021. Abiotic dechlorination in the presence of ferrous minerals. Journal of Contaminant Hydrology, 241, 103839. [https://doi.org/10.1016/j.jconhyd.2021.103839 doi: 10.1016/j.jconhyd.2021.103839]  [[Media: SchaeferEtAl2021.pdf | Open Access Manuscript]]</ref>, filled squares from Schaefer ''et al.'', 2018<ref name="SchaeferEtAl2018"/>, and Schaefer ''et al.'', 2017<ref>Schaefer, C.E., Ho., Gurr, C., Berns, E., Werth, C., 2017. Abiotic dechlorination of chlorinated ethenes in natural clayey soils: impacts of mineralogy and temperature. Journal of Contaminant Hydrology, 206, pp. 10-17. [https://doi.org/10.1016/j.jconhyd.2017.09.007 doi: 10.1016/j.jconhyd.2017.09.007]  [[Media: SchaeferEtAl2017.pdf | Open Access Manuscript]]</ref>, and open squares from Schaefer ''et al.'', 2025<ref name="SchaeferEtAl2025"/>. ]]
[[File: TranFig2.png | thumb | 600 px | Figure 2: Flowchart diagram of field screening procedures]]
The recommended approach builds upon the methodology and findings of a recent study<ref name="SchaeferEtAl2025">Schaefer, C.E., Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils. Groundwater Monitoring and Remediation, 45(2), pp. 31-39. [https://doi.org/10.1111/gwmr.12709 doi: 10.1111/gwmr.12709]</ref>, emphasizing field-based and analytical techniques to quantify abiotic first-order reductive dechlorination rate constants for PCE and TCE in clayey soils under anoxic conditions. Key components of this evaluation are listed below:
#Zone Identification: The focus of the investigation should be to delineate clayey zones adjacent to hydraulically conductive zones.
#Ferrous Mineral Quantification: Assess ferrous mineral context in clay via 1% HCl extraction at ambient temperature over a 10-minute interval.
#Mineralogical Characterization: Conduct XRD analysis with the specific intent of identifying the presence of pyrite and biotite.
#Reduced Gas Analysis: Measurement of reduced gases such as acetylene, ethene, and ethane concentrations in clay samples. Gas-tight sampling devices (e.g., En Core® soil samplers by En Novative Technologies, Inc.) should be used to ensure sample integrity during collection and transport.

Clay samples should be collected within a few centimeters of the high-permeability interface, with optional additional sampling further inward. For mineralogical analysis, a defined interval may be collected and subsequently subsampled. To preserve sample integrity, exposure to air should be minimized during collection, transport, and handling. Homogenization should occur within an anaerobic chamber, and if subsamples are required for external analysis, they must be shipped in gas-tight, anaerobic containers.

Estimation of the abiotic reductive first-order rate constant for PCE and TCE is based on the “reactive” ferrous content in the clay. Reactive ferrous content (Fe(II)r) is estimated as shown in Equation 1:

::'''Equation 1:'''       <big>''Fe(II)r = DA + XRDpyr - XRDbiotite''</big>

where ''DA'' is the ferrous content from the dilute acid (1% HCl) extraction, ''XRDpyr'' is the pyrite content from XRD analysis, and ''XRDbiotite'' is the biotite content from XRD analysis<ref name="SchaeferEtAl2025"/>.

Abiotic dechlorination is unlikely to contribute to mitigating contaminant back-diffusion when reactive ferrous iron (Fe(II)r) concentrations are below 100 mg/kg (Figure 1). For Fe(II)r above 100 mg/kg, the first-order rate constant for PCE and TCE reductive dechlorination can be estimated using the correlation shown in Figure 1<ref name="SchaeferEtAl2018">Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2018. Mechanisms for abiotic dechlorination of trichloroethene by ferrous minerals under oxic and anoxic conditions in natural sediments. Environmental Science and Technology, 52(23), pp.13747-13755. [https://doi.org/10.1021/acs.est.8b04108 doi: 10.1021/acs.est.8b04108]</ref><ref>Borden, R.C., Cha, K.Y., 2021. Evaluating the impact of back diffusion on groundwater cleanup time. Journal of Contaminant Hydrology, 243, Article 103889. [https://doi.org/10.1016/j.jconhyd.2021.103889 doi: 10.1016/j.jconhyd.2021]  [[Media: BordenCha2021.pdf | Open Access Manuscript]]</ref>. The rate constant exhibits a strong positive correlation with the logarithm of reactive Fe(II) content (Pearson’s ''r'' = 0.82), with a slope of 4.7 × 10⁻⁸ L g⁻¹ d⁻¹ (log mg kg⁻¹)⁻¹.

Figure 2 presents a decision flowchart designed to evaluate the significance and extent of abiotic reductive dechlorination. By applying Equation 1 to the dilute acid extractable Fe(II) plus measured mineral species data from clay samples, the reactive ferrous iron content (Fe(II)r) can be quantified, enabling a streamlined assessment of the extent to which abiotic processes are contributing to the mitigation of contaminant back-diffusion.

If Fe(II)r is ≥ 100 mg/kg, a first-order dechlorination rate constant can be estimated and subsequently used within a contaminant fate and transport model. However, if acetylene is detected in the clay, even with Fe(II)r less than 100 mg/kg, then bench-scale testing using methods similar to those described in a recent study<ref name="SchaeferEtAl2025"/> is recommended, as such results would likely be inconsistent with those shown in Figure 1, suggesting some other mechanism might be involved, or that the system mineralogy might be more complex than anticipated. Even if Fe(II)r ≥ 100 mg/kg, confirmatory bench-scale testing may be conducted for additional verification and to refine estimation of the abiotic dechlorination rate constant.

==Summary and Recommendations==
The approach outlined above is intended to serve as a generalized guide for practitioners and site managers to cost-effectively determine the extent to which beneficial abiotic reductive dechlorination reactions are likely occurring in low permeability (e.g., clayey) zones. This approach may be contraindicated if co-contaminants are present. It is currently unclear whether other classes of potentially reactive chemicals, such as trinitrotoluene (TNT) or chlorinated ethanes, could interact competitively with PCE and TCE.

In addition, it remains unclear how other classes of compounds such as per- and polyfluoroalkyl substances (PFAS) may interact or sorb with ferrous minerals and potentially inhibit abiotic dechlorination reactions. Coupling these recommended activities with conventional site investigation tasks would provide an opportunity to perform many of the up-front screening activities with minimal additional project costs. It is important to note that the guidance proposed herein pertains to particularly low permeability media. Sites with complex or varying lithology, where the mineralogy and/or redox conditions may vary, might require evaluation of multiple samples to provide appropriate site-wide information.
 

==References==
<references />

==See Also==
*[https://serdp-estcp.mil/projects/details/a7e3f7b5-ed82-4591-adaa-6196ff33dd60 ESTCP Project ER20-5031 – In Situ Verification and Quantification of Naturally Occurring Dechlorination Rates in Clays: Demonstrating Processes that Mitigate Back-Diffusion and Plume Persistence]

Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions

2026-04-28T20:17:57Z

Admin:

The U.S. Department of Defense (DoD) faces many challenges in restoring aquifers at contaminated sites, often due to back-diffusion of tetrachloroethene (PCE) and trichloroethene (TCE) from low-permeability clay zones. The uptake, storage, and subsequent long-term release of these dissolved contaminants from clays are key processes in understanding the longevity, intensity, and risks associated with many persistent chlorinated ethene groundwater plumes. Although naturally occurring abiotic and biotic dechlorination processes in clays may reduce stored contaminant mass and significantly aid natural attenuation, no standardized field method currently exists to verify or quantify these reactions. It is critical to remediation design efforts to demonstrate and validate a cost-effective ''in situ'' approach for assessing these dechlorination processes using first-order rate constants. An approach was developed and applied across eight DoD sites to support Remedial Project Managers (RPMs) and regulators in evaluating natural attenuation potential in clay-rich environments.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Chlorinated Solvents]]
*[[Monitored Natural Attenuation (MNA)]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
*[[Matrix Diffusion]]
*[[REMChlor - MD]]

'''Contributors:''' Dani Tran, [[Dr. Charles Schaefer]], Dr. Charles Werth

'''Key Resource:'''
*Schaefer, C.E, Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils<ref name="SchaeferEtAl2025"/>

==Introduction==
Cost-effective methods are needed to verify the occurrence of natural dechlorination processes and quantify their dechlorination rates in clays under ambient in situ conditions in order to reliably predict their long-term influence on plume longevity and mass discharge. However, accurately determining these rates is challenging due to slow reaction kinetics, the transient nature of transformation products, and the interplay of biotic and abiotic mechanisms within the clay matrix or at clay-sand interfaces. Tools capable of quantifying these reactions and assessing their role in mitigating plume persistence would be a significant aid for long-term site management.

For reductive abiotic dechlorination under anoxic conditions, a 1% hydrochloric acid (HCl) extraction of a sample of native clay coupled with X-ray diffraction (XRD) data can be used as a screening level tool to estimate reductive dechlorination rate constants. These rate constants can be inserted into fate and transport models such as [[REMChlor - MD]]<ref>Falta, R., and Wang, W., 2017. A semi-analytical method for simulating matrix diffusion in numerical transport models. Journal of Contaminant Hydrology, 197, pp. 39-49. [https://doi.org/10.1016/j.jconhyd.2016.12.007 doi: 10.1016/j.jconhyd.2016.12.007]  [[Media: FaltaWang2017.pdf | Open Access Manuscript]]</ref><ref>Kulkarni, P.R., Adamson, D.T., Popovic, J., Newell, C.J., 2022. Modeling a well-charactized perfluorooctane sulfate (PFOS) source and plume using the REMChlor-MD model to account for matrix diffusion. Journal of Contaminant Hydrology, 247, Article 103986. [https://doi.org/10.1016/j.jconhyd.2022.103986 doi: 10.1016/j.jconhyd.2022.103986]  [[Media: KulkarniEtAl2022.pdf | Open Access Manuscript]]</ref> to quantify abiotic dechlorination impacts within clay aquitards on chlorinated solvent plumes. Thus, determination of the abiotic reductive dechlorination rate constant for a particular clayey soil can be readily utilized to provide a more accurate assessment of aquifer cleanup timeframes for groundwater plumes that are being sustained by contaminant back-diffusion.

==Recommended Approach==
[[File: TranFig1.png | thumb | 500 px | Figure 1: First-order rate constants for abiotic reductive dechlorination of TCE under anaerobic conditions. Circles are data from Schaefer ''et al.'', 2021<ref>Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2021. Abiotic dechlorination in the presence of ferrous minerals. Journal of Contaminant Hydrology, 241, 103839. [https://doi.org/10.1016/j.jconhyd.2021.103839 doi: 10.1016/j.jconhyd.2021.103839]  [[Media: SchaeferEtAl2021.pdf | Open Access Manuscript]]</ref>, filled squares from Schaefer ''et al.'', 2018<ref name="SchaeferEtAl2018"/>, and Schaefer ''et al.'', 2017<ref>Schaefer, C.E., Ho., Gurr, C., Berns, E., Werth, C., 2017. Abiotic dechlorination of chlorinated ethenes in natural clayey soils: impacts of mineralogy and temperature. Journal of Contaminant Hydrology, 206, pp. 10-17. [https://doi.org/10.1016/j.jconhyd.2017.09.007 doi: 10.1016/j.jconhyd.2017.09.007]  [[Media: SchaeferEtAl2017.pdf | Open Access Manuscript]]</ref>, and open squares from Schaefer ''et al.'', 2025<ref name="SchaeferEtAl2025"/>. ]]
[[File: TranFig2.png | thumb | 600 px | Figure 2: Flowchart diagram of field screening procedures]]
The recommended approach builds upon the methodology and findings of a recent study<ref name="SchaeferEtAl2025">Schaefer, C.E., Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils. Groundwater Monitoring and Remediation, 45(2), pp. 31-39. [https://doi.org/10.1111/gwmr.12709 doi: 10.1111/gwmr.12709]</ref>, emphasizing field-based and analytical techniques to quantify abiotic first-order reductive dechlorination rate constants for PCE and TCE in clayey soils under anoxic conditions. Key components of this evaluation are listed below:
#Zone Identification: The focus of the investigation should be to delineate clayey zones adjacent to hydraulically conductive zones.
#Ferrous Mineral Quantification: Assess ferrous mineral context in clay via 1% HCl extraction at ambient temperature over a 10-minute interval.
#Mineralogical Characterization: Conduct XRD analysis with the specific intent of identifying the presence of pyrite and biotite.
#Reduced Gas Analysis: Measurement of reduced gases such as acetylene, ethene, and ethane concentrations in clay samples. Gas-tight sampling devices (e.g., En Core® soil samplers by En Novative Technologies, Inc.) should be used to ensure sample integrity during collection and transport.

Clay samples should be collected within a few centimeters of the high-permeability interface, with optional additional sampling further inward. For mineralogical analysis, a defined interval may be collected and subsequently subsampled. To preserve sample integrity, exposure to air should be minimized during collection, transport, and handling. Homogenization should occur within an anaerobic chamber, and if subsamples are required for external analysis, they must be shipped in gas-tight, anaerobic containers.

Estimation of the abiotic reductive first-order rate constant for PCE and TCE is based on the “reactive” ferrous content in the clay. Reactive ferrous content (Fe(II)r) is estimated as shown in Equation 1:

::'''Equation 1:'''       <big>''Fe(II)r = DA + XRDpyr - XRDbiotite''</big>

where ''DA'' is the ferrous content from the dilute acid (1% HCl) extraction, ''XRDpyr'' is the pyrite content from XRD analysis, and ''XRDbiotite'' is the biotite content from XRD analysis<ref name="SchaeferEtAl2025"/>.

Abiotic dechlorination is unlikely to contribute to mitigating contaminant back-diffusion when reactive ferrous iron (Fe(II)r) concentrations are below 100 mg/kg (Figure 1). For Fe(II)r above 100 mg/kg, the first-order rate constant for PCE and TCE reductive dechlorination can be estimated using the correlation shown in Figure 1<ref name="SchaeferEtAl2018">Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2018. Mechanisms for abiotic dechlorination of trichloroethene by ferrous minerals under oxic and anoxic conditions in natural sediments. Environmental Science and Technology, 52(23), pp.13747-13755. [https://doi.org/10.1021/acs.est.8b04108 doi: 10.1021/acs.est.8b04108]</ref><ref>Borden, R.C., Cha, K.Y., 2021. Evaluating the impact of back diffusion on groundwater cleanup time. Journal of Contaminant Hydrology, 243, Article 103889. [https://doi.org/10.1016/j.jconhyd.2021.103889 doi: 10.1016/j.jconhyd.2021]  [[Media: BordenCha2021.pdf | Open Access Manuscript]]</ref>. The rate constant exhibits a strong positive correlation with the logarithm of reactive Fe(II) content (Pearson’s ''r'' = 0.82), with a slope of 4.7 × 10⁻⁸ L g⁻¹ d⁻¹ (log mg kg⁻¹)⁻¹.

Figure 2 presents a decision flowchart designed to evaluate the significance and extent of abiotic reductive dechlorination. By applying Equation 1 to the dilute acid extractable Fe(II) plus measured mineral species data from clay samples, the reactive ferrous iron content (Fe(II)r) can be quantified, enabling a streamlined assessment of the extent to which abiotic processes are contributing to the mitigation of contaminant back-diffusion.

If Fe(II)r is ≥ 100 mg/kg, a first-order dechlorination rate constant can be estimated and subsequently used within a contaminant fate and transport model. However, if acetylene is detected in the clay, even with Fe(II)r less than 100 mg/kg, then bench-scale testing using methods similar to those described in a recent study<ref name="SchaeferEtAl2025"/> is recommended, as such results would likely be inconsistent with those shown in Figure 1, suggesting some other mechanism might be involved, or that the system mineralogy might be more complex than anticipated. Even if Fe(II)r ≥ 100 mg/kg, confirmatory bench-scale testing may be conducted for additional verification and to refine estimation of the abiotic dechlorination rate constant.

==Summary and Recommendations==
The approach outlined above is intended to serve as a generalized guide for practitioners and site managers to cost-effectively determine the extent to which beneficial abiotic reductive dechlorination reactions are likely occurring in low permeability (e.g., clayey) zones. This approach may be contraindicated if co-contaminants are present. It is currently unclear whether other classes of potentially reactive chemicals, such as trinitrotoluene (TNT) or chlorinated ethanes, could interact competitively with PCE and TCE.

In addition, it remains unclear how other classes of compounds such as per- and polyfluoroalkyl substances (PFAS) may interact or sorb with ferrous minerals and potentially inhibit abiotic dechlorination reactions. Coupling these recommended activities with conventional site investigation tasks would provide an opportunity to perform many of the up-front screening activities with minimal additional project costs. It is important to note that the guidance proposed herein pertains to particularly low permeability media. Sites with complex or varying lithology, where the mineralogy and/or redox conditions may vary, might require evaluation of multiple samples to provide appropriate site-wide information.
 

==References==
<references />

==See Also==
*[https://serdp-estcp.mil/projects/details/a7e3f7b5-ed82-4591-adaa-6196ff33dd60 ESTCP Project ER20-5031 – In Situ Verification and Quantification of Naturally Occurring Dechlorination Rates in Clays: Demonstrating Processes that Mitigate Back-Diffusion and Plume Persistence]

Dr. Charles Schaefer

2026-04-28T20:15:17Z

Admin:

==Work and Contact Information==

EMPLOYER:
:CDM Smith 
:110 Fieldcrest Avenue
:6th Floor 
:Edison, NJ 08837

EMAIL: [mailto:schaeferce@cdmsmith.com schaeferce@cdmsmith.com]

WEBPAGE: https://www.cdmsmith.com/en/experts/charles-schaefer

==About the Contributor==
Charles Schaefer is a chemical engineer with over 25 years of years of experience in laboratory and field evaluations of contaminant transport in subsurface systems and engineered water systems. Dr. Schaefer is the director of CDM Smith’s Research and Testing laboratory located in Bellevue, WA

==Article Contributions==
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]

__NOTOC__

[[Category: Contributors|Schaefer]]

Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions

2026-04-28T20:14:28Z

Admin:

The U.S. Department of Defense (DoD) faces many challenges in restoring aquifers at contaminated sites, often due to back-diffusion of tetrachloroethene (PCE) and trichloroethene (TCE) from low-permeability clay zones. The uptake, storage, and subsequent long-term release of these dissolved contaminants from clays are key processes in understanding the longevity, intensity, and risks associated with many persistent chlorinated ethene groundwater plumes. Although naturally occurring abiotic and biotic dechlorination processes in clays may reduce stored contaminant mass and significantly aid natural attenuation, no standardized field method currently exists to verify or quantify these reactions. It is critical to remediation design efforts to demonstrate and validate a cost-effective ''in situ'' approach for assessing these dechlorination processes using first-order rate constants. An approach was developed and applied across eight DoD sites to support Remedial Project Managers (RPMs) and regulators in evaluating natural attenuation potential in clay-rich environments.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Monitored Natural Attenuation (MNA)]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
*[[Matrix Diffusion]]
*[[REMChlor - MD]]

'''Contributors:''' Dani Tran, [[Dr. Charles Schaefer]], Dr. Charles Werth

'''Key Resource:'''
*Schaefer, C.E, Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils<ref name="SchaeferEtAl2025"/>

==Introduction==
Cost-effective methods are needed to verify the occurrence of natural dechlorination processes and quantify their dechlorination rates in clays under ambient in situ conditions in order to reliably predict their long-term influence on plume longevity and mass discharge. However, accurately determining these rates is challenging due to slow reaction kinetics, the transient nature of transformation products, and the interplay of biotic and abiotic mechanisms within the clay matrix or at clay-sand interfaces. Tools capable of quantifying these reactions and assessing their role in mitigating plume persistence would be a significant aid for long-term site management.

For reductive abiotic dechlorination under anoxic conditions, a 1% hydrochloric acid (HCl) extraction of a sample of native clay coupled with X-ray diffraction (XRD) data can be used as a screening level tool to estimate reductive dechlorination rate constants. These rate constants can be inserted into fate and transport models such as [[REMChlor - MD]]<ref>Falta, R., and Wang, W., 2017. A semi-analytical method for simulating matrix diffusion in numerical transport models. Journal of Contaminant Hydrology, 197, pp. 39-49. [https://doi.org/10.1016/j.jconhyd.2016.12.007 doi: 10.1016/j.jconhyd.2016.12.007]  [[Media: FaltaWang2017.pdf | Open Access Manuscript]]</ref><ref>Kulkarni, P.R., Adamson, D.T., Popovic, J., Newell, C.J., 2022. Modeling a well-charactized perfluorooctane sulfate (PFOS) source and plume using the REMChlor-MD model to account for matrix diffusion. Journal of Contaminant Hydrology, 247, Article 103986. [https://doi.org/10.1016/j.jconhyd.2022.103986 doi: 10.1016/j.jconhyd.2022.103986]  [[Media: KulkarniEtAl2022.pdf | Open Access Manuscript]]</ref> to quantify abiotic dechlorination impacts within clay aquitards on chlorinated solvent plumes. Thus, determination of the abiotic reductive dechlorination rate constant for a particular clayey soil can be readily utilized to provide a more accurate assessment of aquifer cleanup timeframes for groundwater plumes that are being sustained by contaminant back-diffusion.

==Recommended Approach==
[[File: TranFig1.png | thumb | 500 px | Figure 1: First-order rate constants for abiotic reductive dechlorination of TCE under anaerobic conditions. Circles are data from Schaefer ''et al.'', 2021<ref>Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2021. Abiotic dechlorination in the presence of ferrous minerals. Journal of Contaminant Hydrology, 241, 103839. [https://doi.org/10.1016/j.jconhyd.2021.103839 doi: 10.1016/j.jconhyd.2021.103839]  [[Media: SchaeferEtAl2021.pdf | Open Access Manuscript]]</ref>, filled squares from Schaefer ''et al.'', 2018<ref name="SchaeferEtAl2018"/>, and Schaefer ''et al.'', 2017<ref>Schaefer, C.E., Ho., Gurr, C., Berns, E., Werth, C., 2017. Abiotic dechlorination of chlorinated ethenes in natural clayey soils: impacts of mineralogy and temperature. Journal of Contaminant Hydrology, 206, pp. 10-17. [https://doi.org/10.1016/j.jconhyd.2017.09.007 doi: 10.1016/j.jconhyd.2017.09.007]  [[Media: SchaeferEtAl2017.pdf | Open Access Manuscript]]</ref>, and open squares from Schaefer ''et al.'', 2025<ref name="SchaeferEtAl2025"/>. ]]
[[File: TranFig2.png | thumb | 600 px | Figure 2: Flowchart diagram of field screening procedures]]
The recommended approach builds upon the methodology and findings of a recent study<ref name="SchaeferEtAl2025">Schaefer, C.E., Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils. Groundwater Monitoring and Remediation, 45(2), pp. 31-39. [https://doi.org/10.1111/gwmr.12709 doi: 10.1111/gwmr.12709]</ref>, emphasizing field-based and analytical techniques to quantify abiotic first-order reductive dechlorination rate constants for PCE and TCE in clayey soils under anoxic conditions. Key components of this evaluation are listed below:
#Zone Identification: The focus of the investigation should be to delineate clayey zones adjacent to hydraulically conductive zones.
#Ferrous Mineral Quantification: Assess ferrous mineral context in clay via 1% HCl extraction at ambient temperature over a 10-minute interval.
#Mineralogical Characterization: Conduct XRD analysis with the specific intent of identifying the presence of pyrite and biotite.
#Reduced Gas Analysis: Measurement of reduced gases such as acetylene, ethene, and ethane concentrations in clay samples. Gas-tight sampling devices (e.g., En Core® soil samplers by En Novative Technologies, Inc.) should be used to ensure sample integrity during collection and transport.

Clay samples should be collected within a few centimeters of the high-permeability interface, with optional additional sampling further inward. For mineralogical analysis, a defined interval may be collected and subsequently subsampled. To preserve sample integrity, exposure to air should be minimized during collection, transport, and handling. Homogenization should occur within an anaerobic chamber, and if subsamples are required for external analysis, they must be shipped in gas-tight, anaerobic containers.

Estimation of the abiotic reductive first-order rate constant for PCE and TCE is based on the “reactive” ferrous content in the clay. Reactive ferrous content (Fe(II)r) is estimated as shown in Equation 1:

::'''Equation 1:'''       <big>''Fe(II)r = DA + XRDpyr - XRDbiotite''</big>

where ''DA'' is the ferrous content from the dilute acid (1% HCl) extraction, ''XRDpyr'' is the pyrite content from XRD analysis, and ''XRDbiotite'' is the biotite content from XRD analysis<ref name="SchaeferEtAl2025"/>.

Abiotic dechlorination is unlikely to contribute to mitigating contaminant back-diffusion when reactive ferrous iron (Fe(II)r) concentrations are below 100 mg/kg (Figure 1). For Fe(II)r above 100 mg/kg, the first-order rate constant for PCE and TCE reductive dechlorination can be estimated using the correlation shown in Figure 1<ref name="SchaeferEtAl2018">Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2018. Mechanisms for abiotic dechlorination of trichloroethene by ferrous minerals under oxic and anoxic conditions in natural sediments. Environmental Science and Technology, 52(23), pp.13747-13755. [https://doi.org/10.1021/acs.est.8b04108 doi: 10.1021/acs.est.8b04108]</ref><ref>Borden, R.C., Cha, K.Y., 2021. Evaluating the impact of back diffusion on groundwater cleanup time. Journal of Contaminant Hydrology, 243, Article 103889. [https://doi.org/10.1016/j.jconhyd.2021.103889 doi: 10.1016/j.jconhyd.2021]  [[Media: BordenCha2021.pdf | Open Access Manuscript]]</ref>. The rate constant exhibits a strong positive correlation with the logarithm of reactive Fe(II) content (Pearson’s ''r'' = 0.82), with a slope of 4.7 × 10⁻⁸ L g⁻¹ d⁻¹ (log mg kg⁻¹)⁻¹.

Figure 2 presents a decision flowchart designed to evaluate the significance and extent of abiotic reductive dechlorination. By applying Equation 1 to the dilute acid extractable Fe(II) plus measured mineral species data from clay samples, the reactive ferrous iron content (Fe(II)r) can be quantified, enabling a streamlined assessment of the extent to which abiotic processes are contributing to the mitigation of contaminant back-diffusion.

If Fe(II)r is ≥ 100 mg/kg, a first-order dechlorination rate constant can be estimated and subsequently used within a contaminant fate and transport model. However, if acetylene is detected in the clay, even with Fe(II)r less than 100 mg/kg, then bench-scale testing using methods similar to those described in a recent study<ref name="SchaeferEtAl2025"/> is recommended, as such results would likely be inconsistent with those shown in Figure 1, suggesting some other mechanism might be involved, or that the system mineralogy might be more complex than anticipated. Even if Fe(II)r ≥ 100 mg/kg, confirmatory bench-scale testing may be conducted for additional verification and to refine estimation of the abiotic dechlorination rate constant.

==Summary and Recommendations==
The approach outlined above is intended to serve as a generalized guide for practitioners and site managers to cost-effectively determine the extent to which beneficial abiotic reductive dechlorination reactions are likely occurring in low permeability (e.g., clayey) zones. This approach may be contraindicated if co-contaminants are present. It is currently unclear whether other classes of potentially reactive chemicals, such as trinitrotoluene (TNT) or chlorinated ethanes, could interact competitively with PCE and TCE.

In addition, it remains unclear how other classes of compounds such as per- and polyfluoroalkyl substances (PFAS) may interact or sorb with ferrous minerals and potentially inhibit abiotic dechlorination reactions. Coupling these recommended activities with conventional site investigation tasks would provide an opportunity to perform many of the up-front screening activities with minimal additional project costs. It is important to note that the guidance proposed herein pertains to particularly low permeability media. Sites with complex or varying lithology, where the mineralogy and/or redox conditions may vary, might require evaluation of multiple samples to provide appropriate site-wide information.
 

==References==
<references />

==See Also==
*[https://serdp-estcp.mil/projects/details/a7e3f7b5-ed82-4591-adaa-6196ff33dd60 ESTCP Project ER20-5031 – In Situ Verification and Quantification of Naturally Occurring Dechlorination Rates in Clays: Demonstrating Processes that Mitigate Back-Diffusion and Plume Persistence]

Dr. Stephen Richardson

2026-04-28T20:10:32Z

Admin:

==Work and Contact Information==
EMPLOYER:
:GSI Environmental
:9600 Great Hills Trail, Suite 350E
:Austin, TX 78759

EMAIL: [mailto:sdrichardson@gsi-net.com sdrichardson@gsi-net.com]

WEBPAGE: http://www.gsi-net.com/en/people/employees/stephen-d-richardson-ph-d-p-e.html

==About the Contributor==
Dr. Stephen Richardson is a Vice President and Principal Engineer with GSI Environmental in Austin, Texas. Stephen specializes in the application of innovative strategies to treat conventional and emerging contaminants in soil, groundwater, and surface water at a wide range of contaminated sites. He has served as a Principal Investigator on several DoD-sponsored research projects on cometabolic biodegradation of 1,4-Dioxane, innovative approaches for treatment of chlorinated solvents in low permeability zones, anaerobic bioremediation of DNAPL, and treatment of per- and polyfluoroalkyl substances. Stephen has authored more than 15 peer-reviewed journal articles on ''in situ'' bioremediation, PFAS remediation, chemical oxidation, cosolvent flushing, decentralized water treatment, contaminant bioavailability, and water chemistry in areas of oil and gas development. Stephen is a Licensed Professional Engineer in Texas, Louisiana, North Carolina, and Alberta, Canada and holds a doctoral degree in environmental engineering from the University of North Carolina at Chapel Hill, a master’s degree from Louisiana State University, and a bachelor’s degree from the University of Waterloo.

==Article Contributions==
*[[Polycyclic Aromatic Hydrocarbons (PAHs)]]
*[[Amendment Distribution in Low Conductivity Materials]]
*[[PFAS Treatment by Electrical Discharge Plasma]]

__NOTOC__

[[Category: Contributors|Richardson]]

MediaWiki:Sidebar

2026-04-04T00:48:35Z

Admin:

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Articles

2026-03-13T21:31:29Z

Admin:

{| class="wikitable sortable"
|-
!Title!!First Author!!Linking Phrases
|-
|[[Groundwater Sampling - No-Purge/Passive]]||[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]||passive sampling, no purge sampling, grab samplers, diffusion samplers, sorptive samplers
|-
|[[ Long-Term Monitoring (LTM)]]||[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]||long-term monitoring, LTM, LTM objectives, LTM programs, LTM challenges
|-
|[[PFAS Monitored Retention (PMR) and PFAS Enhanced Retention (PER)]]
|[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]
|PFAS, MNA, natural attenuation
|-
|[[Sorption of Organic Contaminants]]||[[Richelle Allen-King|Allen-King, Richelle]]||
|-
|[[PFAS Transport and Fate]]
|[[Dr. Richard Anderson|Anderson, Richard, Ph.D.]]
|PFAS, fate and transport
|-
|[[Mass Flux and Mass Discharge]]||[[Dr. Michael Annable, P.E. |Annable, Michael, Ph.D., P.E.]]||source reduction
|-
|[[PFAS Toxicology and Risk Assessment]]
|[[Jennifer Arblaster|Arblaster, Jennifer]]
|PFAS, toxicology, risk assessment
|-
|[[Metal(loid)s - Small Arms Ranges]]|| Dr. Amanda Barker |[[Dr. Amanda Barker|Barker, Amanda, Ph.D.]]
|
|-
|[[Munitions Constituents – Photolysis|Munitions Constituents - Photolysis]]
|[[Dr. Warren Kadoya|Kadoya, Warren, Ph.D.]]
|
|-
|[[Munitions Constituents - Soil Sampling]]
|[[Dr. Samuel Beal|Beal, Samuel, Ph.D.]]
|
|-
|[[Vapor Intrusion – Sewers and Utility Tunnels as Preferential Pathways|Vapor Intrusion - Sewers and Utility Tunnels as Preferential Pathways]]
|[[Lila Beckley|Beckley, Lila]]
|vapor intrusion
|-
|[[Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill]]
|[[Dr. Barbara Bekins|Bekins, Barbara, Ph.D.]]||
|-
|[[Bioremediation - Anaerobic Secondary Water Quality Impacts]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||secondary impacts, water quality (in regards to anaerobic conditions)
|-
|[[Design Tool - Base Addition for ERD]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||aquifer acidity, base addition
|-
|[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation]]
|[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||
|-
|[[Low pH Inhibition of Reductive Dechlorination]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||low pH inhibition
|-
|[[In Situ Toxicity Identification Evaluation (iTIE)]]||Burton, Allen, P.E.||toxicity evaluation
|-
|[[OPTically-based In-situ Characterization System (OPTICS)]]
|[[Dr. Grace Chang|Chang, Grace, Ph.D.]]
|
|-
|[[Munitions Constituents - Electrochemical Treatment]]
|[[Dr. Brian P. Chaplin|Chaplin, Brian, Ph.D.]]
|munitions constituents remediation
|-
|[[PFAS Sources]]
|[[Dr. Dora Chiang|Chiang, Dora, Ph.D.]]
|
|-
|[[Biodegradation - Cometabolic]]||[[Dr. Kung-Hui (Bella) Chu |Chu, Kung-Hui (Bella), Ph.D]]||cometabolic biodegradation
|-
|[[Munitions Constituents - Composting]]
|[[Harry Craig|Craig, Harry]]
|
|-
|[[Chemical Oxidation (In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||ISCO, chemical oxidation
|-
|[[Chemical Oxidation Oxidant Selection (In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||chemical oxidant, oxidant (in regards to ISCO)
|-
|[[Chemical Oxidation Design Considerations(In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||screening, design, implementation, oxidant delivery (in regards to ISCO)
|-
|[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]||[[Dr. Rula Deeb |Deeb, Rula, Ph.D.]]||PFAS, perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS)
|-
|[[Metal and Metalloid Contaminants]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||metal contaminant(s), metalloid contaminant(s), metal(s), metalloid(s),
|-
|[[Metals and Metalloids - Mobility in Groundwater]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||metal mobility, aqueous speciation, adsorption, precipitation, colloidal transport (in regards to metals)
|-
|[[Monitored Natural Attenuation (MNA) of Metal and Metalloids]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||MNA, attenuation of metal(s), natural attenuation processes, attenuation (in regards to metals)
|-
|[[Metal and Metalloids - Remediation]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||remediation, in situ technologies, contaminant removal (in regards to metals)
|-
|[[pH Buffering in Aquifers]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||ph buffer, natural pH buffer, engineered pH buffer
|-
|[[Munitions Constituents - Sorption]]||[[Dr. Katerina Dontsova |Dontsova, Katerina, Ph.D.]]||energetics, sorption
|-
|[[Biodegradation - Hydrocarbons]]||[[Dr. Elizabeth Edwards |Edwards, Elizabeth, Ph.D.]]||hydrocarbon, biodegradation
|-
|[[Source Zone Modeling]]||[[Dr. Ron Falta |Falta, Ron, Ph.D.]]||source zone modeling
|-
|[[Plume Response Modeling]]||[[Dr. Ron Falta |Falta, Ron, Ph.D.]]||plume response modeling
|-
|[[REMChlor - MD]]
|[[Dr. Ron Falta|Falta, Ron, Ph.D.]]
|
|-
|[[In Situ Groundwater Treatment with Activated Carbon]]
|[[Dr. Dimin Fan|Fan, Dimin, Ph.D.]]
|
|-
|[[LNAPL Remediation Technologies]]||[[Dr. Shahla Farhat |Farhat, Shahla, Ph.D.]]||
|-
|[[Sustainable Remediation]]||[[Paul Favara |Favara, Paul]]||sustainable remediation, social, economic and environmental impacts
|-
|[[Munitions Constituents]]||[[Dr. Kevin Finneran |Finneran, Kevin, Ph.D.]]||Explosives, energetics, insensitive munitions
|-
|[[Biodegradation - Reductive Processes]]||[[Dr. David Freedman |Freedman, David, Ph.D.]]||biotic reduction, biotic reductive processes, hydrogenolysis, dihaloelimination, coupling, organohalide respiration
|-
|[[Remediation of Stormwater Runoff Contaminated by Munition Constituents|Munitions Constituents - Remediation of Stormwater Runoff]]||Fuller, Mark, Ph.D.||energetics, insensitive munitions, stormwater runoff
|-
|[[Subgrade Biogeochemical Reactor (SBGR)]]||[[Jeff Gamlin |Gamlin, Jeff]]||SBGR, subgrade biogeochemical reactor, bioreactor
|-
|[[Thermal Remediation - Smoldering]]||[[Dr. Jason Gerhard |Gerhard, Jason, Ph.D.]]||smouldering remediation, self-sustaining treatment for active remediation, STAR
|-
|[[Contaminated Sediments - Introduction]]
|[[Dr. Upal Ghosh|Ghosh, Upal, Ph.D.]]
|
|-
|[[Contaminated Sediment Risk Assessment]]
|[[Richard Wenning|Wenning, Richard]]
|
|-
|[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
|[[Dr. Upal Ghosh|Ghosh, Upal, Ph.D.]]
|
|-
|[[PFAS Ex Situ Water Treatment]]
|[[Dr. Scott Grieco |Grieco, Scott, Ph.D.]]
|
|-
|[[Stream Restoration]]
|[[Dr. Natalie Griffiths|Griffiths, Natalie, Ph.D.]]
|
|-
|[[Passive Sampling of Sediments]]
|[[Dr. Philip M. Gschwend|Gschwend, Philip]]
|
|-
|[[Phytoplankton (Algae) Blooms]]
|[[Dr. Nathan Hall|Hall, Nathan]]
|
|-
|[[PFAS Soil Remediation Technologies]]||[[James_Hatton |Hatton, Jim]]||PFAS, Soil source zones
|-
|[[Proteomics and Proteogenomics]]
|[[Dr. Kate Kucharzyk|Kucharzyk, Kate, Ph.D.]]
|
|-
|[[N-nitrosodimethylamine (NDMA)]]
|[[Paul Hatzinger|Hatzinger, Paul, Ph.D.]]
|
|-
|[[Alternative Endpoints]]||[[Elisabeth Hawley |Hawley, Elisabeth]]||management of complex sites||
|-
|[[Thermal Remediation]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation, in situ thermal
|-
|[[Thermal Remediation - Steam]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||Steam Enhanced Extraction
|-
|[[Thermal Remediation - Electrical Resistance Heating]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||Electrical Resistance Heating
|-
|[[Thermal Conduction Heating (TCH)]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal desorption
|-
|[[Thermal Remediation - Combined Remedies]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation
|-
|[[Thermal Conduction Heating for Treatment of PFAS-Impacted Soil]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation, PFAS
|-
|[[Predicting Species Responses to Climate Change with Population Models]]
|[[Dr. Brian Hudgens|Hudgens, Brian, Ph.D.]]
|climate change
|-
|[[Infrastructure Resilience]]
|[[Dr. John Hummel|Hummel, John, Ph.D.]]
|
|-
|[[Munitions Constituents- TREECS™ Fate and Risk Modeling|Munitions Constituents - TREECS™ Fate and Risk Modeling]]||[[Dr. Billy E. Johnson |Johnson, Billy, Ph.D.]]||munitions constituents fate and transport modeling, TREECS
|-
|[[Munitions Constituents - Alkaline Degradation]]
|[[Jared Johnson|Johnson, Jared]]
|
|-
|[[Assessing Vapor Intrusion (VI) Impacts in Neighborhoods with Groundwater Contaminated by Chlorinated Volatile Organic Chemicals (CVOCs)|Vapor Intrusion - Assessing VI Impacts in Neighborhoods with Groundwater Contaminated CVOCs]]
|[[Dr. Paul C. Johnson|Johnson, Paul, Ph.D.]]
|vapor intrusion
|-
|[[Munitions Constituents - IM Toxicology]]||-----||insensitive explosives, insensitive munitions, IMX-101, IMX
|-
|[[Landfarming]]
|[[Dr. Roopa Kamath|Kamath, Roopa, Ph.D.]]
|
|-
|[[NAPL Mobility]]
|[[Andrew Kirkman|Kirkman, Andrew]]
|
|-
|[[Climate Change Primer]]
|[[Dr. Rao Kotamarthi|Kotamarthi, Rao, Ph.D.]]
|
|-
|[[Perchlorate]]||[[Thomas Krug |Krug, Thomas]]||perchlorate
|-
|[[Injection Techniques for Liquid Amendments]]||[[Thomas Krug |Krug, Thomas]]||amendment injection
|-
|[[Transition of Aqueous Film Forming Foam (AFFF) Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances (PFAS)]]
|[[Dr. Johnsie Ray Lang|Lang, Johnsie Ray, Ph.D.]]||PFAS
|
|-
|[[Characterization Methods – Hydraulic Conductivity]]
|[[Dr. Gaisheng Liu|Liu, Gaisheng, Ph.D.]]
|
|-
|[[Compound Specific Isotope Analysis (CSIA)]]||[[Dr. Barbara Sherwood Lollar, F.R.S.C. |Lollar, Barbara S., FRSC]]||Compound Specific Isotope Analysis (CSIA)
|-
|[[Passive Sampling of Munitions Constituents|Munitions Constituents - Passive Sampling]]
|[[Dr. Guilherme Lotufo|Lotufo, Guilerme, Ph.D.]]
|
|-
|[[Vapor Intrusion (VI)]]||[[Chris Lutes |Lutes, Chris]]||vapor intrusion
|-
|[[Bioremediation - Anaerobic]]||[[Leah MacKinnon, M.A.Sc., P. Eng.|MacKinnon, Leah]]||anaerobic bioremediation
|-
|[[Bioremediation - Anaerobic Design Considerations]]||[[Leah MacKinnon, M.A.Sc., P. Eng.|MacKinnon, Leah]]||
|-
|[[Biodegradation - 1,4-Dioxane]]
|[[Dr. Shaily Mahendra|Mahendra, Shaily, Ph.D.]]
|
|-
|[[Direct Push (DP) Technology]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push, DP, DP machines, DP technology
|-
|[[Direct Push Sampling]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push sampling, soil sampling, groundwater sampling, well installation, soil vapor sampling (in regards to DP)
|-
|[[Direct Push Logging]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push logging, Cone Penetration Testing, CPT, Electrical Conductivity, EC, Hydraulic Profiling Tool, HPT, Membrane Interface Probe, MIP, Optical Imaging Profiler, OIP
|-
|[[Downscaled High Resolution Datasets for Climate Change Projections]]
|[[Dr. Rao Kotamarthi|Kotamarthi, Rao, Ph.D.]]
|
|-
|[[Remediation Performance Assessment at Chlorinated Solvent Sites]]||[[Travis McGuire|McGuire, Travis]]||multi-site studies
|-
|[[LNAPL Conceptual Site Models]]
|[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]
|
|-
|[[Long-Term Monitoring (LTM) - Data Analysis]]||[[Dr. Thomas McHugh |McHugh, Thomas, Ph.D.]]||data analysis, analysis methods (in regards to LTM)
|-
|[[Long-Term Monitoring (LTM) - Data Variability]]||[[Dr. Thomas McHugh |McHugh, Thomas, Ph.D.]]||data variability (in regards to LTM), LTM evaluation
|-
|-
|[[Munitions Constituents - Abiotic Reduction]]||[[Dr. Jimmy Murillo-Gelvez |Murillo-Gelvez, Jimmy, Ph.D.]]
|-
|[[Supercritical Water Oxidation (SCWO)]]
|Nagar, Kobe
|
|-
|[[Matrix Diffusion]]
|[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]
|
|-
|[[Groundwater Flow and Solute Transport]]||[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]||groundwater flow, advection, dispersion, diffusion, molecular diffusion, mechanical dispersion
|-
|[[Molecular Biological Tools - MBTs]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||MBT, Molecular Biological Tool(s)
|-
|[[Quantitative Polymerase Chain Reaction (qPCR)]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||qPCR, Polymerase Chain Reaction
|-
|[[Sediment Capping]]
|[[Dr. Danny Reible|Reible, Danny]]
|
|-
|[[Stable Isotope Probing (SIP)]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||SIP, Stable Isotope Probing
|-
|[[Metagenomics]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||metagenomics
|-
|[[Natural Source Zone Depletion (NSZD)]]||[[Tom Palaia |Palaia, Tom]]||natural source zone depletion, NSZD
|-
|[[Climate Change Effects on Wildlife]]
|[[Dr. Breanna F. Powers|Powers, Breanna, PhD.]]
|
|-
|[[Amendment Distribution in Low Conductivity Materials]]||[[Dr. Stephen Richardson |Richardson, Stephen, Ph.D.]]||
|-
|[[Polycyclic Aromatic Hydrocarbons (PAHs)]]||[[Dr. Stephen Richardson |Richardson, Stephen, Ph.D.]]||polycyclic aromatic hydrocarbons, PAH(s)
|-
|[[Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)]]
|[[Florent Risacher|Risacher, Florent, M.Sc.]]
|
|-
|[[1,2,3-Trichloropropane]]||[[Dr. Alexandra Salter-Blanc |Salter-Blanc, Alexandra, Ph.D.]]||TCP, trichloropropane
|-
|[[Zerovalent Iron (ZVI) (Chemical Reduction - ISCR)]]||[[Dr. Alexandra Salter-Blanc |Salter-Blanc, Alexandra, Ph.D.]]||ZVI
|-
|[[Mercury in Sediments]]
|[[Dr. Grace Schwartz|Schwartz, Grace, Ph.D.]]
|
|-
|[[Munitions Constituents – Sample Extraction and Analytical Techniques|Munitions Constituents - Sample Extraction and Analytical Techniques]]
|[[Dr. Austin Scircle|Scircle, Austin]]
|
|-
|[[Geophysical Methods]]||[[Dr. Lee Slater |Slater, Lee, Ph.D.]]||geophysics
|-
|[[Geophysical Methods - Case Studies]]||[[Dr. Lee Slater |Slater, Lee, Ph.D.]]||geophysics
|-
|[[PFAS Treatment by Anion Exchange]]
|[[Dr. Timothy J. Strathmann|Strathmann, Timothy, Ph.D.]]
|PFAS
|-
|[[Munitions Constituents - Dissolution]]||[[Dr. Susan Taylor |Taylor, Susan, Ph.D.]]||explosive(s), dissolution,
|-
|[[PFAS Treatment by Electrical Discharge Plasma]]
|[[Dr. Selma Mededovic Thagard|Thagard, Selma Mededovic, Ph.D.]]
|PFAS
|-
|[[Restoration of Ecological Function in Terrestrial Systems Impacted by Invasive Species]]
|Thierry, Hugo, Ph.D.
|climate change, invasive species, restoration ecology
|-
|[[Chemical Reduction (In Situ - ISCR)]]||[[Dr. Paul Tratnyek |Tratnyek, Paul, Ph.D.]]||In Situ Chemical Reduction, ISCR
|-
|[[Injection Techniques - Viscosity Modification]]||[[Michael Truex |Truex, Michael]]||viscosity, viscosity modifiers, viscosity modification
|-
|[[Soil Vapor Extraction (SVE)]]||[[Michael Truex |Truex, Michael]]||soil vapor extraction, SVE
|-
|[[Munitions Constituents - Deposition]]||[[Michael R. Walsh, P.E., M.E.|Walsh, Michael, P.E.]]||explosive deposition, energetics deposition
|-
|[[Vapor Intrusion - Separation Distances from Petroleum Sources]]
|[[Dr. James Weaver|Weaver, James, Ph.D.]]
|vapor intrusion
|-
|[[Zerovalent Iron Permeable Reactive Barriers]]
|[[Dr. Richard Wilkin|Wilkin, Rick, Ph.D.]]
|
|-
|[[Monitored Natural Attenuation (MNA)]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, In Situ MNA, natural attenuation, natural attenuation processes
|-
|[[Monitored Natural Attenuation (MNA) of Fuels]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, natural attenuation, attenuate (when used in context related to petroleum hydrocarbons and fuel components)
|-
|[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, natural attenuation, attenuate (when used in context related to chlorinated solvents)
|-
|[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
|[[Dr. John Wilson|Wilson, John, Ph.D.]]
|MNA, natural attenuation
|-
|[[Chlorinated Solvents]]||[[Dr. Bilgen Yuncu, P.E. |Yuncu, Bilgen, Ph.D., P.E.]]||chlorinated solvents
|-
|[[Petroleum Hydrocarbons (PHCs)]]
|[[Dr. Bilgen Yuncu, P.E.|Yuncu, Bilgen, Ph.D., P.E.]]
|Petroleum Hydrocarbons (PHCs)
|-
|[[Photoactivated Reductive Defluorination - PFAS Destruction]]
|[[Dr. Suzanne Witt|Witt, Suzanne, Ph.D.]]
|PFAS destruction
|
|-
|[[Hydrogeophysical Methods for Characterization and Monitoring of Groundwater-Surface Water Exchanges]]
|[[Dr. Lee Slater|Slater, Lee, Ph.D.]]
|geophysics, hydrogeophysical methods,
|
|-
|[[Hydrothermal Alkaline Treatment (HALT)]]
|[[Dr. Brian Pinkard|Pinkard, Brian]]
|
|-
|[[1,4-Dioxane]]
|[[Matthew Zenker|Zenker, Matthew]]
|
|-
|[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]
|[[Dr. John F. Stults|Stults, Dr. John]]
|PFAS, vadose zone, lysimeter, field investigation
|
|-
|[[PFAS Destruction by Ultraviolet/Sulfite Treatment]]||[[Dr. Yida Fang |Fang, Yida, Ph.D.]]||PFAS destruction
|
|-
|}

Articles

2026-03-13T21:29:10Z

Admin:

{| class="wikitable sortable"
|-
!Title!!First Author!!Linking Phrases
|-
|[[Groundwater Sampling - No-Purge/Passive]]||[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]||passive sampling, no purge sampling, grab samplers, diffusion samplers, sorptive samplers
|-
|[[ Long-Term Monitoring (LTM)]]||[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]||long-term monitoring, LTM, LTM objectives, LTM programs, LTM challenges
|-
|[[PFAS Monitored Retention (PMR) and PFAS Enhanced Retention (PER)]]
|[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]
|PFAS, MNA, natural attenuation
|-
|[[Sorption of Organic Contaminants]]||[[Richelle Allen-King|Allen-King, Richelle]]||
|-
|[[PFAS Transport and Fate]]
|[[Dr. Richard Anderson|Anderson, Richard, Ph.D.]]
|PFAS, fate and transport
|-
|[[Mass Flux and Mass Discharge]]||[[Dr. Michael Annable, P.E. |Annable, Michael, Ph.D., P.E.]]||source reduction
|-
|[[PFAS Toxicology and Risk Assessment]]
|[[Jennifer Arblaster|Arblaster, Jennifer]]
|PFAS, toxicology, risk assessment
|-
|[[Metal(loid)s - Small Arms Ranges]]|| Dr. Amanda Barker |[[Dr. Amanda Barker|Barker, Amanda, Ph.D.]]
|
|-
|[[Munitions Constituents – Photolysis|Munitions Constituents - Photolysis]]
|[[Dr. Warren Kadoya|Kadoya, Warren, Ph.D.]]
|
|-
|[[Munitions Constituents - Soil Sampling]]
|[[Dr. Samuel Beal|Beal, Samuel, Ph.D.]]
|
|-
|[[Vapor Intrusion – Sewers and Utility Tunnels as Preferential Pathways|Vapor Intrusion - Sewers and Utility Tunnels as Preferential Pathways]]
|[[Lila Beckley|Beckley, Lila]]
|vapor intrusion
|-
|[[Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill]]
|[[Dr. Barbara Bekins|Bekins, Barbara, Ph.D.]]||
|-
|[[Bioremediation - Anaerobic Secondary Water Quality Impacts]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||secondary impacts, water quality (in regards to anaerobic conditions)
|-
|[[Design Tool - Base Addition for ERD]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||aquifer acidity, base addition
|-
|[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation]]
|[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||
|-
|[[Low pH Inhibition of Reductive Dechlorination]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||low pH inhibition
|-
|[[In Situ Toxicity Identification Evaluation (iTIE)]]||Burton, Allen, P.E.||toxicity evaluation
|-
|[[OPTically-based In-situ Characterization System (OPTICS)]]
|[[Dr. Grace Chang|Chang, Grace, Ph.D.]]
|
|-
|[[Munitions Constituents - Electrochemical Treatment]]
|[[Dr. Brian P. Chaplin|Chaplin, Brian, Ph.D.]]
|munitions constituents remediation
|-
|[[PFAS Sources]]
|[[Dr. Dora Chiang|Chiang, Dora, Ph.D.]]
|
|-
|[[Biodegradation - Cometabolic]]||[[Dr. Kung-Hui (Bella) Chu |Chu, Kung-Hui (Bella), Ph.D]]||cometabolic biodegradation
|-
|[[Munitions Constituents - Composting]]
|[[Harry Craig|Craig, Harry]]
|
|-
|[[Chemical Oxidation (In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||ISCO, chemical oxidation
|-
|[[Chemical Oxidation Oxidant Selection (In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||chemical oxidant, oxidant (in regards to ISCO)
|-
|[[Chemical Oxidation Design Considerations(In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||screening, design, implementation, oxidant delivery (in regards to ISCO)
|-
|[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]||[[Dr. Rula Deeb |Deeb, Rula, Ph.D.]]||PFAS, perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS)
|-
|[[Metal and Metalloid Contaminants]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||metal contaminant(s), metalloid contaminant(s), metal(s), metalloid(s),
|-
|[[Metals and Metalloids - Mobility in Groundwater]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||metal mobility, aqueous speciation, adsorption, precipitation, colloidal transport (in regards to metals)
|-
|[[Monitored Natural Attenuation (MNA) of Metal and Metalloids]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||MNA, attenuation of metal(s), natural attenuation processes, attenuation (in regards to metals)
|-
|[[Metal and Metalloids - Remediation]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||remediation, in situ technologies, contaminant removal (in regards to metals)
|-
|[[pH Buffering in Aquifers]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||ph buffer, natural pH buffer, engineered pH buffer
|-
|[[Munitions Constituents - Sorption]]||[[Dr. Katerina Dontsova |Dontsova, Katerina, Ph.D.]]||energetics, sorption
|-
|[[Biodegradation - Hydrocarbons]]||[[Dr. Elizabeth Edwards |Edwards, Elizabeth, Ph.D.]]||hydrocarbon, biodegradation
|-
|[[Source Zone Modeling]]||[[Dr. Ron Falta |Falta, Ron, Ph.D.]]||source zone modeling
|-
|[[Plume Response Modeling]]||[[Dr. Ron Falta |Falta, Ron, Ph.D.]]||plume response modeling
|-
|[[REMChlor - MD]]
|[[Dr. Ron Falta|Falta, Ron, Ph.D.]]
|
|-
|[[In Situ Groundwater Treatment with Activated Carbon]]
|[[Dr. Dimin Fan|Fan, Dimin, Ph.D.]]
|
|-
|[[LNAPL Remediation Technologies]]||[[Dr. Shahla Farhat |Farhat, Shahla, Ph.D.]]||
|-
|[[Sustainable Remediation]]||[[Paul Favara |Favara, Paul]]||sustainable remediation, social, economic and environmental impacts
|-
|[[Munitions Constituents]]||[[Dr. Kevin Finneran |Finneran, Kevin, Ph.D.]]||Explosives, energetics, insensitive munitions
|-
|[[Biodegradation - Reductive Processes]]||[[Dr. David Freedman |Freedman, David, Ph.D.]]||biotic reduction, biotic reductive processes, hydrogenolysis, dihaloelimination, coupling, organohalide respiration
|-
|[[Remediation of Stormwater Runoff Contaminated by Munition Constituents|Munitions Constituents - Remediation of Stormwater Runoff]]||Fuller, Mark, Ph.D.||energetics, insensitive munitions, stormwater runoff
|-
|[[Subgrade Biogeochemical Reactor (SBGR)]]||[[Jeff Gamlin |Gamlin, Jeff]]||SBGR, subgrade biogeochemical reactor, bioreactor
|-
|[[Thermal Remediation - Smoldering]]||[[Dr. Jason Gerhard |Gerhard, Jason, Ph.D.]]||smouldering remediation, self-sustaining treatment for active remediation, STAR
|-
|[[Contaminated Sediments - Introduction]]
|[[Dr. Upal Ghosh|Ghosh, Upal, Ph.D.]]
|
|-
|[[Contaminated Sediment Risk Assessment]]
|[[Richard Wenning|Wenning, Richard]]
|
|-
|[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
|[[Dr. Upal Ghosh|Ghosh, Upal, Ph.D.]]
|
|-
|[[PFAS Ex Situ Water Treatment]]
|[[Dr. Scott Grieco |Grieco, Scott, Ph.D.]]
|
|-
|[[Stream Restoration]]
|[[Dr. Natalie Griffiths|Griffiths, Natalie, Ph.D.]]
|
|-
|[[Passive Sampling of Sediments]]
|[[Dr. Philip M. Gschwend|Gschwend, Philip]]
|
|-
|[[Phytoplankton (Algae) Blooms]]
|[[Dr. Nathan Hall|Hall, Nathan]]
|
|-
|[[PFAS Soil Remediation Technologies]]||[[James_Hatton |Hatton, Jim]]||PFAS, Soil source zones
|-
|[[Proteomics and Proteogenomics]]
|[[Dr. Kate Kucharzyk|Kucharzyk, Kate, Ph.D.]]
|
|-
|[[N-nitrosodimethylamine (NDMA)]]
|[[Paul Hatzinger|Hatzinger, Paul, Ph.D.]]
|
|-
|[[Alternative Endpoints]]||[[Elisabeth Hawley |Hawley, Elisabeth]]||management of complex sites||
|-
|[[Thermal Remediation]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation, in situ thermal
|-
|[[Thermal Remediation - Steam]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||Steam Enhanced Extraction
|-
|[[Thermal Remediation - Electrical Resistance Heating]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||Electrical Resistance Heating
|-
|[[Thermal Conduction Heating (TCH)]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal desorption
|-
|[[Thermal Remediation - Combined Remedies]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation
|-
|[[Thermal Conduction Heating for Treatment of PFAS-Impacted Soil]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation, PFAS
|-
|[[Predicting Species Responses to Climate Change with Population Models]]
|[[Dr. Brian Hudgens|Hudgens, Brian, Ph.D.]]
|climate change
|-
|[[Infrastructure Resilience]]
|[[Dr. John Hummel|Hummel, John, Ph.D.]]
|
|-
|[[Munitions Constituents- TREECS™ Fate and Risk Modeling|Munitions Constituents - TREECS™ Fate and Risk Modeling]]||[[Dr. Billy E. Johnson |Johnson, Billy, Ph.D.]]||munitions constituents fate and transport modeling, TREECS
|-
|[[Munitions Constituents - Alkaline Degradation]]
|[[Jared Johnson|Johnson, Jared]]
|
|-
|[[Assessing Vapor Intrusion (VI) Impacts in Neighborhoods with Groundwater Contaminated by Chlorinated Volatile Organic Chemicals (CVOCs)|Vapor Intrusion - Assessing VI Impacts in Neighborhoods with Groundwater Contaminated CVOCs]]
|[[Dr. Paul C. Johnson|Johnson, Paul, Ph.D.]]
|vapor intrusion
|-
|[[Munitions Constituents - IM Toxicology]]||-----||insensitive explosives, insensitive munitions, IMX-101, IMX
|-
|[[Landfarming]]
|[[Dr. Roopa Kamath|Kamath, Roopa, Ph.D.]]
|
|-
|[[NAPL Mobility]]
|[[Andrew Kirkman|Kirkman, Andrew]]
|
|-
|[[Climate Change Primer]]
|[[Dr. Rao Kotamarthi|Kotamarthi, Rao, Ph.D.]]
|
|-
|[[Perchlorate]]||[[Thomas Krug |Krug, Thomas]]||perchlorate
|-
|[[Injection Techniques for Liquid Amendments]]||[[Thomas Krug |Krug, Thomas]]||amendment injection
|-
|[[Transition of Aqueous Film Forming Foam (AFFF) Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances (PFAS)]]
|[[Dr. Johnsie Ray Lang|Lang, Johnsie Ray, Ph.D.]]||PFAS
|
|-
|[[Characterization Methods – Hydraulic Conductivity]]
|[[Dr. Gaisheng Liu|Liu, Gaisheng, Ph.D.]]
|
|-
|[[Compound Specific Isotope Analysis (CSIA)]]||[[Dr. Barbara Sherwood Lollar, F.R.S.C. |Lollar, Barbara S., FRSC]]||Compound Specific Isotope Analysis (CSIA)
|-
|[[Passive Sampling of Munitions Constituents|Munitions Constituents - Passive Sampling]]
|[[Dr. Guilherme Lotufo|Lotufo, Guilerme, Ph.D.]]
|
|-
|[[Vapor Intrusion (VI)]]||[[Chris Lutes |Lutes, Chris]]||vapor intrusion
|-
|[[Bioremediation - Anaerobic]]||[[Leah MacKinnon, M.A.Sc., P. Eng.|MacKinnon, Leah]]||anaerobic bioremediation
|-
|[[Bioremediation - Anaerobic Design Considerations]]||[[Leah MacKinnon, M.A.Sc., P. Eng.|MacKinnon, Leah]]||
|-
|[[Biodegradation - 1,4-Dioxane]]
|[[Dr. Shaily Mahendra|Mahendra, Shaily, Ph.D.]]
|
|-
|[[Direct Push (DP) Technology]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push, DP, DP machines, DP technology
|-
|[[Direct Push Sampling]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push sampling, soil sampling, groundwater sampling, well installation, soil vapor sampling (in regards to DP)
|-
|[[Direct Push Logging]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push logging, Cone Penetration Testing, CPT, Electrical Conductivity, EC, Hydraulic Profiling Tool, HPT, Membrane Interface Probe, MIP, Optical Imaging Profiler, OIP
|-
|[[Downscaled High Resolution Datasets for Climate Change Projections]]
|[[Dr. Rao Kotamarthi|Kotamarthi, Rao, Ph.D.]]
|
|-
|[[Remediation Performance Assessment at Chlorinated Solvent Sites]]||[[Travis McGuire|McGuire, Travis]]||multi-site studies
|-
|[[LNAPL Conceptual Site Models]]
|[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]
|
|-
|[[Long-Term Monitoring (LTM) - Data Analysis]]||[[Dr. Thomas McHugh |McHugh, Thomas, Ph.D.]]||data analysis, analysis methods (in regards to LTM)
|-
|[[Long-Term Monitoring (LTM) - Data Variability]]||[[Dr. Thomas McHugh |McHugh, Thomas, Ph.D.]]||data variability (in regards to LTM), LTM evaluation
|-
|-
|[[Munitions Constituents - Abiotic Reduction]]||[[Dr. Jimmy Murillo-Gelvez |Murillo-Gelvez, Jimmy, Ph.D.]]
|-
|[[Supercritical Water Oxidation (SCWO)]]
|Nagar, Kobe
|
|-
|[[Matrix Diffusion]]
|[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]
|
|-
|[[Groundwater Flow and Solute Transport]]||[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]||groundwater flow, advection, dispersion, diffusion, molecular diffusion, mechanical dispersion
|-
|[[Molecular Biological Tools - MBTs]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||MBT, Molecular Biological Tool(s)
|-
|[[Quantitative Polymerase Chain Reaction (qPCR)]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||qPCR, Polymerase Chain Reaction
|-
|[[Sediment Capping]]
|[[Dr. Danny Reible|Reible, Danny]]
|
|-
|[[Stable Isotope Probing (SIP)]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||SIP, Stable Isotope Probing
|-
|[[Metagenomics]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||metagenomics
|-
|[[Natural Source Zone Depletion (NSZD)]]||[[Tom Palaia |Palaia, Tom]]||natural source zone depletion, NSZD
|-
|[[Climate Change Effects on Wildlife]]
|[[Dr. Breanna F. Powers|Powers, Breanna, PhD.]]
|
|-
|[[Amendment Distribution in Low Conductivity Materials]]||[[Dr. Stephen Richardson |Richardson, Stephen, Ph.D.]]||
|-
|[[Polycyclic Aromatic Hydrocarbons (PAHs)]]||[[Dr. Stephen Richardson |Richardson, Stephen, Ph.D.]]||polycyclic aromatic hydrocarbons, PAH(s)
|-
|[[Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)]]
|[[Florent Risacher|Risacher, Florent, M.Sc.]]
|
|-
|[[1,2,3-Trichloropropane]]||[[Dr. Alexandra Salter-Blanc |Salter-Blanc, Alexandra, Ph.D.]]||TCP, trichloropropane
|-
|[[Zerovalent Iron (ZVI) (Chemical Reduction - ISCR)]]||[[Dr. Alexandra Salter-Blanc |Salter-Blanc, Alexandra, Ph.D.]]||ZVI
|-
|[[Mercury in Sediments]]
|[[Dr. Grace Schwartz|Schwartz, Grace, Ph.D.]]
|
|-
|[[Munitions Constituents – Sample Extraction and Analytical Techniques|Munitions Constituents - Sample Extraction and Analytical Techniques]]
|[[Dr. Austin Scircle|Scircle, Austin]]
|
|-
|[[Geophysical Methods]]||[[Dr. Lee Slater |Slater, Lee, Ph.D.]]||geophysics
|-
|[[Geophysical Methods - Case Studies]]||[[Dr. Lee Slater |Slater, Lee, Ph.D.]]||geophysics
|-
|[[PFAS Treatment by Anion Exchange]]
|[[Dr. Timothy J. Strathmann|Strathmann, Timothy, Ph.D.]]
|PFAS
|-
|[[Munitions Constituents - Dissolution]]||[[Dr. Susan Taylor |Taylor, Susan, Ph.D.]]||explosive(s), dissolution,
|-
|[[PFAS Treatment by Electrical Discharge Plasma]]
|[[Dr. Selma Mededovic Thagard|Thagard, Selma Mededovic, Ph.D.]]
|PFAS
|-
|[[Restoration of Ecological Function in Terrestrial Systems Impacted by Invasive Species]]
|Thierry, Hugo, Ph.D.
|climate change, invasive species, restoration ecology
|-
|[[Chemical Reduction (In Situ - ISCR)]]||[[Dr. Paul Tratnyek |Tratnyek, Paul, Ph.D.]]||In Situ Chemical Reduction, ISCR
|-
|[[Injection Techniques - Viscosity Modification]]||[[Michael Truex |Truex, Michael]]||viscosity, viscosity modifiers, viscosity modification
|-
|[[Soil Vapor Extraction (SVE)]]||[[Michael Truex |Truex, Michael]]||soil vapor extraction, SVE
|-
|[[Munitions Constituents - Deposition]]||[[Michael R. Walsh, P.E., M.E.|Walsh, Michael, P.E.]]||explosive deposition, energetics deposition
|-
|[[Vapor Intrusion - Separation Distances from Petroleum Sources]]
|[[Dr. James Weaver|Weaver, James, Ph.D.]]
|vapor intrusion
|-
|[[Zerovalent Iron Permeable Reactive Barriers]]
|[[Dr. Richard Wilkin|Wilkin, Rick, Ph.D.]]
|
|-
|[[Monitored Natural Attenuation (MNA)]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, In Situ MNA, natural attenuation, natural attenuation processes
|-
|[[Monitored Natural Attenuation (MNA) of Fuels]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, natural attenuation, attenuate (when used in context related to petroleum hydrocarbons and fuel components)
|-
|[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, natural attenuation, attenuate (when used in context related to chlorinated solvents)
|-
|[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
|[[Dr. John Wilson|Wilson, John, Ph.D.]]
|MNA, natural attenuation
|-
|[[Chlorinated Solvents]]||[[Dr. Bilgen Yuncu, P.E. |Yuncu, Bilgen, Ph.D., P.E.]]||chlorinated solvents
|-
|[[Petroleum Hydrocarbons (PHCs)]]
|[[Dr. Bilgen Yuncu, P.E.|Yuncu, Bilgen, Ph.D., P.E.]]
|Petroleum Hydrocarbons (PHCs)
|-
|[[Photoactivated Reductive Defluorination - PFAS Destruction]]
|[[Dr. Suzanne Witt|Witt, Suzanne, Ph.D.]]
|PFAS
|
|-
|[[Hydrogeophysical Methods for Characterization and Monitoring of Groundwater-Surface Water Exchanges]]
|[[Dr. Lee Slater|Slater, Lee, Ph.D.]]
|geophysics, hydrogeophysical methods,
|
|-
|[[Hydrothermal Alkaline Treatment (HALT)]]
|[[Dr. Brian Pinkard|Pinkard, Brian]]
|
|-
|[[1,4-Dioxane]]
|[[Matthew Zenker|Zenker, Matthew]]
|
|-
|[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]
|[[Dr. John F. Stults|Stults, Dr. John]]
|PFAS, vadose zone, lysimeter, field investigation
|
|-
|[[PFAS Destruction by Ultraviolet/Sulfite Treatment]]||[[Dr. Yida Fang |Fang, Yida, Ph.D.]]||
|
|-
|}

Articles

2026-03-13T21:25:52Z

Admin:

{| class="wikitable sortable"
|-
!Title!!First Author!!Linking Phrases
|-
|[[Groundwater Sampling - No-Purge/Passive]]||[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]||passive sampling, no purge sampling, grab samplers, diffusion samplers, sorptive samplers
|-
|[[ Long-Term Monitoring (LTM)]]||[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]||long-term monitoring, LTM, LTM objectives, LTM programs, LTM challenges
|-
|[[PFAS Monitored Retention (PMR) and PFAS Enhanced Retention (PER)]]
|[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]
|PFAS, MNA, natural attenuation
|-
|[[Sorption of Organic Contaminants]]||[[Richelle Allen-King|Allen-King, Richelle]]||
|-
|[[PFAS Transport and Fate]]
|[[Dr. Richard Anderson|Anderson, Richard, Ph.D.]]
|PFAS, fate and transport
|-
|[[Mass Flux and Mass Discharge]]||[[Dr. Michael Annable, P.E. |Annable, Michael, Ph.D., P.E.]]||source reduction
|-
|[[PFAS Toxicology and Risk Assessment]]
|[[Jennifer Arblaster|Arblaster, Jennifer]]
|PFAS, toxicology, risk assessment
|-
|[[Metal(loid)s - Small Arms Ranges]]|| Dr. Amanda Barker |[[Dr. Amanda Barker|Barker, Amanda, Ph.D.]]
|
|-
|[[Munitions Constituents – Photolysis|Munitions Constituents - Photolysis]]
|[[Dr. Warren Kadoya|Kadoya, Warren, Ph.D.]]
|
|-
|[[Munitions Constituents - Soil Sampling]]
|[[Dr. Samuel Beal|Beal, Samuel, Ph.D.]]
|
|-
|[[Vapor Intrusion – Sewers and Utility Tunnels as Preferential Pathways|Vapor Intrusion - Sewers and Utility Tunnels as Preferential Pathways]]
|[[Lila Beckley|Beckley, Lila]]
|vapor intrusion
|-
|[[Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill]]
|[[Dr. Barbara Bekins|Bekins, Barbara, Ph.D.]]||
|-
|[[Bioremediation - Anaerobic Secondary Water Quality Impacts]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||secondary impacts, water quality (in regards to anaerobic conditions)
|-
|[[Design Tool - Base Addition for ERD]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||aquifer acidity, base addition
|-
|[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation]]
|[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||
|-
|[[Low pH Inhibition of Reductive Dechlorination]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||low pH inhibition
|-
|[[In Situ Toxicity Identification Evaluation (iTIE)]]||[[Burton, Allen, P.E.]]||toxicity evaluation
|-
|[[OPTically-based In-situ Characterization System (OPTICS)]]
|[[Dr. Grace Chang|Chang, Grace, Ph.D.]]
|
|-
|[[Munitions Constituents - Electrochemical Treatment]]
|[[Dr. Brian P. Chaplin|Chaplin, Brian, Ph.D.]]
|munitions constituents remediation
|-
|[[PFAS Sources]]
|[[Dr. Dora Chiang|Chiang, Dora, Ph.D.]]
|
|-
|[[Biodegradation - Cometabolic]]||[[Dr. Kung-Hui (Bella) Chu |Chu, Kung-Hui (Bella), Ph.D]]||cometabolic biodegradation
|-
|[[Munitions Constituents - Composting]]
|[[Harry Craig|Craig, Harry]]
|
|-
|[[Chemical Oxidation (In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||ISCO, chemical oxidation
|-
|[[Chemical Oxidation Oxidant Selection (In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||chemical oxidant, oxidant (in regards to ISCO)
|-
|[[Chemical Oxidation Design Considerations(In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||screening, design, implementation, oxidant delivery (in regards to ISCO)
|-
|[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]||[[Dr. Rula Deeb |Deeb, Rula, Ph.D.]]||PFAS, perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS)
|-
|[[Metal and Metalloid Contaminants]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||metal contaminant(s), metalloid contaminant(s), metal(s), metalloid(s),
|-
|[[Metals and Metalloids - Mobility in Groundwater]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||metal mobility, aqueous speciation, adsorption, precipitation, colloidal transport (in regards to metals)
|-
|[[Monitored Natural Attenuation (MNA) of Metal and Metalloids]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||MNA, attenuation of metal(s), natural attenuation processes, attenuation (in regards to metals)
|-
|[[Metal and Metalloids - Remediation]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||remediation, in situ technologies, contaminant removal (in regards to metals)
|-
|[[pH Buffering in Aquifers]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||ph buffer, natural pH buffer, engineered pH buffer
|-
|[[Munitions Constituents - Sorption]]||[[Dr. Katerina Dontsova |Dontsova, Katerina, Ph.D.]]||energetics, sorption
|-
|[[Biodegradation - Hydrocarbons]]||[[Dr. Elizabeth Edwards |Edwards, Elizabeth, Ph.D.]]||hydrocarbon, biodegradation
|-
|[[Source Zone Modeling]]||[[Dr. Ron Falta |Falta, Ron, Ph.D.]]||source zone modeling
|-
|[[Plume Response Modeling]]||[[Dr. Ron Falta |Falta, Ron, Ph.D.]]||plume response modeling
|-
|[[REMChlor - MD]]
|[[Dr. Ron Falta|Falta, Ron, Ph.D.]]
|
|-
|[[In Situ Groundwater Treatment with Activated Carbon]]
|[[Dr. Dimin Fan|Fan, Dimin, Ph.D.]]
|
|-
|[[LNAPL Remediation Technologies]]||[[Dr. Shahla Farhat |Farhat, Shahla, Ph.D.]]||
|-
|[[Sustainable Remediation]]||[[Paul Favara |Favara, Paul]]||sustainable remediation, social, economic and environmental impacts
|-
|[[Munitions Constituents]]||[[Dr. Kevin Finneran |Finneran, Kevin, Ph.D.]]||Explosives, energetics, insensitive munitions
|-
|[[Biodegradation - Reductive Processes]]||[[Dr. David Freedman |Freedman, David, Ph.D.]]||biotic reduction, biotic reductive processes, hydrogenolysis, dihaloelimination, coupling, organohalide respiration
|-
|[[Remediation of Stormwater Runoff Contaminated by Munition Constituents|Munitions Constituents - Remediation of Stormwater Runoff]]||Fuller, Mark, Ph.D.||energetics, insensitive munitions, stormwater runoff
|-
|[[Subgrade Biogeochemical Reactor (SBGR)]]||[[Jeff Gamlin |Gamlin, Jeff]]||SBGR, subgrade biogeochemical reactor, bioreactor
|-
|[[Thermal Remediation - Smoldering]]||[[Dr. Jason Gerhard |Gerhard, Jason, Ph.D.]]||smouldering remediation, self-sustaining treatment for active remediation, STAR
|-
|[[Contaminated Sediments - Introduction]]
|[[Dr. Upal Ghosh|Ghosh, Upal, Ph.D.]]
|
|-
|[[Contaminated Sediment Risk Assessment]]
|[[Richard Wenning|Wenning, Richard]]
|
|-
|[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
|[[Dr. Upal Ghosh|Ghosh, Upal, Ph.D.]]
|
|-
|[[PFAS Ex Situ Water Treatment]]
|[[Dr. Scott Grieco |Grieco, Scott, Ph.D.]]
|
|-
|[[Stream Restoration]]
|[[Dr. Natalie Griffiths|Griffiths, Natalie, Ph.D.]]
|
|-
|[[Passive Sampling of Sediments]]
|[[Dr. Philip M. Gschwend|Gschwend, Philip]]
|
|-
|[[Phytoplankton (Algae) Blooms]]
|[[Dr. Nathan Hall|Hall, Nathan]]
|
|-
|[[PFAS Soil Remediation Technologies]]||[[James_Hatton |Hatton, Jim]]||PFAS, Soil source zones
|-
|[[Proteomics and Proteogenomics]]
|[[Dr. Kate Kucharzyk|Kucharzyk, Kate, Ph.D.]]
|
|-
|[[N-nitrosodimethylamine (NDMA)]]
|[[Paul Hatzinger|Hatzinger, Paul, Ph.D.]]
|
|-
|[[Alternative Endpoints]]||[[Elisabeth Hawley |Hawley, Elisabeth]]||management of complex sites||
|-
|[[Thermal Remediation]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation, in situ thermal
|-
|[[Thermal Remediation - Steam]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||Steam Enhanced Extraction
|-
|[[Thermal Remediation - Electrical Resistance Heating]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||Electrical Resistance Heating
|-
|[[Thermal Conduction Heating (TCH)]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal desorption
|-
|[[Thermal Remediation - Combined Remedies]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation
|-
|[[Thermal Conduction Heating for Treatment of PFAS-Impacted Soil]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation, PFAS
|-
|[[Predicting Species Responses to Climate Change with Population Models]]
|[[Dr. Brian Hudgens|Hudgens, Brian, Ph.D.]]
|climate change
|-
|[[Infrastructure Resilience]]
|[[Dr. John Hummel|Hummel, John, Ph.D.]]
|
|-
|[[Munitions Constituents- TREECS™ Fate and Risk Modeling|Munitions Constituents - TREECS™ Fate and Risk Modeling]]||[[Dr. Billy E. Johnson |Johnson, Billy, Ph.D.]]||munitions constituents fate and transport modeling, TREECS
|-
|[[Munitions Constituents - Alkaline Degradation]]
|[[Jared Johnson|Johnson, Jared]]
|
|-
|[[Assessing Vapor Intrusion (VI) Impacts in Neighborhoods with Groundwater Contaminated by Chlorinated Volatile Organic Chemicals (CVOCs)|Vapor Intrusion - Assessing VI Impacts in Neighborhoods with Groundwater Contaminated CVOCs]]
|[[Dr. Paul C. Johnson|Johnson, Paul, Ph.D.]]
|vapor intrusion
|-
|[[Munitions Constituents - IM Toxicology]]||-----||insensitive explosives, insensitive munitions, IMX-101, IMX
|-
|[[Landfarming]]
|[[Dr. Roopa Kamath|Kamath, Roopa, Ph.D.]]
|
|-
|[[NAPL Mobility]]
|[[Andrew Kirkman|Kirkman, Andrew]]
|
|-
|[[Climate Change Primer]]
|[[Dr. Rao Kotamarthi|Kotamarthi, Rao, Ph.D.]]
|
|-
|[[Perchlorate]]||[[Thomas Krug |Krug, Thomas]]||perchlorate
|-
|[[Injection Techniques for Liquid Amendments]]||[[Thomas Krug |Krug, Thomas]]||amendment injection
|-
|[[Transition of Aqueous Film Forming Foam (AFFF) Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances (PFAS)]]
|[[Dr. Johnsie Ray Lang|Lang, Johnsie Ray, Ph.D.]]||PFAS
|
|-
|[[Characterization Methods – Hydraulic Conductivity]]
|[[Dr. Gaisheng Liu|Liu, Gaisheng, Ph.D.]]
|
|-
|[[Compound Specific Isotope Analysis (CSIA)]]||[[Dr. Barbara Sherwood Lollar, F.R.S.C. |Lollar, Barbara S., FRSC]]||Compound Specific Isotope Analysis (CSIA)
|-
|[[Passive Sampling of Munitions Constituents|Munitions Constituents - Passive Sampling]]
|[[Dr. Guilherme Lotufo|Lotufo, Guilerme, Ph.D.]]
|
|-
|[[Vapor Intrusion (VI)]]||[[Chris Lutes |Lutes, Chris]]||vapor intrusion
|-
|[[Bioremediation - Anaerobic]]||[[Leah MacKinnon, M.A.Sc., P. Eng.|MacKinnon, Leah]]||anaerobic bioremediation
|-
|[[Bioremediation - Anaerobic Design Considerations]]||[[Leah MacKinnon, M.A.Sc., P. Eng.|MacKinnon, Leah]]||
|-
|[[Biodegradation - 1,4-Dioxane]]
|[[Dr. Shaily Mahendra|Mahendra, Shaily, Ph.D.]]
|
|-
|[[Direct Push (DP) Technology]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push, DP, DP machines, DP technology
|-
|[[Direct Push Sampling]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push sampling, soil sampling, groundwater sampling, well installation, soil vapor sampling (in regards to DP)
|-
|[[Direct Push Logging]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push logging, Cone Penetration Testing, CPT, Electrical Conductivity, EC, Hydraulic Profiling Tool, HPT, Membrane Interface Probe, MIP, Optical Imaging Profiler, OIP
|-
|[[Downscaled High Resolution Datasets for Climate Change Projections]]
|[[Dr. Rao Kotamarthi|Kotamarthi, Rao, Ph.D.]]
|
|-
|[[Remediation Performance Assessment at Chlorinated Solvent Sites]]||[[Travis McGuire|McGuire, Travis]]||multi-site studies
|-
|[[LNAPL Conceptual Site Models]]
|[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]
|
|-
|[[Long-Term Monitoring (LTM) - Data Analysis]]||[[Dr. Thomas McHugh |McHugh, Thomas, Ph.D.]]||data analysis, analysis methods (in regards to LTM)
|-
|[[Long-Term Monitoring (LTM) - Data Variability]]||[[Dr. Thomas McHugh |McHugh, Thomas, Ph.D.]]||data variability (in regards to LTM), LTM evaluation
|-
|-
|[[Munitions Constituents - Abiotic Reduction]]||[[Dr. Jimmy Murillo-Gelvez |Murillo-Gelvez, Jimmy, Ph.D.]]
|-
|[[Supercritical Water Oxidation (SCWO)]]
|Nagar, Kobe
|
|-
|[[Matrix Diffusion]]
|[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]
|
|-
|[[Groundwater Flow and Solute Transport]]||[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]||groundwater flow, advection, dispersion, diffusion, molecular diffusion, mechanical dispersion
|-
|[[Molecular Biological Tools - MBTs]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||MBT, Molecular Biological Tool(s)
|-
|[[Quantitative Polymerase Chain Reaction (qPCR)]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||qPCR, Polymerase Chain Reaction
|-
|[[Sediment Capping]]
|[[Dr. Danny Reible|Reible, Danny]]
|
|-
|[[Stable Isotope Probing (SIP)]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||SIP, Stable Isotope Probing
|-
|[[Metagenomics]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||Metagenomics
|-
|[[Natural Source Zone Depletion (NSZD)]]||[[Tom Palaia |Palaia, Tom]]||natural source zone depletion, NSZD
|-
|[[Climate Change Effects on Wildlife]]
|[[Dr. Breanna F. Powers|Powers, Breanna, PhD.]]
|
|-
|[[Amendment Distribution in Low Conductivity Materials]]||[[Dr. Stephen Richardson |Richardson, Stephen, Ph.D.]]||
|-
|[[Polycyclic Aromatic Hydrocarbons (PAHs)]]||[[Dr. Stephen Richardson |Richardson, Stephen, Ph.D.]]||Polycyclic Aromatic Hydrocarbons, PAH(s)
|-
|[[Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)]]
|[[Florent Risacher|Risacher, Florent, M.Sc.]]
|
|-
|[[1,2,3-Trichloropropane]]||[[Dr. Alexandra Salter-Blanc |Salter-Blanc, Alexandra, Ph.D.]]||TCP, trichloropropane
|-
|[[Zerovalent Iron (ZVI) (Chemical Reduction - ISCR)]]||[[Dr. Alexandra Salter-Blanc |Salter-Blanc, Alexandra, Ph.D.]]||ZVI
|-
|[[Mercury in Sediments]]
|[[Dr. Grace Schwartz|Schwartz, Grace, Ph.D.]]
|
|-
|[[Munitions Constituents – Sample Extraction and Analytical Techniques|Munitions Constituents - Sample Extraction and Analytical Techniques]]
|[[Dr. Austin Scircle|Scircle, Austin]]
|
|-
|[[Geophysical Methods]]||[[Dr. Lee Slater |Slater, Lee, Ph.D.]]||geophysics
|-
|[[Geophysical Methods - Case Studies]]||[[Dr. Lee Slater |Slater, Lee, Ph.D.]]||geophysics
|-
|[[PFAS Treatment by Anion Exchange]]
|[[Dr. Timothy J. Strathmann|Strathmann, Timothy, Ph.D.]]
|PFAS
|-
|[[Munitions Constituents - Dissolution]]||[[Dr. Susan Taylor |Taylor, Susan, Ph.D.]]||explosive(s), dissolution,
|-
|[[PFAS Treatment by Electrical Discharge Plasma]]
|[[Dr. Selma Mededovic Thagard|Thagard, Selma Mededovic, Ph.D.]]
|PFAS
|-
|[[Restoration of Ecological Function in Terrestrial Systems Impacted by Invasive Species]]
|Thierry, Hugo, Ph.D.
|climate change, invasive species, restoration ecology
|-
|[[Chemical Reduction (In Situ - ISCR)]]||[[Dr. Paul Tratnyek |Tratnyek, Paul, Ph.D.]]||In Situ Chemical Reduction, ISCR
|-
|[[Injection Techniques - Viscosity Modification]]||[[Michael Truex |Truex, Michael]]||viscosity, viscosity modifiers, viscosity modification
|-
|[[Soil Vapor Extraction (SVE)]]||[[Michael Truex |Truex, Michael]]||soil vapor extraction, SVE
|-
|[[Munitions Constituents - Deposition]]||[[Michael R. Walsh, P.E., M.E.|Walsh, Michael, P.E.]]||explosive deposition, energetics deposition
|-
|[[Vapor Intrusion - Separation Distances from Petroleum Sources]]
|[[Dr. James Weaver|Weaver, James, Ph.D.]]
|vapor intrusion
|-
|[[Zerovalent Iron Permeable Reactive Barriers]]
|[[Dr. Richard Wilkin|Wilkin, Rick, Ph.D.]]
|
|-
|[[Monitored Natural Attenuation (MNA)]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, In Situ MNA, natural attenuation, natural attenuation processes
|-
|[[Monitored Natural Attenuation (MNA) of Fuels]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, natural attenuation, attenuate (when used in context related to petroleum hydrocarbons and fuel components)
|-
|[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, natural attenuation, attenuate (when used in context related to chlorinated solvents)
|-
|[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
|[[Dr. John Wilson|Wilson, John, Ph.D.]]
|MNA, natural attenuation
|-
|[[Chlorinated Solvents]]||[[Dr. Bilgen Yuncu, P.E. |Yuncu, Bilgen, Ph.D., P.E.]]||chlorinated solvents
|-
|[[Petroleum Hydrocarbons (PHCs)]]
|[[Dr. Bilgen Yuncu, P.E.|Yuncu, Bilgen, Ph.D., P.E.]]
|Petroleum Hydrocarbons (PHCs)
|-
|[[Photoactivated Reductive Defluorination - PFAS Destruction]]
|[[Dr. Suzanne Witt|Witt, Suzanne, Ph.D.]]
|PFAS
|
|-
|[[Hydrogeophysical Methods for Characterization and Monitoring of Groundwater-Surface Water Exchanges]]
|[[Dr. Lee Slater|Slater, Lee, Ph.D.]]
|geophysics, hydrogeophysical methods,
|
|-
|[[Hydrothermal Alkaline Treatment (HALT)]]
|[[Dr. Brian Pinkard|Pinkard, Brian]]
|
|-
|[[1,4-Dioxane]]
|[[Matthew Zenker|Zenker, Matthew]]
|
|-
|[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]
|[[Dr. John F. Stults|Stults, Dr. John]]
|PFAS, vadose zone, lysimeter, field investigation
|
|-
|[[PFAS Destruction by Ultraviolet/Sulfite Treatment]]||[[Dr. Yida Fang |Fang, Yida, Ph.D.]]||
|
|-
|}

Articles

2026-03-13T21:23:40Z

Admin:

{| class="wikitable sortable"
|-
!Title!!First Author!!Linking Phrases
|-
|[[Groundwater Sampling - No-Purge/Passive]]||[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]||passive sampling, no purge sampling, grab samplers, diffusion samplers, sorptive samplers
|-
|[[ Long-Term Monitoring (LTM)]]||[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]||long-term monitoring, LTM, LTM objectives, LTM programs, LTM challenges
|-
|[[PFAS Monitored Retention (PMR) and PFAS Enhanced Retention (PER)]]
|[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]
|PFAS, MNA, natural attenuation
|-
|[[Sorption of Organic Contaminants]]||[[Richelle Allen-King|Allen-King, Richelle]]||
|-
|[[PFAS Transport and Fate]]
|[[Dr. Richard Anderson|Anderson, Richard, Ph.D.]]
|PFAS, fate and transport
|-
|[[Mass Flux and Mass Discharge]]||[[Dr. Michael Annable, P.E. |Annable, Michael, Ph.D., P.E.]]||source reduction
|-
|[[PFAS Toxicology and Risk Assessment]]
|[[Jennifer Arblaster|Arblaster, Jennifer]]
|PFAS, toxicology, risk assessment
|-
|[[Metal(loid)s - Small Arms Ranges]]|| Dr. Amanda Barker |[[Dr. Amanda Barker|Barker, Amanda, Ph.D.]]
|
|-
|[[Munitions Constituents – Photolysis|Munitions Constituents - Photolysis]]
|[[Dr. Warren Kadoya|Kadoya, Warren, Ph.D.]]
|
|-
|[[Munitions Constituents - Soil Sampling]]
|[[Dr. Samuel Beal|Beal, Samuel, Ph.D.]]
|
|-
|[[Vapor Intrusion – Sewers and Utility Tunnels as Preferential Pathways|Vapor Intrusion - Sewers and Utility Tunnels as Preferential Pathways]]
|[[Lila Beckley|Beckley, Lila]]
|vapor intrusion
|-
|[[Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill]]
|[[Dr. Barbara Bekins|Bekins, Barbara, Ph.D.]]||
|-
|[[Bioremediation - Anaerobic Secondary Water Quality Impacts]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||secondary impacts, water quality (in regards to anaerobic conditions)
|-
|[[Design Tool - Base Addition for ERD]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||aquifer acidity, base addition
|-
|[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation]]
|[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||
|-
|[[Low pH Inhibition of Reductive Dechlorination]]||[[Dr. G. Allen Burton]]||toxicity evaluation
|-
|[[In Situ Toxicity Identification Evaluation (iTIE)]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||low pH inhibition
|-
|[[OPTically-based In-situ Characterization System (OPTICS)]]
|[[Dr. Grace Chang|Chang, Grace, Ph.D.]]
|
|-
|[[Munitions Constituents - Electrochemical Treatment]]
|[[Dr. Brian P. Chaplin|Chaplin, Brian, Ph.D.]]
|munitions constituents remediation
|-
|[[PFAS Sources]]
|[[Dr. Dora Chiang|Chiang, Dora, Ph.D.]]
|
|-
|[[Biodegradation - Cometabolic]]||[[Dr. Kung-Hui (Bella) Chu |Chu, Kung-Hui (Bella), Ph.D]]||cometabolic biodegradation
|-
|[[Munitions Constituents - Composting]]
|[[Harry Craig|Craig, Harry]]
|
|-
|[[Chemical Oxidation (In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||ISCO, chemical oxidation
|-
|[[Chemical Oxidation Oxidant Selection (In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||chemical oxidant, oxidant (in regards to ISCO)
|-
|[[Chemical Oxidation Design Considerations(In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||screening, design, implementation, oxidant delivery (in regards to ISCO)
|-
|[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]||[[Dr. Rula Deeb |Deeb, Rula, Ph.D.]]||PFAS, perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS)
|-
|[[Metal and Metalloid Contaminants]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||metal contaminant(s), metalloid contaminant(s), metal(s), metalloid(s),
|-
|[[Metals and Metalloids - Mobility in Groundwater]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||metal mobility, aqueous speciation, adsorption, precipitation, colloidal transport (in regards to metals)
|-
|[[Monitored Natural Attenuation (MNA) of Metal and Metalloids]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||MNA, attenuation of metal(s), natural attenuation processes, attenuation (in regards to metals)
|-
|[[Metal and Metalloids - Remediation]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||remediation, in situ technologies, contaminant removal (in regards to metals)
|-
|[[pH Buffering in Aquifers]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||ph buffer, natural pH buffer, engineered pH buffer
|-
|[[Munitions Constituents - Sorption]]||[[Dr. Katerina Dontsova |Dontsova, Katerina, Ph.D.]]||energetics, sorption
|-
|[[Biodegradation - Hydrocarbons]]||[[Dr. Elizabeth Edwards |Edwards, Elizabeth, Ph.D.]]||hydrocarbon, biodegradation
|-
|[[Source Zone Modeling]]||[[Dr. Ron Falta |Falta, Ron, Ph.D.]]||source zone modeling
|-
|[[Plume Response Modeling]]||[[Dr. Ron Falta |Falta, Ron, Ph.D.]]||plume response modeling
|-
|[[REMChlor - MD]]
|[[Dr. Ron Falta|Falta, Ron, Ph.D.]]
|
|-
|[[In Situ Groundwater Treatment with Activated Carbon]]
|[[Dr. Dimin Fan|Fan, Dimin, Ph.D.]]
|
|-
|[[LNAPL Remediation Technologies]]||[[Dr. Shahla Farhat |Farhat, Shahla, Ph.D.]]||
|-
|[[Sustainable Remediation]]||[[Paul Favara |Favara, Paul]]||sustainable remediation, social, economic and environmental impacts
|-
|[[Munitions Constituents]]||[[Dr. Kevin Finneran |Finneran, Kevin, Ph.D.]]||Explosives, energetics, insensitive munitions
|-
|[[Biodegradation - Reductive Processes]]||[[Dr. David Freedman |Freedman, David, Ph.D.]]||biotic reduction, biotic reductive processes, hydrogenolysis, dihaloelimination, coupling, organohalide respiration
|-
|[[Remediation of Stormwater Runoff Contaminated by Munition Constituents|Munitions Constituents - Remediation of Stormwater Runoff]]||Fuller, Mark, Ph.D.||energetics, insensitive munitions, stormwater runoff
|-
|[[Subgrade Biogeochemical Reactor (SBGR)]]||[[Jeff Gamlin |Gamlin, Jeff]]||SBGR, subgrade biogeochemical reactor, bioreactor
|-
|[[Thermal Remediation - Smoldering]]||[[Dr. Jason Gerhard |Gerhard, Jason, Ph.D.]]||smouldering remediation, self-sustaining treatment for active remediation, STAR
|-
|[[Contaminated Sediments - Introduction]]
|[[Dr. Upal Ghosh|Ghosh, Upal, Ph.D.]]
|
|-
|[[Contaminated Sediment Risk Assessment]]
|[[Richard Wenning|Wenning, Richard]]
|
|-
|[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
|[[Dr. Upal Ghosh|Ghosh, Upal, Ph.D.]]
|
|-
|[[PFAS Ex Situ Water Treatment]]
|[[Dr. Scott Grieco |Grieco, Scott, Ph.D.]]
|
|-
|[[Stream Restoration]]
|[[Dr. Natalie Griffiths|Griffiths, Natalie, Ph.D.]]
|
|-
|[[Passive Sampling of Sediments]]
|[[Dr. Philip M. Gschwend|Gschwend, Philip]]
|
|-
|[[Phytoplankton (Algae) Blooms]]
|[[Dr. Nathan Hall|Hall, Nathan]]
|
|-
|[[PFAS Soil Remediation Technologies]]||[[James_Hatton |Hatton, Jim]]||PFAS, Soil source zones
|-
|[[Proteomics and Proteogenomics]]
|[[Dr. Kate Kucharzyk|Kucharzyk, Kate, Ph.D.]]
|
|-
|[[N-nitrosodimethylamine (NDMA)]]
|[[Paul Hatzinger|Hatzinger, Paul, Ph.D.]]
|
|-
|[[Alternative Endpoints]]||[[Elisabeth Hawley |Hawley, Elisabeth]]||management of complex sites||
|-
|[[Thermal Remediation]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation, in situ thermal
|-
|[[Thermal Remediation - Steam]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||Steam Enhanced Extraction
|-
|[[Thermal Remediation - Electrical Resistance Heating]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||Electrical Resistance Heating
|-
|[[Thermal Conduction Heating (TCH)]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal desorption
|-
|[[Thermal Remediation - Combined Remedies]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation
|-
|[[Thermal Conduction Heating for Treatment of PFAS-Impacted Soil]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation, PFAS
|-
|[[Predicting Species Responses to Climate Change with Population Models]]
|[[Dr. Brian Hudgens|Hudgens, Brian, Ph.D.]]
|climate change
|-
|[[Infrastructure Resilience]]
|[[Dr. John Hummel|Hummel, John, Ph.D.]]
|
|-
|[[Munitions Constituents- TREECS™ Fate and Risk Modeling|Munitions Constituents - TREECS™ Fate and Risk Modeling]]||[[Dr. Billy E. Johnson |Johnson, Billy, Ph.D.]]||munitions constituents fate and transport modeling, TREECS
|-
|[[Munitions Constituents - Alkaline Degradation]]
|[[Jared Johnson|Johnson, Jared]]
|
|-
|[[Assessing Vapor Intrusion (VI) Impacts in Neighborhoods with Groundwater Contaminated by Chlorinated Volatile Organic Chemicals (CVOCs)|Vapor Intrusion - Assessing VI Impacts in Neighborhoods with Groundwater Contaminated CVOCs]]
|[[Dr. Paul C. Johnson|Johnson, Paul, Ph.D.]]
|vapor intrusion
|-
|[[Munitions Constituents - IM Toxicology]]||-----||insensitive explosives, insensitive munitions, IMX-101, IMX
|-
|[[Landfarming]]
|[[Dr. Roopa Kamath|Kamath, Roopa, Ph.D.]]
|
|-
|[[NAPL Mobility]]
|[[Andrew Kirkman|Kirkman, Andrew]]
|
|-
|[[Climate Change Primer]]
|[[Dr. Rao Kotamarthi|Kotamarthi, Rao, Ph.D.]]
|
|-
|[[Perchlorate]]||[[Thomas Krug |Krug, Thomas]]||perchlorate
|-
|[[Injection Techniques for Liquid Amendments]]||[[Thomas Krug |Krug, Thomas]]||amendment injection
|-
|[[Transition of Aqueous Film Forming Foam (AFFF) Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances (PFAS)]]
|[[Dr. Johnsie Ray Lang|Lang, Johnsie Ray, Ph.D.]]||PFAS
|
|-
|[[Characterization Methods – Hydraulic Conductivity]]
|[[Dr. Gaisheng Liu|Liu, Gaisheng, Ph.D.]]
|
|-
|[[Compound Specific Isotope Analysis (CSIA)]]||[[Dr. Barbara Sherwood Lollar, F.R.S.C. |Lollar, Barbara S., FRSC]]||Compound Specific Isotope Analysis (CSIA)
|-
|[[Passive Sampling of Munitions Constituents|Munitions Constituents - Passive Sampling]]
|[[Dr. Guilherme Lotufo|Lotufo, Guilerme, Ph.D.]]
|
|-
|[[Vapor Intrusion (VI)]]||[[Chris Lutes |Lutes, Chris]]||vapor intrusion
|-
|[[Bioremediation - Anaerobic]]||[[Leah MacKinnon, M.A.Sc., P. Eng.|MacKinnon, Leah]]||anaerobic bioremediation
|-
|[[Bioremediation - Anaerobic Design Considerations]]||[[Leah MacKinnon, M.A.Sc., P. Eng.|MacKinnon, Leah]]||
|-
|[[Biodegradation - 1,4-Dioxane]]
|[[Dr. Shaily Mahendra|Mahendra, Shaily, Ph.D.]]
|
|-
|[[Direct Push (DP) Technology]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push, DP, DP machines, DP technology
|-
|[[Direct Push Sampling]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push sampling, soil sampling, groundwater sampling, well installation, soil vapor sampling (in regards to DP)
|-
|[[Direct Push Logging]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push logging, Cone Penetration Testing, CPT, Electrical Conductivity, EC, Hydraulic Profiling Tool, HPT, Membrane Interface Probe, MIP, Optical Imaging Profiler, OIP
|-
|[[Downscaled High Resolution Datasets for Climate Change Projections]]
|[[Dr. Rao Kotamarthi|Kotamarthi, Rao, Ph.D.]]
|
|-
|[[Remediation Performance Assessment at Chlorinated Solvent Sites]]||[[Travis McGuire|McGuire, Travis]]||multi-site studies
|-
|[[LNAPL Conceptual Site Models]]
|[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]
|
|-
|[[Long-Term Monitoring (LTM) - Data Analysis]]||[[Dr. Thomas McHugh |McHugh, Thomas, Ph.D.]]||data analysis, analysis methods (in regards to LTM)
|-
|[[Long-Term Monitoring (LTM) - Data Variability]]||[[Dr. Thomas McHugh |McHugh, Thomas, Ph.D.]]||data variability (in regards to LTM), LTM evaluation
|-
|-
|[[Munitions Constituents - Abiotic Reduction]]||[[Dr. Jimmy Murillo-Gelvez |Murillo-Gelvez, Jimmy, Ph.D.]]
|-
|[[Supercritical Water Oxidation (SCWO)]]
|Nagar, Kobe
|
|-
|[[Matrix Diffusion]]
|[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]
|
|-
|[[Groundwater Flow and Solute Transport]]||[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]||groundwater flow, advection, dispersion, diffusion, molecular diffusion, mechanical dispersion
|-
|[[Molecular Biological Tools - MBTs]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||MBT, Molecular Biological Tool(s)
|-
|[[Quantitative Polymerase Chain Reaction (qPCR)]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||qPCR, Polymerase Chain Reaction
|-
|[[Sediment Capping]]
|[[Dr. Danny Reible|Reible, Danny]]
|
|-
|[[Stable Isotope Probing (SIP)]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||SIP, Stable Isotope Probing
|-
|[[Metagenomics]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||Metagenomics
|-
|[[Natural Source Zone Depletion (NSZD)]]||[[Tom Palaia |Palaia, Tom]]||natural source zone depletion, NSZD
|-
|[[Climate Change Effects on Wildlife]]
|[[Dr. Breanna F. Powers|Powers, Breanna, PhD.]]
|
|-
|[[Amendment Distribution in Low Conductivity Materials]]||[[Dr. Stephen Richardson |Richardson, Stephen, Ph.D.]]||
|-
|[[Polycyclic Aromatic Hydrocarbons (PAHs)]]||[[Dr. Stephen Richardson |Richardson, Stephen, Ph.D.]]||Polycyclic Aromatic Hydrocarbons, PAH(s)
|-
|[[Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)]]
|[[Florent Risacher|Risacher, Florent, M.Sc.]]
|
|-
|[[1,2,3-Trichloropropane]]||[[Dr. Alexandra Salter-Blanc |Salter-Blanc, Alexandra, Ph.D.]]||TCP, trichloropropane
|-
|[[Zerovalent Iron (ZVI) (Chemical Reduction - ISCR)]]||[[Dr. Alexandra Salter-Blanc |Salter-Blanc, Alexandra, Ph.D.]]||ZVI
|-
|[[Mercury in Sediments]]
|[[Dr. Grace Schwartz|Schwartz, Grace, Ph.D.]]
|
|-
|[[Munitions Constituents – Sample Extraction and Analytical Techniques|Munitions Constituents - Sample Extraction and Analytical Techniques]]
|[[Dr. Austin Scircle|Scircle, Austin]]
|
|-
|[[Geophysical Methods]]||[[Dr. Lee Slater |Slater, Lee, Ph.D.]]||geophysics
|-
|[[Geophysical Methods - Case Studies]]||[[Dr. Lee Slater |Slater, Lee, Ph.D.]]||geophysics
|-
|[[PFAS Treatment by Anion Exchange]]
|[[Dr. Timothy J. Strathmann|Strathmann, Timothy, Ph.D.]]
|PFAS
|-
|[[Munitions Constituents - Dissolution]]||[[Dr. Susan Taylor |Taylor, Susan, Ph.D.]]||explosive(s), dissolution,
|-
|[[PFAS Treatment by Electrical Discharge Plasma]]
|[[Dr. Selma Mededovic Thagard|Thagard, Selma Mededovic, Ph.D.]]
|PFAS
|-
|[[Restoration of Ecological Function in Terrestrial Systems Impacted by Invasive Species]]
|Thierry, Hugo, Ph.D.
|climate change, invasive species, restoration ecology
|-
|[[Chemical Reduction (In Situ - ISCR)]]||[[Dr. Paul Tratnyek |Tratnyek, Paul, Ph.D.]]||In Situ Chemical Reduction, ISCR
|-
|[[Injection Techniques - Viscosity Modification]]||[[Michael Truex |Truex, Michael]]||viscosity, viscosity modifiers, viscosity modification
|-
|[[Soil Vapor Extraction (SVE)]]||[[Michael Truex |Truex, Michael]]||soil vapor extraction, SVE
|-
|[[Munitions Constituents - Deposition]]||[[Michael R. Walsh, P.E., M.E.|Walsh, Michael, P.E.]]||explosive deposition, energetics deposition
|-
|[[Vapor Intrusion - Separation Distances from Petroleum Sources]]
|[[Dr. James Weaver|Weaver, James, Ph.D.]]
|vapor intrusion
|-
|[[Zerovalent Iron Permeable Reactive Barriers]]
|[[Dr. Richard Wilkin|Wilkin, Rick, Ph.D.]]
|
|-
|[[Monitored Natural Attenuation (MNA)]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, In Situ MNA, natural attenuation, natural attenuation processes
|-
|[[Monitored Natural Attenuation (MNA) of Fuels]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, natural attenuation, attenuate (when used in context related to petroleum hydrocarbons and fuel components)
|-
|[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, natural attenuation, attenuate (when used in context related to chlorinated solvents)
|-
|[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
|[[Dr. John Wilson|Wilson, John, Ph.D.]]
|MNA, natural attenuation
|-
|[[Chlorinated Solvents]]||[[Dr. Bilgen Yuncu, P.E. |Yuncu, Bilgen, Ph.D., P.E.]]||chlorinated solvents
|-
|[[Petroleum Hydrocarbons (PHCs)]]
|[[Dr. Bilgen Yuncu, P.E.|Yuncu, Bilgen, Ph.D., P.E.]]
|Petroleum Hydrocarbons (PHCs)
|-
|[[Photoactivated Reductive Defluorination - PFAS Destruction]]
|[[Dr. Suzanne Witt|Witt, Suzanne, Ph.D.]]
|PFAS
|
|-
|[[Hydrogeophysical Methods for Characterization and Monitoring of Groundwater-Surface Water Exchanges]]
|[[Dr. Lee Slater|Slater, Lee, Ph.D.]]
|geophysics, hydrogeophysical methods,
|
|-
|[[Hydrothermal Alkaline Treatment (HALT)]]
|[[Dr. Brian Pinkard|Pinkard, Brian]]
|
|-
|[[1,4-Dioxane]]
|[[Matthew Zenker|Zenker, Matthew]]
|
|-
|[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]
|[[Dr. John F. Stults|Stults, Dr. John]]
|PFAS, vadose zone, lysimeter, field investigation
|
|-
|[[PFAS Destruction by Ultraviolet/Sulfite Treatment]]||[[Dr. Yida Fang |Fang, Yida, Ph.D.]]||
|
|-
|}

Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)

2026-03-13T21:01:13Z

Admin:

Sediment porewater dialysis passive samplers, also known as “peepers,” are sampling devices that allow the measurement of dissolved inorganic ions in the porewater of a saturated sediment. Peepers function by allowing freely-dissolved ions in sediment porewater to diffuse across a micro-porous membrane towards water contained in an isolated compartment that has been inserted into sediment. Once retrieved after a deployment period, the resulting sample obtained can provide concentrations of freely-dissolved inorganic constituents in sediment, which provides measurements that can be used for understanding contaminant fate and risk. Peepers can also be used in the same manner in surface water, although this article is focused on the use of peepers in sediment.

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'''Related Article(s):'''

*[[Contaminated Sediments - Introduction]]
*[[Contaminated Sediment Risk Assessment]]
*[[In Situ Toxicity Identification Evaluation (iTIE) | In Situ Toxicity Identification Evaluation]]
*[[Mercury in Sediments]]
*[[Passive Sampling of Munitions Constituents]]
*[[Passive Sampling of Sediments]]
*[[Sediment Capping]]

'''Contributor(s):''' [[Florent Risacher|Florent Risacher, M.Sc]]. and [[Dr. Jason Conder|Dr. Jason Conder]]

'''Key Resource(s):'''

*A review of peeper passive sampling approaches to measure the availability of inorganics in sediment porewater<ref>Risacher, F.F., Schneider, H., Drygiannaki, I., Conder, J., Pautler, B.G., and Jackson, A.W., 2023. A Review of Peeper Passive Sampling Approaches to Measure the Availability of Inorganics in Sediment Porewater. Environmental Pollution, 328, Article 121581. [https://doi.org/10.1016/j.envpol.2023.121581 doi: 10.1016/j.envpol.2023.121581]  [//www.enviro.wiki/images/4/4f/RisacherEtAl2023a.pdf Article pdf]</ref>

*Best Practices User’s Guide: Standardizing Sediment Porewater Passive Samplers for Inorganic Constituents of Concern<ref name="RisacherEtAl2023">Risacher, F.F., Nichols, E., Schneider, H., Lawrence, M., Conder, J., Sweett, A., Pautler, B.G., Jackson, W.A., Rosen, G., 2023b. Best Practices User’s Guide: Standardizing Sediment Porewater Passive Samplers for Inorganic Constituents of Concern, ESTCP ER20-5261. [https://serdp-estcp.mil/projects/details/db871313-fbc0-4432-b536-40c64af3627f Project Website]  [//www.enviro.wiki/images/4/42/ER20-5261BPUG.pdf Report.pdf]</ref>

*[https://serdp-estcp.mil/projects/details/db871313-fbc0-4432-b536-40c64af3627f/er20-5261-project-overview Standardizing Sediment Porewater Passive Samplers for Inorganic Constituents of Concern, ESTCP Project ER20-5261]

==Introduction==
Biologically available inorganic constituents associated with sediment toxicity can be quantified by measuring the freely-dissolved fraction of contaminants in the porewater<ref>Conder, J.M., Fuchsman, P.C., Grover, M.M., Magar, V.S., Henning, M.H., 2015. Critical review of mercury SQVs for the protection of benthic invertebrates. Environmental Toxicology and Chemistry, 34(1), pp. 6-21. [https://doi.org/10.1002/etc.2769 doi: 10.1002/etc.2769]   [//www.enviro.wiki/images/8/8d/ConderEtAl2015.pdf Article pdf]</ref><ref name="ClevelandEtAl2017">Cleveland, D., Brumbaugh, W.G., MacDonald, D.D., 2017. A comparison of four porewater sampling methods for metal mixtures and dissolved organic carbon and the implications for sediment toxicity evaluations. Environmental Toxicology and Chemistry, 36(11), pp. 2906-2915. [https://doi.org/10.1002/etc.3884 doi: 10.1002/etc.3884]</ref>. Classical sediment porewater analysis usually consists of collecting large volumes of bulk sediments which are then mechanically squeezed or centrifuged to produce a supernatant, or suction of porewater from intact sediment, followed by filtration and collection<ref name="GruzalskiEtAl2016">Gruzalski, J.G., Markwiese, J.T., Carriker, N.E., Rogers, W.J., Vitale, R.J., Thal, D.I., 2016. Pore Water Collection, Analysis and Evolution: The Need for Standardization. In: Reviews of Environmental Contamination and Toxicology, Vol. 237, pp. 37–51. Springer. [https://doi.org/10.1007/978-3-319-23573-8_2 doi: 10.1007/978-3-319-23573-8_2]</ref>. The extraction and measurement processes present challenges due to the heterogeneity of sediments, physical disturbance, high reactivity of some complexes, and interaction between the solid and dissolved phases, which can impact the measured concentration of dissolved inorganics<ref>Peijnenburg, W.J.G.M., Teasdale, P.R., Reible, D., Mondon, J., Bennett, W.W., Campbell, P.G.C., 2014. Passive Sampling Methods for Contaminated Sediments: State of the Science for Metals. Integrated Environmental Assessment and Management, 10(2), pp. 179–196. [https://doi.org/10.1002/ieam.1502 doi: 10.1002/ieam.1502]  [//www.enviro.wiki/images/9/99/PeijnenburgEtAl2014.pdf Article pdf]</ref>. For example, sampling disturbance can affect redox conditions<ref name="TeasdaleEtAl1995">Teasdale, P.R., Batley, G.E., Apte, S.C., Webster, I.T., 1995. Pore water sampling with sediment peepers. Trends in Analytical Chemistry, 14(6), pp. 250–256. [https://doi.org/10.1016/0165-9936(95)91617-2 doi: 10.1016/0165-9936(95)91617-2]</ref><ref>Schroeder, H., Duester, L., Fabricius, A.L., Ecker, D., Breitung, V., Ternes, T.A., 2020. Sediment water (interface) mobility of metal(loid)s and nutrients under undisturbed conditions and during resuspension. Journal of Hazardous Materials, 394, Article 122543. [https://doi.org/10.1016/j.jhazmat.2020.122543 doi: 10.1016/j.jhazmat.2020.122543] [//www.enviro.wiki/images/6/6d/SchroederEtAl2020.pdf Article pdf]</ref>, which can lead to under or over representation of inorganic chemical concentrations relative to the true dissolved phase concentration in the sediment porewater<ref>Wise, D.E., 2009. Sampling techniques for sediment pore water in evaluation of reactive capping efficacy. Master of Science Thesis. University of New Hampshire Scholars’ Repository. 178 pages. [https://scholars.unh.edu/thesis/502 Website] [//www.enviro.wiki/images/5/57/Wise2009.pdf Report.pdf]</ref><ref name="GruzalskiEtAl2016" />.

To address the complications with mechanical porewater sampling, passive sampling approaches for inorganics have been developed to provide a method that has a low impact on the surrounding geochemistry of sediments and sediment porewater, thus enabling more precise measurements of inorganics<ref name="ClevelandEtAl2017" />. Sediment porewater dialysis passive samplers, also known as “peepers,” were developed more than 45 years ago<ref name="Hesslein1976">Hesslein, R.H., 1976. An in situ sampler for close interval pore water studies. Limnology and Oceanography, 21(6), pp. 912-914. [https://doi.org/10.4319/lo.1976.21.6.0912 doi: 10.4319/lo.1976.21.6.0912] [//www.enviro.wiki/images/c/c7/Hesslein1976.pdf Article pdf]</ref> and refinements to the method such as the use of reverse tracers have been made, improving the acceptance of the technology as decision making tool.

==Peeper Designs==
[[File:RisacherFig1.png|thumb|300px|Figure 1. Conceptual illustration of peeper construction showing (top, left to right) the peeper cap (optional), peeper membrane and peeper chamber, and (bottom) an assembled peeper containing peeper water]]
[[File:RisacherFig2.png | thumb |400px| Figure 2. Example of Hesslein<ref name="Hesslein1976" /> general peeper design (42 peeper chambers), from [https://www.usgs.gov/media/images/peeper-samplers USGS]]]
[[File:RisacherFig3.png | thumb |400px| Figure 3. Peeper deployment structure to allow the measurement of metal availability in different sediment layers using five single-chamber peepers (Photo: Geosyntec Consultants)]]
Peepers (Figure 1) are inert containers with a small volume (typically 1-100 mL) of purified water (“peeper water”) capped with a semi-permeable membrane. Peepers can be manufactured in a wide variety of formats (Figure 2, Figure 3) and deployed in in various ways.

Two designs are commonly used for peepers. Frequently, the designs are close adaptations of the original multi-chamber Hesslein design<ref name="Hesslein1976" /> (Figure 2), which consists of an acrylic sampler body with multiple sample chambers machined into it. Peeper water inside the chambers is separated from the outside environment by a semi-permeable membrane, which is held in place by a top plate fixed to the sampler body using bolts or screws. An alternative design consists of single-chamber peepers constructed using a single sample vial with a membrane secured over the mouth of the vial, as shown in Figure 3, and applied in Teasdale ''et al.''<ref name="TeasdaleEtAl1995" />, Serbst ''et al.''<ref>Serbst, J.R., Burgess, R.M., Kuhn, A., Edwards, P.A., Cantwell, M.G., Pelletier, M.C., Berry, W.J., 2003. Precision of dialysis (peeper) sampling of cadmium in marine sediment interstitial water. Archives of Environmental Contamination and Toxicology, 45(3), pp. 297–305. [https://doi.org/10.1007/s00244-003-0114-5 doi: 10.1007/s00244-003-0114-5]</ref>, Thomas and Arthur<ref name="ThomasArthur2010">Thomas, B., Arthur, M.A., 2010. Correcting porewater concentration measurements from peepers: Application of a reverse tracer. Limnology and Oceanography: Methods, 8(8), pp. 403–413. [https://doi.org/10.4319/lom.2010.8.403 doi: 10.4319/lom.2010.8.403] [//www.enviro.wiki/images/7/7b/ThomasArthur2010.pdf Article pdf]</ref>, Passeport ''et al.''<ref>Passeport, E., Landis, R., Lacrampe-Couloume, G., Lutz, E.J., Erin Mack, E., West, K., Morgan, S., Lollar, B.S., 2016. Sediment Monitored Natural Recovery Evidenced by Compound Specific Isotope Analysis and High-Resolution Pore Water Sampling. Environmental Science and Technology, 50(22), pp. 12197–12204. [https://doi.org/10.1021/acs.est.6b02961 doi: 10.1021/acs.est.6b02961]</ref>, and Risacher ''et al.''<ref name="RisacherEtAl2023" />. The vial is filled with deionized water, and the membrane is held in place using the vial cap or an o-ring. Individual vials are either directly inserted into sediment or are incorporated into a support structure to allow multiple single-chamber peepers to be deployed at once over a given depth profile (Figure 3).

==Peepers Preparation, Deployment and Retrieval==
[[File:RisacherFig4.png | thumb |300px| Figure 4: Conceptual illustration of peeper passive sampling in a sediment matrix, showing peeper immediately after deployment (top) and after equilibration between the porewater and peeper chamber water (bottom)]]
Peepers are often prepared in laboratories but are also commercially available in a variety of designs from several suppliers. Peepers are prepared by first cleaning all materials to remove even trace levels of metals before assembly. The water contained inside the peeper is sometimes deoxygenated, and in some cases the peeper is maintained in a deoxygenated atmosphere until deployment<ref>Carignan, R., St‐Pierre, S., Gachter, R., 1994. Use of diffusion samplers in oligotrophic lake sediments: Effects of free oxygen in sampler material. Limnology and Oceanography, 39(2), pp. 468-474. [https://doi.org/10.4319/lo.1994.39.2.0468 doi: 10.4319/lo.1994.39.2.0468] [//www.enviro.wiki/images/9/9c/CarignanEtAl1994.pdf Article pdf]</ref>. However, recent studies<ref name="RisacherEtAl2023" /> have shown that deoxygenation prior to deployment does not significantly impact sampling results due to oxygen rapidly diffusing out of the peeper during deployment. Once assembled, peepers are usually shipped in a protective bag inside a hard-case cooler for protection.

Peepers are deployed by insertion into sediment for a period of a few days to a few weeks. Insertion into the sediment can be achieved by wading to the location when the water depth is shallow, by using push poles for deeper deployments<ref name="RisacherEtAl2023" />, or by professional divers for the deepest sites. If divers are used, an appropriate boat or ship will be required to accommodate the diver and their equipment. Whichever method is used, peepers should be attached to an anchor or a small buoy to facilitate retrieval at the end of the deployment period.

During deployment, passive sampling is achieved via diffusion of inorganics through the peeper’s semi-permeable membrane, as the enclosed volume of peeper water equilibrates with the surrounding sediment porewater (Figure 4). It is assumed that the peeper insertion does not greatly alter geochemical conditions that affect freely-dissolved inorganics. Additionally, it is assumed that the peeper water equilibrates with freely-dissolved inorganics in sediment in such a way that the concentration of inorganics in the peeper water would be equal to that of the concentration of inorganics in the sediment porewater.

After retrieval, the peepers are brought to the surface and usually preserved until they can be processed. This can be achieved by storing the peepers inside a sealable, airtight bag with either inert gas or oxygen absorbing packets<ref name="RisacherEtAl2023" />. The peeper water can then be processed by quickly pipetting it into an appropriate sample bottle which usually contains a preservative (e.g., nitric acid for metals). This step is generally conducted in the field. Samples are stored on ice to maintain a temperature of less than 4°C and shipped to an analytical laboratory. The samples are then analyzed for inorganics by standard methods (i.e., USEPA SW-846). The results obtained from the analytical laboratory are then used directly or assessed using the equations below if a reverse tracer is used because deployment time is insufficient for all analytes to reach equilibrium.

==Equilibrium Determination (Tracers)==
The equilibration period of peepers can last several weeks and depends on deployment conditions, analyte of interest, and peeper design. In many cases, it is advantageous to use pre-equilibrium methods that can use measurements in peepers deployed for shorter periods to predict concentrations at equilibrium<ref name="USEPA2017">USEPA, 2017. Laboratory, Field, and Analytical Procedures for Using Passive Sampling in the Evaluation of Contaminated Sediments: User’s Manual. EPA/600/R-16/357. [//www.enviro.wiki/images/0/08/EPA_600_R-16_357.pdf Report.pdf]</ref>.

Although the equilibrium concentration of an analyte in sediment can be evaluated by examining analyte results for peepers deployed for several different amounts of time (i.e., a time series), this is impractical for typical field investigations because it would require several mobilizations to the site to retrieve samplers. Alternately, reverse tracers (referred to as a performance reference compound when used with organic compound passive sampling) can be used to evaluate the percentage of equilibrium reached by a passive sampler.

Thomas and Arthur<ref name="ThomasArthur2010" /> studied the use of a reverse tracer to estimate percent equilibrium in lab experiments and a field application. They concluded that bromide can be used to estimate concentrations in porewater using measurements obtained before equilibrium is reached. Further studies were also conducted by Risacher ''et al.''<ref name="RisacherEtAl2023" /> showed that lithium can also be used as a tracer for brackish and saline environments. Both studies included a mathematical model for estimating concentrations of ions in external media (''C0'') based on measured concentrations in the peeper chamber (''Cp,t''), the elimination rate of the target analyte (''K'') and the deployment time (''t''):
 
{|
| ||'''Equation 1:'''
|     [[File: Equation1r.png]]
|-
|Where:|| ||
|-
| ||''C0''||is the freely dissolved concentration of the analyte in the sediment (mg/L or μg/L), sometimes referred to as ''Cfree ''
|-
| ||''Cp,t''||is the measured concentration of the analyte in the peeper at time of retrieval (mg/L or μg/L)
|-
| ||''K''||is the elimination rate of the target analyte
|-
| ||''t''||is the deployment time (days)
|}

The elimination rate of the target analyte (''K'') is calculated using Equation 2:
 
{|
| ||'''Equation 2:'''
|     [[File: Equation2r.png]]
|-
|Where:|| ||
|-
| ||''K''||is the elimination rate of the target analyte
|-
| ||''Ktracer''||is the elimination rate of the tracer
|-
| ||''D''||is the free water diffusivity of the analyte (cm2/s)
|-
| ||''Dtracer''||is the free water diffusivity of the tracer (cm2/s)
|}

The elimination rate of the tracer (''Ktracer'') is calculated using Equation 3:
 
{|
| ||'''Equation 3:'''
|         [[File: Equation3r2.png]]
|-
|Where:|| ||
|-
| ||''Ktracer''||is the elimination rate of the tracer
|-
| ||''Ctracer,i''||is the measured initial concentration of the tracer in the peeper prior to deployment (mg/L or μg/L)
|-
| ||''Ctracer,t''||is the measured final concentration of the tracer in the peeper at time of retrieval (mg/L or μg/L)
|-
| ||''t''||is the deployment time (days)
|}

Using this set of equations allows the calculation of the porewater concentration of the analyte prior to its equilibrium with the peeper water. A template for these calculations can be found in the appendix of Risacher ''et al.''<ref name="RisacherEtAl2023" />.

==Using Peeper Data at a Sediment Site==
Peeper data can be used to enable site specific decision making in a variety of ways. Some of the most common uses for peepers and peeper data are discussed below.

'''Nature and Extent:''' Multiple peepers deployed in sediment can help delineate areas of increased metal availability. Peepers are especially helpful for sites that are comprised of coarse, relatively inert materials that may not be conducive to traditional bulk sediment sampling. Because much of the inorganics present in these types of sediments may be associated with the porewater phase rather than the solid phase, peepers can provide a more representative measurement of C0. Additionally, at sites where tidal pumping or groundwater flux may be influencing the nature and extent of inorganics, peepers can provide a distinct advantage to bulk sediment sampling or other point-in-time measurements, as peepers can provide an average measurement that integrates the variability in the hydrodynamic and chemical conditions over time.

'''Sources and Fate:''' A considerable advantage to using peepers is that C0 results are expressed as concentration in units of mass per volume (e.g., mg/L), providing a common unit of measurement to compare across multiple media. For example, synchronous measurements of C0 using peepers deployed in both surface water and sediment can elucidate the potential flux of inorganics from sediment to surface water. Paired measurements of both C0 and bulk metals in sediment can also allow site specific sediment-porewater partition coefficients to be calculated. These values can be useful in understanding and predicting contaminant fate, especially in situations where the potential dissolution of metals from sediment are critical to predict, such as when sediment is dredged.

'''Direct Toxicity to Aquatic Life:''' Peepers are frequently used to understand the potential direct toxicity to aquatic life, such as benthic invertebrates and fish. A C0 measurement obtained from a peeper deployed in sediment (''in situ'') or surface water (''ex situ''), can be compared to toxicological benchmarks for aquatic life to understand the potential toxicity to aquatic life and to set remediation goals<ref name="USEPA2017" />. C0 measurements can also be incorporated in more sophisticated approaches, such as the Biotic Ligand Model<ref>Santore, C.R., Toll, E.J., DeForest, K.D., Croteau, K., Baldwin, A., Bergquist, B., McPeek, K., Tobiason, K., and Judd, L.N., 2022. Refining our understanding of metal bioavailability in sediments using information from porewater: Application of a multi-metal BLM as an extension of the Equilibrium Partitioning Sediment Benchmarks. Integrated Environmental Assessment and Management, 18(5), pp. 1335–1347. [https://doi.org/10.1002/ieam.4572 doi: 10.1002/ieam.4572]</ref> to understand the potential for toxicity or the need to conduct toxicological testing or ecological evaluations.

'''Bioaccumulation of Inorganics by Aquatic Life:''' Peepers can also be used to understand site specific relationship between C0 and concentrations of inorganics in aquatic life. For example, measuring C0 in sediment from which organisms are collected and analyzed can enable the estimation of a site-specific uptake factor. This C0-to-organism uptake factor (or model) can then be applied for a variety of uses, including predicting the concentration of inorganics in other organisms, or estimating a sediment C0 value that would be safe for consumption by wildlife or humans. Because several decades of research have found that the correlation between C0 measurements and bioavailability is usually better than the correlation between measurements of chemicals in bulk sediment and bioavailability, C0-to-organism uptake factors are likely to be more accurate than uptake factors based on bulk sediment testing.

'''Evaluating Sediment Remediation Efficacy:''' Passive sampling has been used widely to evaluate the efficacy of remedial actions such as active amendments, thin layer placements, and capping to reduce the availability of contaminants at sediment sites. A particularly powerful approach is to compare baseline (pre-remedy) C0 in sediment to C0 in sediment after the sediment remedy has been applied. Peepers can be used in this context for inorganics, allowing the sediment remedy’s success to be evaluated and monitored in laboratory benchtop remedy evaluations, pilot scale remedy evaluations, and full-scale remediation monitoring.

==References==
<references />

==See Also==

*[https://vimeo.com/809180171/c276c1873a Peeper Deployment Video]
*[https://vimeo.com/811073634/303edf2693 Peeper Retrieval Video]
*[https://vimeo.com/811328715/aea3073540 Peeper Processing Video]
*[https://sepub-prod-0001-124733793621-us-gov-west-1.s3.us-gov-west-1.amazonaws.com/s3fs-public/2024-09/ER20-5261%20Fact%20Sheet.pdf?VersionId=malAixSQQM3mWCRiaVaxY8wLdI0jE1PX Fact Sheet]

Passive Sampling of Sediments

2026-03-13T21:00:54Z

Admin:

"Passive sampling" refers to a group of methods used to quantify the availability of organic contaminants to move between different media and/or to react in environmental systems such as indoor air, lake waters, or contaminated sediment beds. To do this, the passive sampling material is deployed in the environmental system and allowed to absorb chemicals of interest via diffusive transfers from the surroundings. Upon recovery of the passive sampler, the accumulated contaminants are measured, and the concentrations in the sampler are interpreted to infer the chemical concentrations in specific surrounding media like porewater in a sediment bed. Such data are then useful inputs for site assessments such as those seeking to quantify fluxes from contaminated sediment beds to overlying waters or to evaluate the risk of significant uptake into benthic infauna and the larger food web.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Contaminated Sediments - Introduction]]
*[[Contaminated Sediment Risk Assessment]]
*[[In Situ Toxicity Identification Evaluation (iTIE) | In Situ Toxicity Identification Evaluation]]
*[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
*[[Passive Sampling of Munitions Constituents]]
*[[Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)]]

'''Contributor(s):''' [[Dr. Philip M. Gschwend]]

'''Key Resource(s):'''

*Validating the Use of Performance Reference Compounds in Passive Samplers to Assess Porewater Concentrations in Sediment Beds<ref name="Apell2014">Apell, J.N. and Gschwend, P.M., 2014. Validating the Use of Performance Reference Compounds in Passive Samplers to Assess Porewater Concentrations in Sediment Beds. Environmental Science and Technology, 48(17), pp. 10301-10307. [https://doi.org/10.1021/es502694g DOI: 10.1021/es502694g]</ref>

*''In situ'' passive sampling of sediments in the Lower Duwamish Waterway Superfund site: Replicability, comparison with ''ex situ'' measurements, and use of data<ref name="Apell2016">Apell, J.N., and Gschwend, P.M., 2016. ''In situ'' passive sampling of sediments in the Lower Duwamish Waterway Superfund site: Replicability, comparison with ''ex situ'' measurements, and use of data. Environmental Pollution, 218, pp. 95-101. [https://doi.org/10.1016/j.envpol.2016.08.023 DOI: 10.1016/j.envpol.2016.08.023]   [//www.enviro.wiki/images/5/53/ApellGschwend2016.pdf Authors’ Manuscript]</ref>

*Laboratory, Field, and Analytical Procedures for Using Passive Sampling in the Evaluation of Contaminated Sediments: User’s Manual<ref name="Burgess2017">Burgess, R.M., Kane Driscoll, S.B., Burton, A., Gschwend, P.M., Ghosh, U., Reible, D., Ahn, S., and Thompson, T., 2017. Laboratory, Field, and Analytical Procedures for Using Passive Sampling in the Evaluation of Contaminated Sediments: User’s Manual, EPA/600/R-16/357. SERDP/ESTCP and U.S. EPA, Office of Research and Development, Washington, DC 20460. [https://cfpub.epa.gov/si/si_public_record_report.cfm?Lab=NHEERL&dirEntryID=308731 Website]   [//www.enviro.wiki/images/c/c5/EPA600R16357.pdf Report.pdf]</ref>

==Introduction==
[[File: Gschwend1w2fig1.png | thumb | 300px | Figure 1. A representation of a clam living in a sediment bed that contains a chemical contaminant (depicted as red hexagons). The contaminant is partly dissolved in the sediment porewater between the solid grains, and partly associated with solid phases, like natural organic matter and "black carbons" such as soots from diesel engines and chars emitted during forest fires. All of these liquid and solid materials can exchange their contaminant loads with one another, with the distributions dependent on the chemical's relative affinity for each material. When an organism like a clam lives in such a system, the chemical contaminant is accumulated into the organism, until the concentration of the chemical in the organism is also equilibrated with the other solids and liquid(s) present.]]
Environmental media such as sediments typically contain many different materials or phases, including liquid solutions (e.g. water, [[Light Non-Aqueous Phase Liquids (LNAPLs)| nonaqueous phase liquids]] like spilled oils) and diverse solids (e.g., quartz, aluminosilicate clays, and combustion-derived soots). Further, the chemical concentration in the porewater medium includes both molecules that are "truly dissolved" in the water and others that are associated with colloids in the porewater<ref name="Brownawell1986">Brownawell, B.J., and Farrington, J.W., 1986. Biogeochemistry of PCBs in interstitial waters of a coastal marine sediment. Geochimica et Cosmochimica Acta, 50(1), pp. 157-169. [https://doi.org/10.1016/0016-7037(86)90061-X DOI: 10.1016/0016-7037(86)90061-X]   Free download available from: [https://semspub.epa.gov/work/01/268631.pdf US EPA].</ref><ref name="Chin1992">Chin, Y.P., and Gschwend, P.M., 1992. Partitioning of Polycyclic Aromatic Hydrocarbons to Marine Porewater Organic Colloids. Environmental Science and Technology, 26(8), pp. 1621-1626. [https://doi.org/10.1021/es00032a020 DOI: 10.1021/es00032a020]</ref><ref name="Achman1996">Achman, D.R., Brownawell, B.J., and Zhang, L., 1996. Exchange of Polychlorinated Biphenyls Between Sediment and Water in the Hudson River Estuary. Estuaries, 19(4), pp. 950-965. [https://doi.org/10.2307/1352310 DOI: 10.2307/1352310]   Free download available from: [https://www.academia.edu/download/55010335/135231020171114-2212-b93vic.pdf Academia.edu]</ref>. As a result, contaminant chemicals distribute among these diverse media (Figure 1) according to their affinity for each and the amount of each phase in the system<ref name="Gustafsson1996">Gustafsson, Ö., Haghseta, F., Chan, C., MacFarlane, J., and Gschwend, P.M., 1996. Quantification of the Dilute Sedimentary Soot Phase: Implications for PAH Speciation and Bioavailability. Environmental Science and Technology, 31(1), pp. 203-209. [https://doi.org/10.1021/es960317s DOI: 10.1021/es960317s]</ref><ref name="Luthy1997">Luthy, R.G., Aiken, G.R., Brusseau, M.L., Cunningham, S.D., Gschwend, P.M., Pignatello, J.J., Reinhard, M., Traina, S.J., Weber, W.J., and Westall, J.C., 1997. Sequestration of Hydrophobic Organic Contaminants by Geosorbents. Environmental Science and Technology, 31(12), pp. 3341-3347. [https://doi.org/10.1021/es970512m DOI: 10.1021/es970512m]</ref><ref name="Lohmann2005">Lohmann, R., MacFarlane, J.K., and Gschwend, P.M., 2005. Importance of Black Carbon to Sorption of Native PAHs, PCBs, and PCDDs in Boston and New York Harbor Sediments. Environmental Science and Technology, 39(1), pp.141-148. [https://doi.org/10.1021/es049424+ DOI: 10.1021/es049424+]</ref><ref name="Cornelissen2005">Cornelissen, G., Gustafsson, Ö., Bucheli, T.D., Jonker, M.T., Koelmans, A.A., and van Noort, P.C., 2005. Extensive Sorption of Organic Compounds to Black Carbon, Coal, and Kerogen in Sediments and Soils: Mechanisms and Consequences for Distribution, Bioaccumulation, and Biodegradation. Environmental Science and Technology, 39(18), pp. 6881-6895. [https://doi.org/10.1021/es050191b DOI: 10.1021/es050191b]</ref><ref name="Koelmans2009">Koelmans, A.A., Kaag, K., Sneekes, A., and Peeters, E.T.H.M., 2009. Triple Domain in Situ Sorption Modeling of Organochlorine Pesticides, Polychlorobiphenyls, Polyaromatic Hydrocarbons, Polychlorinated Dibenzo-p-Dioxins, and Polychlorinated Dibenzofurans in Aquatic Sediments. Environmental Science and Technology, 43(23), pp. 8847-8853. [https://doi.org/10.1021/es9021188 DOI: 10.1021/es9021188]</ref>. As such, the chemical concentration in any one medium (e.g., truly dissolved in porewater) in a multi-material system like sediment is very hard to know from measures of the total sediment concentration, which unfortunately is the information typically found by analyzing for chemicals in sediment samples.

If an animal such as a clam moves into this system (Figure 1), it will also accumulate the chemical in its tissues from the loads in all the other materials. This can lead to exposures of the chemical to other organisms, including humans, who may eat such animals. Predicting the quantity of contaminant in the animal requires knowledge of the relative affinities of the chemical for the animal versus the sediment materials. For example, if one knew the chemical's truly dissolved concentration in the porewater and could reasonably assume the chemical of interest in the animal has mostly accumulated in its lipids (as is often the case for very hydrophobic compounds), then one could estimate the chemical concentration in the animal (''Canimal'', typically in units of μg/kg animal wet weight) using a lipid-water [[Wikipedia: Partition coefficient | partition coefficient]], ''Klipid-water'', typically in units of (μg/kg lipid)'''/'''(μg/L water), and the porewater concentration of the chemical (''Cporewater'', in μg/L) with Equation 1.
{|
|
|-
| ||Equation 1.
| style="text-align:center;" |<big>'''''Canimal '''=''' flipid '''x''' Klipid-water '''x''' Cporewater'''''</big>
|-
|where:
|-
| ||''flipid''||is the fraction lipids contribute to the total wet weight of the animal (kg lipid/kg animal wet weight), and
|-
| ||''Cporewater''||is the freely dissolved contaminant concentration in the porewater surrounding the animal.
|}

While there is a great deal of information on the values of ''Klipid-water'' for many chemicals<ref name="Schwarzenbach2017">Schwarzenbach, R.P., Gschwend, P.M., and Imboden, D.M., 2017. Environmental Organic Chemistry, 3rd edition. Ch. 16: Equilibrium Partitioning from Water and Air to Biota, pp. 469-521. John Wiley and Sons. ISBN: 978-1-118-76723-8</ref>, it is often very inaccurate to estimate truly dissolved porewater concentrations from total sediment concentrations using assumptions about the affinity of those chemicals for the solids in the system<ref name="Gustafsson1996" />. Further, it is difficult to isolate porewater without colloids and/or measure the very low truly dissolved concentrations of hydrophobic contaminants of concern like [[Polycyclic Aromatic Hydrocarbons (PAHs) | polycyclic aromatic hydrocarbons (PAHs)]], [[Wikipedia: Polychlorinated biphenyl | polychlorinated biphenyls (PCBs)]], nonionic pesticides like [[Wikipedia: DDT | dichlorodiphenyltrichloroethane (DDT)]], and [[Wikipedia: Polychlorinated dibenzodioxins | polychlorinated dibenzo-p-dioxins (PCDDs)]]/[[Wikipedia: Polychlorinated dibenzofurans | dibenzofurans (PCDFs)]]<ref name="Hawthorne2005">Hawthorne, S.B., Grabanski, C.B., Miller, D.J., and Kreitinger, J.P., 2005. Solid-Phase Microextraction Measurement of Parent and Alkyl Polycyclic Aromatic Hydrocarbons in Milliliter Sediment Pore Water Samples and Determination of KDOC Values. Environmental Science and Technology, 39(8), pp. 2795-2803. [https://doi.org/10.1021/es0405171 DOI: 10.1021/es0405171]</ref>.

==Passive Samplers==
One approach to address this problem for contaminated sediments is to insert into the sediment films of organic polymers like low density polyethylene (LDPE), polydimethylsiloxane (PDMS), or polyoxymethylene (POM) that can absorb such hydrophobic chemicals from their surroundings<ref name="Mayer2000">Mayer, P., Vaes, W.H., Wijnker, F., Legierse, K.C., Kraaij, R., Tolls, J., and Hermens, J.L., 2000. Sensing Dissolved Sediment Porewater Concentrations of Persistent and Bioaccumulative Pollutants Using Disposable Solid-Phase Microextraction Fibers. Environmental Science and Technology, 34(24), pp. 5177-5183. [https://doi.org/10.1021/es001179g DOI: 10.1021/es001179g]</ref><ref name="Booij2003">Booij, K., Hoedemaker, J.R., and Bakker, J.F., 2003. Dissolved PCBs, PAHs, and HCB in Pore Waters and Overlying Waters of Contaminated Harbor Sediments. Environmental Science and Technology, 37(18), pp. 4213-4220. [https://doi.org/10.1021/es034147c DOI: 10.1021/es034147c]</ref><ref name="Cornelissen2008">Cornelissen, G., Pettersen, A., Broman, D., Mayer, P., and Breedveld, G.D., 2008. Field testing of equilibrium passive samplers to determine freely dissolved native polycyclic aromatic hydrocarbon concentrations. Environmental Toxicology and Chemistry, 27(3), pp. 499-508. [https://doi.org/10.1897/07-253.1 DOI: 10.1897/07-253.1]</ref><ref name="Tomaszewski2008">Tomaszewski, J.E., and Luthy, R.G., 2008. Field Deployment of Polyethylene Devices to Measure PCB Concentrations in Pore Water of Contaminated Sediment. Environmental Science and Technology, 42(16), pp. 6086-6091. [https://doi.org/10.1021/es800582a DOI: 10.1021/es800582a]</ref><ref name="Fernandez2009">Fernandez, L.A., MacFarlane, J.K., Tcaciuc, A.P., and Gschwend, P.M., 2009. Measurement of Freely Dissolved PAH Concentrations in Sediment Beds Using Passive Sampling with Low-Density Polyethylene Strips. Environmental Science and Technology, 43(5), pp. 1430-1436. [https://doi.org/10.1021/es802288w DOI: 10.1021/es802288w]</ref><ref name="Arp2015">Arp, H.P.H., Hale, S.E., Elmquist Kruså, M., Cornelissen, G., Grabanski, C.B., Miller, D.J., and Hawthorne, S.B., 2015. Review of polyoxymethylene passive sampling methods for quantifying freely dissolved porewater concentrations of hydrophobic organic contaminants. Environmental Toxicology and Chemistry, 34(4), pp. 710-720. [https://doi.org/10.1002/etc.2864 DOI: 10.1002/etc.2864]   [https://setac.onlinelibrary.wiley.com/doi/epdf/10.1002/etc.2864 Free access article.]   [//www.enviro.wiki/images/f/f4/Arp2015.pdf Report.pdf]</ref><ref name="Apell2016" />. In this approach, the polymer is inserted in the sediment bed where it absorbs some of the contaminant load via the contaminant's diffusion into the polymer from the surroundings. When the polymer achieves sorptive equilibration with the sediments, the chemical concentration in the polymer, ''Cpolymer'' (μg/kg polymer), can be used to find the corresponding concentration in the porewater, ''Cporewater'' (μg/L), using a polymer-water partition coefficient, ''Kpolymer-water'' ((μg/kg polymer)'''/'''(μg/L water)), that has previously been found in laboratory testing<ref name="Lohmann2012">Lohmann, R., 2012. Critical Review of Low-Density Polyethylene’s Partitioning and Diffusion Coefficients for Trace Organic Contaminants and Implications for Its Use as a Passive Sampler. Environmental Science and Technology, 46(2), pp. 606-618. [https://doi.org/10.1021/es202702y DOI: 10.1021/es202702y]</ref><ref name="Ghosh2014">Ghosh, U., Kane Driscoll, S., Burgess, R.M., Jonker, M.T., Reible, D., Gobas, F., Choi, Y., Apitz, S.E., Maruya, K.A., Gala, W.R., Mortimer, M., and Beegan, C., 2014. Passive Sampling Methods for Contaminated Sediments: Practical Guidance for Selection, Calibration, and Implementation. Integrated Environmental Assessment and Management, 10(2), pp. 210-223. [https://doi.org/10.1002/ieam.1507 DOI: 10.1002/ieam.1507]   [https://setac.onlinelibrary.wiley.com/doi/epdf/10.1002/ieam.1507 Free access article.]   [//www.enviro.wiki/images/3/37/Ghosh2014.pdf Report.pdf]</ref>, as shown in Equation 2.
{|
|
|-
|        ||Equation 2.
| style="width:600px; text-align:center;" |<big>'''''Cporewater '''=''' Cpolymer '''/''' Kpolymer-water'''''</big>
|}

Such “passive uptake” by the polymer also reflects the availability of the chemicals for transport to adjacent systems (e.g., overlying surface waters) and for uptake into organisms (e.g., [[Wikipedia: Bioaccumulation | bioaccumulation]]). Thus, one can use the porewater concentrations to estimate the biotic accumulation of the chemicals, too. For example, for the concentration in the animal equilibrated with the sediment, ''Canimal'' (μg/kg animal), would be found by combining Equations 1 and 2 to get Equation 3.
{|
|
|-
|        ||Equation 3.
| style="width:700px; text-align:center;" |<big>'''''Canimal '''=''' flipid '''x''' Klipid-water '''x''' Cpolymer '''/''' Kpolymer-water'''''</big>
|}
[[File: Gschwend1w2fig2a.PNG | thumb | 300px | Figure 2a. Schematic plot of the initial concentrations of a PRC (green lines) in a polyethylene (PE) film inserted in a sediment showing constant concentration across the PE and zero concentration outside the PE. At the same time, a target contaminant of interest (red lines) initially has a constant concentration in the sediment outside the PE and zero concentration inside the PE.]][[File: Gschwend1w2fig2b.PNG | thumb | 300px | Figure 2b. After the PE has been deployed for a time, the PRC is depleted from the PE (green lines), especially near the surfaces contacting the sediment, and its concentration is building up outside the PE and diffusing away into the sediment. Meanwhile, the target chemical leaves the sediment and begins to diffuse into the PE (red lines). The "jumps" in concentration at the PE-sediment boundary reflect the equilibrium partitioning coefficient, ''KPE-sed = CPE '''/''' Csediment''.]]

==Performance Reference Compounds (PRCs)==
Perhaps unsurprisingly, pollutants with low water solubility like PAHs, PCBs, etc. do not diffuse quickly through sediment beds. As a result, their accumulation in polymeric materials in sediments can take a long time to achieve equilibration<ref name="Fernandez2009b">Fernandez, L. A., Harvey, C.F., and Gschwend, P.M., 2009. Using Performance Reference Compounds in Polyethylene Passive Samplers to Deduce Sediment Porewater Concentrations for Numerous Target Chemicals. Environmental Science and Technology, 43(23), pp. 8888-8894. [https://doi.org/10.1021/es901877a DOI: 10.1021/es901877a]</ref><ref name="Lampert2015">Lampert, D.J., Thomas, C., and Reible, D.D., 2015. Internal and external transport significance for predicting contaminant uptake rates in passive samplers. Chemosphere, 119, pp. 910-916. [https://doi.org/10.1016/j.chemosphere.2014.08.063 DOI: 10.1016/j.chemosphere.2014.08.063]   Free download available from: [https://www.academia.edu/download/44146586/chemosphere_2014.pdf Academia.edu]</ref><ref name="Apell2016b">Apell, J.N., Tcaciuc, A.P., and Gschwend, P.M., 2016. Understanding the rates of nonpolar organic chemical accumulation into passive samplers deployed in the environment: Guidance for passive sampler deployments. Integrated Environmental Assessment and Management, 12(3), pp. 486-492. [https://doi.org/10.1002/ieam.1697 DOI: 10.1002/ieam.1697]</ref>. This problem was recognized previously for passive samplers called [[Wikipedia: Semipermeable membrane devices | semipermeable membrane devices]] (SPMDs, e.g. polyethylene bags filled with triolein<ref name="Huckins2002">Huckins, J.N., Petty, J.D., Lebo, J.A., Almeida, F.V., Booij, K., Alvarez, D.A., Cranor, W.L., Clark, R.C., and Mogensen, B.B., 2002. Development of the Permeability/Performance Reference Compound Approach for In Situ Calibration of Semipermeable Membrane Devices. Environmental Science and Technology, 36(1), pp. 85-91. [https://doi.org/10.1021/es010991w DOI: 10.1021/es010991w]</ref>) that were deployed in surface waters. As a result, representative chemicals called performance reference compounds (PRCs) were uniformly impregnated into the samplers before their deployment in the environment, and the PRCs' diffusive losses out of the SPMD could then be used to quantify the fractional approach toward equilibration of the sampler with its environmental surroundings<ref name="Booij2002">Booij, K., Smedes, F., and van Weerlee, E.M., 2002. Spiking of performance reference compounds in low density polyethylene and silicone passive water samplers. Chemosphere 46(8), pp.1157-1161. [https://doi.org/10.1016/S0045-6535(01)00200-4 DOI: 10.1016/S0045-6535(01)00200-4]</ref><ref name="Huckins2002" />. A similar approach can be used for polymers inserted in sediment beds<ref name="Fernandez2009b" /><ref name="Apell2014" />. Commonly, isotopically labeled forms of the compounds of interest such as deuterated or 13C-labelled PAHs or PCBs are homogeneously impregnated into the polymers before their deployments. Upon insertion of the polymer into the sediment bed (or overlying waters or even air), the initially evenly distributed PRCs begin to diffuse out of the sampling polymer and into the surroundings (Figure 2).

Assuming the contaminants of interest undergo the same mass transfer restrictions limiting their rates of uptake into the polymer (e.g., diffusion through the sedimentary porous medium) that are also limiting transfers of the PRCs out of the polymer<ref name="Fernandez2009b" /><ref name="Apell2014" />, then fractional losses of the PRCs during a particular deployment can be used to adjust the accumulated contaminant loads to what they would have been at equilibrium with their surroundings with Equation 4.
{|
|
|-
| ||Equation 4.
| style="text-align:center;" |<big>'''''C(∞)polymer '''=''' C(t)polymer '''/''' fPRC lost'''''</big>
|-
|where:
|-
| ||''fPRC lost''||is the fraction of the PRC lost to outward diffusion,
|-
| ||''C(∞)polymer''||is the concentration of the contaminant in the polymer at equilibrium, and
|-
| ||''C(t)polymer''||is the concentration of the contaminant in the polymer after deployment time, t.                     
|}

Since investigators are commonly interested in many chemicals at the same time, it is impractical to have a PRC for each contaminant of interest. Instead, a representative set of PRCs is used to characterize the rates of polymer-environment exchange as a function of the PRCs' properties (e.g., diffusivities, partition coefficients), the characteristics of the sediments (e.g., porosity), and the nature of the polymer used (e.g., film thickness, affinity for the chemicals)<ref name="Fernandez2009b" /><ref name="Lampert2015" />. The resulting mass transfer model fit can then be used to estimate the fractional approaches to equilibrium for many other contaminants, whose diffusive and partitioning properties are also known. And these fractions can be used to adjust the target chemical concentrations that have accumulated from the sediment into the same polymeric sampler to find the equilibrated results<ref name="Apell2014" />. Finally, these equilibrated concentrations can be used in Eq. 2 to estimate truly dissolved contaminant concentrations in the sediment's porewater.

==Field Applications==
[[File: Gschwend1w2fig3.png | thumb |left| 450px | Figure 3. Passive sampler system made of polyethylene film loaded into an aluminum sheet metal frame, before (left), during (middle), and after (right) deployment in sediment.]]
Polymeric materials can be deployed in sediment in various ways<ref name="Burgess2017" />. PDMS-coated silica fibers, called SPMEs (solid phase micro extraction devices), can be incorporated into slotted rods, while thin films of polymers like LDPE or POM can be incorporated into sheet metal frames. In both cases, such hardware is used to insert the polymers into sediment beds (Figure 3).

Deployment of the assembled passive samplers can be accomplished via poles from a boat<ref name="Apell2014" />, by divers<ref name="Apell2016" />, or by attaching the samplers to a sampling platform lowered off a vessel<ref name="Fernandez2012">Fernandez, L.A., Lao, W., Maruya, K.A., White, C., Burgess, R.M., 2012. Passive Sampling to Measure Baseline Dissolved Persistent Organic Pollutant Concentrations in the Water Column of the Palos Verdes Shelf Superfund Site. Environmental Science and Technology, 46(21), pp. 11937-11947. [https://doi.org/10.1021/es302139y DOI: 10.1021/es302139y]</ref>. Typically, the method used depends on the water depth. Small buoys on short lines, sometimes with associated water-sampling polymeric materials in mesh bags (see right panel of Figure 3), are attached to the samplers to facilitate the sampler recoveries. After recovery, the samplers are wiped to remove any adhering sediment, biofilm, or precipitates and returned to the laboratory for PRC and target contaminant analyses. The resulting measurements of the accumulated target chemical concentrations can be adjusted using the observed PRC losses and publicly available software programs<ref name="Gschwend2014">Gschwend, P.M., Tcaciuc, A.P., and Apell, J.N., 2014. Guidance Document: Passive PE Sampling in Support of In Situ Remediation of Contaminated Sediments – Passive Sampler PRC Calculation Software User’s Guide, US Department of Defense, Environmental Security Technology Certification Program Project ER-200915. Available from: [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Sediments/Bioavailability/ER-200915 ESTCP].</ref><ref name="Thompson2015">Thompson, J.M., Hsieh, C.H. and Luthy, R.G., 2015. Modeling Uptake of Hydrophobic Organic Contaminants into Polyethylene Passive Samplers. Environmental Science and Technology, 49(4), pp. 2270-2277. [https://doi.org/10.1021/es504442s DOI: 10.1021/es504442s]</ref>.

Subsequently, since the passive sampling reveals the concentrations of contaminants in a sediment bed's porewater and the overlying bottom water<ref name="Booij2003" />, the data can be used to estimate bed-to-water column diffusive fluxes of contaminants<ref name="Koelmans2010">Koelmans, A.A., Poot, A., De Lange, H.J., Velzeboer, I., Harmsen, J., and van Noort, P.C.M., 2010. Estimation of In Situ Sediment-to-Water Fluxes of Polycyclic Aromatic Hydrocarbons, Polychlorobiphenyls and Polybrominated Diphenylethers. Environmental Science and Technology, 44(8), pp. 3014-3020. [https://doi.org/10.1021/es903938z DOI: 10.1021/es903938z]</ref><ref name="Fernandez2012" /> and bioirrigation-affected fluxes<ref name="Apell2018">Apell, J.N., Shull, D.H., Hoyt, A.M., and Gschwend, P.M., 2018. Investigating the Effect of Bioirrigation on In Situ Porewater Concentrations and Fluxes of Polychlorinated Biphenyls Using Passive Samplers. Environmental Science and Technology, 52(8), pp. 4565-4573. [https://doi.org/10.1021/acs.est.7b05809 DOI: 10.1021/acs.est.7b05809]</ref>. The data are also useful for assessing the tendency of the contaminants to accumulate in benthic organisms<ref name="Vinturella2004">Vinturella, A.E., Burgess, R.M., Coull, B.A., Thompson, K.M., and Shine, J.P., 2004. Use of Passive Samplers to Mimic Uptake of Polycyclic Aromatic Hydrocarbons by Benthic Polychaetes. Environmental Science and Technology, 38(4), pp. 1154-1160. [https://doi.org/10.1021/es034706f DOI: 10.1021/es034706f]</ref><ref name="Yates2011">Yates, K., Pollard, P., Davies, I.M., Webster, L., and Moffat, C.F., 2011. Application of silicone rubber passive samplers to investigate the bioaccumulation of PAHs by Nereis virens from marine sediments. Environmental Pollution, 159(12), pp. 3351-3356. [https://doi.org/10.1016/j.envpol.2011.08.038 DOI: 10.1016/j.envpol.2011.08.038]</ref><ref name="Fernandez2015">Fernandez, L.A. and Gschwend, P.M., 2015. Predicting bioaccumulation of polycyclic aromatic hydrocarbons in soft-shelled clams (Mya arenaria) using field deployments of polyethylene passive samplers. Environmental Toxicology and Chemistry, 34(5), pp. 993-1000. [https://doi.org/10.1002/etc.2892 DOI: 10.1002/etc.2892]</ref>, and by extension into food webs that include such benthic species<ref name="vonStackelberg2017">von Stackelberg, K., Williams, M.A., Clough, J., and Johnson, M.S., 2017. Spatially explicit bioaccumulation modeling in aquatic environments: Results from 2 demonstration sites. Integrated Environmental Assessment and Management, 13(6), pp. 1023-1037. [https://doi.org/10.1002/ieam.1927 DOI: 10.1002/ieam.1927]</ref>. Furthermore, recent efforts have found that passive sampling observations can be used to infer ''in situ'' transformations of substances like nitro aromatic compounds<ref name="Belles2016">Belles, A., Alary, C., Criquet, J., and Billon, G., 2016. A new application of passive samplers as indicators of in-situ biodegradation processes. Chemosphere, 164, pp. 347-354. [https://doi.org/10.1016/j.chemosphere.2016.08.111 DOI: 10.1016/j.chemosphere.2016.08.111]</ref> and DDT<ref name="Tcaciuc2018">Tcaciuc, A.P., Borrelli, R., Zaninetta, L.M., and Gschwend, P.M., 2018. Passive sampling of DDT, DDE and DDD in sediments: accounting for degradation processes with reaction–diffusion modeling. Environmental Science: Processes and Impacts, 20(1), pp. 220-231. [https://doi.org/10.1039/C7EM00501F DOI: 10.1039/C7EM00501F]   Open access article available from: [https://pubs.rsc.org/--/content/articlehtml/2018/em/c7em00501f Royal Society of Chemistry].</ref>.
 

==References==
<references />

==See Also==

[https://www.serdp-estcp.org/Tools-and-Training/Tools/PRC-Correction-Calculator A PRC Correction Calculator for LDPE deployed in sediments]

Contaminated Sediment Risk Assessment

2026-03-13T21:00:34Z

Admin:

[[Contaminated Sediments - Introduction | Contaminated sediments]] in rivers and streams, lakes, coastal harbors, and estuaries have the potential to pose ecological and human health risks. The goals of risk assessment applied to contaminated sediments are to characterize the nature and magnitude of the current and potential threats to human health, wildlife and ecosystem functioning posed by contamination; identify the key factors contributing to the potential health and ecological risks; evaluate how implementation of one or more remedy actions will mitigate the risks in the short and long term; and evaluate the risks and impacts from sediment management, both during and after any dredging or other remedy construction activities.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Contaminated Sediments - Introduction]]
*[[In Situ Toxicity Identification Evaluation (iTIE) | In Situ Toxicity Identification Evaluation]]
*[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
*[[Passive Sampling of Sediments]]
*[[Sediment Capping]]

'''Contributor(s):''' [[Richard Wenning]] and [[Dr. Sabine E. Apitz]]

'''Key Resource(s):'''

*Contaminated Sediment Remediation Guidance for Hazardous Waste Sites<ref name="USEPA2005">United States Environmental Protection Agency (USEPA), 2005. Contaminated Sediment Remediation Guidance for Hazardous Waste Sites. Office of Solid Waste and Emergency Response, Washington, D.C. EPA-540-R-05-012. OSWER 9355.0-85. [//www.enviro.wiki/images/7/7e/2005-USEPA-Contaminated_Sediment_Remediation_Guidance_for_Hazardous_Waste_Sites.pdf Report pdf]</ref>

*Principles for Environmental Risk Assessment of the Sediment Compartment<ref name="Tarazona2014">Tarazona, J.V., Versonnen, B., Janssen, C., De Laender, F., Vangheluwe, M. and Knight, D., 2014. Principles for Environmental Risk Assessment of the Sediment Compartment: Proceedings of the Topical Scientific Workshop. 7-8 May 2013. European Chemicals Agency, Helsinki. Document ECHA-14-R-13-EN. [//www.enviro.wiki/images/c/cc/ECHA-14-R-13-EN.pdf Report pdf]</ref>

*Assessing and managing contaminated sediments:

::Part I, Developing an Effective Investigation and Risk Evaluation Strategy<ref name="Apitz2005a">Apitz, S.E., Davis, J.W., Finkelstein, K., Hohreiter, D.W., Hoke, R., Jensen, R.H., Jersak, J., Kirtay, V.J., Mack, E.E., Magar, V.S. and Moore, D., 2005. Assessing and Managing Contaminated Sediments: Part I, Developing an Effective Investigation and Risk Evaluation Strategy. Integrated Environmental Assessment and Management, 1(1), pp. 2-8. [https://doi.org/10.1897/IEAM_2004a-002.1 DOI: 10.1897/IEAM_2004a-002.1] [//www.enviro.wiki/images/3/3f/Apitz2005a.pdf Article pdf]</ref>
::Part II, Evaluating Risk and Monitoring Sediment Remedy Effectiveness<ref name="Apitz2005b">Apitz, S.E., Davis, J.W., Finkelstein, K., Hohreiter, D.W., Hoke, R., Jensen, R.H., Jersak, J., Kirtay, V.J., Mack, E.E., Magar, V.S. and Moore, D., 2005b. Assessing and Managing Contaminated Sediments: Part II, Evaluating Risk and Monitoring Sediment Remedy Effectiveness. Integrated Environmental Assessment and Management, 1(1), pp.e1-e14. [https://doi.org/10.1897/IEAM_2004a-002e.1 DOI: 10.1897/IEAM_2004a-002e.1]</ref>

==Introduction==
Improving the management of [[Contaminated Sediments - Introduction | contaminated sediments]] is of growing concern globally. Sediment processes in both marine and freshwater environments are important to the function of aquatic ecosystems<ref name="Apitz2012">Apitz, S.E., 2012. Conceptualizing the role of sediment in sustaining ecosystem services: Sediment-Ecosystem Regional Assessment (SEcoRA), Science of the Total Environment, 415, pp. 9-30. [https://doi.org/10.1016/j.scitotenv.2011.05.060 DOI:10.1016/j.scitotenv.2011.05.060]</ref>, and many organisms rely on certain sediment quality and quantity characteristics for their life cycle<ref name="Hauer2018">Hauer, C., Leitner, P., Unfer, G., Pulg, U., Habersack, H. and Graf, W., 2018. The Role of Sediment and Sediment Dynamics in the Aquatic Environment. In: Schmutz S., Sendzimir J. (ed.s) Riverine Ecosystem Management. Aquatic Ecology Series, vol. 8, pp. 151-169. Springer. [https://doi.org/10.1007/978-3-319-73250-3_8 DOI: 10.1007/978-3-319-73250-3_8] [https://library.oapen.org/bitstream/handle/20.500.12657/27726/1002280.pdf?seque#page=153 Book pdf]</ref>. Human health can also be affected by sediment conditions, either via direct contact, as a result of sediment impacts on water quality, or because of the strong influence sediments can have on the quality of fish and shellfish consumed by people<ref name="Greenfield2015">Greenfield, B.K., Melwani, A.R. and Bay, S.M., 2015. A Tiered Assessment Framework to Evaluate Human Health Risk of Contaminated Sediment. Integrated Environmental Assessment and Management, 11(3), pp. 459-473. [https://doi.org/10.1002/ieam.1610 DOI: 10.1002/ieam.1610]</ref>.

Science-based methods for assessing sediment quality and use of risk-based decision-making in sediment management are important for identifying conditions suspected to adversely affect ecological and human services provided by sediments, and predicting the likely consequences of different sediment management actions<ref name="Bridges2006">Bridges, T.S., Apitz, S.E., Evison, L., Keckler, K., Logan, M., Nadeau, S. and Wenning, R.J., 2006. Risk‐Based Decision Making to Manage Contaminated Sediments. Integrated Environmental Assessment and Management, 2(1), pp. 51-58. [https://doi.org/10.1002/ieam.5630020110 DOI: 10.1002/ieam.5630020110] [https://setac.onlinelibrary.wiley.com/doi/epdf/10.1002/ieam.5630020110 Open access article.]</ref><ref name="Apitz2011">Apitz, S.E., 2011. Integrated Risk Assessments for the Management of Contaminated Sediments in Estuaries and Coastal Systems. In: Wolanski, E. and McLusky, D.S. (eds.) Treatise on Estuarine and Coastal Science, Vol 4, pp. 311–338. Waltham: Academic Press. ISBN: 9780123747112</ref>.A common approach to achieving the explicit management goals inherent in different sediment assessment frameworks in North America and elsewhere is the use of the ecological risk assessment (ERA)<ref name="USEPA1997a">US Environmental Protection Agency (USEPA), 1997. The Incidence and Severity of Sediment Contamination in Surface Waters of the United States: Volume 1, National Sediment Quality Survey. EPA-823R-97-006. Washington, DC. [//www.enviro.wiki/images/5/5a/EPA-823-R-97-006.pdf Report pdf]</ref>. An ERA “evaluates the likelihood and magnitude of adverse effects from exposure to a chemical for organisms, such as animals, plants, or microbes, in the environment”<ref name="SETAC2018">Society of Environmental Toxicology and Chemistry (SETAC), 2018. Technical Issue Paper: Environmental Risk Assessment of Chemicals. SETAC, Pensacola, FL. 5 pp. [//www.enviro.wiki/images/8/84/Setac_tip_era2018.pdf Article pdf]</ref>. An ERA provides information relevant to sediment management decision-making <ref name="Stahl2001">Stahl, R.G., Bachman, R., Barton, A., Clark, J., deFur, P., Ells, S., Pittinger, C., Slimak, M., Wentsel, R., 2001. Risk Management: Ecological Risk-Based Decision Making. SETAC Press, Pensacola, FL, 222 pp. ISBN: 978-1-880611-26-5</ref>. It should be based on sound science and performed in a technically defensible manner that is cost-effective and aimed at protecting human health and the environment<ref name="CNO1999">Chief of Naval Operations (CNO), 1999. Navy Policy for Conducting Ecological Risk Assessments, Letter 5090, Ser N453E/9U595355, dated 05 April 99. Department of the Navy, Washington, DC. [//www.enviro.wiki/images/5/56/CNO1999.pdf Report pdf]</ref>.

Sediment risk assessment is a specific application of ERA. It may include aspects of a human health risk assessment, as well, to examine the direct and indirect consequences of sediment conditions on human health. It is increasingly used by governmental agencies to support sediment management in freshwater, estuarine, and marine environments. Strategies for sediment management encompass a wide variety of actions, from removal, capping or treatment of contaminated sediment to the monitoring of natural processes, including sedimentation, binding, and bio- and photo-degradation that serve to reduce the potential threat to aquatic life over time. It is not uncommon to revisit a sediment risk assessment periodically to check how changed environmental conditions reflected in sediment and biotic sampling work has either reduced or exacerbated the threats identified in the initial assessment.

At present, several countries lack common recommendations specific to conducting risk assessment of contaminated sediments<ref name="Bruce2020">Bruce, P., Sobek, A., Ohlsson, Y. and Bradshaw, C., 2020. Risk assessments of contaminated sediments from the perspective of weight of evidence strategies – a Swedish case study. Human and Ecological Risk Assessment, 27(5), pp. 1366-1387. [https://doi.org/10.1080/10807039.2020.1848414 DOI: 10.1080/10807039.2020.1848414] </ref>. In the European Union, sediment has played a secondary role in the Water Framework Directive (WFD), with most quality standards being focused on water with the option for the development of national standards for sediment and biota for bioaccumulative compounds. The Common Implementation Strategy (CIS) in 2010 provided guidance on the monitoring of contaminants in sediments and biota, but not on risk-based decision-making<ref name="EC2010">European Commission, 2010. Common Implementation Strategy For The Water Framework Directive (2000/60/EC), Technical Report - 2010 – 041; Guidance document No. 25 On Chemical Monitoring Of Sediment And Biota Under The Water Framework Directive. 82pp. ISBN 978-92-79-16224-4. [https://op.europa.eu/en/publication-detail/-/publication/5ff7a8ec-995b-4d90-a140-0cc9b4bf980d Open access article.]</ref>. Additional changes to the strategy were initiated in 2021 to incorporate guidance for management of contaminated sediment <ref name="Brils2020">Brils, J., 2020. Including sediment in European River Basin Management Plans: Twenty years of work by SedNet. Journal of Soils and Sediments, 20(12), pp.4229-4237. [https://doi.org/10.1007/s11368-020-02782-1 DOI: 10.1007/s11368-020-02782-1] [https://link.springer.com/content/pdf/10.1007/s11368-020-02782-1.pdf Article pdf]</ref>. Sediment risk assessment guidance from Norway, Canada, the Netherlands, and the US are most often referenced when assessing the risks from contaminated sediments<ref name="Bruce2020" /><ref name="Birch2018">Birch, G.F., 2018. A review of chemical-based sediment quality assessment methodologies for the marine environment. Marine Pollution Bulletin, 133, pp.218-232. [https://doi.org/10.1016/j.marpolbul.2018.05.039 DOI: 10.1016/j.marpolbul.2018.05.039]</ref><ref name="Kwok2014">Kwok, K.W., Batley, G.E., Wenning, R.J., Zhu, L., Vangheluwe, M. and Lee, S., 2014. Sediment quality guidelines: challenges and opportunities for improving sediment management. Environmental Science and Pollution Research, 21(1), pp. 17-27. [https://doi.org/10.1007/s11356-013-1778-7 DOI: 10.1007/s11356-013-1778-7] [https://www.researchgate.net/profile/Graeme-Batley/publication/236836992_Sediment_quality_guidelines_Challenges_and_opportunities_for_improving_sediment_management/links/0c96052b8a8f5ad0c6000000/Sediment-quality-guidelines-Challenges-and-opportunities-for-improving-sediment-management.pdf Article pdf]</ref>. Some European countries, such as Norway, have focused their risk assessment guidance on the assessment of sediment conditions relative to general chemical thresholds, while in North America, risk assessment guidance focuses on site- or region-specific conditions<ref name="Apitz2008">Apitz, S.E., 2008. Is risk-based, sustainable sediment management consistent with European policy?. Journal of Soils and Sediments, 8(6), p.461-466. [https://doi.org/10.1007/s11368-008-0039-8 DOI: 10.1007/s11368-008-0039-8]</ref>.

There is general consensus from a regulatory perspective, globally, on the importance of sediment risk assessment. Technical guidance documents prepared by Canada<ref name="Fletcher2008">Fletcher, R., Welsh, P. and Fletcher, T., 2008. Guidelines for Identifying, Assessing, and Managing Contaminated Sediments in Ontario. [https://www.ontario.ca/page/ministry-environment-conservation-parks Ontario Ministry of the Environment]. PIBS6658e.</ref><ref name="HealthCanada2017">Health Canada, 2017. Supplemental Guidance on Human Health Risk Assessment of Contaminated Sediments: Direct Contact Pathway, Federal Contaminated Site Risk Assessment in Canada. ISBN: 978-0-660-07989-9. Cat.: H144-41/2017E-PDF. Pub. 160382. [//www.enviro.wiki/images/1/10/HealthCanada2017.pdf Report pdf]</ref> , the European Union<ref name="Tarazona2014" />, and the United States Environmental Protection Agency (USEPA)<ref name="USEPA2005" /> advise a flexible, tiered approach for sediment risk assessment. Sediment quality guidelines in many countries reflect the scientific importance of including certain sediment-specific measurement and biotic assessment endpoints, as well as certain physical sediment processes and chemical transformation processes potentially affecting biotic responses to contaminant exposure in the sediment<ref name="Wenning2005">Wenning, R.J. Batley, G.E., Ingersoll, C.G., and Moore, D.W., (eds), 2005. Use Of Sediment Quality Guidelines And Related Tools For The Assessment Of Contaminated Sediments. SETAC, Pensacola, FL. 815 pp. ISBN 1-880611-71-6.</ref>. New risk assessment methods continue to emerge in the scientific literature<ref name="Benson2018">Benson, N.U., Adedapo, A.E., Fred-Ahmadu, O.H., Williams, A.B., Udosen, E.D., Ayejuyo, O.O. and Olajire, A.A., 2018. A new method for assessment of sediment-associated contamination risks using multivariate statistical approach. MethodsX, 5, pp. 268-276. [https://doi.org/10.1016/j.mex.2018.03.005 DOI: 10.1016/j.mex.2018.03.005] [//www.enviro.wiki/images/7/7f/Benson2018.pdf Article pdf]</ref><ref name="Saeedi2015">Saeedi, M. and Jamshidi-Zanjani, A., 2015. Development of a new aggregative index to assess potential effect of metals pollution in aquatic sediments. Ecological Indicators, 58, pp. 235-243. [https://doi.org/10.1016/j.ecolind.2015.05.047 DOI: 10.1016/j.ecolind.2015.05.047]</ref><ref name="Vaananen2018">Väänänen, K., Leppänen, M.T., Chen, X. and Akkanen, J., 2018. Metal bioavailability in ecological risk assessment of freshwater ecosystems: from science to environmental management. Ecotoxicology and Environmental Safety, 147, pp. 430-446. [https://doi.org/10.1016/j.ecoenv.2017.08.064 DOI: 10.1016/j.ecoenv.2017.08.064]</ref>. These new methods, however, are likely to be considered supplemental to the more generalized framework shared globally.

==Fundamentals of Sediment Risk Assessment==
[[File: SedRiskFig1.PNG | thumb |700px|Figure 1. Schematic of the sediment risk assessment process]]
Whereas there is strong evidence of anthropogenic impacts on the benthic community at many sediment sites, the degree of toxicity (or even its presence or absence) cannot be predicted with absolute certainty using contaminant concentrations alone<ref name="Apitz2011" />. A sediment ERA should include lines of evidence (LOEs) derived from several different investigations<ref name="Wenning2005" />. One common approach to develop several of these LOEs in a decision framework is the triad approach. Triad-based assessment frameworks require evidence based on sediment chemistry, toxicity, and benthic community structure (possibly including evidence of bioaccumulation) to designate sediment as toxic and requiring management or control<ref name="Chapman1996">Chapman, P.M., Paine, M.D., Arthur, A.D., Taylor, L.A., 1996. A triad study of sediment quality associated with a major, relatively untreated marine sewage discharge. Marine Pollution Bulletin 32(1), pp. 47–64. [https://doi.org/10.1016/0025-326X(95)00108-Y DOI: 10.1016/0025-326X(95)00108-Y]</ref>. In some decision frameworks, particularly those used to establish and rank risks in national or regional programs, all components of the triad are carried out simultaneously, with the various LOEs combined to support weight of evidence (WOE) decision making. In other frameworks, LOEs are tiered to minimize costs by collecting only the data required to make a decision and leaving some potential consequences and uncertainties unresolved.

Figure 1 provides an overview of a sediment risk assessment process. The first step, and a fundamental requirement, in sediment risk assessment involves scoping and planning prior to undertaking work. This is important for optimizing the available assessment resource and conducting an assessment at the appropriate level of detail that is transparent and free, to the extent possible, of any bias or preconceived beliefs concerning the outcome<ref name="Hill2000">Hill, R.A., Chapman, P.M., Mann, G.S. and Lawrence, G.S., 2000. Level of Detail in Ecological Risk Assessments. Marine Pollution Bulletin, 40(6), pp. 471-477. [https://doi.org/10.1016/S0025-326X(00)00036-9 DOI: 10.1016/S0025-326X(00)00036-9]</ref>.

===Screening-Level Risk Assessment (SLRA)===
Technical guidance in many countries strongly encourages sediment risk assessment to begin with a Screening-Level Risk Assessment (SLRA)<ref name="USEPA2005" /><ref name="Tarazona2014" /><ref name="Fletcher2008" />. The SLRA is a simplified risk assessment conducted using limited data and often assuming certain, generally conservative and generic, sediment characteristics, sediment contaminant levels, and exposure parameters in the absence of sufficient readily available information<ref name="Hope2006">Hope, B.K., 2006. An examination of ecological risk assessment and management practices. Environment International, 32(8), pp. 983-995. [https://doi.org/10.1016/j.envint.2006.06.005 DOI: 10.1016/j.envint.2006.06.005]</ref><ref name="Weinstein2010">Weinstein, J.E., Crawford, K.D., Garner, T.R. and Flemming, A.J., 2010. Screening-level ecological and human health risk assessment of polycyclic aromatic hydrocarbons in stormwater detention pond sediments of Coastal South Carolina, USA. Journal of Hazardous Materials, 178(1-3), pp. 906-916. [https://doi.org/10.1016/j.jhazmat.2010.02.024 DOI: 10.1016/j.jhazmat.2010.02.024]</ref><ref name="Rak2008">Rak, A., Maly, M.E., Tracey, G., 2008. A Guide to Screening Level Ecological Risk Assessment, TG-090801. Tri-Services Ecological Risk Assessment Working Group (TSERAWG), U.S. Army Biological Technical Assistance Group (BTAG), Aberdeen Proving Ground, MD. 26 pp. [//www.enviro.wiki/images/6/67/Rak2008.pdf Report pdf]</ref><ref name="USEPA2001">US Environmental Protection Agency (USEPA), 2001. ECO Update. The Role of Screening-Level Risk Assessments and Refining Contaminants of Concern in Baseline Ecological Risk Assessments. EPA 540/F-01/014. Washington, D.C. [//www.enviro.wiki/images/3/3f/EPA_540_F-01_014.pdf Report pdf]</ref>.

The analysis is often semi-quantitative, and typically includes comparisons of various chemical and physical sediment conditions to threshold limits established in national or international regulations or by generally accepted scientific interpretations. US technical guidance encourages the comparison of contaminant measurements in water, sediment, or soil to National Oceanographic and Atmospheric Administration (NOAA) sediment screening quick reference tables, or SQuiRT cards, which list quality guidelines from a range of sources, based on differing narrative intent<ref name="Buchman2008">Buchman, M.F., 2008. Screening Quick Reference Tables (SQuiRTs), NOAA OR&R Report 08-1. National Oceanographic and Atmospheric Administration (NOAA), Coastal Protection and Restoration Protection Division. 34 pp. [//www.enviro.wiki/images/d/de/SQuiRTs2008.pdf Report.pdf]</ref>.

The screening level approach is intended to minimize the chances of concluding that there is no risk when, in fact, risk may exist. Thus, the results of an SLRA may indicate contaminants or sediments in certain locations in the original study area initially thought to be of concern are acceptable (i.e., contaminant levels are below threshold levels), or that contaminant levels are high enough to indicate immediate action without further assessment (e.g., contaminant levels are well above probable-effects guidelines). In other cases, or at other locations, SLRA may indicate the need for further examination. Further study may apply site-specific, rather than generic and conservative assumptions, to reduce uncertainty.

===Detailed Risk Assessment===
Detailed sediment risk assessment is conducted when SLRA results indicate one or more sediment contaminants exceed background conditions or regulatory threshold limits. For some contaminants, such as the dioxins and other persistent, bioaccumulative, and toxic substances (PBTs), technical guidance may mandate further examination, regardless of whether threshold levels are exceeded<ref name="Solomon2013">Solomon, K., Matthies, M., and Vighi, M., 2013. Assessment of PBTs in the European Union: a critical assessment of the proposed evaluation scheme with reference to plant protection products. Environmental Sciences Europe, 25(1), pp. 1-17. [https://doi.org/10.1186/2190-4715-25-10 DOI: 10.1186/2190-4715-25-10]  [https://enveurope.springeropen.com/articles/10.1186/2190-4715-25-10 Open access article.]</ref><ref name="Matthies2016">Matthies, M., Solomon, K., Vighi, M., Gilman, A. and Tarazona, J.V., 2016. The origin and evolution of assessment criteria for persistent, bioaccumulative and toxic (PBT) chemicals and persistent organic pollutants (POPs). Environmental Science: Processes and Impacts, 18(9), pp. 1114-1128. [https://doi.org/10.1039/C6EM00311G DOI: 10.1039/C6EM00311G]</ref>. Detailed sediment risk assessment typically follows a three-step framework similar to that described for ecological risk assessment - problem formulation, analysis, and risk characterization<ref name="Suter2008">Suter, G.W., 2008. Ecological Risk Assessment in the United States Environmental Protection Agency: A Historical Overview. Integrated Environmental Assessment And Management, 4(3), pp. 285-289. [https://doi.org/10.1897/IEAM_2007-062.1 DOI: 10.1897/IEAM_2007-062.1]</ref>.

US sediment management guidance describes a detailed risk assessment process similar to that followed for US ecological risk assessment<ref name="USEPA2005" />. The first step is problem formulation. It involves defining chemical and physical conditions, delineating the spatial footprint of the sediment area to be examined, and identifying the human uses and ecological features of the sediment. Historical data are included in this initial step to better understand the results of biota, sediment, and water sampling as well as laboratory toxicity testing results. The SLRA is often included as a part of problem formulation.

The second step is analysis, which includes both an exposure assessment and an effects assessment. The exposure assessment includes the identification of pathways by which human and aquatic organisms might directly or indirectly contact contaminants in the sediment. The exposure route (i.e., ingestion, dermal, or inhalation of particulates or gaseous emissions) and both the frequency and duration of contact (i.e., hourly, daily, or seasonally) are determined for each contaminant exposure pathway and human and ecological receptor. The environmental fate of the contaminant, factors affecting uptake, and the overall exposure dose are included in the calculation of the level of contaminant exposure. The effects assessment identifies the possible short-term (acute) and long-term (chronic) biological responses associated with different levels of exposure for each contaminant and human and ecological receptor.

The third step is risk-characterization. It involves calculating the risks for each human and ecological receptor posed by each sediment contaminant, as well as the cumulative risk associated with the combined exposure to all contaminants exerting similar biological effects. An uncertainty analysis is often included in this step of the risk assessment to convey where knowledge or data are lacking regarding the presence of the contaminant in the sediment, the biological response associated with exposure to the contaminant, or the behavior of the receptor with respect to contact with the sediment. A sensitivity analysis also may be conducted to convey the range of exposures (lowest, typical, and worst-case) and risks associated with changes in key assumptions and parameter values used in the exposure calculations and effects assessment.

==Key Considerations==
===Stakeholder Engagement===
Stakeholder involvement is widely acknowledged as an important element of [[Wikipedia: Dredging |dredged]] material management<ref name="Collier2014">Collier, Z.A., Bates, M.E., Wood, M.D. and Linkov, I., 2014. Stakeholder engagement in dredged material management decisions. Science of the Total Environment, 496, pp. 248-256. [https://doi.org/10.1016/j.scitotenv.2014.07.044 DOI: 10.1016/j.scitotenv.2014.07.044] [https://www.researchgate.net/profile/Matthew-Bates-9/publication/264460412_Stakeholder_Engagement_in_Dredged_Material_Management_Decisions/links/5a9d50fbaca2721e3f32adea/Stakeholder-Engagement-in-Dredged-Material-Management-Decisions.pdf Article pdf]</ref>, sediment remediation<ref name="Oen2010">Oen, A.M.P., Sparrevik, M., Barton, D.N., Nagothu, U.S., Ellen, G.J., Breedveld, G.D., Skei, J. and Slob, A., 2010. Sediment and society: an approach for assessing management of contaminated sediments and stakeholder involvement in Norway. Journal of Soils and Sediments, 10(2), pp. 202-208. [https://doi.org/10.1007/s11368-009-0182-x DOI: 10.1007/s11368-009-0182-x]</ref>, and other environmental and sediment related activities<ref name="Gerrits2004">Gerrits, L. and Edelenbos, J., 2004. Management of Sediments Through Stakeholder Involvement. Journal of Soils and Sediments, 4(4), pp. 239-246. [https://doi.org/10.1007/BF02991120 DOI: 10.1007/BF02991120]</ref><ref name="Braun2019">Braun, A.B., da Silva Trentin, A.W., Visentin, C. and Thomé, A., 2019. Sustainable remediation through the risk management perspective and stakeholder involvement: A systematic and bibliometric view of the literature. Environmental Pollution, 255(1), p.113221. [https://doi.org/10.1016/j.envpol.2019.113221 DOI: 10.1016/j.envpol.2019.113221]</ref>.

Sediment management, particularly at the watershed or river basin scale, involves a wide variety of different environmental, governmental, and societal issues<ref name="Liu2018">Liu, C., Walling, D.E. and He, Y., 2018. The International Sediment Initiative case studies of sediment problems in river basins and their management. International Journal of Sediment Research, 33(2), pp. 216-219. [https://doi.org/10.1016/j.ijsrc.2017.05.005 DOI: 10.1016/j.ijsrc.2017.05.005] [https://www.researchgate.net/profile/Cheng-Liu-43/publication/317032034_Review_The_International_Sediment_Initiative_Case_Studies_of_sediment_problems_in_river_basins_and_their_management/links/5f4f37d2299bf13a319703df/Review-The-International-Sediment-Initiative-Case-Studies-of-sediment-problems-in-river-basins-and-their-management.pdf Article pdf]</ref>. Incorporating these different views, interests, and perspectives into a form that builds consensus for whatever actions and goals are in mind (e.g., commercial ports and shipping, navigation, flood protection, or habitat restoration) necessitates a formal stakeholder engagement process<ref name="Slob2008">Slob, A.F.L., Ellen, G.J. and Gerrits, L., 2008. Sediment management and stakeholder involvement. In: Sustainable Management of Sediment Resources, Vol. 4: Sediment Management at the River Basin Scale, Owens, P.N. (ed.), pp. 199-216. Elsevier. [https://doi.org/10.1016/S1872-1990(08)80009-8 DOI: 10.1016/S1872-1990(08)80009-8]</ref>.

Results from a three-year (2008-2010) [https://www.ngi.no/eng/Projects/Sediment-and-society Sediment and Society] research project funded by the Norwegian Research Council point to three important challenges that must be resolved for successful stakeholder engagement: (1) including people who have important management information and local knowledge, but not much influence in the decision-making process; (2) securing resources to ensure participation and (3) engaging and motivating stakeholders to participate early in the sediment remediation planning process<ref name="Oen2010" />.

===Conceptual Site Model===
The preparation of a conceptual site model (CSM) is a fundamental component of problem formulation and the first step in sediment risk assessment. The CSM is a narrative and/or illustrative representation of the physical, chemical and biological processes that control the transport, migration and actual or potential impacts of sediment contamination to human and/or ecological receptors<ref name="NJDEP2019">New Jersey Department of Environmental Protection, 2019. Technical Guidance for Preparation and Submission of a Conceptual Site Model. Version 1.1. Site Remediation and Waste Management Program, Trenton, NJ. 46 pp. [https://www.nj.gov/dep/srp/guidance/srra/csm_tech_guidance.pdf Report pdf].</ref><ref name="USEPA2011">US Environmental Protection Agency, 2011. Guidance for the Development of Conceptual Models for a Problem Formulation Developed for Registration Review. Environmental Fate and Effects Division, Office of Pesticide Programs, Washington, D.C. [https://www.epa.gov/pesticide-science-and-assessing-pesticide-risks/guidance-development-conceptual-models-problem Website]</ref>. The CSM should include a “food web” because the aquatic food web is an important exposure pathway by which contaminants in the sediment reach humans and pelagic aquatic life<ref name="Arnot2004">Arnot, J.A. and Gobas, F.A., 2004. A Food Web Bioaccumulation Model for Organic Chemicals in Aquatic Ecosystems. Environmental Toxicology and Chemistry, 23(10), pp. 2343-2355. [https://doi.org/10.1897/03-438 DOI: 10.1897/03-438]</ref>.

The CSM provides an early opportunity for critical examination of the interactions between sediment and the water column and the influence of groundwater inputs, surface runoff, and hydrodynamics. For example, there are situations where impacts in the aquatic food web can be driven by ongoing inputs to the water column from upstream sources, but mistakenly connected to polluted sediments. Other considerations included in a CSM can be socio-economic and include linkages to the ecosystem services provided by sediments<ref name="Broszeit2019">Broszeit, S., Beaumont, N.J., Hooper, T.L., Somerfield, P.J. and Austen, M.C., 2019. Developing conceptual models that link multiple ecosystem services to ecological research to aid management and policy, the UK marine example. Marine Pollution Bulletin, 141, pp.236-243. [https://doi.org/10.1016/j.marpolbul.2019.02.051 DOI: 10.1016/j.marpolbul.2019.02.051] [https://www.sciencedirect.com/science/article/pii/S0025326X19301511/pdfft?md5=34993d6c3a57b6fb18a8b6329597fcb9&pid=1-s2.0-S0025326X19301511-main.pdf Article pdf]</ref><ref name="Wang2021">Wang, J., Lautz, L.S., Nolte, T.M., Posthuma, L., Koopman, K.R., Leuven, R.S. and Hendriks, A.J., 2021. Towards a systematic method for assessing the impact of chemical pollution on ecosystem services of water systems. Journal of Environmental Management, 281, p. 111873. [https://doi.org/10.1016/j.jenvman.2020.111873 DOI: 10.1016/j.jenvman.2020.111873]  [https://www.sciencedirect.com/science/article/pii/S0301479720317989/pdfft?md5=daff5e94f8aed44ffce6508afef2308c&pid=1-s2.0-S0301479720317989-main.pdf Article pdf.]</ref>, or the social, economic and environmental impacts of sediment management alternatives. In such cases where the sediment risk assessment is intended to address the longer-term societal benefits of different management actions (including no action), the CSM could be viewed as part of a sustainable development strategy, or SustCSM<ref name="McNally2020">McNally, A.D., Fitzpatrick, A.G., Harrison, D., Busey, A., and Apitz, S.E., 2020. Tiered approach to sustainability analysis in sediment remediation decision making. Remediation Journal, 31(1), pp. 29-44. [https://doi.org/10.1002/rem.21661 DOI: 10.1002/rem.21661] [https://onlinelibrary.wiley.com/doi/epdf/10.1002/rem.21661 Open access article].</ref><ref name="Holland2011">Holland, K.S., Lewis, R.E., Tipton, K., Karnis, S., Dona, C., Petrovskis, E., and Hook, C., 2011. Framework for Integrating Sustainability Into Remediation Projects. Remediation Journal, 21(3), pp. 7-38. [https://doi.org/10.1002/rem.20288 DOI: 10.1002/rem.20288].</ref>. At a minimum, however, the purpose of the CSM is to illustrate the scope of the risk assessment and guide the quantification of exposure and risk.

===Environmental Fate===
An important consideration in exposure analysis is the determination of the bioavailable fraction of the contaminant in the sediment. There are two considerations. First, the adverse condition may be buried deep enough in sediments to be below the biologically available zone; typically, conditions in sediment below a depth of 5 cm will not contact burrowing benthic organisms<ref name="Anderson2010">Anderson, R.H., Prues, A.G. and Kravitz, M.J., 2010. Determination of the biologically relevant sampling depth for terrestrial ecological risk assessments. Geoderma, 154(3-4), pp.336-339. [https://doi.org/10.1016/j.geoderma.2009.11.004 DOI: 10.1016/j.geoderma.2009.11.004]</ref>. If there is no prospect for the adverse condition to come closer to the surface, then the risk assessment could conclude the risk of exposure is insignificant. The second consideration relates to chemistry and the factors involved in the binding to sediment particles or the chemical form of the substance in the sediment<ref name="Eggleton2004">Eggleton, J. and Thomas, K.V., 2004. A review of factors affecting the release and bioavailability of contaminants during sediment disturbance events. Environment International, 30(7), pp. 973-980. [https://doi.org/10.1016/j.envint.2004.03.001 DOI: 10.1016/j.envint.2004.03.001]</ref>. However, these assumptions should be examined in the context of [[Climate Change Primer | climate change]], and the likelihood of more frequent and extreme events, putting burial at risk, higher temperatures and changing biogeochemical conditions, which may alter environmental fate of contaminants, compared to historical studies.

The above contaminant bioavailability considerations are important factors influencing assumptions in the risk assessment about contaminant exposure<ref name="Peijnenburg2020">Peijnenburg, W.J., 2020. Implementation of bioavailability in prospective and retrospective risk assessment of chemicals in soils and sediments. In: The Handbook of Environmental Chemistry, vol 100, Bioavailability of Organic Chemicals in Soil and Sediment, Ortega-Calvo, J.J., Parsons, J.R. (ed.s), pp.391-422. Springer. [https://doi.org/10.1007/698_2020_516 DOI: 10.1007/698_2020_516]</ref><ref name="Ortega-Calvo2015">Ortega-Calvo, J.J., Harmsen, J., Parsons, J.R., Semple, K.T., Aitken, M.D., Ajao, C., Eadsforth, C., Galay-Burgos, M., Naidu, R., Oliver, R. and Peijnenburg, W.J., 2015. From Bioavailability Science to Regulation of Organic Chemicals. Environmental Science and Technology, 49, 10255−10264. [https://doi.org/10.1021/acs.est.5b02412 DOI: 10.1021/acs.est.5b02412] [https://pubs.acs.org/doi/pdf/10.1021/acs.est.5b02412 Open access article].</ref>. There have been recent advances in the use of sorbent amendments applied to contaminated sediments that alter sediment geochemistry, increase contaminant binding, and reduce contaminant exposure risks to people and the environment<ref name="Ghosh2011">Ghosh, U., Luthy, R.G., Cornelissen, G., Werner, D. and Menzie, C.A., 2011. In-situ sorbent amendments: a new direction in contaminated sediment management. Environmental Science and Technology, 45, 4, 1163–1168. [https://doi.org/10.1021/es102694h DOI: 10.1021/es102694h] [https://pubs.acs.org/doi/pdf/10.1021/es102694h Open access article.]</ref>. [[Passive Sampling of Sediments | Passive sampling techniques]] have emerged to quantify chemical binding to sediment and determine the freely dissolved concentration that is bioavailable.

===Assessment and Measurement Endpoints===
Assessment and measurement endpoints used in sediment risk assessment are comparable to those described in USEPA ecological risk assessment guidance<ref name="USEPA2005" /><ref name="USEPA1992">US Environmental Protection Agency (USEPA), 1992. Framework for Ecological Risk Assessment, EPA/630/R-92/001. Risk Assessment Forum, Washington DC. [//www.enviro.wiki/images/9/94/EPA-630-R-92-001.pdf Report pdf]</ref><ref name="USEPA1996">US Environmental Protection Agency (USEPA), 1996. Eco Update: Ecological Significance and Selection of Candidate Assessment Endpoints. EPA/540/F-95/037. Office of Solid Waste and Emergency Response, Washington DC. [//www.enviro.wiki/images/f/fa/EPA_540-F-95-037.pdf Report pdf]</ref><ref name="USEPA1997b">US Environmental Protection Agency (USEPA), 1997. Ecological Risk Assessment Guidance for Superfund: Process for Designing and Conducting Ecological Risk Assessments - Interim Final, EPA 540/R-97/006. Office of Solid Waste and Emergency Response, Washington DC. [//www.enviro.wiki/images/7/72/EPA_540-R-97-006.pdf Report pdf]</ref><ref name="USEPA1998">US Environmental Protection Agency (USEPA), 1998. Guidelines for Ecological Risk Assessment. EPA/630/R-95/002F. Risk Assessment Forum, Washington DC. [//www.enviro.wiki/images/5/55/EPA_630-R-95-002F.pdf Report pdf]</ref>. A sediment risk assessment, and ecological risk assessments more broadly, must have clearly defined endpoints that are socially and biologically relevant, accessible to prediction and measurement, and susceptible to the hazard being assessed<ref name="USEPA1992" />.

Assessment endpoints for humans include both carcinogenic and noncarcinogenic effects. Due to their assumed higher levels of exposure, human receptors in sediment risk assessment typically include recreational, commercial, and subsistence fishermen, i.e., people who might be at increased risk from eating fish or contacting the sediment or water on a regular basis such as indigenous peoples, immigrants from fishing cultures, and subsistence fishers who rely upon fish as a major source of protein. Special considerations are given to women of child-bearing age, pregnant women and young children. Assessment endpoints for ecological receptors focus on benthic organisms, resident fish, piscivorous and other predatory birds and marine mammals. Endpoints typically include mortality, reproductive success and population susceptibility to disease or similar adverse chronic conditions.

Measurement endpoints are related quantitatively to each assessment endpoint. Whenever practical, multiple measurement endpoints are chosen to provide additional lines of evidence for each assessment endpoint. For example, for humans, it might be possible to measure contaminant levels in both food items and human blood or tissue. For predatory fish, birds and mammals, it might be possible to measure contaminants in both prey and predator tissues. Measurement endpoints can be selected to assess non-chemical stressors as well, such as habitat alteration and water turbidity. Typically, measurement endpoints are compared to measurements at a reference site to ascertain the degree of departure from local natural or background conditions.

===Sediment Toxicity Testing===
Sediment bioassays are an integral part of effects characterization when assessing the risks posed by contaminated sediments and developing sediment quality guidelines<ref name="USEPA2014">US Environmental Protection Agency (USEPA), 2014. Toxicity Testing and Ecological Risk Assessment Guidance for Benthic Invertebrates. Office of Chemical Safety and Pollution Prevention, Washington DC. [//www.enviro.wiki/images/d/d0/USEPA2014.pdf Report pdf]</ref><ref name="Simpson2016a">Simpson, S., Campana, O., Ho, K., 2016. Chapter 7, Sediment Toxicity Testing. In: J. Blasco, P.M. Chapman, O. Campana, M. Hampel (ed.s), Marine Ecotoxicology: Current Knowledge and Future Issues. Academic Press Incorporated. pp. 199-237. [https://doi.org/10.1016/B978-0-12-803371-5.00007-2 DOI: 10.1016/B978-0-12-803371-5.00007-2]</ref>. The selection of appropriate sediment bioassays is dependent on the questions being addressed, the physical and chemical characteristics of the sediment matrix, the nature of the contaminant(s) of concern, and preferences of the supervising regulatory authority for the test method and test organisms<ref name="Amiard-Triquet2015">Amiard-Triquet, C., Amiard, J.C. and Mouneyrac, C. (ed.s), 2015. Aquatic Ecotoxicology: Advancing Tools For Dealing With Emerging Risks. Academic Press, NY. ISBN #9780128009499. [https://doi.org/10.1016/B978-0-12-800949-9.12001-7 DOI: 10.1016/B978-0-12-800949-9.12001-7]</ref>. Bioassay procedures have been standardized in several countries, and it is not unusual for different test methods to be required in different countries for the same sediment management purpose<ref name="DelValls2004">DelValls, T.A., Andres, A., Belzunce, M.J., Buceta, J.L., Casado-Martinez, M.C., Castro, R., Riba, I., Viguri, J.R. and Blasco, J., 2004. Chemical and ecotoxicological guidelines for managing disposal of dredged material. TrAC Trends in Analytical Chemistry, 23(10-11), pp. 819-828. [https://doi.org/10.1016/j.trac.2004.07.014 DOI: 10.1016/j.trac.2004.07.014]</ref>. Guidance documents in Australia, Canada, Europe and the US cover the wide range of sediment bioassay procedures most often used in risk assessment<ref name="Bat2005">Bat, L., 2005. A Review of Sediment Toxicity Bioassays Using the Amphipods and Polychaetes. Turkish Journal of Fisheries and Aquatic Sciences, 5(2), pp. 119-139. [//www.enviro.wiki/images/8/84/Bat2005.pdf Article pdf]</ref><ref name="Keddy1995">Keddy, C.J., Greene, J.C. and Bonnell, M.A., 1995. Review of Whole-Organism Bioassays: Soil, Freshwater Sediment, and Freshwater Assessment in Canada. Ecotoxicology and Environmental Safety, 30(3), pp. 221-251. [https://doi.org/10.1006/eesa.1995.1027 DOI: 10.1006/eesa.1995.1027]</ref><ref name="Giesy1990">Giesy, J.P., Rosiu, C.J., Graney, R.L. and Henry, M.G., 1990. Benthic invertebrate bioassays with toxic sediment and pore water. Environmental Toxicology and Chemistry, 9(2), pp. 233-248. [https://doi.org/10.1002/etc.5620090214 DOI: 10.1002/etc.5620090214]</ref><ref name="Simpson2016b">Simpson, S. and Batley, G. (ed.s), 2016. Sediment Quality Assessment: A Practical Guide, Second Edition. 358 pp. CSIRO Publishing, Australia. ISBN # 9781486303847.</ref><ref name="Moore2019">Moore, D.W., Farrar, D., Altman, S. and Bridges, T.S., 2019. Comparison of Acute and Chronic Toxicity Laboratory Bioassay Endpoints with Benthic Community Responses in Field‐Exposed Contaminated Sediments. Environmental Toxicology and Chemistry, 38(8), pp. 1784-1802. [https://doi.org/10.1002/etc.4454 DOI: 10.1002/etc.4454]</ref>.

In general, sediment toxicity tests focus on either (acute) lethality in whole organisms (typically benthic infaunal species such as amphipods and polychaetes) following short-term or acute exposures (<14 days) or (chronic) sublethal responses (e.g., reduced growth or reproduction or both) following longer-term exposures<ref name="Simpson2016a" />. It is not unusual in sediment risk assessment to rely on more than one sediment bioassay. Both acute and chronic tests involving either solid-phase or pore-water sediment fractions can be useful to discern the contributions of different contaminants in whole sediment by examining the response of different endpoints in different test organisms<ref name="Keddy1995" /><ref name="Giesy1990" />. The application of more specialized techniques such as toxicity identification evaluations (TIEs) have also proved useful to help identify contaminants or contaminant classes most likely responsible for toxicity and to exclude potentially confounding factors such as ammonia<ref name="Ho2013">Ho, K.T. and Burgess, R.M., 2013. What's causing toxicity in sediments? Results of 20 years of toxicity identification and evaluations. Environmental Toxicology and Chemistry, 32(11), pp. 2424-2432. [https://doi.org/10.1002/etc.2359 DOI: 10.1002/etc.2359]</ref><ref name="Bailey2016">Bailey, H.C., Curran, C.A., Arth, P., Lo, B.P. and Gossett, R., 2016. Application of sediment toxicity identification evaluation techniques to a site with multiple contaminants. Environmental Toxicology and Chemistry, 35(10), pp. 2456-2465. [https://doi.org/10.1002/etc.3488 DOI: 10.1002/etc.3488]</ref>.

===Uncertainty===
As part of the overall analysis of risk from exposure to certain sediment conditions, it is generally understood there is a moderate degree of uncertainty associated with sampling and the environmental fate of contaminants; an order of magnitude of uncertainty associated with ecological exposure and dose-response; and greater than an order of magnitude of uncertainty associated with the quantification of potential human health effects<ref name="DiGuardo2018">Di Guardo, A., Gouin, T., MacLeod, M. and Scheringer, M., 2018. Environmental fate and exposure models: advances and challenges in 21st century chemical risk assessment. Environmental Science: Processes and Impacts, 20(1), pp. 58-71. [https://doi.org/10.1039/C7EM00568G DOI: 10.1039/C7EM00568G] [https://pubs.rsc.org/en/content/articlehtml/2018/em/c7em00568g Open access article.]</ref>. The sources of uncertainty and significance to sediment risk assessment can vary widely, thereby affecting confidence in the decisions made based on risk assessment<ref name="Reckhow1994">Reckhow, K.H., 1994. Water quality simulation modeling and uncertainty analysis for risk assessment and decision making. Ecological Modelling, 72(1-2), pp.1-20. [https://doi.org/10.1016/0304-3800(94)90143-0 DOI: 10.1016/0304-3800(94)90143-0]</ref><ref name="Chapman2002">Chapman, P.M., Ho, K.T., Munns Jr, W.R., Solomon, K. and Weinstein, M.P., 2002. Issues in sediment toxicity and ecological risk assessment. Marine Pollution Bulletin, 44(4), pp. 271-278. [https://doi.org/10.1016/S0025-326X(01)00329-0 DOI: 10.1016/S0025-326X(01)00329-0]</ref>.

Consequently, technical guidance in several countries encourages including a quantitative uncertainty analysis in sediment risk assessment<ref name="USEPA2005" /><ref name="Tarazona2014" /><ref name="Apitz2005a" /><ref name="Apitz2005b" />. The aim of uncertainty analysis is to express either quantitatively or qualitatively the limitations inherent in predicting exposures and effects and, ultimately, the level of overall risk posed by sediment conditions<ref name="Batley2002">Batley, G.E., Burton, G.A., Chapman, P.M. and Forbes, V.E., 2002. Uncertainties in Sediment Quality Weight-of-Evidence (WOE) Assessments. Human and Ecological Risk Assessment, 8(7), pp. 1517-1547. [https://doi.org/10.1080/20028091057466 DOI: 10.1080/20028091057466]</ref>. Sediment risk assessment increasingly relies on a weight-of-evidence process to improve the certainty of conclusions about whether or not impairment exists due to sediment contamination, and, if so, which stressors and biological species (or ecological responses) are of greatest concern<ref name="Burton2002">Burton, G.A., Batley, G.E., Chapman, P.M., Forbes, V.E., Smith, E.P., Reynoldson, T., Schlekat, C.E., Besten, P.J.D., Bailer, A.J., Green, A.S. and Dwyer, R.L., 2002. A Weight-of-Evidence Framework for Assessing Sediment (or Other) Contamination: Improving Certainty in the Decision-Making Process. Human and Ecological Risk Assessment, 8(7), pp. 1675-1696. [https://doi.org/10.1080/20028091056854 DOI: 10.1080/20028091056854]</ref>. Recent advancements, including the use of Bayesian networks and geographic information systems, also help capture the range of variability in both measured and predicted exposures and responses<ref name="Holsman2017">Holsman, K., Samhouri, J., Cook, G., Hazen, E., Olsen, E., Dillard, M., Kasperski, S., Gaichas, S., Kelble, C.R., Fogarty, M. and Andrews, K., 2017. An ecosystem‐based approach to marine risk assessment. Ecosystem Health and Sustainability, 3(1), p. e01256. [https://doi.org/10.1002/ehs2.1256 DOI: 10.1002/ehs2.1256]  [https://www.tandfonline.com/doi/pdf/10.1002/ehs2.1256?needAccess=true Open access article.]</ref><ref name="Marcot2019">Marcot, B.G. and Penman, T.D., 2019. Advances in Bayesian network modelling: Integration of modelling technologies. Environmental Modelling and Software, 111, pp. 386-393. [https://doi.org/10.1016/j.envsoft.2018.09.016 DOI: 10.1016/j.envsoft.2018.09.016]</ref><ref name="Men2019">Men, C., Liu, R., Wang, Q., Guo, L., Miao, Y. and Shen, Z., 2019. Uncertainty analysis in source apportionment of heavy metals in road dust based on positive matrix factorization model and geographic information system. Science of The Total Environment, 652, pp. 27-39. [https://doi.org/10.1016/j.scitotenv.2018.10.212 DOI: 10.1016/j.scitotenv.2018.10.212]</ref>. The level of sophistication applied to the uncertainty analysis is a subjective consideration and often decided by regulatory pressures, public perceptions, and the likely cost (not only economic, but also social and environmental) of mitigating or removing the contamination.

==Role in Sediment Management==
Whether or not remediation of contaminated sediments is warranted depends on the magnitude of direct or indirect health risks to humans, ecological threats to aquatic biota, and the extent of risk reduction that can be achieved by removal or containment of the contamination<ref name="Kvasnicka2020">Kvasnicka, J., Burton Jr, G.A., Semrau, J. and Jolliet, O., 2020. Dredging Contaminated Sediments: Is it Worth the Risks? Environmental Toxicology and Chemistry, 39(3), pp. 515-516. [https://setac.onlinelibrary.wiley.com/doi/pdfdirect/10.1002/etc.4679 DOI: 10.1002/etc.4679]  [https://setac.onlinelibrary.wiley.com/doi/pdfdirect/10.1002/etc.4679 Open access article.]</ref>. As all sediment management also introduces risk pathways, such as sediment re-suspension leading to contaminant release, possible impacts due to land, water and energy usage, and risk to workers, remedial decision-making should also consider the risks posed by the remedial process. There are two types of remediation risks inherent in sediment remediation - engineering and biological. Sediment remedy implementation risks are predominantly short-term engineering issues associated with applying the remedy such that worker and community health and safety are protected, and equipment failures and accidents are minimized<ref name="Wenning2006">Wenning, R.J., Sorensen, M. and Magar, V.S., 2006. Importance of Implementation and Residual Risk Analyses in Sediment Remediation. Integrated Environmental Assessment and Management, 2(1), pp. 59-65. [https://doi.org/10.1002/ieam.5630020111 DOI: 10.1002/ieam.5630020111] [https://setac.onlinelibrary.wiley.com/doi/epdf/10.1002/ieam.5630020111 Open access article.]</ref>. Sediment residual risks are predominantly longer-term concerns associated with the consequences of residual chronic exposures and effects to humans, aquatic biota, and wildlife after the remedy has been implemented<ref name="Wenning2006" />.

In addition to evaluating sediment conditions prior to remediation, sediment risk assessment can be useful to predict the extent to which engineering risks, contaminant exposure pathways, and different human and wildlife populations at risk might change with different remediation options<ref name="NRC2001">National Research Council (NRC), 2001. A Risk‐Management Strategy For PCB Contaminated Sediments. Committee On Remediation Of PCB‐Contaminated Sediments, Board On Environmental Studies And Toxicology. National Academies Press, Washington DC. 452 pp. ISBN: 0-309-58873-1 [https://doi.org/10.17226/10041 DOI: 10.17226/10041] [//www.enviro.wiki/images/b/b4/10041.pdf Article pdf]</ref>. Decision tools such as multi-criteria decision analysis (MCDA), or sustainability assessment<ref name="Apitz2018">Apitz, S.E., Fitzpatrick, A., McNally, A., Harrison, D., Coughlin, C., and Edwards, D.A., 2018. Stakeholder Value-Linked Sustainability Assessment: Evaluating Remedial Alternatives for the Portland Harbor Superfund Site, Portland, Oregon, USA. Integrated Environmental Assessment and Management, 14(1), pp. 43-62. [https://doi.org/10.1002/ieam.1998 DOI: 10.1002/ieam.1998] [https://setac.onlinelibrary.wiley.com/doi/epdf/10.1002/ieam.1998 Open access article.]</ref><ref name="Fitzpatrick2018">Fitzpatrick, A., Apitz, S.E., Harrison, D., Ruffle, B., and Edwards, D.A., 2018. The Portland Harbor Superfund Site Sustainability Project: Introduction. Integrated Environmental Assessment and Management, 14(1), pp. 17-21. [https://doi.org/10.1002/ieam.1997 DOI: 10.1002/ieam.197]  [https://setac.onlinelibrary.wiley.com/doi/epdf/10.1002/ieam.1997 Open access article.]</ref>, for example, incorporate elements from sediment risk assessment to support remediation decision making<ref name="Linkov2006a">Linkov, I., Satterstrom, F.K., Kiker, G., Seager, T.P., Bridges, T., Gardner, K.H., Rogers, S.H., Belluck, D.A. and Meyer, A., 2006. Multicriteria Decision Analysis: A Comprehensive Decision Approach for Management of Contaminated Sediments. Risk Analysis, 26(1), pp. 61-78. [https://doi.org/10.1111/j.1539-6924.2006.00713.x DOI: 10.1111/j.1539-6924.2006.00713.x]  [https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1164&context=usarmyceomaha Open access article.]</ref>. Sediment risk assessment also plays an important role in the implementation of monitored natural recovery (MNR) as a remediation strategy<ref name="Magar2006">Magar, V.S. and Wenning, R.J., 2006. The role of monitored natural recovery in sediment remediation. Integrated Environmental Assessment and Management, 2(1), pp. 66-74. [https://doi.org/10.1002/ieam.5630020112 DOI: 10.1002/ieam.5630020112]  [https://setac.onlinelibrary.wiley.com/doi/epdf/10.1002/ieam.5630020112 Open access article.]</ref>. Insofar as ecological recovery is affected by surface‐sediment‐contaminant concentrations, the primary recovery processes for MNR are natural sediment burial and transformation of the contaminant to less toxic forms by biological or chemical processes<ref name="Magar2009">Magar, V.S., Chadwick, D.B., Bridges, T.S., Fuchsman, P.C., Conder, J.M., Dekker, T.J., Steevens, J.A., Gustavson, K.E. and Mills, M.A., 2009. Technical Guide: Monitored Natural Recovery at Contaminated Sediment Sites. Environmental Security Technology Certification Program (ESTCP) Project [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Sediments/In-place-Remediation/ER-200622/(language)/eng-US ER-0622]. 277 pp. [https://apps.dtic.mil/sti/pdfs/ADA512822.pdf Report pdf]</ref>.

Since risk reduction is the long‐term goal of contaminated sediment management<ref name="Apitz2002">Apitz, S.E. and Power, E.A., 2002. From Risk Assessment to Sediment Management: An International Perspective. Journal of Soils and Sediments, 2(2), pp. 61-66. [https://doi.org/10.1007/BF02987872 DOI: 10.1007/BF02987872]   Free download from: [https://www.researchgate.net/profile/Sabine-Apitz/publication/225649107_From_risk_assessment_to_sediment_management_An_international_perspective/links/09e4150cb2df7c6331000000/From-risk-assessment-to-sediment-management-An-international-perspective.pdf ResearchGate].</ref>, predicting the rate at which contaminant exposures and risks are mitigated by sedimentation and degradation over time can be aided by including parameters in the risk assessment that calculate the rate of contaminant removal or decay in the sediment. Evaluating sediment management options in terms of risk reduction involves assessing risks for the plausible range of environmental conditions expected in the affected waterbody, which includes the current state of the site as well as the conditions that might occur during the remedy implementation and long after the work is complete and the ecosystem stabilizes<ref name="Linkov2006b">Linkov, I., Satterstrom, F.K., Kiker, G.A., Bridges, T.S., Benjamin, S.L. and Belluck, D.A., 2006. From Optimization to Adaptation: Shifting Paradigms in Environmental Management and Their Application to Remedial Decisions. Integrated Environmental Assessment and Management, 2(1), pp. 92-98. [https://doi.org/10.1002/ieam.5630020116 DOI: 10.1002/ieam.5630020116] [https://setac.onlinelibrary.wiley.com/doi/epdf/10.1002/ieam.5630020116 Article pdf]</ref><ref name="Reible2003">Reible, D., Hayes, D., Lue-Hing, C., Patterson, J., Bhowmik, N., Johnson, M. and Teal, J., 2003. Comparison of the Long-Term Risks of Removal and In Situ Management of Contaminated Sediments in the Fox River. Soil and Sediment Contamination, 12(3), pp. 325-344. [https://doi.org/10.1080/713610975 DOI: 10.1080/713610975]</ref>.

==Summary==
Effective sediment risk assessment begins with an initial scoping and planning exercise. The work proceeds to a SLRA and, if warranted, detailed risk assessment using a process comparable to an ERA. The key elements of sediment risk assessment must include a well‐designed and site‐specific CSM; a transparent and well‐thought‐out biological and chemical data collection and analysis plan; carefully selected reference sites and decision criteria; and an explicit discussion of uncertainty. If the sediment risk assessment concludes that unacceptable risks exist, the plausible risk management strategies must be identified, evaluated, selected, implemented, and their success monitored relative to the outcomes predicted in the risk assessment.

Sediment risk assessments are designed to simulate and predict plausible interactions between contaminants or other stressors and both ecological and human receptors. The intent is to derive meaningful insights that provide conclusions that are both rational and protective, in that they err on the side of over-estimating the likely environmental risks. Although conservative assumptions should always be used early in the sediment risk assessment process, final decisions should be supported by refined, realistic estimates of risk provided by site‐specific data and sound analytical approaches. It is increasingly evident after nearly 50 years of application that sediment risk assessment is most useful when supported by a well‐designed, site‐specific, and tiered assessment process<ref name="Bridges2005">Bridges, T., Berry, W., Della Sala, S., Dorn, P., Ells, S., Gries, T., Ireland, S., Maher, E., Menzie, C., Porebski, L., and Stronkhorst, J., 2005. Chapter 6: A framework for assessing and managing risks from contaminated sediments. In: Use of sediment quality guidelines and related tools for the assessment of contaminated sediments. Wenning, Batley, Ingersoll, and Moore, editors. Society of Environmental Toxicology and Chemistry (SETAC), pp. 227–266. ISBN: 1-880611-71-6</ref>.

==References==
<references />

==See Also==

Contaminated Sediments - Introduction

2026-03-13T21:00:04Z

Admin:

Sediments are unconsolidated particulate materials found at the bottom of natural and manmade surface water bodies. They may include clay, silt, sand, gravel, decaying organic matter, or shells. Discharge of contaminants to surface water can result in contamination of sediments and potentially adverse impacts to receptors including [[Wikipedia: Benthic zone | benthic]] and water-column invertebrates, fish, wildlife, plants, and human populations. Contaminant sources include contaminated wastewater, surface water runoff, stormwater discharge, or groundwater, as well as atmospheric deposition, and spills and releases. Common contaminants include petroleum products, [[wikipedia:Polychlorinated biphenyl | polychlorinated biphenyls (PCBs)]], [[Polycyclic Aromatic Hydrocarbons (PAHs) | polycyclic aromatic hydrocarbons (PAHs)]], [[Wikipedia:Dioxins_and_dioxin-like_compounds | dioxins]], pesticides, metals, radionuclides, and excess nutrients<ref name="USEPA2019S">U.S. Environmental Protection Agency (USEPA), 2019. Superfund Contaminated Sediments: Guidance and Technical Support [https://www.epa.gov/superfund/superfund-contaminated-sediments-guidance-and-technical-support website]</ref><ref name="ITRC2011">ITRC, 2011. Incorporating Bioavailability Considerations into the Evaluation of Contaminated Sediment Sites. [//www.enviro.wiki/images/3/30/2011-ITRC_incorporating_bioavailability_Considerations_into_the_Evaluation_of_Contaminated_Sediment_Sites.pdf Report.pdf]</ref>.

This article provides a brief introduction to the major topics associated with contaminated sediment management and remediation. It is not intended to be an exhaustive treatise on the subject of contaminated sediments, but rather to be a curated resource on key aspects of contaminated sediments that have seen major innovations in recent years. It also provides links to more in-depth enviro.wiki discussions on the key topics as well as links to major resources on the subject of contaminated sediments.

<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s)''':

*[[Contaminated Sediment Risk Assessment]]
*[[In Situ Toxicity Identification Evaluation (iTIE) | In Situ Toxicity Identification Evaluation]]
*[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
*[[Mercury in Sediments]]
*[[Passive Sampling of Sediments]]
*[[Sediment Capping]]
*[[Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)]]

'''Contributor(s):''' [[Dr. Upal Ghosh]]

'''Key Resource(s)''':

*[https://www.itrcweb.org/contseds_remedy-selection/ ITRC - Contaminated Sediments Remediation]<ref name="ITRC2014">ITRC, 2014. Contaminated Sediments Remediation [https://www.itrcweb.org/contseds_remedy-selection/Content/2%20Remedy%20Evaluation%20Framework.htm Website]</ref>
*[//www.enviro.wiki/images/6/6f/2002-USEPA-_Principles_for_Managing_Contaminated_Sediment_Risks_at_Hazardous_Waste_Sites.pdf USEPA – Sediment Risk Management Principles]<ref>USEPA, 2002. Principles for Managing Contaminated Sediment Risks at Hazardous Waste Sites. OSWER Directive 9285.6–08. [//www.enviro.wiki/images/6/6f/2002-USEPA-_Principles_for_Managing_Contaminated_Sediment_Risks_at_Hazardous_Waste_Sites.pdf Report.pdf]</ref>
*[https://www.epa.gov/superfund/superfund-contaminated-sediments-guidance-and-technical-support USEPA - Superfund Contaminated Sediments: Guidance and Technical Support]<ref name="USEPA2019S" />

==Introduction==
[[File:Ghosh1w2Fig1.png|thumb|450px|Figure 1. Key Exposure Pathways for Human Health Risk at Contaminated Sediment Sites. [https://www.itrcweb.org/contseds-bioavailability/index.htm Source]]]
Discharge of contaminants to lakes, rivers, and estuaries can result in contamination of the underlying sediments and potential adverse impacts to critical receptors including benthic and water-column invertebrates, fish, wildlife, plants and human populations. Contaminated sediments are often located in sensitive aquatic environments and sometimes may require corrective measures to reduce exposure to human and ecological receptors<ref name="ITRC2011" />.

In most cases, the Contaminants of Potential Concern (COPC) in sediments are relatively immobile and long-lived. This includes petroleum products, [[wikipedia:Polychlorinated biphenyl | polychlorinated biphenyls (PCBs)]], [[Polycyclic_Aromatic_Hydrocarbons_(PAHs)| polycyclic aromatic hydrocarbons (PAHs)]], [[Wikipedia:Dioxins_and_dioxin-like_compounds | dioxins]], metals (mercury, copper, cadmium, lead, nickel, zinc, tin), radionuclides, and excess nutrients<ref name="ITRC2011" />. Some of the contaminants (like metals and PAHs) primarily pose a risk to benthic organisms present in the sediments, while the bioaccumulative chemicals (PCBs, dioxins) are more likely to impact higher trophic organisms such as fish and humans.

==Sediment Risk Assessment and Management==
[[File:Ghosh1w2Fig2.png|thumb|450px|Figure 2.Physical Transport and Ecological Receptor Processes for Contaminants of Potential Concern (COPC) in a Freshwater System. [https://www.itrcweb.org/contseds-bioavailability/index.htm Source]]]
Effective risk management and remediation of contaminated sediments requires an understanding of how contaminants are released from sediment, transported, and taken up by receptors<ref name="ITRC2011" />. A clear understanding of important exposure pathways based on site-specific information is needed for the formulation of a robust site conceptual model. Figure 1 illustrates typical transport pathways between sediment and receptors in a freshwater system. A substantial portion of the total mass of a contaminant present in sediment is often not available to potential receptors due to a variety of “…physical, chemical, and biological interactions that determine the exposure of plants and animals to chemicals associated with soils and sediments” <ref>National Research Council, 2003. Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications. Washington, DC: The National Academies Press. 432p. https://doi.org/10.17226/10523 [https://www.nap.edu/download/10523 free download]</ref>. As a result, contaminant “bioavailability” must be incorporated into Conceptual Site Models (CSMs) and risk assessments<ref name="ITRC2011" />.

Important processes controlling contaminant bioavailability include physical (advection/diffusion, resuspension/deposition, burial, bioturbation, and ebullition/gas transport), chemical (sorption/desorption, transformation/degradation, oxidation/reduction) and biological (uptake, bioconcentration or bioaccumulation, biotransformation)<ref name="ITRC2011" />. Important physical transport and ecological receptor processes for contaminants of potential concern (COPCs) in a typical freshwater ecosystem are illustrated in Figure 2.

==Risk Management and Remedy Selection==
Risk management at contaminated sediment sites typically follows the guidance established by the U.S. Environmental Protection Agency (USEPA) Superfund program. Once the nature and magnitude of risk has been established, several options exist for the management of the risk including institutional controls such as site access control or fish consumption advisories, relying on the natural process of attenuation, and/or active remedies of the contaminated sediments. The USEPA has developed [https://www.epa.gov/superfund/superfund-contaminated-sediments-guidance-and-technical-support technical guidance] to facilitate characterization, risk management, and remediation of contaminated sediment sites and encourage national consistency in these processes.

During selection of a remedy to address contaminated sediment, the USEPA recommends a risk-based approach that accounts for short-term and long-term risks. In addition, the remedy selected must maintain consistency with the [https://www.epa.gov/emergency-response/national-oil-and-hazardous-substances-pollution-contingency-plan-ncp-overview National Oil and Hazardous Substances Pollution Contingency Plan’s] nine remedy selection criteria set forth in [https://www.law.cornell.edu/cfr/text/40/300.430 40 CFR Part 300.430(e)9(iii)] as follows:

#Protection of human health and the environment,
#Compliance with Applicable or Relevant and Appropriate Requirements,
#Long-term effectiveness and permanence,
#Toxicity, mobility or volume reduction through treatment,
#Short-term effectiveness,
#Implementability,
#Cost,
#State agency acceptance, and
#Community acceptance.

[[File:Ghosh1w2Fig3.png|thumb|450px|Figure 3. Sediment removal being conducted at the Milltown Reservoir Sediments Superfund Site in Missoula County, Montana. [https://commons.wikimedia.org/wiki/File:Excavation.jpg Source]]]
The Interstate Technology and Regulatory Council (ITRC) has also developed a [https://www.itrcweb.org/contseds_remedy-selection/Content/2%20Remedy%20Evaluation%20Framework.htm remedy selection framework] to help project managers evaluate remedial technologies and develop alternatives based on site-specific data. This framework includes:

#Review of site characteristics,
#Remedial zone identification and mapping,
#Screening remedial technologies,
#Detailed evaluation remedial technologies,
#Development of remedial action alternatives, and
#Evaluation of remedial action alternatives<ref name="ITRC2014" />.

==Remedial Technologies==
Commonly employed technologies for sediment remediation include monitored natural recovery, enhanced monitored natural recovery, ''in situ'' treatment, capping, and removal<ref name="ITRC2014" />.

'''Monitored natural recovery (MNR) ''' is defined as a remediation practice that relies on natural processes to decrease chemical contaminants in sediment to acceptable concentrations within a reasonable time frame <ref>National Research Council, 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. https://doi.org/10.17226/9792 [https://www.nap.edu/download/9792 free download]</ref>. Enhanced MNR (EMNR) applies material or amendments to enhance these natural recovery processes (such as the addition of a thin-layer cap or a carbon amendment). Parallel natural or enhanced processes, taken together with observed and predicted reductions of contaminant concentrations in fish tissue, sediments, and water provide multiple lines of evidence to support the selection of MNR/EMNR as the primary remedial strategy<ref>Magar, V.S., Chadwick, D.B., Bridges, T.S., Fuchsman, P.C., Conder, J.M., Dekker, T.J., Steevens, J.A., Gustavson, K.E. and Mills, M.A., 2009. Monitored natural recovery at contaminated sediment sites. Environ International Corp Arlington Va. [//www.enviro.wiki/images/f/fd/2009-Magar-Technical_Guide.pdf Report.pdf]</ref>. Important processes to consider in assessments of MNR/EMNR include contaminant burial, dispersion, sorption, precipitation, and chemical, biological and radioactive transformations<ref name="ITRC2014" />.

'''''In situ'' treatment''' commonly involves the addition of treatment amendments to accelerate contaminant removal and/or immobilize the contaminant<ref name="ITRC2014" />. Amendments that have been considered for sediment treatment include organophilic clay, zeolites, bauxite, iron oxide/hydroxide, apatite, and zero valent iron<ref>O'Day, P.A. and Vlassopoulos, D., 2010. Mineral-based amendments for remediation. Elements, 6(6), pp.375-381 [https://doi.org/10.2113/gselements.6.6.375 doi: 10.2113/gselements.6.6.375]</ref>. However, the most common approach is addition of activated carbon (AC) as a thin-layer cap or incorporated into the sediment<ref>Ghosh, U., Luthy, R.G., Cornelissen, G., Werner, D. and Menzie, C.A., 2011. In-situ sorbent amendments: a new direction in contaminated sediment management. [https://doi.org/10.1021/es102694h doi: 10.1021/es102694h]</ref>. 

'''Capping''' is the process of placing a clean layer of sand, sediments or other material over contaminated sediments in order to mitigate risk posed by those sediments<ref name="ITRC2014" />. The cap can include geotextiles and armoring layers to improve stability and enhance habitat. 

'''Removal (dredging or excavation) ''' physically removes the contaminated sediment from the ecosystem and is most effective for hot spots and major sources but may be less effective for overall risk reduction because of resuspension and residual contamination<ref name="ITRC2014" />. Once removed, the contaminated sediments are treated or disposed at an offsite location. Figure 3 shows sediment excavation at the Milltown Reservoir Sediments Superfund Site.

==References==

<references />

==See Also==

*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Sediments Managing Contaminated Sediments]
*[https://semspub.epa.gov/work/11/140524.pdf EPA’s Contaminated Sediment Management Strategy]
*[https://semspub.epa.gov/work/HQ/174471.pdf#1819 Contaminated Sediment Remediation Guidance for Hazardous Waste Sites]
*[https://semspub.epa.gov/work/HQ/196834.pdf Remediating Contaminated Sediment Sites – Clarification of Several Key Remedial Investigation/Feasibility Study and Risk Management Recommendations, and Updated Contaminated Sediment Technical Advisory Group Operating Procedures]

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[[File:WH Picture1.JPG|thumb|center|x350px|link=Matrix Diffusion|Molecular diffusion slowly transports solutes into clay-rich, lower permeability zones]]
[[File:WH Picture2.JPG|thumb|center|x350px|link=Subgrade Biogeochemical Reactor (SBGR)|Typical subgrade biogeochemical reactor (SBGR) layout. The SBGR is an in situ remediation technology for treatment of contaminated source areas and groundwater plume hot spots ]]
[[File:WH Picture3.JPG|thumb|center|x350px|link=Direct Push Logging|An Hydraulic Profiling Tool (HPT) log with electrical conductivity (EC) on left, injection pressure in middle, and flow rate on the right]]
[[File:WH Picture4.JPG|thumb|center|x350px|link=PH Buffering in Aquifers|Diagram of mineral surface exchanging hydrogen ions with varying pH. The surface of most aquifer minerals carries an electrical charge that varies with pH]]
[[File:WH Picture5.JPG|thumb|center|x350px|link=Biodegradation - Hydrocarbons|Comparison of the longitudinal redox zonation concept (A) and the plume fringe concept (B). Both concepts describe the spatial distribution of electron acceptors and respiration processes in a hydrocarbon contaminant plume]]
[[File:WH Picture6.JPG|thumb|center|x350px|link=Direct Push Logging|Schematic of an Hydraulic Profiling Tool (HPT) probe. HPT were developed to better understand formation permeability and the distribution of permeable and low permeability zones in unconsolidated formations]]
[[File:WH Picture7.JPG|thumb|center|x350px|link=Chemical Oxidation Design Considerations(In Situ - ISCO)|In situ chemical oxidation using (a) direct-push injection probes or (b) well-to-well flushing to delivery oxidants (shown in blue) into a target treatment zone of groundwater contaminated by dense nonaqueous phase liquid compounds (shown in red)]]
[[File:WH Picture8.JPG|thumb|center|x350px|link=Geophysical Methods - Case_Studies|High-resolution 3D cross-borehole electrical imaging of contaminated fractured rock at the former Naval Air Warfare Center in New Jersey. Cross-borehole resistivity tomography imaging is a geophysical technique that can be used for site characterization and monitoring by observing variations in the electrical properties of subsurface materials]]
[[File:WH Picture9.JPG|thumb|center|x350px|link=Stable_Isotope_Probing_(SIP)|Stable isotope probing (SIP) in use: Loading, deployment and recovery of Bio-Trap® passive sampler with 13C-labeled benzene. Stable isotope probing (SIP) is used to conclusively determine whether in situ biodegradation of a contaminant is occurring]]
[[File:WH Picture10.JPG|thumb|center|x350px|link=1,2,3-Trichloropropane|Summary of anticipated, primary reaction pathways for degradation of 1,2,3-Trichloropropane (TCP). TCP is a man-made chemical that was used in the past primarily as a solvent and extractive agent, a paint and varnish remover, and as a cleaning and degreasing agent]]
[[File:WH Picture11.JPG|thumb|center|x350px|link=Monitored Natural Attenuation (MNA) of Fuels|Distribution of BTEX plume lengths from 604 hydrocarbon sites. Monitored Natural Attenuation (MNA) is one of the most commonly used remediation approaches for groundwater contaminated with petroleum hydrocarbons (PHCs) and certain fuel additives such as fuel oxygenates or lead scavengers]]
[[File:WH Picture12.JPG|thumb|center|x350px|link=Groundwater Sampling - No-Purge/Passive|No-purge and passive sampling methods eliminate the pre-purging step for groundwater sample collection and represent alternatives to conventional sampling methods that rely on low-flow purging of a well prior to collection. The Snap SamplerTM is an example of a passive grab sampler]]
[[File:WH Picture13.JPG|thumb|center|x350px|link=Natural Source Zone Depletion (NSZD)|Conceptualization of Vapor Transport-related Natural Source Zone Depletion (NSZD) processes at a Petroleum Release Site]]
[[File:WH Picture14.JPG|thumb|center|x350px|link=Soil Vapor Extraction (SVE)|Conceptual diagram of basic Soil Vapor Extraction (SVE) system for vadose zone remediation. (SVE) is a common and typically effective physical treatment process for remediation of volatile contaminants in vadose zone (unsaturated) soils]]
[[File:WH Picture15.JPG|thumb|center|x350px|link=Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation|Emulsified Vegetable Oil (EVO) mixed in field during early pilot test. EVO is commonly added as a slowly fermentable substrate to stimulate the in situ anaerobic bioremediation of chlorinated solvents, explosives, perchlorate, chromate, and other contaminants]]
[[File:WH Picture16.JPG|thumb|center|x350px|link=Vapor_Intrusion_(VI)|Key elements of vapor intrusion pathways]]
[[File:WH Picture17.JPG|thumb|center|x350px|link=Sorption_of_Organic_Contaminants|Batch reactor experiments to generate points on a sorption isotherm]]
[[File:WH Picture18.JPG|thumb|center|x350px|link=Metagenomics|Results for metagenomic analysis of a groundwater sample obtained from a site impacted with petroleum hydrocarbons]]
[[File:WH Picture19.JPG|thumb|center|x350px|link=Perchlorate|Perchlorate releases and drinking water detections]]
[[File:WH Picture20.JPG|thumb|center|x350px|link=Mass_Flux_and_Mass_Discharge|Data input screen for ESTCP Mass Flux Toolkit]]
[[File:WH Picture21.JPG|thumb|center|x350px|link=Bioremediation_-_Anaerobic_Design_Considerations|Amendment addition for biobarrier]]
[[File:WH Picture22.JPG|thumb|center|x350px|link=Thermal Conduction Heating (TCH)|Thermal Remediation - Desorption schematic]]
[[File:WH_Picture23.jpg|thumb|center|x350px|link=Contaminated_Sediments_-_Introduction |Key exposure pathways for human health risk from contaminated sediments]]
[[File:WH_Picture24.jpg|thumb|center|x350px|link=Perfluoroalkyl_and_Polyfluoroalkyl_Substances_(PFAS)| The PFAS family of compounds]]

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'''[[Transport & Attenuation Processes | Attenuation & Transport Processes]]'''

*[[Biodegradation - 1,4-Dioxane]]
*[[Biodegradation - Cometabolic]]
*[[Biodegradation - Hydrocarbons]]
*[[Biodegradation - Reductive Processes]]
*[[Groundwater Flow and Solute Transport]]
*[[Matrix Diffusion]]
*[[Metals and Metalloids - Mobility in Groundwater | Mobility of Metals and Metalloids]]
*[[pH Buffering in Aquifers]]
*[[Sorption of Organic Contaminants]]
*[[Vapor Intrusion (VI)]]
**[[Vapor Intrusion - Separation Distances from Petroleum Sources]]
**[[Vapor Intrusion – Sewers and Utility Tunnels as Preferential Pathways|Vapor Intrusion - Sewers and Utility Tunnels as Preferential Pathways]]
**[[Assessing Vapor Intrusion (VI) Impacts in Neighborhoods with Groundwater Contaminated by Chlorinated Volatile Organic Chemicals (CVOCs)|Vapor Intrusion - Assessing VI Impacts in Neighborhoods with Groundwater Contaminated CVOCs]]

'''[[Characterization, Assessment & Monitoring]]'''

*[[Characterization Methods – Hydraulic Conductivity]]
*[[Compound Specific Isotope Analysis (CSIA)|Compound Specific Isotope Analysis (CSIA)]]
*[[Direct Push (DP) Technology]]
**[[Direct Push Logging |Direct Push Logging]]
**[[Direct Push Sampling |Direct Push Sampling]]
*[[Geophysical Methods | Geophysical Methods]]
**[[Geophysical Methods - Case Studies |Case Studies]]
**[[Hydrogeophysical Methods for Characterization and Monitoring of Groundwater-Surface Water Exchanges]]
*[[Groundwater Sampling - No-Purge/Passive]]
*[[Long-Term Monitoring (LTM)|Long-Term Monitoring (LTM)]]
**[[Long-Term Monitoring (LTM) - Data Analysis |LTM Data Analysis]]
**[[Long-Term Monitoring (LTM) - Data Variability |LTM Data Variability]]
*[[Molecular Biological Tools - MBTs |Molecular Biological Tools (MBTs)]]
**[[Metagenomics]]
**[[Proteomics and Proteogenomics]]
**[[Quantitative Polymerase Chain Reaction (qPCR)]]
**[[Stable Isotope Probing (SIP)]]
*[[Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill |Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill]]
*[[OPTically-based In-situ Characterization System (OPTICS)]]

'''[[Climate Change Primer | Climate Change]]'''

*[[Climate Change Effects on Wildlife]]
*[[Downscaled High Resolution Datasets for Climate Change Projections]]
*[[Infrastructure Resilience]]
*[[Predicting Species Responses to Climate Change with Population Models]]
*[[Restoration of Ecological Function in Terrestrial Systems Impacted by Invasive Species]]

'''[[Coastal and Estuarine Ecology]]'''

*[[Phytoplankton (Algae) Blooms]]

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'''[[Contaminated Sediments - Introduction | Contaminated Sediments]]'''

*[[Contaminated Sediment Risk Assessment]]
*[[In Situ Toxicity Identification Evaluation (iTIE) | In Situ Toxicity Identification Evaluation]]
*[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
*[[Mercury in Sediments]]
*[[Passive Sampling of Sediments]]
**[[Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)]]
*[[Sediment Capping]]

'''[[Light Non-Aqueous Phase Liquids (LNAPLs)]]'''

*[[LNAPL Conceptual Site Models]]
*[[LNAPL Remediation Technologies]]
*[[NAPL Mobility]]

'''[[Munitions Constituents]]'''

*[[Munitions Constituents - Abiotic Reduction|Abiotic Reduction]]
*[[Munitions Constituents - Alkaline Degradation|Alkaline Degradation]]
**[[Pyrogenic Carbonaceous Matter Enhanced Alkaline Hydrolysis]]
*[[Munitions Constituents - Composting|Composting]]
*[[Munitions Constituents - Deposition |Deposition]]
*[[Munitions Constituents - Dissolution |Dissolution]]
*[[Munitions Constituents - Electrochemical Treatment|Electrochemical Treatment]]
*[[Metal(loid)s - Small Arms Ranges]]
*[[Passive Sampling of Munitions Constituents|Passive Sampling]]
*[[Munitions Constituents – Photolysis |Photolysis]]
*[[Remediation of Stormwater Runoff Contaminated by Munition Constituents |Remediation of Stormwater Runoff ]]
*[[Munitions Constituents – Sample Extraction and Analytical Techniques|Sample Extraction and Analytical Techniques]]
*[[Munitions Constituents - Soil Sampling |Soil Sampling]]
*[[Munitions Constituents - Sorption |Sorption]]
*[[Munitions Constituents - IM Toxicology |Toxicology]]
*[[Munitions Constituents- TREECS™ Fate and Risk Modeling|TREECS™]]

'''[[Monitored Natural Attenuation (MNA)]]'''

*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| MNA of Chlorinated Solvents]]
*[[Monitored Natural Attenuation (MNA) of Fuels| MNA of Fuels]]
*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| MNA of Metals and Metalloids]]
*[[Natural Source Zone Depletion (NSZD)]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies| Transitioning from Active Remedies]]

'''[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]'''

*[[Hydrothermal Alkaline Treatment (HALT)]]
*[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]
*[[PFAS Destruction by Ultraviolet/Sulfite Treatment]]
*[[PFAS Ex Situ Water Treatment]]
**[[PFAS Treatment by Anion Exchange]]
*[[PFAS Monitored Retention (PMR) and PFAS Enhanced Retention (PER)]]
*[[PFAS Soil Remediation Technologies]]
*[[PFAS Sources]]
*[[PFAS Toxicology and Risk Assessment]]
*[[PFAS Transport and Fate]]
*[[PFAS Treatment by Electrical Discharge Plasma]]
*[[Photoactivated Reductive Defluorination - PFAS Destruction | Photoactivated Reductive Defluorination]]
*[[Reverse Osmosis and Nanofiltration Membrane Filtration Systems for PFAS Removal]]
*[[Thermal Conduction Heating for Treatment of PFAS-Impacted Soil]]
*[[Transition of Aqueous Film Forming Foam (AFFF) Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances (PFAS)| Transition of Aqueous Film Forming Foam Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances]]

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'''[[Regulatory Issues and Site Management]]'''

*[[Alternative Endpoints]]
*[[Mass Flux and Mass Discharge]]
*[[Plume Response Modeling]]
*[[REMChlor - MD | REMChlor-MD]]
*[[Source Zone Modeling]]
*[[Sustainable Remediation]]

'''[[Remediation Technologies]]'''
*[[Amendment Distribution in Low Conductivity Materials]]
*[[Bioremediation - Anaerobic|Anaerobic Bioremediation]]
**[[Bioremediation - Anaerobic Design Considerations | Design Considerations]]
**[[Design Tool - Base Addition for ERD]]
**[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation]]
**[[Low pH Inhibition of Reductive Dechlorination]]
**[[Bioremediation - Anaerobic Secondary Water Quality Impacts | Secondary Water Quality Impacts]]
*[[Chemical Oxidation (In Situ - ISCO) | In Situ Chemical Oxidation (ISCO)]]
**[[Chemical Oxidation Design Considerations(In Situ - ISCO) | Design Considerations]]
**[[Chemical Oxidation Oxidant Selection (In Situ - ISCO) | Oxidant Selection]]
*[[Chemical Reduction (In Situ - ISCR) | In Situ Chemical Reduction (ISCR)]]
**[[Zerovalent Iron (ZVI) (Chemical Reduction - ISCR) | Zero-Valent Iron (ZVI)]]
**[[Zerovalent Iron Permeable Reactive Barriers]]
*[[In Situ Groundwater Treatment with Activated Carbon]]
*[[Injection Techniques for Liquid Amendments]]
*[[Injection Techniques - Viscosity Modification]]
*[[Landfarming]]
*[[Metal and Metalloids - Remediation | Remediation of Metals and Metalloids]]
*[[Remediation Performance Assessment at Chlorinated Solvent Sites]]
*[[Soil Vapor Extraction (SVE)]]
*[[Stream Restoration]]
*[[Subgrade Biogeochemical Reactor (SBGR)]]
*[[Supercritical Water Oxidation (SCWO)]]
*[[Thermal Remediation]]
**[[Thermal Remediation - Combined Remedies | Combined Remedies]]
**[[Thermal Remediation - Electrical Resistance Heating | Electrical Resistance Heating (ERH)]]
**[[Thermal Remediation - Smoldering | Smoldering]]
**[[Thermal Remediation - Steam | Steam Enhanced Extraction (SEE)]]
**[[Thermal Conduction Heating (TCH)]]

'''[[Soil & Groundwater Contaminants]]'''

*[[1,2,3-Trichloropropane]]
*[[1,4-Dioxane]]
*[[Chlorinated Solvents]]
*[[Metal and Metalloid Contaminants|Metals and Metalloids]]
*[[N-nitrosodimethylamine (NDMA)]]
*[[Perchlorate|Perchlorate]]
*[[Petroleum Hydrocarbons (PHCs)]]
*[[Polycyclic Aromatic Hydrocarbons (PAHs)]]
|}
|}
|}

Main Page

2026-03-03T22:48:51Z

Admin:

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| style="width:55%;" |<center><big>Welcome to '''ENVIRO''' '''Wiki'''</big> Peer Reviewed. Accessible. Written By Experts</center>
| style="width:40%;" |<center> ''Developed and brought to you by '' [[File:MainLogo-serdp-estcp.png|link=https://www.serdp-estcp.org |frameless|center|350px]]''Your Environmental Information Gateway''
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| The goal of ENVIRO Wiki is to make scientific and engineering research results more accessible to environmental professionals, facilitating the permitting, design and implementation of environmental projects. Articles are written and edited by invited experts (see [[Contributors]]) to summarize current knowledge for the target audience on an array of topics, with cross-linked references to reports and technical literature. 
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| id="mp-left" class="MainPageBG" style="width:55%; padding:0; vertical-align:top; color:#000;" |
<h2 id="mp-tfa-h2" style="margin:0.5em; background:#cef2e0; font-family:inherit; font-size:120%; font-weight:bold; border:1px solid #a3bfb1; color:#000; padding:0.2em 0.4em;"> Featured article: PFAS Destruction by Ultraviolet/Sulfite Treatment</h2>
<div id="mp-tfa" style="padding:0.0em 1.0em;">[[File:XiongFig1.png|400px|left|link=PFAS Destruction by Ultraviolet/Sulfite Treatment]]<dailyfeaturedpage></dailyfeaturedpage>  

[[PFAS Destruction by Ultraviolet/Sulfite Treatment|(Full article...)]] </div>

| style="border:1px solid transparent;" |


| id="mp-right" class="MainPageBG" style="width:40%; padding:0; horizontal-align:center; vertical-align:top;" |
<h2 id="mp-itn-h2" style="margin:0.5em; background:#cedff2; font-family:inherit; font-size:120%; font-weight:bold; border:1px solid #a3b0bf; color:#000; padding:0.2em 0.4em;">Enviro Wiki Highlights</h2>
<div id="mp-itn" style="padding:0.0em 0.5em;">

<slideshow sequence="random" transition="fade" refresh="7500">

[[File:WH Picture1.JPG|thumb|center|x350px|link=Matrix Diffusion|Molecular diffusion slowly transports solutes into clay-rich, lower permeability zones]]
[[File:WH Picture2.JPG|thumb|center|x350px|link=Subgrade Biogeochemical Reactor (SBGR)|Typical subgrade biogeochemical reactor (SBGR) layout. The SBGR is an in situ remediation technology for treatment of contaminated source areas and groundwater plume hot spots ]]
[[File:WH Picture3.JPG|thumb|center|x350px|link=Direct Push Logging|An Hydraulic Profiling Tool (HPT) log with electrical conductivity (EC) on left, injection pressure in middle, and flow rate on the right]]
[[File:WH Picture4.JPG|thumb|center|x350px|link=PH Buffering in Aquifers|Diagram of mineral surface exchanging hydrogen ions with varying pH. The surface of most aquifer minerals carries an electrical charge that varies with pH]]
[[File:WH Picture5.JPG|thumb|center|x350px|link=Biodegradation - Hydrocarbons|Comparison of the longitudinal redox zonation concept (A) and the plume fringe concept (B). Both concepts describe the spatial distribution of electron acceptors and respiration processes in a hydrocarbon contaminant plume]]
[[File:WH Picture6.JPG|thumb|center|x350px|link=Direct Push Logging|Schematic of an Hydraulic Profiling Tool (HPT) probe. HPT were developed to better understand formation permeability and the distribution of permeable and low permeability zones in unconsolidated formations]]
[[File:WH Picture7.JPG|thumb|center|x350px|link=Chemical Oxidation Design Considerations(In Situ - ISCO)|In situ chemical oxidation using (a) direct-push injection probes or (b) well-to-well flushing to delivery oxidants (shown in blue) into a target treatment zone of groundwater contaminated by dense nonaqueous phase liquid compounds (shown in red)]]
[[File:WH Picture8.JPG|thumb|center|x350px|link=Geophysical Methods - Case_Studies|High-resolution 3D cross-borehole electrical imaging of contaminated fractured rock at the former Naval Air Warfare Center in New Jersey. Cross-borehole resistivity tomography imaging is a geophysical technique that can be used for site characterization and monitoring by observing variations in the electrical properties of subsurface materials]]
[[File:WH Picture9.JPG|thumb|center|x350px|link=Stable_Isotope_Probing_(SIP)|Stable isotope probing (SIP) in use: Loading, deployment and recovery of Bio-Trap® passive sampler with 13C-labeled benzene. Stable isotope probing (SIP) is used to conclusively determine whether in situ biodegradation of a contaminant is occurring]]
[[File:WH Picture10.JPG|thumb|center|x350px|link=1,2,3-Trichloropropane|Summary of anticipated, primary reaction pathways for degradation of 1,2,3-Trichloropropane (TCP). TCP is a man-made chemical that was used in the past primarily as a solvent and extractive agent, a paint and varnish remover, and as a cleaning and degreasing agent]]
[[File:WH Picture11.JPG|thumb|center|x350px|link=Monitored Natural Attenuation (MNA) of Fuels|Distribution of BTEX plume lengths from 604 hydrocarbon sites. Monitored Natural Attenuation (MNA) is one of the most commonly used remediation approaches for groundwater contaminated with petroleum hydrocarbons (PHCs) and certain fuel additives such as fuel oxygenates or lead scavengers]]
[[File:WH Picture12.JPG|thumb|center|x350px|link=Groundwater Sampling - No-Purge/Passive|No-purge and passive sampling methods eliminate the pre-purging step for groundwater sample collection and represent alternatives to conventional sampling methods that rely on low-flow purging of a well prior to collection. The Snap SamplerTM is an example of a passive grab sampler]]
[[File:WH Picture13.JPG|thumb|center|x350px|link=Natural Source Zone Depletion (NSZD)|Conceptualization of Vapor Transport-related Natural Source Zone Depletion (NSZD) processes at a Petroleum Release Site]]
[[File:WH Picture14.JPG|thumb|center|x350px|link=Soil Vapor Extraction (SVE)|Conceptual diagram of basic Soil Vapor Extraction (SVE) system for vadose zone remediation. (SVE) is a common and typically effective physical treatment process for remediation of volatile contaminants in vadose zone (unsaturated) soils]]
[[File:WH Picture15.JPG|thumb|center|x350px|link=Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation|Emulsified Vegetable Oil (EVO) mixed in field during early pilot test. EVO is commonly added as a slowly fermentable substrate to stimulate the in situ anaerobic bioremediation of chlorinated solvents, explosives, perchlorate, chromate, and other contaminants]]
[[File:WH Picture16.JPG|thumb|center|x350px|link=Vapor_Intrusion_(VI)|Key elements of vapor intrusion pathways]]
[[File:WH Picture17.JPG|thumb|center|x350px|link=Sorption_of_Organic_Contaminants|Batch reactor experiments to generate points on a sorption isotherm]]
[[File:WH Picture18.JPG|thumb|center|x350px|link=Metagenomics|Results for metagenomic analysis of a groundwater sample obtained from a site impacted with petroleum hydrocarbons]]
[[File:WH Picture19.JPG|thumb|center|x350px|link=Perchlorate|Perchlorate releases and drinking water detections]]
[[File:WH Picture20.JPG|thumb|center|x350px|link=Mass_Flux_and_Mass_Discharge|Data input screen for ESTCP Mass Flux Toolkit]]
[[File:WH Picture21.JPG|thumb|center|x350px|link=Bioremediation_-_Anaerobic_Design_Considerations|Amendment addition for biobarrier]]
[[File:WH Picture22.JPG|thumb|center|x350px|link=Thermal Conduction Heating (TCH)|Thermal Remediation - Desorption schematic]]
[[File:WH_Picture23.jpg|thumb|center|x350px|link=Contaminated_Sediments_-_Introduction |Key exposure pathways for human health risk from contaminated sediments]]
[[File:WH_Picture24.jpg|thumb|center|x350px|link=Perfluoroalkyl_and_Polyfluoroalkyl_Substances_(PFAS)| The PFAS family of compounds]]

</slideshow>
</div>
|}

{| id="mp-upper" style="width: 95%; margin:3px 0 0 0; "
| class="MainPageBG" style="width:50%; background:#f5faff; vertical-align:top; color:#000;" |
{| id="mp-left" style="width:100%; vertical-align:top; background:#f9f9f9;"
| style="padding:2px;" |<h2 id="mp-tfa-h2_2" style="margin:3px; background:#cef2e0; font-family:inherit; font-size:120%; font-weight:bold; border:1px solid #a3bfb1; text-align:center; color:#000; padding:0.2em 0.4em;">Table of Contents </h2>
{| style="width:100%; vertical-align:top;"
| style="vertical-align:top;" |
'''[[Transport & Attenuation Processes | Attenuation & Transport Processes]]'''

*[[Biodegradation - 1,4-Dioxane]]
*[[Biodegradation - Cometabolic]]
*[[Biodegradation - Hydrocarbons]]
*[[Biodegradation - Reductive Processes]]
*[[Groundwater Flow and Solute Transport]]
*[[Matrix Diffusion]]
*[[Metals and Metalloids - Mobility in Groundwater | Mobility of Metals and Metalloids]]
*[[pH Buffering in Aquifers]]
*[[Sorption of Organic Contaminants]]
*[[Vapor Intrusion (VI)]]
**[[Vapor Intrusion - Separation Distances from Petroleum Sources]]
**[[Vapor Intrusion – Sewers and Utility Tunnels as Preferential Pathways|Vapor Intrusion - Sewers and Utility Tunnels as Preferential Pathways]]
**[[Assessing Vapor Intrusion (VI) Impacts in Neighborhoods with Groundwater Contaminated by Chlorinated Volatile Organic Chemicals (CVOCs)|Vapor Intrusion - Assessing VI Impacts in Neighborhoods with Groundwater Contaminated CVOCs]]

'''[[Characterization, Assessment & Monitoring]]'''

*[[Characterization Methods – Hydraulic Conductivity]]
*[[Compound Specific Isotope Analysis (CSIA)|Compound Specific Isotope Analysis (CSIA)]]
*[[Direct Push (DP) Technology]]
**[[Direct Push Logging |Direct Push Logging]]
**[[Direct Push Sampling |Direct Push Sampling]]
*[[Geophysical Methods | Geophysical Methods]]
**[[Geophysical Methods - Case Studies |Case Studies]]
**[[Hydrogeophysical Methods for Characterization and Monitoring of Groundwater-Surface Water Exchanges]]
*[[Groundwater Sampling - No-Purge/Passive]]
*[[Long-Term Monitoring (LTM)|Long-Term Monitoring (LTM)]]
**[[Long-Term Monitoring (LTM) - Data Analysis |LTM Data Analysis]]
**[[Long-Term Monitoring (LTM) - Data Variability |LTM Data Variability]]
*[[Molecular Biological Tools - MBTs |Molecular Biological Tools (MBTs)]]
**[[Metagenomics]]
**[[Proteomics and Proteogenomics]]
**[[Quantitative Polymerase Chain Reaction (qPCR)]]
**[[Stable Isotope Probing (SIP)]]
*[[Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill |Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill]]
*[[OPTically-based In-situ Characterization System (OPTICS)]]

'''[[Climate Change Primer | Climate Change]]'''

*[[Climate Change Effects on Wildlife]]
*[[Downscaled High Resolution Datasets for Climate Change Projections]]
*[[Infrastructure Resilience]]
*[[Predicting Species Responses to Climate Change with Population Models]]
*[[Restoration of Ecological Function in Terrestrial Systems Impacted by Invasive Species]]

'''[[Coastal and Estuarine Ecology]]'''

*[[Phytoplankton (Algae) Blooms]]

| style="width:33%; vertical-align:top; " |
'''[[Contaminated Sediments - Introduction | Contaminated Sediments]]'''

*[[Contaminated Sediment Risk Assessment]]
*[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
*[[Mercury in Sediments]]
*[[Passive Sampling of Sediments]]
**[[Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)]]
*[[Sediment Capping]]

'''[[Light Non-Aqueous Phase Liquids (LNAPLs)]]'''

*[[LNAPL Conceptual Site Models]]
*[[LNAPL Remediation Technologies]]
*[[NAPL Mobility]]

'''[[Munitions Constituents]]'''

*[[Munitions Constituents - Abiotic Reduction|Abiotic Reduction]]
*[[Munitions Constituents - Alkaline Degradation|Alkaline Degradation]]
**[[Pyrogenic Carbonaceous Matter Enhanced Alkaline Hydrolysis]]
*[[Munitions Constituents - Composting|Composting]]
*[[Munitions Constituents - Deposition |Deposition]]
*[[Munitions Constituents - Dissolution |Dissolution]]
*[[Munitions Constituents - Electrochemical Treatment|Electrochemical Treatment]]
*[[Metal(loid)s - Small Arms Ranges]]
*[[Passive Sampling of Munitions Constituents|Passive Sampling]]
*[[Munitions Constituents – Photolysis |Photolysis]]
*[[Remediation of Stormwater Runoff Contaminated by Munition Constituents |Remediation of Stormwater Runoff ]]
*[[Munitions Constituents – Sample Extraction and Analytical Techniques|Sample Extraction and Analytical Techniques]]
*[[Munitions Constituents - Soil Sampling |Soil Sampling]]
*[[Munitions Constituents - Sorption |Sorption]]
*[[Munitions Constituents - IM Toxicology |Toxicology]]
*[[Munitions Constituents- TREECS™ Fate and Risk Modeling|TREECS™]]

'''[[Monitored Natural Attenuation (MNA)]]'''

*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| MNA of Chlorinated Solvents]]
*[[Monitored Natural Attenuation (MNA) of Fuels| MNA of Fuels]]
*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| MNA of Metals and Metalloids]]
*[[Natural Source Zone Depletion (NSZD)]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies| Transitioning from Active Remedies]]

'''[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]'''

*[[Hydrothermal Alkaline Treatment (HALT)]]
*[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]
*[[PFAS Destruction by Ultraviolet/Sulfite Treatment]]
*[[PFAS Ex Situ Water Treatment]]
**[[PFAS Treatment by Anion Exchange]]
*[[PFAS Monitored Retention (PMR) and PFAS Enhanced Retention (PER)]]
*[[PFAS Soil Remediation Technologies]]
*[[PFAS Sources]]
*[[PFAS Toxicology and Risk Assessment]]
*[[PFAS Transport and Fate]]
*[[PFAS Treatment by Electrical Discharge Plasma]]
*[[Photoactivated Reductive Defluorination - PFAS Destruction | Photoactivated Reductive Defluorination]]
*[[Reverse Osmosis and Nanofiltration Membrane Filtration Systems for PFAS Removal]]
*[[Thermal Conduction Heating for Treatment of PFAS-Impacted Soil]]
*[[Transition of Aqueous Film Forming Foam (AFFF) Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances (PFAS)| Transition of Aqueous Film Forming Foam Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances]]

| style="width:33%; vertical-align:top; " |
'''[[Regulatory Issues and Site Management]]'''

*[[Alternative Endpoints]]
*[[Mass Flux and Mass Discharge]]
*[[Plume Response Modeling]]
*[[REMChlor - MD | REMChlor-MD]]
*[[Source Zone Modeling]]
*[[Sustainable Remediation]]

'''[[Remediation Technologies]]'''
*[[Amendment Distribution in Low Conductivity Materials]]
*[[Bioremediation - Anaerobic|Anaerobic Bioremediation]]
**[[Bioremediation - Anaerobic Design Considerations | Design Considerations]]
**[[Design Tool - Base Addition for ERD]]
**[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation]]
**[[Low pH Inhibition of Reductive Dechlorination]]
**[[Bioremediation - Anaerobic Secondary Water Quality Impacts | Secondary Water Quality Impacts]]
*[[Chemical Oxidation (In Situ - ISCO) | In Situ Chemical Oxidation (ISCO)]]
**[[Chemical Oxidation Design Considerations(In Situ - ISCO) | Design Considerations]]
**[[Chemical Oxidation Oxidant Selection (In Situ - ISCO) | Oxidant Selection]]
*[[Chemical Reduction (In Situ - ISCR) | In Situ Chemical Reduction (ISCR)]]
**[[Zerovalent Iron (ZVI) (Chemical Reduction - ISCR) | Zero-Valent Iron (ZVI)]]
**[[Zerovalent Iron Permeable Reactive Barriers]]
*[[In Situ Groundwater Treatment with Activated Carbon]]
*[[Injection Techniques for Liquid Amendments]]
*[[Injection Techniques - Viscosity Modification]]
*[[Landfarming]]
*[[Metal and Metalloids - Remediation | Remediation of Metals and Metalloids]]
*[[Remediation Performance Assessment at Chlorinated Solvent Sites]]
*[[Soil Vapor Extraction (SVE)]]
*[[Stream Restoration]]
*[[Subgrade Biogeochemical Reactor (SBGR)]]
*[[Supercritical Water Oxidation (SCWO)]]
*[[Thermal Remediation]]
**[[Thermal Remediation - Combined Remedies | Combined Remedies]]
**[[Thermal Remediation - Electrical Resistance Heating | Electrical Resistance Heating (ERH)]]
**[[Thermal Remediation - Smoldering | Smoldering]]
**[[Thermal Remediation - Steam | Steam Enhanced Extraction (SEE)]]
**[[Thermal Conduction Heating (TCH)]]

'''[[Soil & Groundwater Contaminants]]'''

*[[1,2,3-Trichloropropane]]
*[[1,4-Dioxane]]
*[[Chlorinated Solvents]]
*[[Metal and Metalloid Contaminants|Metals and Metalloids]]
*[[N-nitrosodimethylamine (NDMA)]]
*[[Perchlorate|Perchlorate]]
*[[Petroleum Hydrocarbons (PHCs)]]
*[[Polycyclic Aromatic Hydrocarbons (PAHs)]]
|}
|}
|}

Main Page

2026-03-03T22:48:10Z

Admin:

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| style="width:55%;" |<center><big>Welcome to '''ENVIRO''' '''Wiki'''</big> Peer Reviewed. Accessible. Written By Experts</center>
| style="width:40%;" |<center> ''Developed and brought to you by '' [[File:MainLogo-serdp-estcp.png|link=https://www.serdp-estcp.org |frameless|center|350px]]''Your Environmental Information Gateway''
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| The goal of ENVIRO Wiki is to make scientific and engineering research results more accessible to environmental professionals, facilitating the permitting, design and implementation of environmental projects. Articles are written and edited by invited experts (see [[Contributors]]) to summarize current knowledge for the target audience on an array of topics, with cross-linked references to reports and technical literature. 
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| id="mp-left" class="MainPageBG" style="width:55%; padding:0; vertical-align:top; color:#000;" |
<h2 id="mp-tfa-h2" style="margin:0.5em; background:#cef2e0; font-family:inherit; font-size:120%; font-weight:bold; border:1px solid #a3bfb1; color:#000; padding:0.2em 0.4em;"> Featured article: PFAS Destruction by Ultraviolet/Sulfite Treatment</h2>
<div id="mp-tfa" style="padding:0.0em 1.0em;">[[File:XiongFig1.png|400px|left|link=PFAS Destruction by Ultraviolet/Sulfite Treatment]]<dailyfeaturedpage></dailyfeaturedpage>  

[[PFAS Destruction by Ultraviolet/Sulfite Treatment|(Full article...)]] </div>

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| id="mp-right" class="MainPageBG" style="width:40%; padding:0; horizontal-align:center; vertical-align:top;" |
<h2 id="mp-itn-h2" style="margin:0.5em; background:#cedff2; font-family:inherit; font-size:120%; font-weight:bold; border:1px solid #a3b0bf; color:#000; padding:0.2em 0.4em;">Enviro Wiki Highlights</h2>
<div id="mp-itn" style="padding:0.0em 0.5em;">

<slideshow sequence="random" transition="fade" refresh="7500">

[[File:WH Picture1.JPG|thumb|center|x350px|link=Matrix Diffusion|Molecular diffusion slowly transports solutes into clay-rich, lower permeability zones]]
[[File:WH Picture2.JPG|thumb|center|x350px|link=Subgrade Biogeochemical Reactor (SBGR)|Typical subgrade biogeochemical reactor (SBGR) layout. The SBGR is an in situ remediation technology for treatment of contaminated source areas and groundwater plume hot spots ]]
[[File:WH Picture3.JPG|thumb|center|x350px|link=Direct Push Logging|An Hydraulic Profiling Tool (HPT) log with electrical conductivity (EC) on left, injection pressure in middle, and flow rate on the right]]
[[File:WH Picture4.JPG|thumb|center|x350px|link=PH Buffering in Aquifers|Diagram of mineral surface exchanging hydrogen ions with varying pH. The surface of most aquifer minerals carries an electrical charge that varies with pH]]
[[File:WH Picture5.JPG|thumb|center|x350px|link=Biodegradation - Hydrocarbons|Comparison of the longitudinal redox zonation concept (A) and the plume fringe concept (B). Both concepts describe the spatial distribution of electron acceptors and respiration processes in a hydrocarbon contaminant plume]]
[[File:WH Picture6.JPG|thumb|center|x350px|link=Direct Push Logging|Schematic of an Hydraulic Profiling Tool (HPT) probe. HPT were developed to better understand formation permeability and the distribution of permeable and low permeability zones in unconsolidated formations]]
[[File:WH Picture7.JPG|thumb|center|x350px|link=Chemical Oxidation Design Considerations(In Situ - ISCO)|In situ chemical oxidation using (a) direct-push injection probes or (b) well-to-well flushing to delivery oxidants (shown in blue) into a target treatment zone of groundwater contaminated by dense nonaqueous phase liquid compounds (shown in red)]]
[[File:WH Picture8.JPG|thumb|center|x350px|link=Geophysical Methods - Case_Studies|High-resolution 3D cross-borehole electrical imaging of contaminated fractured rock at the former Naval Air Warfare Center in New Jersey. Cross-borehole resistivity tomography imaging is a geophysical technique that can be used for site characterization and monitoring by observing variations in the electrical properties of subsurface materials]]
[[File:WH Picture9.JPG|thumb|center|x350px|link=Stable_Isotope_Probing_(SIP)|Stable isotope probing (SIP) in use: Loading, deployment and recovery of Bio-Trap® passive sampler with 13C-labeled benzene. Stable isotope probing (SIP) is used to conclusively determine whether in situ biodegradation of a contaminant is occurring]]
[[File:WH Picture10.JPG|thumb|center|x350px|link=1,2,3-Trichloropropane|Summary of anticipated, primary reaction pathways for degradation of 1,2,3-Trichloropropane (TCP). TCP is a man-made chemical that was used in the past primarily as a solvent and extractive agent, a paint and varnish remover, and as a cleaning and degreasing agent]]
[[File:WH Picture11.JPG|thumb|center|x350px|link=Monitored Natural Attenuation (MNA) of Fuels|Distribution of BTEX plume lengths from 604 hydrocarbon sites. Monitored Natural Attenuation (MNA) is one of the most commonly used remediation approaches for groundwater contaminated with petroleum hydrocarbons (PHCs) and certain fuel additives such as fuel oxygenates or lead scavengers]]
[[File:WH Picture12.JPG|thumb|center|x350px|link=Groundwater Sampling - No-Purge/Passive|No-purge and passive sampling methods eliminate the pre-purging step for groundwater sample collection and represent alternatives to conventional sampling methods that rely on low-flow purging of a well prior to collection. The Snap SamplerTM is an example of a passive grab sampler]]
[[File:WH Picture13.JPG|thumb|center|x350px|link=Natural Source Zone Depletion (NSZD)|Conceptualization of Vapor Transport-related Natural Source Zone Depletion (NSZD) processes at a Petroleum Release Site]]
[[File:WH Picture14.JPG|thumb|center|x350px|link=Soil Vapor Extraction (SVE)|Conceptual diagram of basic Soil Vapor Extraction (SVE) system for vadose zone remediation. (SVE) is a common and typically effective physical treatment process for remediation of volatile contaminants in vadose zone (unsaturated) soils]]
[[File:WH Picture15.JPG|thumb|center|x350px|link=Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation|Emulsified Vegetable Oil (EVO) mixed in field during early pilot test. EVO is commonly added as a slowly fermentable substrate to stimulate the in situ anaerobic bioremediation of chlorinated solvents, explosives, perchlorate, chromate, and other contaminants]]
[[File:WH Picture16.JPG|thumb|center|x350px|link=Vapor_Intrusion_(VI)|Key elements of vapor intrusion pathways]]
[[File:WH Picture17.JPG|thumb|center|x350px|link=Sorption_of_Organic_Contaminants|Batch reactor experiments to generate points on a sorption isotherm]]
[[File:WH Picture18.JPG|thumb|center|x350px|link=Metagenomics|Results for metagenomic analysis of a groundwater sample obtained from a site impacted with petroleum hydrocarbons]]
[[File:WH Picture19.JPG|thumb|center|x350px|link=Perchlorate|Perchlorate releases and drinking water detections]]
[[File:WH Picture20.JPG|thumb|center|x350px|link=Mass_Flux_and_Mass_Discharge|Data input screen for ESTCP Mass Flux Toolkit]]
[[File:WH Picture21.JPG|thumb|center|x350px|link=Bioremediation_-_Anaerobic_Design_Considerations|Amendment addition for biobarrier]]
[[File:WH Picture22.JPG|thumb|center|x350px|link=Thermal Conduction Heating (TCH)|Thermal Remediation - Desorption schematic]]
[[File:WH_Picture23.jpg|thumb|center|x350px|link=Contaminated_Sediments_-_Introduction |Key exposure pathways for human health risk from contaminated sediments]]
[[File:WH_Picture24.jpg|thumb|center|x350px|link=Perfluoroalkyl_and_Polyfluoroalkyl_Substances_(PFAS)| The PFAS family of compounds]]

</slideshow>
</div>
|}

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'''[[Transport & Attenuation Processes | Attenuation & Transport Processes]]'''

*[[Biodegradation - 1,4-Dioxane]]
*[[Biodegradation - Cometabolic]]
*[[Biodegradation - Hydrocarbons]]
*[[Biodegradation - Reductive Processes]]
*[[Groundwater Flow and Solute Transport]]
*[[Matrix Diffusion]]
*[[Metals and Metalloids - Mobility in Groundwater | Mobility of Metals and Metalloids]]
*[[pH Buffering in Aquifers]]
*[[Sorption of Organic Contaminants]]
*[[Vapor Intrusion (VI)]]
**[[Vapor Intrusion - Separation Distances from Petroleum Sources]]
**[[Vapor Intrusion – Sewers and Utility Tunnels as Preferential Pathways|Vapor Intrusion - Sewers and Utility Tunnels as Preferential Pathways]]
**[[Assessing Vapor Intrusion (VI) Impacts in Neighborhoods with Groundwater Contaminated by Chlorinated Volatile Organic Chemicals (CVOCs)|Vapor Intrusion - Assessing VI Impacts in Neighborhoods with Groundwater Contaminated CVOCs]]

'''[[Characterization, Assessment & Monitoring]]'''

*[[Characterization Methods – Hydraulic Conductivity]]
*[[Compound Specific Isotope Analysis (CSIA)|Compound Specific Isotope Analysis (CSIA)]]
*[[Direct Push (DP) Technology]]
**[[Direct Push Logging |Direct Push Logging]]
**[[Direct Push Sampling |Direct Push Sampling]]
*[[Geophysical Methods | Geophysical Methods]]
**[[Geophysical Methods - Case Studies |Case Studies]]
**[[Hydrogeophysical Methods for Characterization and Monitoring of Groundwater-Surface Water Exchanges]]
*[[Groundwater Sampling - No-Purge/Passive]]
*[[Long-Term Monitoring (LTM)|Long-Term Monitoring (LTM)]]
**[[Long-Term Monitoring (LTM) - Data Analysis |LTM Data Analysis]]
**[[Long-Term Monitoring (LTM) - Data Variability |LTM Data Variability]]
*[[Molecular Biological Tools - MBTs |Molecular Biological Tools (MBTs)]]
**[[Metagenomics]]
**[[Proteomics and Proteogenomics]]
**[[Quantitative Polymerase Chain Reaction (qPCR)]]
**[[Stable Isotope Probing (SIP)]]
*[[Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill |Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill]]
*[[OPTically-based In-situ Characterization System (OPTICS)]]

'''[[Climate Change Primer | Climate Change]]'''

*[[Climate Change Effects on Wildlife]]
*[[Downscaled High Resolution Datasets for Climate Change Projections]]
*[[Infrastructure Resilience]]
*[[Predicting Species Responses to Climate Change with Population Models]]
*[[Restoration of Ecological Function in Terrestrial Systems Impacted by Invasive Species]]

'''[[Coastal and Estuarine Ecology]]'''

*[[Phytoplankton (Algae) Blooms]]

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'''[[Contaminated Sediments - Introduction | Contaminated Sediments]]'''

*[[Contaminated Sediment Risk Assessment]]
*[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
*[[Mercury in Sediments]]
*[[Passive Sampling of Sediments]]
**[[Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)]]
*[[Sediment Capping]]

'''[[Light Non-Aqueous Phase Liquids (LNAPLs)]]'''

*[[LNAPL Conceptual Site Models]]
*[[LNAPL Remediation Technologies]]
*[[NAPL Mobility]]

'''[[Munitions Constituents]]'''

*[[Munitions Constituents - Abiotic Reduction|Abiotic Reduction]]
*[[Munitions Constituents - Alkaline Degradation|Alkaline Degradation]]
**[[Pyrogenic Carbonaceous Matter Enhanced Alkaline Hydrolysis]]
*[[Munitions Constituents - Composting|Composting]]
*[[Munitions Constituents - Deposition |Deposition]]
*[[Munitions Constituents - Dissolution |Dissolution]]
*[[Munitions Constituents - Electrochemical Treatment|Electrochemical Treatment]]
*[[Metal(loid)s - Small Arms Ranges]]
*[[Passive Sampling of Munitions Constituents|Passive Sampling]]
*[[Munitions Constituents – Photolysis |Photolysis]]
*[[Remediation of Stormwater Runoff Contaminated by Munition Constituents |Remediation of Stormwater Runoff ]]
*[[Munitions Constituents – Sample Extraction and Analytical Techniques|Sample Extraction and Analytical Techniques]]
*[[Munitions Constituents - Soil Sampling |Soil Sampling]]
*[[Munitions Constituents - Sorption |Sorption]]
*[[Munitions Constituents - IM Toxicology |Toxicology]]
*[[Munitions Constituents- TREECS™ Fate and Risk Modeling|TREECS™]]

'''[[Monitored Natural Attenuation (MNA)]]'''

*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| MNA of Chlorinated Solvents]]
*[[Monitored Natural Attenuation (MNA) of Fuels| MNA of Fuels]]
*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| MNA of Metals and Metalloids]]
*[[Natural Source Zone Depletion (NSZD)]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies| Transitioning from Active Remedies]]

'''[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]'''

*[[Hydrothermal Alkaline Treatment (HALT)]]
*[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]
*[[PFAS Destruction by Ultraviolet/Sulfite Treatment]]
*[[PFAS Ex Situ Water Treatment]]
**[[PFAS Treatment by Anion Exchange]]
*[[PFAS Monitored Retention (PMR) and PFAS Enhanced Retention (PER)]]
*[[PFAS Soil Remediation Technologies]]
*[[PFAS Sources]]
*[[PFAS Toxicology and Risk Assessment]]
*[[PFAS Transport and Fate]]
*[[PFAS Treatment by Electrical Discharge Plasma]]
*[[Photoactivated Reductive Defluorination - PFAS Destruction | Photoactivated Reductive Defluorination]]
*[[Reverse Osmosis and Nanofiltration Membrane Filtration Systems for PFAS Removal]]
*[[Thermal Conduction Heating for Treatment of PFAS-Impacted Soil]]
*[[Transition of Aqueous Film Forming Foam (AFFF) Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances (PFAS)| Transition of Aqueous Film Forming Foam Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances]]

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'''[[Regulatory Issues and Site Management]]'''

*[[Alternative Endpoints]]
*[[Mass Flux and Mass Discharge]]
*[[Plume Response Modeling]]
*[[REMChlor - MD | REMChlor-MD]]
*[[Source Zone Modeling]]
*[[Sustainable Remediation]]

'''[[Remediation Technologies]]'''
*[[Amendment Distribution in Low Conductivity Materials]]
*[[Bioremediation - Anaerobic|Anaerobic Bioremediation]]
**[[Bioremediation - Anaerobic Design Considerations | Design Considerations]]
**[[Design Tool - Base Addition for ERD]]
**[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation]]
**[[Low pH Inhibition of Reductive Dechlorination]]
**[[Bioremediation - Anaerobic Secondary Water Quality Impacts | Secondary Water Quality Impacts]]
*[[Chemical Oxidation (In Situ - ISCO) | In Situ Chemical Oxidation (ISCO)]]
**[[Chemical Oxidation Design Considerations(In Situ - ISCO) | Design Considerations]]
**[[Chemical Oxidation Oxidant Selection (In Situ - ISCO) | Oxidant Selection]]
*[[Chemical Reduction (In Situ - ISCR) | In Situ Chemical Reduction (ISCR)]]
**[[Zerovalent Iron (ZVI) (Chemical Reduction - ISCR) | Zero-Valent Iron (ZVI)]]
**[[Zerovalent Iron Permeable Reactive Barriers]]
*[[In Situ Groundwater Treatment with Activated Carbon]]
*[[Injection Techniques for Liquid Amendments]]
*[[Injection Techniques - Viscosity Modification]]
*[[Landfarming]]
*[[Metal and Metalloids - Remediation | Remediation of Metals and Metalloids]]
*[[Remediation Performance Assessment at Chlorinated Solvent Sites]]
*[[Soil Vapor Extraction (SVE)]]
*[[Stream Restoration]]
*[[Subgrade Biogeochemical Reactor (SBGR)]]
*[[Supercritical Water Oxidation (SCWO)]]
*[[Thermal Remediation]]
**[[Thermal Remediation - Combined Remedies | Combined Remedies]]
**[[Thermal Remediation - Electrical Resistance Heating | Electrical Resistance Heating (ERH)]]
**[[Thermal Remediation - Smoldering | Smoldering]]
**[[Thermal Remediation - Steam | Steam Enhanced Extraction (SEE)]]
**[[Thermal Conduction Heating (TCH)]]

'''[[Soil & Groundwater Contaminants]]'''

*[[1,2,3-Trichloropropane]]
*[[1,4-Dioxane]]
*[[Chlorinated Solvents]]
*[[Metal and Metalloid Contaminants|Metals and Metalloids]]
*[[N-nitrosodimethylamine (NDMA)]]
*[[Perchlorate|Perchlorate]]
*[[Petroleum Hydrocarbons (PHCs)]]
*[[Polycyclic Aromatic Hydrocarbons (PAHs)]]
|}
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Passive Sampling of Sediments

2026-03-03T22:47:23Z

Admin:

"Passive sampling" refers to a group of methods used to quantify the availability of organic contaminants to move between different media and/or to react in environmental systems such as indoor air, lake waters, or contaminated sediment beds. To do this, the passive sampling material is deployed in the environmental system and allowed to absorb chemicals of interest via diffusive transfers from the surroundings. Upon recovery of the passive sampler, the accumulated contaminants are measured, and the concentrations in the sampler are interpreted to infer the chemical concentrations in specific surrounding media like porewater in a sediment bed. Such data are then useful inputs for site assessments such as those seeking to quantify fluxes from contaminated sediment beds to overlying waters or to evaluate the risk of significant uptake into benthic infauna and the larger food web.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Contaminated Sediments - Introduction]]
*[[Contaminated Sediment Risk Assessment]]
*[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
*[[Passive Sampling of Munitions Constituents]]
*[[Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)]]

'''Contributor(s):''' [[Dr. Philip M. Gschwend]]

'''Key Resource(s):'''

*Validating the Use of Performance Reference Compounds in Passive Samplers to Assess Porewater Concentrations in Sediment Beds<ref name="Apell2014">Apell, J.N. and Gschwend, P.M., 2014. Validating the Use of Performance Reference Compounds in Passive Samplers to Assess Porewater Concentrations in Sediment Beds. Environmental Science and Technology, 48(17), pp. 10301-10307. [https://doi.org/10.1021/es502694g DOI: 10.1021/es502694g]</ref>

*''In situ'' passive sampling of sediments in the Lower Duwamish Waterway Superfund site: Replicability, comparison with ''ex situ'' measurements, and use of data<ref name="Apell2016">Apell, J.N., and Gschwend, P.M., 2016. ''In situ'' passive sampling of sediments in the Lower Duwamish Waterway Superfund site: Replicability, comparison with ''ex situ'' measurements, and use of data. Environmental Pollution, 218, pp. 95-101. [https://doi.org/10.1016/j.envpol.2016.08.023 DOI: 10.1016/j.envpol.2016.08.023]   [//www.enviro.wiki/images/5/53/ApellGschwend2016.pdf Authors’ Manuscript]</ref>

*Laboratory, Field, and Analytical Procedures for Using Passive Sampling in the Evaluation of Contaminated Sediments: User’s Manual<ref name="Burgess2017">Burgess, R.M., Kane Driscoll, S.B., Burton, A., Gschwend, P.M., Ghosh, U., Reible, D., Ahn, S., and Thompson, T., 2017. Laboratory, Field, and Analytical Procedures for Using Passive Sampling in the Evaluation of Contaminated Sediments: User’s Manual, EPA/600/R-16/357. SERDP/ESTCP and U.S. EPA, Office of Research and Development, Washington, DC 20460. [https://cfpub.epa.gov/si/si_public_record_report.cfm?Lab=NHEERL&dirEntryID=308731 Website]   [//www.enviro.wiki/images/c/c5/EPA600R16357.pdf Report.pdf]</ref>

==Introduction==
[[File: Gschwend1w2fig1.png | thumb | 300px | Figure 1. A representation of a clam living in a sediment bed that contains a chemical contaminant (depicted as red hexagons). The contaminant is partly dissolved in the sediment porewater between the solid grains, and partly associated with solid phases, like natural organic matter and "black carbons" such as soots from diesel engines and chars emitted during forest fires. All of these liquid and solid materials can exchange their contaminant loads with one another, with the distributions dependent on the chemical's relative affinity for each material. When an organism like a clam lives in such a system, the chemical contaminant is accumulated into the organism, until the concentration of the chemical in the organism is also equilibrated with the other solids and liquid(s) present.]]
Environmental media such as sediments typically contain many different materials or phases, including liquid solutions (e.g. water, [[Light Non-Aqueous Phase Liquids (LNAPLs)| nonaqueous phase liquids]] like spilled oils) and diverse solids (e.g., quartz, aluminosilicate clays, and combustion-derived soots). Further, the chemical concentration in the porewater medium includes both molecules that are "truly dissolved" in the water and others that are associated with colloids in the porewater<ref name="Brownawell1986">Brownawell, B.J., and Farrington, J.W., 1986. Biogeochemistry of PCBs in interstitial waters of a coastal marine sediment. Geochimica et Cosmochimica Acta, 50(1), pp. 157-169. [https://doi.org/10.1016/0016-7037(86)90061-X DOI: 10.1016/0016-7037(86)90061-X]   Free download available from: [https://semspub.epa.gov/work/01/268631.pdf US EPA].</ref><ref name="Chin1992">Chin, Y.P., and Gschwend, P.M., 1992. Partitioning of Polycyclic Aromatic Hydrocarbons to Marine Porewater Organic Colloids. Environmental Science and Technology, 26(8), pp. 1621-1626. [https://doi.org/10.1021/es00032a020 DOI: 10.1021/es00032a020]</ref><ref name="Achman1996">Achman, D.R., Brownawell, B.J., and Zhang, L., 1996. Exchange of Polychlorinated Biphenyls Between Sediment and Water in the Hudson River Estuary. Estuaries, 19(4), pp. 950-965. [https://doi.org/10.2307/1352310 DOI: 10.2307/1352310]   Free download available from: [https://www.academia.edu/download/55010335/135231020171114-2212-b93vic.pdf Academia.edu]</ref>. As a result, contaminant chemicals distribute among these diverse media (Figure 1) according to their affinity for each and the amount of each phase in the system<ref name="Gustafsson1996">Gustafsson, Ö., Haghseta, F., Chan, C., MacFarlane, J., and Gschwend, P.M., 1996. Quantification of the Dilute Sedimentary Soot Phase: Implications for PAH Speciation and Bioavailability. Environmental Science and Technology, 31(1), pp. 203-209. [https://doi.org/10.1021/es960317s DOI: 10.1021/es960317s]</ref><ref name="Luthy1997">Luthy, R.G., Aiken, G.R., Brusseau, M.L., Cunningham, S.D., Gschwend, P.M., Pignatello, J.J., Reinhard, M., Traina, S.J., Weber, W.J., and Westall, J.C., 1997. Sequestration of Hydrophobic Organic Contaminants by Geosorbents. Environmental Science and Technology, 31(12), pp. 3341-3347. [https://doi.org/10.1021/es970512m DOI: 10.1021/es970512m]</ref><ref name="Lohmann2005">Lohmann, R., MacFarlane, J.K., and Gschwend, P.M., 2005. Importance of Black Carbon to Sorption of Native PAHs, PCBs, and PCDDs in Boston and New York Harbor Sediments. Environmental Science and Technology, 39(1), pp.141-148. [https://doi.org/10.1021/es049424+ DOI: 10.1021/es049424+]</ref><ref name="Cornelissen2005">Cornelissen, G., Gustafsson, Ö., Bucheli, T.D., Jonker, M.T., Koelmans, A.A., and van Noort, P.C., 2005. Extensive Sorption of Organic Compounds to Black Carbon, Coal, and Kerogen in Sediments and Soils: Mechanisms and Consequences for Distribution, Bioaccumulation, and Biodegradation. Environmental Science and Technology, 39(18), pp. 6881-6895. [https://doi.org/10.1021/es050191b DOI: 10.1021/es050191b]</ref><ref name="Koelmans2009">Koelmans, A.A., Kaag, K., Sneekes, A., and Peeters, E.T.H.M., 2009. Triple Domain in Situ Sorption Modeling of Organochlorine Pesticides, Polychlorobiphenyls, Polyaromatic Hydrocarbons, Polychlorinated Dibenzo-p-Dioxins, and Polychlorinated Dibenzofurans in Aquatic Sediments. Environmental Science and Technology, 43(23), pp. 8847-8853. [https://doi.org/10.1021/es9021188 DOI: 10.1021/es9021188]</ref>. As such, the chemical concentration in any one medium (e.g., truly dissolved in porewater) in a multi-material system like sediment is very hard to know from measures of the total sediment concentration, which unfortunately is the information typically found by analyzing for chemicals in sediment samples.

If an animal such as a clam moves into this system (Figure 1), it will also accumulate the chemical in its tissues from the loads in all the other materials. This can lead to exposures of the chemical to other organisms, including humans, who may eat such animals. Predicting the quantity of contaminant in the animal requires knowledge of the relative affinities of the chemical for the animal versus the sediment materials. For example, if one knew the chemical's truly dissolved concentration in the porewater and could reasonably assume the chemical of interest in the animal has mostly accumulated in its lipids (as is often the case for very hydrophobic compounds), then one could estimate the chemical concentration in the animal (''Canimal'', typically in units of μg/kg animal wet weight) using a lipid-water [[Wikipedia: Partition coefficient | partition coefficient]], ''Klipid-water'', typically in units of (μg/kg lipid)'''/'''(μg/L water), and the porewater concentration of the chemical (''Cporewater'', in μg/L) with Equation 1.
{|
|
|-
| ||Equation 1.
| style="text-align:center;" |<big>'''''Canimal '''=''' flipid '''x''' Klipid-water '''x''' Cporewater'''''</big>
|-
|where:
|-
| ||''flipid''||is the fraction lipids contribute to the total wet weight of the animal (kg lipid/kg animal wet weight), and
|-
| ||''Cporewater''||is the freely dissolved contaminant concentration in the porewater surrounding the animal.
|}

While there is a great deal of information on the values of ''Klipid-water'' for many chemicals<ref name="Schwarzenbach2017">Schwarzenbach, R.P., Gschwend, P.M., and Imboden, D.M., 2017. Environmental Organic Chemistry, 3rd edition. Ch. 16: Equilibrium Partitioning from Water and Air to Biota, pp. 469-521. John Wiley and Sons. ISBN: 978-1-118-76723-8</ref>, it is often very inaccurate to estimate truly dissolved porewater concentrations from total sediment concentrations using assumptions about the affinity of those chemicals for the solids in the system<ref name="Gustafsson1996" />. Further, it is difficult to isolate porewater without colloids and/or measure the very low truly dissolved concentrations of hydrophobic contaminants of concern like [[Polycyclic Aromatic Hydrocarbons (PAHs) | polycyclic aromatic hydrocarbons (PAHs)]], [[Wikipedia: Polychlorinated biphenyl | polychlorinated biphenyls (PCBs)]], nonionic pesticides like [[Wikipedia: DDT | dichlorodiphenyltrichloroethane (DDT)]], and [[Wikipedia: Polychlorinated dibenzodioxins | polychlorinated dibenzo-p-dioxins (PCDDs)]]/[[Wikipedia: Polychlorinated dibenzofurans | dibenzofurans (PCDFs)]]<ref name="Hawthorne2005">Hawthorne, S.B., Grabanski, C.B., Miller, D.J., and Kreitinger, J.P., 2005. Solid-Phase Microextraction Measurement of Parent and Alkyl Polycyclic Aromatic Hydrocarbons in Milliliter Sediment Pore Water Samples and Determination of KDOC Values. Environmental Science and Technology, 39(8), pp. 2795-2803. [https://doi.org/10.1021/es0405171 DOI: 10.1021/es0405171]</ref>.

==Passive Samplers==
One approach to address this problem for contaminated sediments is to insert into the sediment films of organic polymers like low density polyethylene (LDPE), polydimethylsiloxane (PDMS), or polyoxymethylene (POM) that can absorb such hydrophobic chemicals from their surroundings<ref name="Mayer2000">Mayer, P., Vaes, W.H., Wijnker, F., Legierse, K.C., Kraaij, R., Tolls, J., and Hermens, J.L., 2000. Sensing Dissolved Sediment Porewater Concentrations of Persistent and Bioaccumulative Pollutants Using Disposable Solid-Phase Microextraction Fibers. Environmental Science and Technology, 34(24), pp. 5177-5183. [https://doi.org/10.1021/es001179g DOI: 10.1021/es001179g]</ref><ref name="Booij2003">Booij, K., Hoedemaker, J.R., and Bakker, J.F., 2003. Dissolved PCBs, PAHs, and HCB in Pore Waters and Overlying Waters of Contaminated Harbor Sediments. Environmental Science and Technology, 37(18), pp. 4213-4220. [https://doi.org/10.1021/es034147c DOI: 10.1021/es034147c]</ref><ref name="Cornelissen2008">Cornelissen, G., Pettersen, A., Broman, D., Mayer, P., and Breedveld, G.D., 2008. Field testing of equilibrium passive samplers to determine freely dissolved native polycyclic aromatic hydrocarbon concentrations. Environmental Toxicology and Chemistry, 27(3), pp. 499-508. [https://doi.org/10.1897/07-253.1 DOI: 10.1897/07-253.1]</ref><ref name="Tomaszewski2008">Tomaszewski, J.E., and Luthy, R.G., 2008. Field Deployment of Polyethylene Devices to Measure PCB Concentrations in Pore Water of Contaminated Sediment. Environmental Science and Technology, 42(16), pp. 6086-6091. [https://doi.org/10.1021/es800582a DOI: 10.1021/es800582a]</ref><ref name="Fernandez2009">Fernandez, L.A., MacFarlane, J.K., Tcaciuc, A.P., and Gschwend, P.M., 2009. Measurement of Freely Dissolved PAH Concentrations in Sediment Beds Using Passive Sampling with Low-Density Polyethylene Strips. Environmental Science and Technology, 43(5), pp. 1430-1436. [https://doi.org/10.1021/es802288w DOI: 10.1021/es802288w]</ref><ref name="Arp2015">Arp, H.P.H., Hale, S.E., Elmquist Kruså, M., Cornelissen, G., Grabanski, C.B., Miller, D.J., and Hawthorne, S.B., 2015. Review of polyoxymethylene passive sampling methods for quantifying freely dissolved porewater concentrations of hydrophobic organic contaminants. Environmental Toxicology and Chemistry, 34(4), pp. 710-720. [https://doi.org/10.1002/etc.2864 DOI: 10.1002/etc.2864]   [https://setac.onlinelibrary.wiley.com/doi/epdf/10.1002/etc.2864 Free access article.]   [//www.enviro.wiki/images/f/f4/Arp2015.pdf Report.pdf]</ref><ref name="Apell2016" />. In this approach, the polymer is inserted in the sediment bed where it absorbs some of the contaminant load via the contaminant's diffusion into the polymer from the surroundings. When the polymer achieves sorptive equilibration with the sediments, the chemical concentration in the polymer, ''Cpolymer'' (μg/kg polymer), can be used to find the corresponding concentration in the porewater, ''Cporewater'' (μg/L), using a polymer-water partition coefficient, ''Kpolymer-water'' ((μg/kg polymer)'''/'''(μg/L water)), that has previously been found in laboratory testing<ref name="Lohmann2012">Lohmann, R., 2012. Critical Review of Low-Density Polyethylene’s Partitioning and Diffusion Coefficients for Trace Organic Contaminants and Implications for Its Use as a Passive Sampler. Environmental Science and Technology, 46(2), pp. 606-618. [https://doi.org/10.1021/es202702y DOI: 10.1021/es202702y]</ref><ref name="Ghosh2014">Ghosh, U., Kane Driscoll, S., Burgess, R.M., Jonker, M.T., Reible, D., Gobas, F., Choi, Y., Apitz, S.E., Maruya, K.A., Gala, W.R., Mortimer, M., and Beegan, C., 2014. Passive Sampling Methods for Contaminated Sediments: Practical Guidance for Selection, Calibration, and Implementation. Integrated Environmental Assessment and Management, 10(2), pp. 210-223. [https://doi.org/10.1002/ieam.1507 DOI: 10.1002/ieam.1507]   [https://setac.onlinelibrary.wiley.com/doi/epdf/10.1002/ieam.1507 Free access article.]   [//www.enviro.wiki/images/3/37/Ghosh2014.pdf Report.pdf]</ref>, as shown in Equation 2.
{|
|
|-
|        ||Equation 2.
| style="width:600px; text-align:center;" |<big>'''''Cporewater '''=''' Cpolymer '''/''' Kpolymer-water'''''</big>
|}

Such “passive uptake” by the polymer also reflects the availability of the chemicals for transport to adjacent systems (e.g., overlying surface waters) and for uptake into organisms (e.g., [[Wikipedia: Bioaccumulation | bioaccumulation]]). Thus, one can use the porewater concentrations to estimate the biotic accumulation of the chemicals, too. For example, for the concentration in the animal equilibrated with the sediment, ''Canimal'' (μg/kg animal), would be found by combining Equations 1 and 2 to get Equation 3.
{|
|
|-
|        ||Equation 3.
| style="width:700px; text-align:center;" |<big>'''''Canimal '''=''' flipid '''x''' Klipid-water '''x''' Cpolymer '''/''' Kpolymer-water'''''</big>
|}
[[File: Gschwend1w2fig2a.PNG | thumb | 300px | Figure 2a. Schematic plot of the initial concentrations of a PRC (green lines) in a polyethylene (PE) film inserted in a sediment showing constant concentration across the PE and zero concentration outside the PE. At the same time, a target contaminant of interest (red lines) initially has a constant concentration in the sediment outside the PE and zero concentration inside the PE.]][[File: Gschwend1w2fig2b.PNG | thumb | 300px | Figure 2b. After the PE has been deployed for a time, the PRC is depleted from the PE (green lines), especially near the surfaces contacting the sediment, and its concentration is building up outside the PE and diffusing away into the sediment. Meanwhile, the target chemical leaves the sediment and begins to diffuse into the PE (red lines). The "jumps" in concentration at the PE-sediment boundary reflect the equilibrium partitioning coefficient, ''KPE-sed = CPE '''/''' Csediment''.]]

==Performance Reference Compounds (PRCs)==
Perhaps unsurprisingly, pollutants with low water solubility like PAHs, PCBs, etc. do not diffuse quickly through sediment beds. As a result, their accumulation in polymeric materials in sediments can take a long time to achieve equilibration<ref name="Fernandez2009b">Fernandez, L. A., Harvey, C.F., and Gschwend, P.M., 2009. Using Performance Reference Compounds in Polyethylene Passive Samplers to Deduce Sediment Porewater Concentrations for Numerous Target Chemicals. Environmental Science and Technology, 43(23), pp. 8888-8894. [https://doi.org/10.1021/es901877a DOI: 10.1021/es901877a]</ref><ref name="Lampert2015">Lampert, D.J., Thomas, C., and Reible, D.D., 2015. Internal and external transport significance for predicting contaminant uptake rates in passive samplers. Chemosphere, 119, pp. 910-916. [https://doi.org/10.1016/j.chemosphere.2014.08.063 DOI: 10.1016/j.chemosphere.2014.08.063]   Free download available from: [https://www.academia.edu/download/44146586/chemosphere_2014.pdf Academia.edu]</ref><ref name="Apell2016b">Apell, J.N., Tcaciuc, A.P., and Gschwend, P.M., 2016. Understanding the rates of nonpolar organic chemical accumulation into passive samplers deployed in the environment: Guidance for passive sampler deployments. Integrated Environmental Assessment and Management, 12(3), pp. 486-492. [https://doi.org/10.1002/ieam.1697 DOI: 10.1002/ieam.1697]</ref>. This problem was recognized previously for passive samplers called [[Wikipedia: Semipermeable membrane devices | semipermeable membrane devices]] (SPMDs, e.g. polyethylene bags filled with triolein<ref name="Huckins2002">Huckins, J.N., Petty, J.D., Lebo, J.A., Almeida, F.V., Booij, K., Alvarez, D.A., Cranor, W.L., Clark, R.C., and Mogensen, B.B., 2002. Development of the Permeability/Performance Reference Compound Approach for In Situ Calibration of Semipermeable Membrane Devices. Environmental Science and Technology, 36(1), pp. 85-91. [https://doi.org/10.1021/es010991w DOI: 10.1021/es010991w]</ref>) that were deployed in surface waters. As a result, representative chemicals called performance reference compounds (PRCs) were uniformly impregnated into the samplers before their deployment in the environment, and the PRCs' diffusive losses out of the SPMD could then be used to quantify the fractional approach toward equilibration of the sampler with its environmental surroundings<ref name="Booij2002">Booij, K., Smedes, F., and van Weerlee, E.M., 2002. Spiking of performance reference compounds in low density polyethylene and silicone passive water samplers. Chemosphere 46(8), pp.1157-1161. [https://doi.org/10.1016/S0045-6535(01)00200-4 DOI: 10.1016/S0045-6535(01)00200-4]</ref><ref name="Huckins2002" />. A similar approach can be used for polymers inserted in sediment beds<ref name="Fernandez2009b" /><ref name="Apell2014" />. Commonly, isotopically labeled forms of the compounds of interest such as deuterated or 13C-labelled PAHs or PCBs are homogeneously impregnated into the polymers before their deployments. Upon insertion of the polymer into the sediment bed (or overlying waters or even air), the initially evenly distributed PRCs begin to diffuse out of the sampling polymer and into the surroundings (Figure 2).

Assuming the contaminants of interest undergo the same mass transfer restrictions limiting their rates of uptake into the polymer (e.g., diffusion through the sedimentary porous medium) that are also limiting transfers of the PRCs out of the polymer<ref name="Fernandez2009b" /><ref name="Apell2014" />, then fractional losses of the PRCs during a particular deployment can be used to adjust the accumulated contaminant loads to what they would have been at equilibrium with their surroundings with Equation 4.
{|
|
|-
| ||Equation 4.
| style="text-align:center;" |<big>'''''C(∞)polymer '''=''' C(t)polymer '''/''' fPRC lost'''''</big>
|-
|where:
|-
| ||''fPRC lost''||is the fraction of the PRC lost to outward diffusion,
|-
| ||''C(∞)polymer''||is the concentration of the contaminant in the polymer at equilibrium, and
|-
| ||''C(t)polymer''||is the concentration of the contaminant in the polymer after deployment time, t.                     
|}

Since investigators are commonly interested in many chemicals at the same time, it is impractical to have a PRC for each contaminant of interest. Instead, a representative set of PRCs is used to characterize the rates of polymer-environment exchange as a function of the PRCs' properties (e.g., diffusivities, partition coefficients), the characteristics of the sediments (e.g., porosity), and the nature of the polymer used (e.g., film thickness, affinity for the chemicals)<ref name="Fernandez2009b" /><ref name="Lampert2015" />. The resulting mass transfer model fit can then be used to estimate the fractional approaches to equilibrium for many other contaminants, whose diffusive and partitioning properties are also known. And these fractions can be used to adjust the target chemical concentrations that have accumulated from the sediment into the same polymeric sampler to find the equilibrated results<ref name="Apell2014" />. Finally, these equilibrated concentrations can be used in Eq. 2 to estimate truly dissolved contaminant concentrations in the sediment's porewater.

==Field Applications==
[[File: Gschwend1w2fig3.png | thumb |left| 450px | Figure 3. Passive sampler system made of polyethylene film loaded into an aluminum sheet metal frame, before (left), during (middle), and after (right) deployment in sediment.]]
Polymeric materials can be deployed in sediment in various ways<ref name="Burgess2017" />. PDMS-coated silica fibers, called SPMEs (solid phase micro extraction devices), can be incorporated into slotted rods, while thin films of polymers like LDPE or POM can be incorporated into sheet metal frames. In both cases, such hardware is used to insert the polymers into sediment beds (Figure 3).

Deployment of the assembled passive samplers can be accomplished via poles from a boat<ref name="Apell2014" />, by divers<ref name="Apell2016" />, or by attaching the samplers to a sampling platform lowered off a vessel<ref name="Fernandez2012">Fernandez, L.A., Lao, W., Maruya, K.A., White, C., Burgess, R.M., 2012. Passive Sampling to Measure Baseline Dissolved Persistent Organic Pollutant Concentrations in the Water Column of the Palos Verdes Shelf Superfund Site. Environmental Science and Technology, 46(21), pp. 11937-11947. [https://doi.org/10.1021/es302139y DOI: 10.1021/es302139y]</ref>. Typically, the method used depends on the water depth. Small buoys on short lines, sometimes with associated water-sampling polymeric materials in mesh bags (see right panel of Figure 3), are attached to the samplers to facilitate the sampler recoveries. After recovery, the samplers are wiped to remove any adhering sediment, biofilm, or precipitates and returned to the laboratory for PRC and target contaminant analyses. The resulting measurements of the accumulated target chemical concentrations can be adjusted using the observed PRC losses and publicly available software programs<ref name="Gschwend2014">Gschwend, P.M., Tcaciuc, A.P., and Apell, J.N., 2014. Guidance Document: Passive PE Sampling in Support of In Situ Remediation of Contaminated Sediments – Passive Sampler PRC Calculation Software User’s Guide, US Department of Defense, Environmental Security Technology Certification Program Project ER-200915. Available from: [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Sediments/Bioavailability/ER-200915 ESTCP].</ref><ref name="Thompson2015">Thompson, J.M., Hsieh, C.H. and Luthy, R.G., 2015. Modeling Uptake of Hydrophobic Organic Contaminants into Polyethylene Passive Samplers. Environmental Science and Technology, 49(4), pp. 2270-2277. [https://doi.org/10.1021/es504442s DOI: 10.1021/es504442s]</ref>.

Subsequently, since the passive sampling reveals the concentrations of contaminants in a sediment bed's porewater and the overlying bottom water<ref name="Booij2003" />, the data can be used to estimate bed-to-water column diffusive fluxes of contaminants<ref name="Koelmans2010">Koelmans, A.A., Poot, A., De Lange, H.J., Velzeboer, I., Harmsen, J., and van Noort, P.C.M., 2010. Estimation of In Situ Sediment-to-Water Fluxes of Polycyclic Aromatic Hydrocarbons, Polychlorobiphenyls and Polybrominated Diphenylethers. Environmental Science and Technology, 44(8), pp. 3014-3020. [https://doi.org/10.1021/es903938z DOI: 10.1021/es903938z]</ref><ref name="Fernandez2012" /> and bioirrigation-affected fluxes<ref name="Apell2018">Apell, J.N., Shull, D.H., Hoyt, A.M., and Gschwend, P.M., 2018. Investigating the Effect of Bioirrigation on In Situ Porewater Concentrations and Fluxes of Polychlorinated Biphenyls Using Passive Samplers. Environmental Science and Technology, 52(8), pp. 4565-4573. [https://doi.org/10.1021/acs.est.7b05809 DOI: 10.1021/acs.est.7b05809]</ref>. The data are also useful for assessing the tendency of the contaminants to accumulate in benthic organisms<ref name="Vinturella2004">Vinturella, A.E., Burgess, R.M., Coull, B.A., Thompson, K.M., and Shine, J.P., 2004. Use of Passive Samplers to Mimic Uptake of Polycyclic Aromatic Hydrocarbons by Benthic Polychaetes. Environmental Science and Technology, 38(4), pp. 1154-1160. [https://doi.org/10.1021/es034706f DOI: 10.1021/es034706f]</ref><ref name="Yates2011">Yates, K., Pollard, P., Davies, I.M., Webster, L., and Moffat, C.F., 2011. Application of silicone rubber passive samplers to investigate the bioaccumulation of PAHs by Nereis virens from marine sediments. Environmental Pollution, 159(12), pp. 3351-3356. [https://doi.org/10.1016/j.envpol.2011.08.038 DOI: 10.1016/j.envpol.2011.08.038]</ref><ref name="Fernandez2015">Fernandez, L.A. and Gschwend, P.M., 2015. Predicting bioaccumulation of polycyclic aromatic hydrocarbons in soft-shelled clams (Mya arenaria) using field deployments of polyethylene passive samplers. Environmental Toxicology and Chemistry, 34(5), pp. 993-1000. [https://doi.org/10.1002/etc.2892 DOI: 10.1002/etc.2892]</ref>, and by extension into food webs that include such benthic species<ref name="vonStackelberg2017">von Stackelberg, K., Williams, M.A., Clough, J., and Johnson, M.S., 2017. Spatially explicit bioaccumulation modeling in aquatic environments: Results from 2 demonstration sites. Integrated Environmental Assessment and Management, 13(6), pp. 1023-1037. [https://doi.org/10.1002/ieam.1927 DOI: 10.1002/ieam.1927]</ref>. Furthermore, recent efforts have found that passive sampling observations can be used to infer ''in situ'' transformations of substances like nitro aromatic compounds<ref name="Belles2016">Belles, A., Alary, C., Criquet, J., and Billon, G., 2016. A new application of passive samplers as indicators of in-situ biodegradation processes. Chemosphere, 164, pp. 347-354. [https://doi.org/10.1016/j.chemosphere.2016.08.111 DOI: 10.1016/j.chemosphere.2016.08.111]</ref> and DDT<ref name="Tcaciuc2018">Tcaciuc, A.P., Borrelli, R., Zaninetta, L.M., and Gschwend, P.M., 2018. Passive sampling of DDT, DDE and DDD in sediments: accounting for degradation processes with reaction–diffusion modeling. Environmental Science: Processes and Impacts, 20(1), pp. 220-231. [https://doi.org/10.1039/C7EM00501F DOI: 10.1039/C7EM00501F]   Open access article available from: [https://pubs.rsc.org/--/content/articlehtml/2018/em/c7em00501f Royal Society of Chemistry].</ref>.
 

==References==
<references />

==See Also==

[https://www.serdp-estcp.org/Tools-and-Training/Tools/PRC-Correction-Calculator A PRC Correction Calculator for LDPE deployed in sediments]

Passive Sampling of Sediments

2026-03-03T22:46:49Z

Admin:

"Passive sampling" refers to a group of methods used to quantify the availability of organic contaminants to move between different media and/or to react in environmental systems such as indoor air, lake waters, or contaminated sediment beds. To do this, the passive sampling material is deployed in the environmental system and allowed to absorb chemicals of interest via diffusive transfers from the surroundings. Upon recovery of the passive sampler, the accumulated contaminants are measured, and the concentrations in the sampler are interpreted to infer the chemical concentrations in specific surrounding media like porewater in a sediment bed. Such data are then useful inputs for site assessments such as those seeking to quantify fluxes from contaminated sediment beds to overlying waters or to evaluate the risk of significant uptake into benthic infauna and the larger food web.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Contaminated Sediments - Introduction]]
*[[Contaminated Sediment Risk Assessment]]
*[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
*[[Passive Sampling of Munitions Constituents]]
*[[Sediment Capping]]
*[[Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)]]

'''Contributor(s):''' [[Dr. Philip M. Gschwend]]

'''Key Resource(s):'''

*Validating the Use of Performance Reference Compounds in Passive Samplers to Assess Porewater Concentrations in Sediment Beds<ref name="Apell2014">Apell, J.N. and Gschwend, P.M., 2014. Validating the Use of Performance Reference Compounds in Passive Samplers to Assess Porewater Concentrations in Sediment Beds. Environmental Science and Technology, 48(17), pp. 10301-10307. [https://doi.org/10.1021/es502694g DOI: 10.1021/es502694g]</ref>

*''In situ'' passive sampling of sediments in the Lower Duwamish Waterway Superfund site: Replicability, comparison with ''ex situ'' measurements, and use of data<ref name="Apell2016">Apell, J.N., and Gschwend, P.M., 2016. ''In situ'' passive sampling of sediments in the Lower Duwamish Waterway Superfund site: Replicability, comparison with ''ex situ'' measurements, and use of data. Environmental Pollution, 218, pp. 95-101. [https://doi.org/10.1016/j.envpol.2016.08.023 DOI: 10.1016/j.envpol.2016.08.023]   [//www.enviro.wiki/images/5/53/ApellGschwend2016.pdf Authors’ Manuscript]</ref>

*Laboratory, Field, and Analytical Procedures for Using Passive Sampling in the Evaluation of Contaminated Sediments: User’s Manual<ref name="Burgess2017">Burgess, R.M., Kane Driscoll, S.B., Burton, A., Gschwend, P.M., Ghosh, U., Reible, D., Ahn, S., and Thompson, T., 2017. Laboratory, Field, and Analytical Procedures for Using Passive Sampling in the Evaluation of Contaminated Sediments: User’s Manual, EPA/600/R-16/357. SERDP/ESTCP and U.S. EPA, Office of Research and Development, Washington, DC 20460. [https://cfpub.epa.gov/si/si_public_record_report.cfm?Lab=NHEERL&dirEntryID=308731 Website]   [//www.enviro.wiki/images/c/c5/EPA600R16357.pdf Report.pdf]</ref>

==Introduction==
[[File: Gschwend1w2fig1.png | thumb | 300px | Figure 1. A representation of a clam living in a sediment bed that contains a chemical contaminant (depicted as red hexagons). The contaminant is partly dissolved in the sediment porewater between the solid grains, and partly associated with solid phases, like natural organic matter and "black carbons" such as soots from diesel engines and chars emitted during forest fires. All of these liquid and solid materials can exchange their contaminant loads with one another, with the distributions dependent on the chemical's relative affinity for each material. When an organism like a clam lives in such a system, the chemical contaminant is accumulated into the organism, until the concentration of the chemical in the organism is also equilibrated with the other solids and liquid(s) present.]]
Environmental media such as sediments typically contain many different materials or phases, including liquid solutions (e.g. water, [[Light Non-Aqueous Phase Liquids (LNAPLs)| nonaqueous phase liquids]] like spilled oils) and diverse solids (e.g., quartz, aluminosilicate clays, and combustion-derived soots). Further, the chemical concentration in the porewater medium includes both molecules that are "truly dissolved" in the water and others that are associated with colloids in the porewater<ref name="Brownawell1986">Brownawell, B.J., and Farrington, J.W., 1986. Biogeochemistry of PCBs in interstitial waters of a coastal marine sediment. Geochimica et Cosmochimica Acta, 50(1), pp. 157-169. [https://doi.org/10.1016/0016-7037(86)90061-X DOI: 10.1016/0016-7037(86)90061-X]   Free download available from: [https://semspub.epa.gov/work/01/268631.pdf US EPA].</ref><ref name="Chin1992">Chin, Y.P., and Gschwend, P.M., 1992. Partitioning of Polycyclic Aromatic Hydrocarbons to Marine Porewater Organic Colloids. Environmental Science and Technology, 26(8), pp. 1621-1626. [https://doi.org/10.1021/es00032a020 DOI: 10.1021/es00032a020]</ref><ref name="Achman1996">Achman, D.R., Brownawell, B.J., and Zhang, L., 1996. Exchange of Polychlorinated Biphenyls Between Sediment and Water in the Hudson River Estuary. Estuaries, 19(4), pp. 950-965. [https://doi.org/10.2307/1352310 DOI: 10.2307/1352310]   Free download available from: [https://www.academia.edu/download/55010335/135231020171114-2212-b93vic.pdf Academia.edu]</ref>. As a result, contaminant chemicals distribute among these diverse media (Figure 1) according to their affinity for each and the amount of each phase in the system<ref name="Gustafsson1996">Gustafsson, Ö., Haghseta, F., Chan, C., MacFarlane, J., and Gschwend, P.M., 1996. Quantification of the Dilute Sedimentary Soot Phase: Implications for PAH Speciation and Bioavailability. Environmental Science and Technology, 31(1), pp. 203-209. [https://doi.org/10.1021/es960317s DOI: 10.1021/es960317s]</ref><ref name="Luthy1997">Luthy, R.G., Aiken, G.R., Brusseau, M.L., Cunningham, S.D., Gschwend, P.M., Pignatello, J.J., Reinhard, M., Traina, S.J., Weber, W.J., and Westall, J.C., 1997. Sequestration of Hydrophobic Organic Contaminants by Geosorbents. Environmental Science and Technology, 31(12), pp. 3341-3347. [https://doi.org/10.1021/es970512m DOI: 10.1021/es970512m]</ref><ref name="Lohmann2005">Lohmann, R., MacFarlane, J.K., and Gschwend, P.M., 2005. Importance of Black Carbon to Sorption of Native PAHs, PCBs, and PCDDs in Boston and New York Harbor Sediments. Environmental Science and Technology, 39(1), pp.141-148. [https://doi.org/10.1021/es049424+ DOI: 10.1021/es049424+]</ref><ref name="Cornelissen2005">Cornelissen, G., Gustafsson, Ö., Bucheli, T.D., Jonker, M.T., Koelmans, A.A., and van Noort, P.C., 2005. Extensive Sorption of Organic Compounds to Black Carbon, Coal, and Kerogen in Sediments and Soils: Mechanisms and Consequences for Distribution, Bioaccumulation, and Biodegradation. Environmental Science and Technology, 39(18), pp. 6881-6895. [https://doi.org/10.1021/es050191b DOI: 10.1021/es050191b]</ref><ref name="Koelmans2009">Koelmans, A.A., Kaag, K., Sneekes, A., and Peeters, E.T.H.M., 2009. Triple Domain in Situ Sorption Modeling of Organochlorine Pesticides, Polychlorobiphenyls, Polyaromatic Hydrocarbons, Polychlorinated Dibenzo-p-Dioxins, and Polychlorinated Dibenzofurans in Aquatic Sediments. Environmental Science and Technology, 43(23), pp. 8847-8853. [https://doi.org/10.1021/es9021188 DOI: 10.1021/es9021188]</ref>. As such, the chemical concentration in any one medium (e.g., truly dissolved in porewater) in a multi-material system like sediment is very hard to know from measures of the total sediment concentration, which unfortunately is the information typically found by analyzing for chemicals in sediment samples.

If an animal such as a clam moves into this system (Figure 1), it will also accumulate the chemical in its tissues from the loads in all the other materials. This can lead to exposures of the chemical to other organisms, including humans, who may eat such animals. Predicting the quantity of contaminant in the animal requires knowledge of the relative affinities of the chemical for the animal versus the sediment materials. For example, if one knew the chemical's truly dissolved concentration in the porewater and could reasonably assume the chemical of interest in the animal has mostly accumulated in its lipids (as is often the case for very hydrophobic compounds), then one could estimate the chemical concentration in the animal (''Canimal'', typically in units of μg/kg animal wet weight) using a lipid-water [[Wikipedia: Partition coefficient | partition coefficient]], ''Klipid-water'', typically in units of (μg/kg lipid)'''/'''(μg/L water), and the porewater concentration of the chemical (''Cporewater'', in μg/L) with Equation 1.
{|
|
|-
| ||Equation 1.
| style="text-align:center;" |<big>'''''Canimal '''=''' flipid '''x''' Klipid-water '''x''' Cporewater'''''</big>
|-
|where:
|-
| ||''flipid''||is the fraction lipids contribute to the total wet weight of the animal (kg lipid/kg animal wet weight), and
|-
| ||''Cporewater''||is the freely dissolved contaminant concentration in the porewater surrounding the animal.
|}

While there is a great deal of information on the values of ''Klipid-water'' for many chemicals<ref name="Schwarzenbach2017">Schwarzenbach, R.P., Gschwend, P.M., and Imboden, D.M., 2017. Environmental Organic Chemistry, 3rd edition. Ch. 16: Equilibrium Partitioning from Water and Air to Biota, pp. 469-521. John Wiley and Sons. ISBN: 978-1-118-76723-8</ref>, it is often very inaccurate to estimate truly dissolved porewater concentrations from total sediment concentrations using assumptions about the affinity of those chemicals for the solids in the system<ref name="Gustafsson1996" />. Further, it is difficult to isolate porewater without colloids and/or measure the very low truly dissolved concentrations of hydrophobic contaminants of concern like [[Polycyclic Aromatic Hydrocarbons (PAHs) | polycyclic aromatic hydrocarbons (PAHs)]], [[Wikipedia: Polychlorinated biphenyl | polychlorinated biphenyls (PCBs)]], nonionic pesticides like [[Wikipedia: DDT | dichlorodiphenyltrichloroethane (DDT)]], and [[Wikipedia: Polychlorinated dibenzodioxins | polychlorinated dibenzo-p-dioxins (PCDDs)]]/[[Wikipedia: Polychlorinated dibenzofurans | dibenzofurans (PCDFs)]]<ref name="Hawthorne2005">Hawthorne, S.B., Grabanski, C.B., Miller, D.J., and Kreitinger, J.P., 2005. Solid-Phase Microextraction Measurement of Parent and Alkyl Polycyclic Aromatic Hydrocarbons in Milliliter Sediment Pore Water Samples and Determination of KDOC Values. Environmental Science and Technology, 39(8), pp. 2795-2803. [https://doi.org/10.1021/es0405171 DOI: 10.1021/es0405171]</ref>.

==Passive Samplers==
One approach to address this problem for contaminated sediments is to insert into the sediment films of organic polymers like low density polyethylene (LDPE), polydimethylsiloxane (PDMS), or polyoxymethylene (POM) that can absorb such hydrophobic chemicals from their surroundings<ref name="Mayer2000">Mayer, P., Vaes, W.H., Wijnker, F., Legierse, K.C., Kraaij, R., Tolls, J., and Hermens, J.L., 2000. Sensing Dissolved Sediment Porewater Concentrations of Persistent and Bioaccumulative Pollutants Using Disposable Solid-Phase Microextraction Fibers. Environmental Science and Technology, 34(24), pp. 5177-5183. [https://doi.org/10.1021/es001179g DOI: 10.1021/es001179g]</ref><ref name="Booij2003">Booij, K., Hoedemaker, J.R., and Bakker, J.F., 2003. Dissolved PCBs, PAHs, and HCB in Pore Waters and Overlying Waters of Contaminated Harbor Sediments. Environmental Science and Technology, 37(18), pp. 4213-4220. [https://doi.org/10.1021/es034147c DOI: 10.1021/es034147c]</ref><ref name="Cornelissen2008">Cornelissen, G., Pettersen, A., Broman, D., Mayer, P., and Breedveld, G.D., 2008. Field testing of equilibrium passive samplers to determine freely dissolved native polycyclic aromatic hydrocarbon concentrations. Environmental Toxicology and Chemistry, 27(3), pp. 499-508. [https://doi.org/10.1897/07-253.1 DOI: 10.1897/07-253.1]</ref><ref name="Tomaszewski2008">Tomaszewski, J.E., and Luthy, R.G., 2008. Field Deployment of Polyethylene Devices to Measure PCB Concentrations in Pore Water of Contaminated Sediment. Environmental Science and Technology, 42(16), pp. 6086-6091. [https://doi.org/10.1021/es800582a DOI: 10.1021/es800582a]</ref><ref name="Fernandez2009">Fernandez, L.A., MacFarlane, J.K., Tcaciuc, A.P., and Gschwend, P.M., 2009. Measurement of Freely Dissolved PAH Concentrations in Sediment Beds Using Passive Sampling with Low-Density Polyethylene Strips. Environmental Science and Technology, 43(5), pp. 1430-1436. [https://doi.org/10.1021/es802288w DOI: 10.1021/es802288w]</ref><ref name="Arp2015">Arp, H.P.H., Hale, S.E., Elmquist Kruså, M., Cornelissen, G., Grabanski, C.B., Miller, D.J., and Hawthorne, S.B., 2015. Review of polyoxymethylene passive sampling methods for quantifying freely dissolved porewater concentrations of hydrophobic organic contaminants. Environmental Toxicology and Chemistry, 34(4), pp. 710-720. [https://doi.org/10.1002/etc.2864 DOI: 10.1002/etc.2864]   [https://setac.onlinelibrary.wiley.com/doi/epdf/10.1002/etc.2864 Free access article.]   [//www.enviro.wiki/images/f/f4/Arp2015.pdf Report.pdf]</ref><ref name="Apell2016" />. In this approach, the polymer is inserted in the sediment bed where it absorbs some of the contaminant load via the contaminant's diffusion into the polymer from the surroundings. When the polymer achieves sorptive equilibration with the sediments, the chemical concentration in the polymer, ''Cpolymer'' (μg/kg polymer), can be used to find the corresponding concentration in the porewater, ''Cporewater'' (μg/L), using a polymer-water partition coefficient, ''Kpolymer-water'' ((μg/kg polymer)'''/'''(μg/L water)), that has previously been found in laboratory testing<ref name="Lohmann2012">Lohmann, R., 2012. Critical Review of Low-Density Polyethylene’s Partitioning and Diffusion Coefficients for Trace Organic Contaminants and Implications for Its Use as a Passive Sampler. Environmental Science and Technology, 46(2), pp. 606-618. [https://doi.org/10.1021/es202702y DOI: 10.1021/es202702y]</ref><ref name="Ghosh2014">Ghosh, U., Kane Driscoll, S., Burgess, R.M., Jonker, M.T., Reible, D., Gobas, F., Choi, Y., Apitz, S.E., Maruya, K.A., Gala, W.R., Mortimer, M., and Beegan, C., 2014. Passive Sampling Methods for Contaminated Sediments: Practical Guidance for Selection, Calibration, and Implementation. Integrated Environmental Assessment and Management, 10(2), pp. 210-223. [https://doi.org/10.1002/ieam.1507 DOI: 10.1002/ieam.1507]   [https://setac.onlinelibrary.wiley.com/doi/epdf/10.1002/ieam.1507 Free access article.]   [//www.enviro.wiki/images/3/37/Ghosh2014.pdf Report.pdf]</ref>, as shown in Equation 2.
{|
|
|-
|        ||Equation 2.
| style="width:600px; text-align:center;" |<big>'''''Cporewater '''=''' Cpolymer '''/''' Kpolymer-water'''''</big>
|}

Such “passive uptake” by the polymer also reflects the availability of the chemicals for transport to adjacent systems (e.g., overlying surface waters) and for uptake into organisms (e.g., [[Wikipedia: Bioaccumulation | bioaccumulation]]). Thus, one can use the porewater concentrations to estimate the biotic accumulation of the chemicals, too. For example, for the concentration in the animal equilibrated with the sediment, ''Canimal'' (μg/kg animal), would be found by combining Equations 1 and 2 to get Equation 3.
{|
|
|-
|        ||Equation 3.
| style="width:700px; text-align:center;" |<big>'''''Canimal '''=''' flipid '''x''' Klipid-water '''x''' Cpolymer '''/''' Kpolymer-water'''''</big>
|}
[[File: Gschwend1w2fig2a.PNG | thumb | 300px | Figure 2a. Schematic plot of the initial concentrations of a PRC (green lines) in a polyethylene (PE) film inserted in a sediment showing constant concentration across the PE and zero concentration outside the PE. At the same time, a target contaminant of interest (red lines) initially has a constant concentration in the sediment outside the PE and zero concentration inside the PE.]][[File: Gschwend1w2fig2b.PNG | thumb | 300px | Figure 2b. After the PE has been deployed for a time, the PRC is depleted from the PE (green lines), especially near the surfaces contacting the sediment, and its concentration is building up outside the PE and diffusing away into the sediment. Meanwhile, the target chemical leaves the sediment and begins to diffuse into the PE (red lines). The "jumps" in concentration at the PE-sediment boundary reflect the equilibrium partitioning coefficient, ''KPE-sed = CPE '''/''' Csediment''.]]

==Performance Reference Compounds (PRCs)==
Perhaps unsurprisingly, pollutants with low water solubility like PAHs, PCBs, etc. do not diffuse quickly through sediment beds. As a result, their accumulation in polymeric materials in sediments can take a long time to achieve equilibration<ref name="Fernandez2009b">Fernandez, L. A., Harvey, C.F., and Gschwend, P.M., 2009. Using Performance Reference Compounds in Polyethylene Passive Samplers to Deduce Sediment Porewater Concentrations for Numerous Target Chemicals. Environmental Science and Technology, 43(23), pp. 8888-8894. [https://doi.org/10.1021/es901877a DOI: 10.1021/es901877a]</ref><ref name="Lampert2015">Lampert, D.J., Thomas, C., and Reible, D.D., 2015. Internal and external transport significance for predicting contaminant uptake rates in passive samplers. Chemosphere, 119, pp. 910-916. [https://doi.org/10.1016/j.chemosphere.2014.08.063 DOI: 10.1016/j.chemosphere.2014.08.063]   Free download available from: [https://www.academia.edu/download/44146586/chemosphere_2014.pdf Academia.edu]</ref><ref name="Apell2016b">Apell, J.N., Tcaciuc, A.P., and Gschwend, P.M., 2016. Understanding the rates of nonpolar organic chemical accumulation into passive samplers deployed in the environment: Guidance for passive sampler deployments. Integrated Environmental Assessment and Management, 12(3), pp. 486-492. [https://doi.org/10.1002/ieam.1697 DOI: 10.1002/ieam.1697]</ref>. This problem was recognized previously for passive samplers called [[Wikipedia: Semipermeable membrane devices | semipermeable membrane devices]] (SPMDs, e.g. polyethylene bags filled with triolein<ref name="Huckins2002">Huckins, J.N., Petty, J.D., Lebo, J.A., Almeida, F.V., Booij, K., Alvarez, D.A., Cranor, W.L., Clark, R.C., and Mogensen, B.B., 2002. Development of the Permeability/Performance Reference Compound Approach for In Situ Calibration of Semipermeable Membrane Devices. Environmental Science and Technology, 36(1), pp. 85-91. [https://doi.org/10.1021/es010991w DOI: 10.1021/es010991w]</ref>) that were deployed in surface waters. As a result, representative chemicals called performance reference compounds (PRCs) were uniformly impregnated into the samplers before their deployment in the environment, and the PRCs' diffusive losses out of the SPMD could then be used to quantify the fractional approach toward equilibration of the sampler with its environmental surroundings<ref name="Booij2002">Booij, K., Smedes, F., and van Weerlee, E.M., 2002. Spiking of performance reference compounds in low density polyethylene and silicone passive water samplers. Chemosphere 46(8), pp.1157-1161. [https://doi.org/10.1016/S0045-6535(01)00200-4 DOI: 10.1016/S0045-6535(01)00200-4]</ref><ref name="Huckins2002" />. A similar approach can be used for polymers inserted in sediment beds<ref name="Fernandez2009b" /><ref name="Apell2014" />. Commonly, isotopically labeled forms of the compounds of interest such as deuterated or 13C-labelled PAHs or PCBs are homogeneously impregnated into the polymers before their deployments. Upon insertion of the polymer into the sediment bed (or overlying waters or even air), the initially evenly distributed PRCs begin to diffuse out of the sampling polymer and into the surroundings (Figure 2).

Assuming the contaminants of interest undergo the same mass transfer restrictions limiting their rates of uptake into the polymer (e.g., diffusion through the sedimentary porous medium) that are also limiting transfers of the PRCs out of the polymer<ref name="Fernandez2009b" /><ref name="Apell2014" />, then fractional losses of the PRCs during a particular deployment can be used to adjust the accumulated contaminant loads to what they would have been at equilibrium with their surroundings with Equation 4.
{|
|
|-
| ||Equation 4.
| style="text-align:center;" |<big>'''''C(∞)polymer '''=''' C(t)polymer '''/''' fPRC lost'''''</big>
|-
|where:
|-
| ||''fPRC lost''||is the fraction of the PRC lost to outward diffusion,
|-
| ||''C(∞)polymer''||is the concentration of the contaminant in the polymer at equilibrium, and
|-
| ||''C(t)polymer''||is the concentration of the contaminant in the polymer after deployment time, t.                     
|}

Since investigators are commonly interested in many chemicals at the same time, it is impractical to have a PRC for each contaminant of interest. Instead, a representative set of PRCs is used to characterize the rates of polymer-environment exchange as a function of the PRCs' properties (e.g., diffusivities, partition coefficients), the characteristics of the sediments (e.g., porosity), and the nature of the polymer used (e.g., film thickness, affinity for the chemicals)<ref name="Fernandez2009b" /><ref name="Lampert2015" />. The resulting mass transfer model fit can then be used to estimate the fractional approaches to equilibrium for many other contaminants, whose diffusive and partitioning properties are also known. And these fractions can be used to adjust the target chemical concentrations that have accumulated from the sediment into the same polymeric sampler to find the equilibrated results<ref name="Apell2014" />. Finally, these equilibrated concentrations can be used in Eq. 2 to estimate truly dissolved contaminant concentrations in the sediment's porewater.

==Field Applications==
[[File: Gschwend1w2fig3.png | thumb |left| 450px | Figure 3. Passive sampler system made of polyethylene film loaded into an aluminum sheet metal frame, before (left), during (middle), and after (right) deployment in sediment.]]
Polymeric materials can be deployed in sediment in various ways<ref name="Burgess2017" />. PDMS-coated silica fibers, called SPMEs (solid phase micro extraction devices), can be incorporated into slotted rods, while thin films of polymers like LDPE or POM can be incorporated into sheet metal frames. In both cases, such hardware is used to insert the polymers into sediment beds (Figure 3).

Deployment of the assembled passive samplers can be accomplished via poles from a boat<ref name="Apell2014" />, by divers<ref name="Apell2016" />, or by attaching the samplers to a sampling platform lowered off a vessel<ref name="Fernandez2012">Fernandez, L.A., Lao, W., Maruya, K.A., White, C., Burgess, R.M., 2012. Passive Sampling to Measure Baseline Dissolved Persistent Organic Pollutant Concentrations in the Water Column of the Palos Verdes Shelf Superfund Site. Environmental Science and Technology, 46(21), pp. 11937-11947. [https://doi.org/10.1021/es302139y DOI: 10.1021/es302139y]</ref>. Typically, the method used depends on the water depth. Small buoys on short lines, sometimes with associated water-sampling polymeric materials in mesh bags (see right panel of Figure 3), are attached to the samplers to facilitate the sampler recoveries. After recovery, the samplers are wiped to remove any adhering sediment, biofilm, or precipitates and returned to the laboratory for PRC and target contaminant analyses. The resulting measurements of the accumulated target chemical concentrations can be adjusted using the observed PRC losses and publicly available software programs<ref name="Gschwend2014">Gschwend, P.M., Tcaciuc, A.P., and Apell, J.N., 2014. Guidance Document: Passive PE Sampling in Support of In Situ Remediation of Contaminated Sediments – Passive Sampler PRC Calculation Software User’s Guide, US Department of Defense, Environmental Security Technology Certification Program Project ER-200915. Available from: [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Sediments/Bioavailability/ER-200915 ESTCP].</ref><ref name="Thompson2015">Thompson, J.M., Hsieh, C.H. and Luthy, R.G., 2015. Modeling Uptake of Hydrophobic Organic Contaminants into Polyethylene Passive Samplers. Environmental Science and Technology, 49(4), pp. 2270-2277. [https://doi.org/10.1021/es504442s DOI: 10.1021/es504442s]</ref>.

Subsequently, since the passive sampling reveals the concentrations of contaminants in a sediment bed's porewater and the overlying bottom water<ref name="Booij2003" />, the data can be used to estimate bed-to-water column diffusive fluxes of contaminants<ref name="Koelmans2010">Koelmans, A.A., Poot, A., De Lange, H.J., Velzeboer, I., Harmsen, J., and van Noort, P.C.M., 2010. Estimation of In Situ Sediment-to-Water Fluxes of Polycyclic Aromatic Hydrocarbons, Polychlorobiphenyls and Polybrominated Diphenylethers. Environmental Science and Technology, 44(8), pp. 3014-3020. [https://doi.org/10.1021/es903938z DOI: 10.1021/es903938z]</ref><ref name="Fernandez2012" /> and bioirrigation-affected fluxes<ref name="Apell2018">Apell, J.N., Shull, D.H., Hoyt, A.M., and Gschwend, P.M., 2018. Investigating the Effect of Bioirrigation on In Situ Porewater Concentrations and Fluxes of Polychlorinated Biphenyls Using Passive Samplers. Environmental Science and Technology, 52(8), pp. 4565-4573. [https://doi.org/10.1021/acs.est.7b05809 DOI: 10.1021/acs.est.7b05809]</ref>. The data are also useful for assessing the tendency of the contaminants to accumulate in benthic organisms<ref name="Vinturella2004">Vinturella, A.E., Burgess, R.M., Coull, B.A., Thompson, K.M., and Shine, J.P., 2004. Use of Passive Samplers to Mimic Uptake of Polycyclic Aromatic Hydrocarbons by Benthic Polychaetes. Environmental Science and Technology, 38(4), pp. 1154-1160. [https://doi.org/10.1021/es034706f DOI: 10.1021/es034706f]</ref><ref name="Yates2011">Yates, K., Pollard, P., Davies, I.M., Webster, L., and Moffat, C.F., 2011. Application of silicone rubber passive samplers to investigate the bioaccumulation of PAHs by Nereis virens from marine sediments. Environmental Pollution, 159(12), pp. 3351-3356. [https://doi.org/10.1016/j.envpol.2011.08.038 DOI: 10.1016/j.envpol.2011.08.038]</ref><ref name="Fernandez2015">Fernandez, L.A. and Gschwend, P.M., 2015. Predicting bioaccumulation of polycyclic aromatic hydrocarbons in soft-shelled clams (Mya arenaria) using field deployments of polyethylene passive samplers. Environmental Toxicology and Chemistry, 34(5), pp. 993-1000. [https://doi.org/10.1002/etc.2892 DOI: 10.1002/etc.2892]</ref>, and by extension into food webs that include such benthic species<ref name="vonStackelberg2017">von Stackelberg, K., Williams, M.A., Clough, J., and Johnson, M.S., 2017. Spatially explicit bioaccumulation modeling in aquatic environments: Results from 2 demonstration sites. Integrated Environmental Assessment and Management, 13(6), pp. 1023-1037. [https://doi.org/10.1002/ieam.1927 DOI: 10.1002/ieam.1927]</ref>. Furthermore, recent efforts have found that passive sampling observations can be used to infer ''in situ'' transformations of substances like nitro aromatic compounds<ref name="Belles2016">Belles, A., Alary, C., Criquet, J., and Billon, G., 2016. A new application of passive samplers as indicators of in-situ biodegradation processes. Chemosphere, 164, pp. 347-354. [https://doi.org/10.1016/j.chemosphere.2016.08.111 DOI: 10.1016/j.chemosphere.2016.08.111]</ref> and DDT<ref name="Tcaciuc2018">Tcaciuc, A.P., Borrelli, R., Zaninetta, L.M., and Gschwend, P.M., 2018. Passive sampling of DDT, DDE and DDD in sediments: accounting for degradation processes with reaction–diffusion modeling. Environmental Science: Processes and Impacts, 20(1), pp. 220-231. [https://doi.org/10.1039/C7EM00501F DOI: 10.1039/C7EM00501F]   Open access article available from: [https://pubs.rsc.org/--/content/articlehtml/2018/em/c7em00501f Royal Society of Chemistry].</ref>.
 

==References==
<references />

==See Also==

[https://www.serdp-estcp.org/Tools-and-Training/Tools/PRC-Correction-Calculator A PRC Correction Calculator for LDPE deployed in sediments]

Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)

2026-03-03T22:46:07Z

Admin:

Sediment porewater dialysis passive samplers, also known as “peepers,” are sampling devices that allow the measurement of dissolved inorganic ions in the porewater of a saturated sediment. Peepers function by allowing freely-dissolved ions in sediment porewater to diffuse across a micro-porous membrane towards water contained in an isolated compartment that has been inserted into sediment. Once retrieved after a deployment period, the resulting sample obtained can provide concentrations of freely-dissolved inorganic constituents in sediment, which provides measurements that can be used for understanding contaminant fate and risk. Peepers can also be used in the same manner in surface water, although this article is focused on the use of peepers in sediment.

<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Contaminated Sediments - Introduction]]
*[[Contaminated Sediment Risk Assessment]]
*[[Mercury in Sediments]]
*[[Passive Sampling of Munitions Constituents]]
*[[Passive Sampling of Sediments]]
*[[Sediment Capping]]

'''Contributor(s):''' [[Florent Risacher|Florent Risacher, M.Sc]]. and [[Dr. Jason Conder|Dr. Jason Conder]]

'''Key Resource(s):'''

*A review of peeper passive sampling approaches to measure the availability of inorganics in sediment porewater<ref>Risacher, F.F., Schneider, H., Drygiannaki, I., Conder, J., Pautler, B.G., and Jackson, A.W., 2023. A Review of Peeper Passive Sampling Approaches to Measure the Availability of Inorganics in Sediment Porewater. Environmental Pollution, 328, Article 121581. [https://doi.org/10.1016/j.envpol.2023.121581 doi: 10.1016/j.envpol.2023.121581]  [//www.enviro.wiki/images/4/4f/RisacherEtAl2023a.pdf Article pdf]</ref>

*Best Practices User’s Guide: Standardizing Sediment Porewater Passive Samplers for Inorganic Constituents of Concern<ref name="RisacherEtAl2023">Risacher, F.F., Nichols, E., Schneider, H., Lawrence, M., Conder, J., Sweett, A., Pautler, B.G., Jackson, W.A., Rosen, G., 2023b. Best Practices User’s Guide: Standardizing Sediment Porewater Passive Samplers for Inorganic Constituents of Concern, ESTCP ER20-5261. [https://serdp-estcp.mil/projects/details/db871313-fbc0-4432-b536-40c64af3627f Project Website]  [//www.enviro.wiki/images/4/42/ER20-5261BPUG.pdf Report.pdf]</ref>

*[https://serdp-estcp.mil/projects/details/db871313-fbc0-4432-b536-40c64af3627f/er20-5261-project-overview Standardizing Sediment Porewater Passive Samplers for Inorganic Constituents of Concern, ESTCP Project ER20-5261]

==Introduction==
Biologically available inorganic constituents associated with sediment toxicity can be quantified by measuring the freely-dissolved fraction of contaminants in the porewater<ref>Conder, J.M., Fuchsman, P.C., Grover, M.M., Magar, V.S., Henning, M.H., 2015. Critical review of mercury SQVs for the protection of benthic invertebrates. Environmental Toxicology and Chemistry, 34(1), pp. 6-21. [https://doi.org/10.1002/etc.2769 doi: 10.1002/etc.2769]   [//www.enviro.wiki/images/8/8d/ConderEtAl2015.pdf Article pdf]</ref><ref name="ClevelandEtAl2017">Cleveland, D., Brumbaugh, W.G., MacDonald, D.D., 2017. A comparison of four porewater sampling methods for metal mixtures and dissolved organic carbon and the implications for sediment toxicity evaluations. Environmental Toxicology and Chemistry, 36(11), pp. 2906-2915. [https://doi.org/10.1002/etc.3884 doi: 10.1002/etc.3884]</ref>. Classical sediment porewater analysis usually consists of collecting large volumes of bulk sediments which are then mechanically squeezed or centrifuged to produce a supernatant, or suction of porewater from intact sediment, followed by filtration and collection<ref name="GruzalskiEtAl2016">Gruzalski, J.G., Markwiese, J.T., Carriker, N.E., Rogers, W.J., Vitale, R.J., Thal, D.I., 2016. Pore Water Collection, Analysis and Evolution: The Need for Standardization. In: Reviews of Environmental Contamination and Toxicology, Vol. 237, pp. 37–51. Springer. [https://doi.org/10.1007/978-3-319-23573-8_2 doi: 10.1007/978-3-319-23573-8_2]</ref>. The extraction and measurement processes present challenges due to the heterogeneity of sediments, physical disturbance, high reactivity of some complexes, and interaction between the solid and dissolved phases, which can impact the measured concentration of dissolved inorganics<ref>Peijnenburg, W.J.G.M., Teasdale, P.R., Reible, D., Mondon, J., Bennett, W.W., Campbell, P.G.C., 2014. Passive Sampling Methods for Contaminated Sediments: State of the Science for Metals. Integrated Environmental Assessment and Management, 10(2), pp. 179–196. [https://doi.org/10.1002/ieam.1502 doi: 10.1002/ieam.1502]  [//www.enviro.wiki/images/9/99/PeijnenburgEtAl2014.pdf Article pdf]</ref>. For example, sampling disturbance can affect redox conditions<ref name="TeasdaleEtAl1995">Teasdale, P.R., Batley, G.E., Apte, S.C., Webster, I.T., 1995. Pore water sampling with sediment peepers. Trends in Analytical Chemistry, 14(6), pp. 250–256. [https://doi.org/10.1016/0165-9936(95)91617-2 doi: 10.1016/0165-9936(95)91617-2]</ref><ref>Schroeder, H., Duester, L., Fabricius, A.L., Ecker, D., Breitung, V., Ternes, T.A., 2020. Sediment water (interface) mobility of metal(loid)s and nutrients under undisturbed conditions and during resuspension. Journal of Hazardous Materials, 394, Article 122543. [https://doi.org/10.1016/j.jhazmat.2020.122543 doi: 10.1016/j.jhazmat.2020.122543] [//www.enviro.wiki/images/6/6d/SchroederEtAl2020.pdf Article pdf]</ref>, which can lead to under or over representation of inorganic chemical concentrations relative to the true dissolved phase concentration in the sediment porewater<ref>Wise, D.E., 2009. Sampling techniques for sediment pore water in evaluation of reactive capping efficacy. Master of Science Thesis. University of New Hampshire Scholars’ Repository. 178 pages. [https://scholars.unh.edu/thesis/502 Website] [//www.enviro.wiki/images/5/57/Wise2009.pdf Report.pdf]</ref><ref name="GruzalskiEtAl2016" />.

To address the complications with mechanical porewater sampling, passive sampling approaches for inorganics have been developed to provide a method that has a low impact on the surrounding geochemistry of sediments and sediment porewater, thus enabling more precise measurements of inorganics<ref name="ClevelandEtAl2017" />. Sediment porewater dialysis passive samplers, also known as “peepers,” were developed more than 45 years ago<ref name="Hesslein1976">Hesslein, R.H., 1976. An in situ sampler for close interval pore water studies. Limnology and Oceanography, 21(6), pp. 912-914. [https://doi.org/10.4319/lo.1976.21.6.0912 doi: 10.4319/lo.1976.21.6.0912] [//www.enviro.wiki/images/c/c7/Hesslein1976.pdf Article pdf]</ref> and refinements to the method such as the use of reverse tracers have been made, improving the acceptance of the technology as decision making tool.

==Peeper Designs==
[[File:RisacherFig1.png|thumb|300px|Figure 1. Conceptual illustration of peeper construction showing (top, left to right) the peeper cap (optional), peeper membrane and peeper chamber, and (bottom) an assembled peeper containing peeper water]]
[[File:RisacherFig2.png | thumb |400px| Figure 2. Example of Hesslein<ref name="Hesslein1976" /> general peeper design (42 peeper chambers), from [https://www.usgs.gov/media/images/peeper-samplers USGS]]]
[[File:RisacherFig3.png | thumb |400px| Figure 3. Peeper deployment structure to allow the measurement of metal availability in different sediment layers using five single-chamber peepers (Photo: Geosyntec Consultants)]]
Peepers (Figure 1) are inert containers with a small volume (typically 1-100 mL) of purified water (“peeper water”) capped with a semi-permeable membrane. Peepers can be manufactured in a wide variety of formats (Figure 2, Figure 3) and deployed in in various ways.

Two designs are commonly used for peepers. Frequently, the designs are close adaptations of the original multi-chamber Hesslein design<ref name="Hesslein1976" /> (Figure 2), which consists of an acrylic sampler body with multiple sample chambers machined into it. Peeper water inside the chambers is separated from the outside environment by a semi-permeable membrane, which is held in place by a top plate fixed to the sampler body using bolts or screws. An alternative design consists of single-chamber peepers constructed using a single sample vial with a membrane secured over the mouth of the vial, as shown in Figure 3, and applied in Teasdale ''et al.''<ref name="TeasdaleEtAl1995" />, Serbst ''et al.''<ref>Serbst, J.R., Burgess, R.M., Kuhn, A., Edwards, P.A., Cantwell, M.G., Pelletier, M.C., Berry, W.J., 2003. Precision of dialysis (peeper) sampling of cadmium in marine sediment interstitial water. Archives of Environmental Contamination and Toxicology, 45(3), pp. 297–305. [https://doi.org/10.1007/s00244-003-0114-5 doi: 10.1007/s00244-003-0114-5]</ref>, Thomas and Arthur<ref name="ThomasArthur2010">Thomas, B., Arthur, M.A., 2010. Correcting porewater concentration measurements from peepers: Application of a reverse tracer. Limnology and Oceanography: Methods, 8(8), pp. 403–413. [https://doi.org/10.4319/lom.2010.8.403 doi: 10.4319/lom.2010.8.403] [//www.enviro.wiki/images/7/7b/ThomasArthur2010.pdf Article pdf]</ref>, Passeport ''et al.''<ref>Passeport, E., Landis, R., Lacrampe-Couloume, G., Lutz, E.J., Erin Mack, E., West, K., Morgan, S., Lollar, B.S., 2016. Sediment Monitored Natural Recovery Evidenced by Compound Specific Isotope Analysis and High-Resolution Pore Water Sampling. Environmental Science and Technology, 50(22), pp. 12197–12204. [https://doi.org/10.1021/acs.est.6b02961 doi: 10.1021/acs.est.6b02961]</ref>, and Risacher ''et al.''<ref name="RisacherEtAl2023" />. The vial is filled with deionized water, and the membrane is held in place using the vial cap or an o-ring. Individual vials are either directly inserted into sediment or are incorporated into a support structure to allow multiple single-chamber peepers to be deployed at once over a given depth profile (Figure 3).

==Peepers Preparation, Deployment and Retrieval==
[[File:RisacherFig4.png | thumb |300px| Figure 4: Conceptual illustration of peeper passive sampling in a sediment matrix, showing peeper immediately after deployment (top) and after equilibration between the porewater and peeper chamber water (bottom)]]
Peepers are often prepared in laboratories but are also commercially available in a variety of designs from several suppliers. Peepers are prepared by first cleaning all materials to remove even trace levels of metals before assembly. The water contained inside the peeper is sometimes deoxygenated, and in some cases the peeper is maintained in a deoxygenated atmosphere until deployment<ref>Carignan, R., St‐Pierre, S., Gachter, R., 1994. Use of diffusion samplers in oligotrophic lake sediments: Effects of free oxygen in sampler material. Limnology and Oceanography, 39(2), pp. 468-474. [https://doi.org/10.4319/lo.1994.39.2.0468 doi: 10.4319/lo.1994.39.2.0468] [//www.enviro.wiki/images/9/9c/CarignanEtAl1994.pdf Article pdf]</ref>. However, recent studies<ref name="RisacherEtAl2023" /> have shown that deoxygenation prior to deployment does not significantly impact sampling results due to oxygen rapidly diffusing out of the peeper during deployment. Once assembled, peepers are usually shipped in a protective bag inside a hard-case cooler for protection.

Peepers are deployed by insertion into sediment for a period of a few days to a few weeks. Insertion into the sediment can be achieved by wading to the location when the water depth is shallow, by using push poles for deeper deployments<ref name="RisacherEtAl2023" />, or by professional divers for the deepest sites. If divers are used, an appropriate boat or ship will be required to accommodate the diver and their equipment. Whichever method is used, peepers should be attached to an anchor or a small buoy to facilitate retrieval at the end of the deployment period.

During deployment, passive sampling is achieved via diffusion of inorganics through the peeper’s semi-permeable membrane, as the enclosed volume of peeper water equilibrates with the surrounding sediment porewater (Figure 4). It is assumed that the peeper insertion does not greatly alter geochemical conditions that affect freely-dissolved inorganics. Additionally, it is assumed that the peeper water equilibrates with freely-dissolved inorganics in sediment in such a way that the concentration of inorganics in the peeper water would be equal to that of the concentration of inorganics in the sediment porewater.

After retrieval, the peepers are brought to the surface and usually preserved until they can be processed. This can be achieved by storing the peepers inside a sealable, airtight bag with either inert gas or oxygen absorbing packets<ref name="RisacherEtAl2023" />. The peeper water can then be processed by quickly pipetting it into an appropriate sample bottle which usually contains a preservative (e.g., nitric acid for metals). This step is generally conducted in the field. Samples are stored on ice to maintain a temperature of less than 4°C and shipped to an analytical laboratory. The samples are then analyzed for inorganics by standard methods (i.e., USEPA SW-846). The results obtained from the analytical laboratory are then used directly or assessed using the equations below if a reverse tracer is used because deployment time is insufficient for all analytes to reach equilibrium.

==Equilibrium Determination (Tracers)==
The equilibration period of peepers can last several weeks and depends on deployment conditions, analyte of interest, and peeper design. In many cases, it is advantageous to use pre-equilibrium methods that can use measurements in peepers deployed for shorter periods to predict concentrations at equilibrium<ref name="USEPA2017">USEPA, 2017. Laboratory, Field, and Analytical Procedures for Using Passive Sampling in the Evaluation of Contaminated Sediments: User’s Manual. EPA/600/R-16/357. [//www.enviro.wiki/images/0/08/EPA_600_R-16_357.pdf Report.pdf]</ref>.

Although the equilibrium concentration of an analyte in sediment can be evaluated by examining analyte results for peepers deployed for several different amounts of time (i.e., a time series), this is impractical for typical field investigations because it would require several mobilizations to the site to retrieve samplers. Alternately, reverse tracers (referred to as a performance reference compound when used with organic compound passive sampling) can be used to evaluate the percentage of equilibrium reached by a passive sampler.

Thomas and Arthur<ref name="ThomasArthur2010" /> studied the use of a reverse tracer to estimate percent equilibrium in lab experiments and a field application. They concluded that bromide can be used to estimate concentrations in porewater using measurements obtained before equilibrium is reached. Further studies were also conducted by Risacher ''et al.''<ref name="RisacherEtAl2023" /> showed that lithium can also be used as a tracer for brackish and saline environments. Both studies included a mathematical model for estimating concentrations of ions in external media (''C0'') based on measured concentrations in the peeper chamber (''Cp,t''), the elimination rate of the target analyte (''K'') and the deployment time (''t''):
 
{|
| ||'''Equation 1:'''
|     [[File: Equation1r.png]]
|-
|Where:|| ||
|-
| ||''C0''||is the freely dissolved concentration of the analyte in the sediment (mg/L or μg/L), sometimes referred to as ''Cfree ''
|-
| ||''Cp,t''||is the measured concentration of the analyte in the peeper at time of retrieval (mg/L or μg/L)
|-
| ||''K''||is the elimination rate of the target analyte
|-
| ||''t''||is the deployment time (days)
|}

The elimination rate of the target analyte (''K'') is calculated using Equation 2:
 
{|
| ||'''Equation 2:'''
|     [[File: Equation2r.png]]
|-
|Where:|| ||
|-
| ||''K''||is the elimination rate of the target analyte
|-
| ||''Ktracer''||is the elimination rate of the tracer
|-
| ||''D''||is the free water diffusivity of the analyte (cm2/s)
|-
| ||''Dtracer''||is the free water diffusivity of the tracer (cm2/s)
|}

The elimination rate of the tracer (''Ktracer'') is calculated using Equation 3:
 
{|
| ||'''Equation 3:'''
|         [[File: Equation3r2.png]]
|-
|Where:|| ||
|-
| ||''Ktracer''||is the elimination rate of the tracer
|-
| ||''Ctracer,i''||is the measured initial concentration of the tracer in the peeper prior to deployment (mg/L or μg/L)
|-
| ||''Ctracer,t''||is the measured final concentration of the tracer in the peeper at time of retrieval (mg/L or μg/L)
|-
| ||''t''||is the deployment time (days)
|}

Using this set of equations allows the calculation of the porewater concentration of the analyte prior to its equilibrium with the peeper water. A template for these calculations can be found in the appendix of Risacher ''et al.''<ref name="RisacherEtAl2023" />.

==Using Peeper Data at a Sediment Site==
Peeper data can be used to enable site specific decision making in a variety of ways. Some of the most common uses for peepers and peeper data are discussed below.

'''Nature and Extent:''' Multiple peepers deployed in sediment can help delineate areas of increased metal availability. Peepers are especially helpful for sites that are comprised of coarse, relatively inert materials that may not be conducive to traditional bulk sediment sampling. Because much of the inorganics present in these types of sediments may be associated with the porewater phase rather than the solid phase, peepers can provide a more representative measurement of C0. Additionally, at sites where tidal pumping or groundwater flux may be influencing the nature and extent of inorganics, peepers can provide a distinct advantage to bulk sediment sampling or other point-in-time measurements, as peepers can provide an average measurement that integrates the variability in the hydrodynamic and chemical conditions over time.

'''Sources and Fate:''' A considerable advantage to using peepers is that C0 results are expressed as concentration in units of mass per volume (e.g., mg/L), providing a common unit of measurement to compare across multiple media. For example, synchronous measurements of C0 using peepers deployed in both surface water and sediment can elucidate the potential flux of inorganics from sediment to surface water. Paired measurements of both C0 and bulk metals in sediment can also allow site specific sediment-porewater partition coefficients to be calculated. These values can be useful in understanding and predicting contaminant fate, especially in situations where the potential dissolution of metals from sediment are critical to predict, such as when sediment is dredged.

'''Direct Toxicity to Aquatic Life:''' Peepers are frequently used to understand the potential direct toxicity to aquatic life, such as benthic invertebrates and fish. A C0 measurement obtained from a peeper deployed in sediment (''in situ'') or surface water (''ex situ''), can be compared to toxicological benchmarks for aquatic life to understand the potential toxicity to aquatic life and to set remediation goals<ref name="USEPA2017" />. C0 measurements can also be incorporated in more sophisticated approaches, such as the Biotic Ligand Model<ref>Santore, C.R., Toll, E.J., DeForest, K.D., Croteau, K., Baldwin, A., Bergquist, B., McPeek, K., Tobiason, K., and Judd, L.N., 2022. Refining our understanding of metal bioavailability in sediments using information from porewater: Application of a multi-metal BLM as an extension of the Equilibrium Partitioning Sediment Benchmarks. Integrated Environmental Assessment and Management, 18(5), pp. 1335–1347. [https://doi.org/10.1002/ieam.4572 doi: 10.1002/ieam.4572]</ref> to understand the potential for toxicity or the need to conduct toxicological testing or ecological evaluations.

'''Bioaccumulation of Inorganics by Aquatic Life:''' Peepers can also be used to understand site specific relationship between C0 and concentrations of inorganics in aquatic life. For example, measuring C0 in sediment from which organisms are collected and analyzed can enable the estimation of a site-specific uptake factor. This C0-to-organism uptake factor (or model) can then be applied for a variety of uses, including predicting the concentration of inorganics in other organisms, or estimating a sediment C0 value that would be safe for consumption by wildlife or humans. Because several decades of research have found that the correlation between C0 measurements and bioavailability is usually better than the correlation between measurements of chemicals in bulk sediment and bioavailability, C0-to-organism uptake factors are likely to be more accurate than uptake factors based on bulk sediment testing.

'''Evaluating Sediment Remediation Efficacy:''' Passive sampling has been used widely to evaluate the efficacy of remedial actions such as active amendments, thin layer placements, and capping to reduce the availability of contaminants at sediment sites. A particularly powerful approach is to compare baseline (pre-remedy) C0 in sediment to C0 in sediment after the sediment remedy has been applied. Peepers can be used in this context for inorganics, allowing the sediment remedy’s success to be evaluated and monitored in laboratory benchtop remedy evaluations, pilot scale remedy evaluations, and full-scale remediation monitoring.

==References==
<references />

==See Also==

*[https://vimeo.com/809180171/c276c1873a Peeper Deployment Video]
*[https://vimeo.com/811073634/303edf2693 Peeper Retrieval Video]
*[https://vimeo.com/811328715/aea3073540 Peeper Processing Video]
*[https://sepub-prod-0001-124733793621-us-gov-west-1.s3.us-gov-west-1.amazonaws.com/s3fs-public/2024-09/ER20-5261%20Fact%20Sheet.pdf?VersionId=malAixSQQM3mWCRiaVaxY8wLdI0jE1PX Fact Sheet]

Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)

2026-03-03T22:45:53Z

Admin:

Sediment porewater dialysis passive samplers, also known as “peepers,” are sampling devices that allow the measurement of dissolved inorganic ions in the porewater of a saturated sediment. Peepers function by allowing freely-dissolved ions in sediment porewater to diffuse across a micro-porous membrane towards water contained in an isolated compartment that has been inserted into sediment. Once retrieved after a deployment period, the resulting sample obtained can provide concentrations of freely-dissolved inorganic constituents in sediment, which provides measurements that can be used for understanding contaminant fate and risk. Peepers can also be used in the same manner in surface water, although this article is focused on the use of peepers in sediment.

<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Contaminated Sediments - Introduction]]
*[[Contaminated Sediment Risk Assessment]]
*[[Mercury in Sediments]]
*[[Passive Sampling of Munitions Constituents]]
*[[Passive Sampling of Sediments]]
*[[Sediment Capping]]

'''Contributor(s):''' [[Florent Risacher|Florent Risacher, M.Sc]]. and [[Dr. Jason Conder|Dr. Jason Conder]]

'''Key Resource(s):'''

*A review of peeper passive sampling approaches to measure the availability of inorganics in sediment porewater<ref>Risacher, F.F., Schneider, H., Drygiannaki, I., Conder, J., Pautler, B.G., and Jackson, A.W., 2023. A Review of Peeper Passive Sampling Approaches to Measure the Availability of Inorganics in Sediment Porewater. Environmental Pollution, 328, Article 121581. [https://doi.org/10.1016/j.envpol.2023.121581 doi: 10.1016/j.envpol.2023.121581]  [//www.enviro.wiki/images/4/4f/RisacherEtAl2023a.pdf Article pdf]</ref>

*Best Practices User’s Guide: Standardizing Sediment Porewater Passive Samplers for Inorganic Constituents of Concern<ref name="RisacherEtAl2023">Risacher, F.F., Nichols, E., Schneider, H., Lawrence, M., Conder, J., Sweett, A., Pautler, B.G., Jackson, W.A., Rosen, G., 2023b. Best Practices User’s Guide: Standardizing Sediment Porewater Passive Samplers for Inorganic Constituents of Concern, ESTCP ER20-5261. [https://serdp-estcp.mil/projects/details/db871313-fbc0-4432-b536-40c64af3627f Project Website]  [//www.enviro.wiki/images/4/42/ER20-5261BPUG.pdf Report.pdf]</ref>

*[https://serdp-estcp.mil/projects/details/db871313-fbc0-4432-b536-40c64af3627f/er20-5261-project-overview Standardizing Sediment Porewater Passive Samplers for Inorganic Constituents of Concern, ESTCP Project ER20-5261]

==Introduction==
Biologically available inorganic constituents associated with sediment toxicity can be quantified by measuring the freely-dissolved fraction of contaminants in the porewater<ref>Conder, J.M., Fuchsman, P.C., Grover, M.M., Magar, V.S., Henning, M.H., 2015. Critical review of mercury SQVs for the protection of benthic invertebrates. Environmental Toxicology and Chemistry, 34(1), pp. 6-21. [https://doi.org/10.1002/etc.2769 doi: 10.1002/etc.2769]   [//www.enviro.wiki/images/8/8d/ConderEtAl2015.pdf Article pdf]</ref><ref name="ClevelandEtAl2017">Cleveland, D., Brumbaugh, W.G., MacDonald, D.D., 2017. A comparison of four porewater sampling methods for metal mixtures and dissolved organic carbon and the implications for sediment toxicity evaluations. Environmental Toxicology and Chemistry, 36(11), pp. 2906-2915. [https://doi.org/10.1002/etc.3884 doi: 10.1002/etc.3884]</ref>. Classical sediment porewater analysis usually consists of collecting large volumes of bulk sediments which are then mechanically squeezed or centrifuged to produce a supernatant, or suction of porewater from intact sediment, followed by filtration and collection<ref name="GruzalskiEtAl2016">Gruzalski, J.G., Markwiese, J.T., Carriker, N.E., Rogers, W.J., Vitale, R.J., Thal, D.I., 2016. Pore Water Collection, Analysis and Evolution: The Need for Standardization. In: Reviews of Environmental Contamination and Toxicology, Vol. 237, pp. 37–51. Springer. [https://doi.org/10.1007/978-3-319-23573-8_2 doi: 10.1007/978-3-319-23573-8_2]</ref>. The extraction and measurement processes present challenges due to the heterogeneity of sediments, physical disturbance, high reactivity of some complexes, and interaction between the solid and dissolved phases, which can impact the measured concentration of dissolved inorganics<ref>Peijnenburg, W.J.G.M., Teasdale, P.R., Reible, D., Mondon, J., Bennett, W.W., Campbell, P.G.C., 2014. Passive Sampling Methods for Contaminated Sediments: State of the Science for Metals. Integrated Environmental Assessment and Management, 10(2), pp. 179–196. [https://doi.org/10.1002/ieam.1502 doi: 10.1002/ieam.1502]  [//www.enviro.wiki/images/9/99/PeijnenburgEtAl2014.pdf Article pdf]</ref>. For example, sampling disturbance can affect redox conditions<ref name="TeasdaleEtAl1995">Teasdale, P.R., Batley, G.E., Apte, S.C., Webster, I.T., 1995. Pore water sampling with sediment peepers. Trends in Analytical Chemistry, 14(6), pp. 250–256. [https://doi.org/10.1016/0165-9936(95)91617-2 doi: 10.1016/0165-9936(95)91617-2]</ref><ref>Schroeder, H., Duester, L., Fabricius, A.L., Ecker, D., Breitung, V., Ternes, T.A., 2020. Sediment water (interface) mobility of metal(loid)s and nutrients under undisturbed conditions and during resuspension. Journal of Hazardous Materials, 394, Article 122543. [https://doi.org/10.1016/j.jhazmat.2020.122543 doi: 10.1016/j.jhazmat.2020.122543] [//www.enviro.wiki/images/6/6d/SchroederEtAl2020.pdf Article pdf]</ref>, which can lead to under or over representation of inorganic chemical concentrations relative to the true dissolved phase concentration in the sediment porewater<ref>Wise, D.E., 2009. Sampling techniques for sediment pore water in evaluation of reactive capping efficacy. Master of Science Thesis. University of New Hampshire Scholars’ Repository. 178 pages. [https://scholars.unh.edu/thesis/502 Website] [//www.enviro.wiki/images/5/57/Wise2009.pdf Report.pdf]</ref><ref name="GruzalskiEtAl2016" />.

To address the complications with mechanical porewater sampling, passive sampling approaches for inorganics have been developed to provide a method that has a low impact on the surrounding geochemistry of sediments and sediment porewater, thus enabling more precise measurements of inorganics<ref name="ClevelandEtAl2017" />. Sediment porewater dialysis passive samplers, also known as “peepers,” were developed more than 45 years ago<ref name="Hesslein1976">Hesslein, R.H., 1976. An in situ sampler for close interval pore water studies. Limnology and Oceanography, 21(6), pp. 912-914. [https://doi.org/10.4319/lo.1976.21.6.0912 doi: 10.4319/lo.1976.21.6.0912] [//www.enviro.wiki/images/c/c7/Hesslein1976.pdf Article pdf]</ref> and refinements to the method such as the use of reverse tracers have been made, improving the acceptance of the technology as decision making tool.

==Peeper Designs==
[[File:RisacherFig1.png|thumb|300px|Figure 1. Conceptual illustration of peeper construction showing (top, left to right) the peeper cap (optional), peeper membrane and peeper chamber, and (bottom) an assembled peeper containing peeper water]]
[[File:RisacherFig2.png | thumb |400px| Figure 2. Example of Hesslein<ref name="Hesslein1976" /> general peeper design (42 peeper chambers), from [https://www.usgs.gov/media/images/peeper-samplers USGS]]]
[[File:RisacherFig3.png | thumb |400px| Figure 3. Peeper deployment structure to allow the measurement of metal availability in different sediment layers using five single-chamber peepers (Photo: Geosyntec Consultants)]]
Peepers (Figure 1) are inert containers with a small volume (typically 1-100 mL) of purified water (“peeper water”) capped with a semi-permeable membrane. Peepers can be manufactured in a wide variety of formats (Figure 2, Figure 3) and deployed in in various ways.

Two designs are commonly used for peepers. Frequently, the designs are close adaptations of the original multi-chamber Hesslein design<ref name="Hesslein1976" /> (Figure 2), which consists of an acrylic sampler body with multiple sample chambers machined into it. Peeper water inside the chambers is separated from the outside environment by a semi-permeable membrane, which is held in place by a top plate fixed to the sampler body using bolts or screws. An alternative design consists of single-chamber peepers constructed using a single sample vial with a membrane secured over the mouth of the vial, as shown in Figure 3, and applied in Teasdale ''et al.''<ref name="TeasdaleEtAl1995" />, Serbst ''et al.''<ref>Serbst, J.R., Burgess, R.M., Kuhn, A., Edwards, P.A., Cantwell, M.G., Pelletier, M.C., Berry, W.J., 2003. Precision of dialysis (peeper) sampling of cadmium in marine sediment interstitial water. Archives of Environmental Contamination and Toxicology, 45(3), pp. 297–305. [https://doi.org/10.1007/s00244-003-0114-5 doi: 10.1007/s00244-003-0114-5]</ref>, Thomas and Arthur<ref name="ThomasArthur2010">Thomas, B., Arthur, M.A., 2010. Correcting porewater concentration measurements from peepers: Application of a reverse tracer. Limnology and Oceanography: Methods, 8(8), pp. 403–413. [https://doi.org/10.4319/lom.2010.8.403 doi: 10.4319/lom.2010.8.403] [//www.enviro.wiki/images/7/7b/ThomasArthur2010.pdf Article pdf]</ref>, Passeport ''et al.''<ref>Passeport, E., Landis, R., Lacrampe-Couloume, G., Lutz, E.J., Erin Mack, E., West, K., Morgan, S., Lollar, B.S., 2016. Sediment Monitored Natural Recovery Evidenced by Compound Specific Isotope Analysis and High-Resolution Pore Water Sampling. Environmental Science and Technology, 50(22), pp. 12197–12204. [https://doi.org/10.1021/acs.est.6b02961 doi: 10.1021/acs.est.6b02961]</ref>, and Risacher ''et al.''<ref name="RisacherEtAl2023" />. The vial is filled with deionized water, and the membrane is held in place using the vial cap or an o-ring. Individual vials are either directly inserted into sediment or are incorporated into a support structure to allow multiple single-chamber peepers to be deployed at once over a given depth profile (Figure 3).

==Peepers Preparation, Deployment and Retrieval==
[[File:RisacherFig4.png | thumb |300px| Figure 4: Conceptual illustration of peeper passive sampling in a sediment matrix, showing peeper immediately after deployment (top) and after equilibration between the porewater and peeper chamber water (bottom)]]
Peepers are often prepared in laboratories but are also commercially available in a variety of designs from several suppliers. Peepers are prepared by first cleaning all materials to remove even trace levels of metals before assembly. The water contained inside the peeper is sometimes deoxygenated, and in some cases the peeper is maintained in a deoxygenated atmosphere until deployment<ref>Carignan, R., St‐Pierre, S., Gachter, R., 1994. Use of diffusion samplers in oligotrophic lake sediments: Effects of free oxygen in sampler material. Limnology and Oceanography, 39(2), pp. 468-474. [https://doi.org/10.4319/lo.1994.39.2.0468 doi: 10.4319/lo.1994.39.2.0468] [//www.enviro.wiki/images/9/9c/CarignanEtAl1994.pdf Article pdf]</ref>. However, recent studies<ref name="RisacherEtAl2023" /> have shown that deoxygenation prior to deployment does not significantly impact sampling results due to oxygen rapidly diffusing out of the peeper during deployment. Once assembled, peepers are usually shipped in a protective bag inside a hard-case cooler for protection.

Peepers are deployed by insertion into sediment for a period of a few days to a few weeks. Insertion into the sediment can be achieved by wading to the location when the water depth is shallow, by using push poles for deeper deployments<ref name="RisacherEtAl2023" />, or by professional divers for the deepest sites. If divers are used, an appropriate boat or ship will be required to accommodate the diver and their equipment. Whichever method is used, peepers should be attached to an anchor or a small buoy to facilitate retrieval at the end of the deployment period.

During deployment, passive sampling is achieved via diffusion of inorganics through the peeper’s semi-permeable membrane, as the enclosed volume of peeper water equilibrates with the surrounding sediment porewater (Figure 4). It is assumed that the peeper insertion does not greatly alter geochemical conditions that affect freely-dissolved inorganics. Additionally, it is assumed that the peeper water equilibrates with freely-dissolved inorganics in sediment in such a way that the concentration of inorganics in the peeper water would be equal to that of the concentration of inorganics in the sediment porewater.

After retrieval, the peepers are brought to the surface and usually preserved until they can be processed. This can be achieved by storing the peepers inside a sealable, airtight bag with either inert gas or oxygen absorbing packets<ref name="RisacherEtAl2023" />. The peeper water can then be processed by quickly pipetting it into an appropriate sample bottle which usually contains a preservative (e.g., nitric acid for metals). This step is generally conducted in the field. Samples are stored on ice to maintain a temperature of less than 4°C and shipped to an analytical laboratory. The samples are then analyzed for inorganics by standard methods (i.e., USEPA SW-846). The results obtained from the analytical laboratory are then used directly or assessed using the equations below if a reverse tracer is used because deployment time is insufficient for all analytes to reach equilibrium.

==Equilibrium Determination (Tracers)==
The equilibration period of peepers can last several weeks and depends on deployment conditions, analyte of interest, and peeper design. In many cases, it is advantageous to use pre-equilibrium methods that can use measurements in peepers deployed for shorter periods to predict concentrations at equilibrium<ref name="USEPA2017">USEPA, 2017. Laboratory, Field, and Analytical Procedures for Using Passive Sampling in the Evaluation of Contaminated Sediments: User’s Manual. EPA/600/R-16/357. [//www.enviro.wiki/images/0/08/EPA_600_R-16_357.pdf Report.pdf]</ref>.

Although the equilibrium concentration of an analyte in sediment can be evaluated by examining analyte results for peepers deployed for several different amounts of time (i.e., a time series), this is impractical for typical field investigations because it would require several mobilizations to the site to retrieve samplers. Alternately, reverse tracers (referred to as a performance reference compound when used with organic compound passive sampling) can be used to evaluate the percentage of equilibrium reached by a passive sampler.

Thomas and Arthur<ref name="ThomasArthur2010" /> studied the use of a reverse tracer to estimate percent equilibrium in lab experiments and a field application. They concluded that bromide can be used to estimate concentrations in porewater using measurements obtained before equilibrium is reached. Further studies were also conducted by Risacher ''et al.''<ref name="RisacherEtAl2023" /> showed that lithium can also be used as a tracer for brackish and saline environments. Both studies included a mathematical model for estimating concentrations of ions in external media (''C0'') based on measured concentrations in the peeper chamber (''Cp,t''), the elimination rate of the target analyte (''K'') and the deployment time (''t''):
 
{|
| ||'''Equation 1:'''
|     [[File: Equation1r.png]]
|-
|Where:|| ||
|-
| ||''C0''||is the freely dissolved concentration of the analyte in the sediment (mg/L or μg/L), sometimes referred to as ''Cfree ''
|-
| ||''Cp,t''||is the measured concentration of the analyte in the peeper at time of retrieval (mg/L or μg/L)
|-
| ||''K''||is the elimination rate of the target analyte
|-
| ||''t''||is the deployment time (days)
|}

The elimination rate of the target analyte (''K'') is calculated using Equation 2:
 
{|
| ||'''Equation 2:'''
|     [[File: Equation2r.png]]
|-
|Where:|| ||
|-
| ||''K''||is the elimination rate of the target analyte
|-
| ||''Ktracer''||is the elimination rate of the tracer
|-
| ||''D''||is the free water diffusivity of the analyte (cm2/s)
|-
| ||''Dtracer''||is the free water diffusivity of the tracer (cm2/s)
|}

The elimination rate of the tracer (''Ktracer'') is calculated using Equation 3:
 
{|
| ||'''Equation 3:'''
|         [[File: Equation3r2.png]]
|-
|Where:|| ||
|-
| ||''Ktracer''||is the elimination rate of the tracer
|-
| ||''Ctracer,i''||is the measured initial concentration of the tracer in the peeper prior to deployment (mg/L or μg/L)
|-
| ||''Ctracer,t''||is the measured final concentration of the tracer in the peeper at time of retrieval (mg/L or μg/L)
|-
| ||''t''||is the deployment time (days)
|}

Using this set of equations allows the calculation of the porewater concentration of the analyte prior to its equilibrium with the peeper water. A template for these calculations can be found in the appendix of Risacher ''et al.''<ref name="RisacherEtAl2023" />.

==Using Peeper Data at a Sediment Site==
Peeper data can be used to enable site specific decision making in a variety of ways. Some of the most common uses for peepers and peeper data are discussed below.

'''Nature and Extent:''' Multiple peepers deployed in sediment can help delineate areas of increased metal availability. Peepers are especially helpful for sites that are comprised of coarse, relatively inert materials that may not be conducive to traditional bulk sediment sampling. Because much of the inorganics present in these types of sediments may be associated with the porewater phase rather than the solid phase, peepers can provide a more representative measurement of C0. Additionally, at sites where tidal pumping or groundwater flux may be influencing the nature and extent of inorganics, peepers can provide a distinct advantage to bulk sediment sampling or other point-in-time measurements, as peepers can provide an average measurement that integrates the variability in the hydrodynamic and chemical conditions over time.

'''Sources and Fate:''' A considerable advantage to using peepers is that C0 results are expressed as concentration in units of mass per volume (e.g., mg/L), providing a common unit of measurement to compare across multiple media. For example, synchronous measurements of C0 using peepers deployed in both surface water and sediment can elucidate the potential flux of inorganics from sediment to surface water. Paired measurements of both C0 and bulk metals in sediment can also allow site specific sediment-porewater partition coefficients to be calculated. These values can be useful in understanding and predicting contaminant fate, especially in situations where the potential dissolution of metals from sediment are critical to predict, such as when sediment is dredged.

'''Direct Toxicity to Aquatic Life:''' Peepers are frequently used to understand the potential direct toxicity to aquatic life, such as benthic invertebrates and fish. A C0 measurement obtained from a peeper deployed in sediment (''in situ'') or surface water (''ex situ''), can be compared to toxicological benchmarks for aquatic life to understand the potential toxicity to aquatic life and to set remediation goals<ref name="USEPA2017" />. C0 measurements can also be incorporated in more sophisticated approaches, such as the Biotic Ligand Model<ref>Santore, C.R., Toll, E.J., DeForest, K.D., Croteau, K., Baldwin, A., Bergquist, B., McPeek, K., Tobiason, K., and Judd, L.N., 2022. Refining our understanding of metal bioavailability in sediments using information from porewater: Application of a multi-metal BLM as an extension of the Equilibrium Partitioning Sediment Benchmarks. Integrated Environmental Assessment and Management, 18(5), pp. 1335–1347. [https://doi.org/10.1002/ieam.4572 doi: 10.1002/ieam.4572]</ref> to understand the potential for toxicity or the need to conduct toxicological testing or ecological evaluations.

'''Bioaccumulation of Inorganics by Aquatic Life:''' Peepers can also be used to understand site specific relationship between C0 and concentrations of inorganics in aquatic life. For example, measuring C0 in sediment from which organisms are collected and analyzed can enable the estimation of a site-specific uptake factor. This C0-to-organism uptake factor (or model) can then be applied for a variety of uses, including predicting the concentration of inorganics in other organisms, or estimating a sediment C0 value that would be safe for consumption by wildlife or humans. Because several decades of research have found that the correlation between C0 measurements and bioavailability is usually better than the correlation between measurements of chemicals in bulk sediment and bioavailability, C0-to-organism uptake factors are likely to be more accurate than uptake factors based on bulk sediment testing.

'''Evaluating Sediment Remediation Efficacy:''' Passive sampling has been used widely to evaluate the efficacy of remedial actions such as active amendments, thin layer placements, and capping to reduce the availability of contaminants at sediment sites. A particularly powerful approach is to compare baseline (pre-remedy) C0 in sediment to C0 in sediment after the sediment remedy has been applied. Peepers can be used in this context for inorganics, allowing the sediment remedy’s success to be evaluated and monitored in laboratory benchtop remedy evaluations, pilot scale remedy evaluations, and full-scale remediation monitoring.

==References==
<references />

==See Also==

*[https://vimeo.com/809180171/c276c1873a Peeper Deployment Video]
*[https://vimeo.com/811073634/303edf2693 Peeper Retrieval Video]
*[https://vimeo.com/811328715/aea3073540 Peeper Processing Video]
*[https://sepub-prod-0001-124733793621-us-gov-west-1.s3.us-gov-west-1.amazonaws.com/s3fs-public/2024-09/ER20-5261%20Fact%20Sheet.pdf?VersionId=malAixSQQM3mWCRiaVaxY8wLdI0jE1PX Fact Sheet]

In Situ Toxicity Identification Evaluation (iTIE)

2026-03-03T22:16:42Z

Admin:

The ''in situ'' Toxicity Identification Evaluation system is a tool to incorporate in weight-of-evidence studies at sites with numerous chemical toxicant classes present. The technology works by continuously sampling site water, immediately fractionating the water using diagnostic sorptive resins, and then exposing test organisms to the water to observe toxicity responses with minimal sample manipulation. It is compatible with various resins, test organisms, and common acute and chronic toxicity tests, and can be deployed at sites with a wide variety of physical and logistical considerations.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Contaminated Sediments - Introduction]]
*[[Contaminated Sediment Risk Assessment]]
*[[Passive Sampling of Sediments]]
*[[Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)]]

'''Contributor(s):''' Dr. G. Allen Burton Jr. and Austin Crane

'''Key Resource(s):'''
*A Novel In Situ Toxicity Identification Evaluation (iTIE) System for Determining which Chemicals Drive Impairments at Contaminated Sites<ref name="BurtonEtAl2020">Burton, G.A., Cervi, E.C., Meyer, K., Steigmeyer, A., Verhamme, E., Daley, J., Hudson, M., Colvin, M., Rosen, G., 2020. A novel In Situ Toxicity Identification Evaluation (iTIE) System for Determining which Chemicals Drive Impairments at Contaminated Sites. Environmental Toxicology and Chemistry, 39(9), pp. 1746-1754. [https://doi.org/10.1002/etc.4799 doi: 10.1002/etc.4799]</ref>
*An in situ toxicity identification and evaluation water analysis system: Laboratory validation<ref name="SteigmeyerEtAl2017">Steigmeyer, A.J., Zhang, J., Daley, J.M., Zhang, X., Burton, G.A. Jr., 2017. An in situ toxicity identification and evaluation water analysis system: Laboratory validation. Environmental Toxicology and Chemistry, 36(6), pp. 1636-1643. [https://doi.org/10.1002/etc.3696 doi: 10.1002/etc.3696]</ref>
*Sediment Toxicity Identification Evaluation (TIE) Phases I, II, and III Guidance Document<ref>United States Environmental Protection Agency, 2007. Sediment Toxicity Identification Evaluation (TIE) Phases I, II, and III Guidance Document, EPA/600/R-07/080. 145 pages. [https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P1003GR1.txt Free Download]  [[Media: EPA2007.pdf | Report pdf]]</ref>
*[https://serdp-estcp.mil/projects/details/88a8f9ba-542b-4b98-bfa4-f693435535cd/er18-1181-project-overview In Situ Toxicity Identification Evaluation (iTIE) Technology for Assessing Contaminated Sediments, Remediation Success, Recontamination and Source Identification - ESTCP Project ER18-1181]<ref>In Situ Toxicity Identification Evaluation (iTIE) Technology for Assessing Contaminated Sediments, Remediation Success, Recontamination and Source Identification- ESTCP Project ER18-1181 [[Media: ER18-1181Ph.II.pdf | Final Report]]</ref>

==Introduction==
In waterways impacted by numerous naturally occurring and anthropogenic chemical stressors, it is crucial for environmental practitioners to be able to identify which chemical classes are causing the highest degrees of toxicity to aquatic life. Previously developed methods, including the Toxicity Identification Evaluation (TIE) protocol developed by the US Environmental Protection Agency (EPA)<ref>Norberg-King, T., Mount, D.I., Amato, J.R., Jensen, D.A., Thompson, J.A., 1992. Toxicity identification evaluation: Characterization of chronically toxic effluents: Phase I. Publication No. EPA/600/6-91/005F. U.S. Environmental Protection Agency, Office of Research and Development. [https://www.epa.gov/sites/default/files/2015-09/documents/owm0255.pdf Free Download from US EPA]  [[Media: usepa1992.pdf | Report pdf]]</ref>, can be confounded by sample manipulation artifacts and temporal limitations of ''ex situ'' organism exposures<ref name="BurtonEtAl2020"/>. These factors may disrupt causal linkages and mislead investigators during site characterization and management decision-making. The ''in situ'' Toxicity Identification Evaluation (iTIE) technology was developed to allow users to strengthen stressor-causality linkages and rank chemical classes of concern at impaired sites, with high degrees of ecological realism.

The technology has undergone a series of improvements in recent years, with the most recent prototype being robust, operable in a wide variety of site conditions, and cost-effective compared to alternative site characterization methods<ref>Burton, G.A. Jr., Nordstrom, J.F., 2004. An in situ toxicity identification evaluation method part I: Laboratory validation. Environmental Toxicology and Chemistry, 23(12), pp. 2844-2850. [https://doi.org/10.1897/03-409.1 doi: 10.1897/03-409.1]</ref><ref>Burton, G.A. Jr., Nordstrom, J.F., 2004. An in situ toxicity identification evaluation method part II: Field validation. Environmental Toxicology and Chemistry, 23(12), pp. 2851-2855. [https://doi.org/10.1897/03-468.1 doi: 10.1897/03-468.1]</ref><ref name="BurtonEtAl2020"/><ref name="SteigmeyerEtAl2017"/>. The latest prototype can be used in any of the following settings: in marine, estuarine, or freshwater sites; to study surface water or sediment pore water; in shallow waters easily accessible by foot or in deep waters only accessible by pier or boat. It can be used to study sites impacted by a wide variety of stressors including ammonia, [[Metal and Metalloid Contaminants | metals]], pesticides, polychlorinated biphenyls (PCB), [[Polycyclic Aromatic Hydrocarbons (PAHs) | polycyclic aromatic hydrocarbons (PAH)]], and [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | per- and polyfluoroalkyl substances (PFAS)]], among others. The technology is applicable to studies of acute toxicity via organism survival or of chronic toxicity via responses in growth, reproduction, or gene expression<ref name="BurtonEtAl2020"/>.

==System Components and Validation==
[[File: CraneFig1.png | thumb | 600 px | Figure 1: A schematic diagram of the iTIE system prototype. The system is divided into three sub-systems: 1) the Pore Water/Surface Water Collection Sub-System (blue); 2) the Pumping Sub-System (red); and 3) the iTIE Resin, Exposure, and Sampling Sub-System (green). Water first enters the system through the Pore Water/Surface Water Collection Sub-System. Porewater can be collected using Trident-style probes, or surface water can be collected using a simple weighted probe. The water is composited in a manifold before being pumped to the rest of the iTIE system by the booster pump. Once in the iTIE Resin, Exposure, and Sampling Sub-System, the water is gently oxygenated by the Oxygen Coil, separated from gas bubbles by the Drip Chamber, and diverted to separate iTIE Resin and Exposure Chambers (or iTIE units) by the Splitting Manifold. Water movement through each iTIE unit is controlled by a dedicated Regulation Pump. Finally, the water is gathered in Sample Collection bottles for analysis.]]
The latest iTIE prototype consists of an array of sorptive resins that differentially fractionate sampled water, and a series of corresponding flow-through organism chambers that receive the treated water ''in situ''. Resin treatments can be selected depending on the chemicals suspected to be present at each site to selectively sequester certain chemical of concern (CoC) classes from the whole water, leaving a smaller subset of chemicals in the resulting water fraction for chemical and toxicological characterization. Test organism species and life stages can also be chosen depending on factors including site characteristics and study goals. In the full iTIE protocol, site water is continuously sampled either from the sediment pore spaces or the water column at a site, gently oxygenated, diverted to different iTIE units for fractionation and organism exposure, and collected in sample bottles for off-site chemical analysis (Figure 1). All iTIE system components are housed within waterproof Pelican cases, which allow for ease of transport and temperature control.

===Porewater and Surface Water Collection Sub-system===
[[File: CraneFig2.png | thumb | 600 px | Figure 2: a) Trident probe with auxiliary sensors attached, b) a Trident probe with end caps removed (the red arrow identifies the intermediate space where glass beads are packed to filter suspended solids), c) a Trident probe being installed using a series of push poles and a fence post driver]]
Given the importance of sediment porewater to ecosystem structure and function, investigators may employ the iTIE system to evaluate the toxic effects associated with the impacted sediment porewater. To accomplish this, the iTIE system utilizes the Trident probe, originally developed for Department of Defense site characterization studies<ref>Chadwick, D.B., Harre, B., Smith, C.F., Groves, J.G., Paulsen, R.J., 2003. Coastal Contaminant Migration Monitoring: The Trident Probe and UltraSeep System. Hardware Description, Protocols, and Procedures. Technical Report 1902. Space and Naval Warfare Systems Center.</ref>. The main body of the Trident is comprised of a stainless-steel frame with six hollow probes (Figure 2). Each probe contains a layer of inert glass beads, which filters suspended solids from the sampled water. The water is drawn through each probe into a composite manifold and transported to the rest of the iTIE system using a high-precision peristaltic pump.

The Trident also includes an adjustable stopper plate, which forms a seal against the sediment and prevents the inadvertent dilution of porewater samples with surface water. (Figure 2). Preliminary laboratory results indicate that the Trident is extremely effective in collecting porewater samples with minimal surface water infiltration in sediments ranging from coarse sand to fine clay. Underwater cameras, sensors, passive samplers, and other auxiliary equipment can be attached to the Trident probe frame to provide supplemental data.

Alternatively, practitioners may employ the iTIE system to evaluate site surface water. To sample surface water, weighted intake tubes can collect surface water from the water column using a peristaltic pump.

===Oxygen Coil, Overflow Bag and Drip Chamber===
[[File: CraneFig3.png | thumb | left | 400 px | Figure 3. Contents of the iTIE system cooler. The pictured HDPE rack (47.6 cm length x 29.7 cm width x 33.7 cm height) is removable from the iTIE cooler. Water enters the system at the red circle, flows through the oxygen coil, and then travels to each of the individual iTIE units where diagnostic resins and organisms are located. The water then briefly leaves the cooler to travel through peristaltic regulation pumps before being gathered in sample collection bottles.]]
Porewater is naturally anoxic due to limited mixing with aerated surface water and high oxygen demand of sediments, which may cause organism mortality and interfere with iTIE results. To preclude this, sampled porewater is exposed to an oxygen coil. This consists of an interior silicone tube connected to a pressurized oxygen canister, threaded through an exterior reinforced PVC tube through which water is slowly pumped (Figure 3). Pump rates are optimized to ensure adequate aeration of the water. In addition to elevating DO levels, the oxygen coil facilitates the oxidation of dissolved sulfides, which naturally occur in some marine sediments and may otherwise cause toxicity to organisms if left in its reduced form.

Gas bubbles may form in the oxygen coil over the course of a deployment. These can be disruptive, decreasing water sample volumes and posing a danger to sensitive organisms like daphnids. To account for this, the water travels to a drip chamber after exiting the oxygen coil, which allows gas bubbles to be separated and diverted to an overflow system. The sample water then flows to a manifold which divides the flow into different paths to each of the treatment units for fractionation and organism exposure.

===iTIE Units: Fractionation and Organism Exposure Chambers===
[[File: CraneFig4.png | thumb | 300px | Figure 4. A diagram of the iTIE prototype. Water flows upward into each resin chamber through the unit bottom. After being chemically fractionated in the resin chamber, water travels into the organism chamber, where test organisms have been placed. Water is drawn through the units by high-precision peristaltic pumps.]]
At the core of the iTIE system are separate dual-chamber iTIE units, each with a resin fractionation chamber and an organism exposure chamber (Figure 4). Developed by Burton ''et al.''<ref name="BurtonEtAl2020"/>, the iTIE prototype is constructed from acrylic, with rubber O-rings to connect each piece. Each iTIE unit can contain a different diagnostic resin matrix, customizable to remove specific chemical classes from the water. Sampled water flows into each unit through the bottom and is differentially fractionated by the resin matrix as it travels upward. Then it reaches the organism chamber, where test organisms are placed for exposure. The organism chamber inlet and outlet are covered by mesh to prevent the escape of the test organisms. This continuous flow-through design allows practitioners to capture the temporal heterogeneity of ambient water conditions over the duration of an ''in situ'' exposure. Currently, the iTIE system can support four independent iTIE treatment units.

After being exposed to test organisms, water is collected in sample bottles. The bottles can be pre-loaded with preservation reagents to allow for later chemical analysis. Sample bottles can be composed of polyethylene, glass or other materials depending on the CoC.

===Pumping Sub-system===
[[File: CraneFig5.png | thumb | 300px | Figure 5. The iTIE system pumping sub-system. The sub-system consists of: A) a single booster pump, which is directly connected to the water sampling device and feeds water to the rest of the iTIE system, and B) a set of four regulation pumps, which each connect to the outflow of an individual iTIE unit. Each pump set is housed in a waterproof case with self-contained rechargeable battery power. A tablet is mounted inside the lid of the four pump case, which can be used to program and operate all of the pumps when connected to the internet.]]
Water movement through the system is driven by a series of high-precision, programmable peristaltic pumps ([https://ecotechmarine.com/ EcoTech Marine]). Each pump set is housed in a Pelican storm travel case. Power is supplied to each pump by internal rechargeable lithium-iron phosphate batteries ([https://www.bioennopower.com/ Bioenno Power]).

First, water is supplied to the system by a booster pump (Figure 5A). This pump is situated between the water sampling sub-system and the oxygen coil. The booster pump: 1) facilitates pore water collection, especially from sediments with high fine particle fractions; 2) helps water overcome vertical lifts to travel to the iTIE system; and 3) prevents vacuums from forming in the iTIE system interior, which can accelerate the formation of disruptive gas bubbles in the oxygen coil. The booster pump should be programmed to supply an excess of water to prevent vacuum formation.

Second, a set of four regulation pumps ensure precise flow rates through each independent iTIE unit (Figure 5B). Each regulation pump pulls water from the top of an iTIE unit and then dispenses that water into a sample bottle for further analysis.

==Study Design Considerations==
===Diagnostic Resin Treatments===
Several commercially available resins have been verified for use in the iTIE system. Investigators can select resins based on stressor classes of interest at each site. Each resin selectively removes a CoC class from site water prior to organism exposure.
*[https://www.dupont.com/products/ambersorb560.html DuPont Ambersorb 560] for removal of 1,4-dioxane and other organic chemicals<ref>Woodard, S., Mohr, T., Nickelsen, M.G., 2014. Synthetic media: A promising new treatment technology for 1,4-dioxane. Remediation Journal, 24(4), pp. 27-40. [https://doi.org/10.1002/rem.21402 doi: 10.1002/rem.21402]</ref>
*C18 for nonpolar organic chemicals
*[https://www.bio-rad.com/en-us Bio-Rad] [https://www.bio-rad.com/en-us/product/chelex-100-resin?ID=6448ab3e-b96a-4162-9124-7b7d2330288e Chelex] for metals
*Granular activated carbon for metals, general organic chemicals, sulfide<ref>Lemos, B.R.S., Teixeira, I.F., de Mesquita, J.P., Ribeiro, R.R., Donnici, C.L., Lago, R.M., 2012. Use of modified activated carbon for the oxidation of aqueous sulfide. Carbon, 50(3), pp. 1386-1393. [https://doi.org/10.1016/j.carbon.2011.11.011 doi: 10.1016/j.carbon.2011.11.011]</ref>
*[https://www.waters.com/nextgen/us/en.html Waters] [https://www.waters.com/nextgen/us/en/search.html?category=Shop&isocode=en_US&keyword=oasis%20hlb&multiselect=true&page=1&rows=12&sort=best-sellers&xcid=ppc-ppc_23916&gad_source=1&gad_campaignid=14746094146&gbraid=0AAAAAD_uR00nhlNwrhhegNh06pBODTgiN&gclid=CjwKCAiAtLvMBhB_EiwA1u6_PsppE0raci2IhvGnAAe5ijciNcetLaGZo5qA3g3r4Z_La7YAPJtzShoC6LoQAvD_BwE Oasis HLB] for general organic chemicals<ref name="SteigmeyerEtAl2017"/>
*[https://www.waters.com/nextgen/us/en.html Waters] [https://www.waters.com/nextgen/us/en/search.html?category=All&enableHL=true&isocode=en_US&keyword=Oasis%20WAX%20&multiselect=true&page=1&rows=12&sort=most-relevant Oasis WAX] for PFAS, organic chemicals of mixed polarity<ref>Iannone, A., Carriera, F., Di Fiore, C., Avino, P., 2024. Poly- and Perfluoroalkyl Substance (PFAS) Analysis in Environmental Matrices: An Overview of the Extraction and Chromatographic Detection Methods. Analytica, 5(2), pp. 187-202. [https://doi.org/10.3390/analytica5020012 doi: 10.3390/analytica5020012]  [[Media: IannoneEtAl2024.pdf | Article pdf]]</ref>
*Zeolite for ammonia, other organic chemicals

Resins must be adequately conditioned prior to use. Otherwise, they may inadequately adsorb toxicants or cause stress to organisms. New resins should be tested for efficacy and toxicity before being used in an iTIE system.

===Test Organism Species and Life Stages===
Practitioners can also select different organism species and life stages for use in the iTIE system, depending on site characteristics and study goals. The iTIE system can accommodate various small test organisms, including embryo-stage fish and most macroinvertebrates. The following common toxicity tests can be adapted for application within iTIE systems<ref>U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, 1994. Catalogue of Standard Toxicity Tests for Ecological Risk Assessment. ECO Update, 2(2), 4 pages. Publication No. 9345.0.05I [https://www.epa.gov/sites/default/files/2015-09/documents/v2no2.pdf Free Download]  [[Media: usepa1994.pdf | Report pdf]]</ref>.
<ul>Freshwater acute toxicity:</ul>
*[[Wikipedia: Daphnia magna | ''Daphnia magna'']] or [[Wikipedia: Daphnia pulex | ''Daphnia pulex'']] 24-, 48-, and 96-hour survival
<ul>Freshwater chronic toxicity:</ul>
*[[Wikipedia: Ceriodaphnia dubia | ''Ceriodaphnia dubia'']] 7-day survival and reproduction
*''D. magna'' 7-day survival and reproduction
*[[Wikipedia: Fathead minnow | ''Pimephales promelas'']] 7-day embryo-larval survival and teratogenicity
*[[Wikipedia: Hyalella azteca | ''Hyalella Azteca'']] 10- or 30-day survival and reproduction
<ul>Marine acute toxicity:</ul>
*[[Wikipedia: Americamysis bahia | ''Americamysis bahia'']] 24- and 48-hour survival
<ul>Marine chronic toxicity:</ul>
*''Americamysis'' survival, growth and fecundity
*[[Wikipedia: Topsmelt silverside | ''Atherinops affinis'']] embryo-larval survival and growth

Acute toxicity is quantifiable via organism survival rates immediately following the termination of an iTIE system field deployment. Chronic toxicity can be quantified by continuing to culture and observe test organisms in-lab. Common chronic endpoints include stunted growth, altered development such as teratogenicity in larval fish, decreased reproduction rates, and changes in gene expression.

Several gene expression endpoints have been detectable in bioassays following an iTIE system deployment and in-lab culturing period. Steigmeyer ''et al.''<ref name="SteigmeyerEtAl2017"/> were able to detect changes in the expression of two genes in ''D. magna'' after a 24-hour exposure to bisphenol A. In a separate study, Nichols<ref>Nichols, E., 2023. Methods for Identification and Prioritization of Stressors at Impaired Sites. Masters thesis, University of Michigan. University of Michigan Library Deep Blue Documents. [https://deepblue.lib.umich.edu/bitstream/handle/2027.42/176142/Nichols_Elizabeth_thesis.pdf?sequence=1 Free Download]  [[Media: Nichols2023.pdf | Report pdf]]</ref> found a significant decline in acetylcholinesterase activity in ''H. azteca'' after a 24-hour exposure to chlorpyrifos. These results indicate a potential to adapt other gene expression bioassays for use in conjunction with iTIE system field exposures to prove stressor-causality linkages.

===Cost Effectiveness Study===
Burton ''et al.''<ref name="BurtonEtAl2020"/> conducted a cost effectiveness study comparing the iTIE technology with the traditional US EPA Phase 1 TIE method. Comparisons were based on the estimated time required to complete various sub-tasks within each method. Sub-tasks included organism care, equipment preparation, mobilization and deployment, test maintenance, test termination, demobilization, and test termination analyses. It was ultimately estimated that the iTIE protocol requires 47% less time (67 fewer hours) to complete than the Phase 1 TIE method, with the largest time differences in equipment preparation, deployment, test maintenance, and demobilization. It is important to note that the iTIE method may require additional initial costs for equipment and training.

==Field Application==
[[File: CraneFig6.png | thumb | left | 400px | Figure 6. iTIES deployment at the Rouge River, Detroit, MI. In the foreground is the iTIE Cooler Sub-System, which contains iTIE resin treatments and test organism groups, as well as the oxygenation coil and sample collection bottles. Next to the iTIE Cooler are the two pump cases. The Trident can be seen above the pump cases, installed in the river channel near shore.]]
The iTIE system has been successfully deployed at a variety of marine and freshwater sites during the proof-of-concept phase of prototype development. One example is the 2024 iTIE system deployment completed near the mouth of the Rouge River in Detroit, MI (Figure 6). The Rouge River watershed has a long history of industrialization, with a legacy of chemical dumping, channelization, damming, and urban runoff<ref>Ridgway, J., Cave, K., DeMaria, A., O’Meara, J., Hartig, J. H., 2018. The Rouge River Area of Concern—A multi-year, multi-level successful approach to restoration of Impaired Beneficial Uses. Aquatic Ecosystem Health and Management, 21(4), pp. 398-408. [https://doi.org/10.1080/14634988.2018.1528816 doi: 10.1080/14634988.2018.1528816]</ref>. This has led to degraded environmental conditions, with previous detections of a wide range of chemicals including heavy metals and various organics.

[[File: CraneFig7.png | thumb | 300px | Figure 7. Survival and healthy development of ''P. promelas'' embryos and larvae following a 48-hour iTIE exposure near the mouth of the Rouge River. Organisms were exposed to site porewater as embryos for 48 hours and cultured post-exposure for an additional 5 days.]]
[[File: CraneFig8.png | thumb | 300px | Figure 8. Survival of ''C. dilutus'' larvae after an iTIE exposure near the mouth of the Rouge River. Organisms were exposed to site porewater for 48 hours and cultured post-exposure for an additional 5 days. Error bars show standard deviation.]]
An iTIE system deployment was designed and completed to determine which chemical classes are most responsible for causing toxicity at the site. Resin treatments included glass wool (inert, non-fractionating substance), Chelex (metals sorption), Oasis HLB (general organics sorption), and Oasis WAX (organics sorption, with a high affinity for PFAS). The study utilized fathead minnow (''P. promelas'') embryos, due to their relative sensitivity to metals and PAHs, as well as second-instar midge ([[Wikipedia: Chironomus |''Chironomus dilutus'']]) larvae due to their relative sensitivity to PFAS.

The test organisms were exposed to fractionated porewater ''in situ'' for 48 hours. Following exposure, organisms were cultured for an additional five days, and survival was recorded (Figures 7 and 8). Moderate declines in survival were seen in both species in the glass wool treatment, indicating toxicity at the site. For ''P. promelas'', the highest proportion of healthy development occurred in the Chelex treatment, supporting the hypothesis that metals are a dominant cause of toxicity. ''C. dilutus'' had the greatest survival in the Oasis WAX treatment, suggesting that an organic stressor class like PFAS is also present at harmful concentrations in the river.

Water chemical analyses of fractionated and unfractionated water samples were completed to support biological results. Analyses were conducted for a range of stressor classes including metals, PAHs, PCBs, an organophosphate pesticide (chlorpyrifos), a PFAS compound (PFOS) and a pyrethroid insecticide (permethrin). Of these analytes, only heavy metals and PFOS were detected. Some chemical classes including PAHs and PCBs were not detected at the site.
To reach similar conclusions using traditional Phase 1 TIE methods, one would need to complete the following tests: baseline toxicity, filtration, aeration, EDTA, C18 SPE, and methanol elution of C18 SPE. The iTIE method allows the same conclusions to be drawn with significantly less time and effort required.

==Summary==
The ''in situ'' Toxicity Identification Evaluation technology and protocol is a powerful tool that investigators can use to strengthen causal linkages between chemical stressors and ecological toxicity. By fractionating sampled water and exposing test organisms ''in situ'', investigators can gather toxicity response data while minimizing sample manipulation and accurately representing environmental conditions.
 

==References==
<references />

==See Also==