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A Conceptual Site Model (CSM) is a collection of information about a contaminated site that integrates the available evidence regarding its hydrogeologic setting, contaminant sources, exposure pathways, potential receptors, and site history.  A CSM for a [[Wikipedia: Light non-aqueous phase liquid | Light Non-Aqueous Phase Liquid (LNAPL)]] site focuses on several key concepts:  the stage in the LNAPL site life cycle, LNAPL distribution in the subsurface and the resulting mobility of the LNAPL, LNAPL as a source of dissolved and vapor plumes, and the attenuation of LNAPL sources over time.
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==Assessing Vapor Intrusion (VI) Impacts in Neighborhoods with Groundwater Contaminated by Chlorinated Volatile Organic Chemicals (CVOCs)==
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The VI Diagnosis Toolkit<ref name="JohnsonEtAl2020">Johnson, P.C., Guo, Y., Dahlen, P., 2020.  The VI Diagnosis Toolkit for Assessing Vapor Intrusion Pathways and Mitigating Impacts in Neighborhoods Overlying Dissolved Chlorinated Solvent Plumes.  ESTCP Project ER-201501, Final Report. [https://serdp-estcp.mil/projects/details/a0d8bafd-c158-4742-b9fe-5f03d002af71 Project Website]&nbsp;&nbsp; [//www.enviro.wiki/images/2/28/ER-201501.pdf  Final Report.pdf]</ref> is a set of tools that can be used individually or in combination to assess vapor intrusion (VI) impacts at one or more buildings overlying regional-scale dissolved chlorinated solvent impacted groundwater plumes. The strategic use of these tools can lead to confident and efficient neighborhood-scale VI pathway assessments.
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<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
 
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
  
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'''Related Article(s)'''
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'''Related Article(s):'''
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* [[LNAPL Remediation Technologies]]
 
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* [[NAPL Mobility]]
 
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* [[Natural Source Zone Depletion (NSZD)]]
 
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* [[Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill]]
 
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* [[Monitored Natural Attenuation (MNA)]]
 
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* [[Biodegradation - Hydrocarbons]]
 
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'''CONTRIBUTOR(S):''' [[Dr. Charles Newell, P.E. | Charles Newell]]
 
  
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'''Key Resource(s):'''
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*[[Vapor Intrusion (VI)]]
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* LNAPL Site Management: LCSM Evolution, Decision Process, and Remedial Technologies. LNAPL-3. ITRC.<ref name="LNAPL-3">Interstate Technology and Regulatory Council (ITRC), 2018. LNAPL Site Management: LCSM Evolution, Decision Process, and Remedial Technologies. LNAPL-3. ITRC, LNAPL Update Team, Washington, DC.  [https://lnapl-3.itrcweb.org LNAPL-3 Website]</ref>
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*[[Vapor Intrusion – Sewers and Utility Tunnels as Preferential Pathways]]
  
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* Managing Risk at LNAPL Sites - Frequently Asked Questions, 2nd Edition. API.<ref name="Sale2018"> Sale, T., Hopkins, H., and Kirkman, A., 2018.  Managing Risk at LNAPL Sites - Frequently Asked Questions, 2nd Edition. American Petroleum Institute (API), Washington, DC. 72 pages. [https://www.api.org/oil-and-natural-gas/environment/clean-water/ground-water/lnapl/lnapl-faqs Free download from API.] [https://www.enviro.wiki/index.php?title=File:Sale-2018_LNAPL_FAQs_2nd_ed.pdf Report.pdf]</ref>
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'''Contributor(s):'''
  
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==Life Cycle of LNAPL Sites==
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*Paul C. Johnson, Ph.D.
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[[File:Newell1w2Fig1.png |thumb|left|250px| Figure 1. Early, Middle, and Late Stage LNAPL releases<ref name= "Sale2018"/>. The key distinctions are the presence of continuous LNAPL that can be mobile and the amount of time that has elapsed for NSZD to remove LNAPL.]]
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*Paul Dahlen, Ph.D.
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A Conceptual Site Model (CSM) is a collection of information about a contaminated site that integrates the available evidence regarding its hydrogeologic setting, contaminant sources, exposure pathways, potential receptors, and site history (see ASTM E1689-95(2014)<ref name="ASTM2014a"> ASTM, 2014. Standard Guide for Developing Conceptual Site Models for Contaminated Sites. ASTM E1689-95(2014), ASTM International, West Conshohocken, PA. [https://doi.org/10.1520/E1689-95R14 DOI: 10.1520/E1689-95R14]  http://www.astm.org/cgi-bin/resolver.cgi?E1689</ref> and ASTM E2531-06(2014)<ref name="ASTM2014b"> ASTM, 2014. Standard Guide for Development of Conceptual Site Models and Remediation Strategies for Light Nonaqueous-Phase Liquids Released to the Subsurface. ASTM E2531-06(2014), ASTM International, West Conshohocken, PA. [https://doi.org/10.1520/E2531-06R14  DOI: 10.1520/E2531-06R14]  http://www.astm.org/cgi-bin/resolver.cgi?E2531</ref>).  When developing a CSM for an LNAPL site, it is important to understand that LNAPL releases evolve and change from what are referred to as Early Stage sites to Middle Stage and then to Late Stage sites<ref name="Sale2018"/> (Figure 1).
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*Yuanming Guo, Ph.D.
  
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An Early Stage site is characterized by the presence of a continuous LNAPL zone where a thick layer of LNAPL accumulation (also known as free product) is observed in monitoring wells. The continuous LNAPL zone (or LNAPL body) may be mobile at Early Stage sites, migrating into previously non-impacted areas. Removal of significant LNAPL mass by active pumping may be feasible at these sites. Early Stage sites are now relatively rare in the United States due to stringent environmental regulations enacted in the 1980s which emphasized preventing releases.
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'''Key Resource(s):'''
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[[File:Newell1w2Fig2a.png |thumb|500px| Figure 2a.  Time lapse conceptualization of the formation of an LNAPL body<ref name="ITRC2019"> Interstate Technology and Regulatory Council (ITRC), 2019. LNAPL Training: Connecting the Science to Managing Sites. Part 1: Understanding LNAPL Behavior in the Subsurface. ITRC, Washington, DC. [[Media: ITRC2019_LNAPLtrainingPart1.pdf | Slides.pdf]]</ref>.]]
 
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[[File:Newell1w2Fig2b.png |thumb|500px| Figure 2b.  Sand tank experiment of an LNAPL release<ref name="ITRC2019"/>.]]
 
  
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Many sites in the U.S. are now considered to be in the Middle Stage, where the LNAPL thickness in wells has been largely depleted by natural spreading of the LNAPL body, [[Natural Source Zone Depletion (NSZD)]], smearing of the water table, and/or active remediation, and where the LNAPL bodies are stable or shrinking<ref name="LNAPL-3"/><ref name="Sale2018"/> (Figure 1).  Active pumping characteristically only recovers LNAPL at relatively low rates of under 100 gallons per acre per year at Middle Stage sites, but NSZD rates may be much higher, on the order of 100s to 1,000s of gallons per acre per year.  Middle Stage dissolved phase plumes, typically comprised of monoaromatics such as benzene, toluene, ethyl benzene, and xylenes, are stable or shrinking over time.
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*The VI Diagnosis Toolkit for Assessing Vapor Intrusion Pathways and Impacts in Neighborhoods Overlying Dissolved Chlorinated Solvent Plumes, ESTCP Project ER-201501, Final Report<ref name="JohnsonEtAl2020" />
  
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Late Stage sites only have a sparse distribution of residual (trapped) LNAPL due to long-term NSZD and any active remediation that has been performed at the site.  The potential risks to receptors are typically low at Late Stage sites due to relatively low concentrations of LNAPL constituents in the dissolved phase and/or vapor plumes.
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*CPM Test Guidelines: Use of Controlled Pressure Method Testing for Vapor Intrusion Pathway Assessment, ESTCP Project ER-201501, Technical Report<ref name="JohnsonEtAl2021">Johnson, P.C., Guo, Y., Dahlen, P., 2021.  CPM Test Guidelines: Use of Controlled Pressure Method Testing for Vapor Intrusion Pathway Assessment.  ESTCP ER-201501, Technical Report. [https://serdp-estcp.mil/projects/details/a0d8bafd-c158-4742-b9fe-5f03d002af71 Project Website]&nbsp;&nbsp; [//www.enviro.wiki/images/c/c7/ER-201501_Technical_Report.pdf  Technical_Report.pdf]</ref>
  
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==LNAPL Body Formation==
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*VI Diagnosis Toolkit User Guide, ESTCP Project ER-201501<ref name="JohnsonEtAl2022">Johnson, P.C., Guo, Y., and Dahlen, P., 2022. VI Diagnosis Toolkit User Guide, ESTCP ER-201501, User Guide.  [https://serdp-estcp.mil/projects/details/a0d8bafd-c158-4742-b9fe-5f03d002af71 Project Website]&nbsp;&nbsp; [//www.enviro.wiki/images/2/22/ER-201501_User_Guide.pdf  User_Guide.pdf]</ref>
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LNAPLs released from tanks, pits, pipelines, or other sources will percolate downwards under the influence of gravity through permeable pathways in the unsaturated zone (e.g., soil pore space, fractures, and macropores) depending on the volume and pressure head of the LNAPL release, until encountering an impermeable layer or the water table, causing the LNAPL body to spread laterally.  The Interstate Technology and Regulatory Council (ITRC)<ref name="LNAPL-3"/> describes this downward movement toward the water table this way:
 
  
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<blockquote>''During the downward movement of LNAPL through the soil, the presence of confining layers, subsurface heterogeneities, or other preferential pathways may result in irregular and complex lateral spreading and/or perching of LNAPL before the water table is encountered. Once at the water table, the LNAPL will spread laterally in a radial fashion as well as penetrate vertically downward into the saturated zone, displacing water to some depth proportional to the driving force of the vertical LNAPL column (or LNAPL head). The vertical penetration of LNAPL into the saturated zone will continue to occur as long as the downward force produced by the LNAPL head or pressure from the LNAPL release exceeds the counteracting forces produced by the resistance of the soil matrix and the buoyancy resulting from the density difference between LNAPL and groundwater.''<ref name="LNAPL-3"/></blockquote>
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==Introduction==
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Most federal, state, and local regulatory guidance for assessing and mitigating the [[Vapor Intrusion (VI) | vapor intrusion]] pathway reflects USEPA’s ''Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air''<ref name="USEPA2015">USEPA, 2015. OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air. U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, OSWER Publication No. 9200.2-154, 267 pages. [https://www.epa.gov/vaporintrusion/technical-guide-assessing-and-mitigating-vapor-intrusion-pathway-subsurface-vapor USEPA Website]&nbsp;&nbsp; [//www.enviro.wiki/images/0/06/USEPA2015.pdf  Report.pdf]</ref>. The paradigm outlined by that guidance includes: 1) a preliminary and mostly qualitative analysis that looks for site conditions that suggest vapor intrusion might occur (e.g., the presence of vapor-forming chemicals in close proximity to buildings); 2) a multi-step and more detailed quantitative screening analysis that involves site-specific data collection and their comparison to screening levels to identify buildings of potential VI concern; and 3) selection and design of mitigation systems or continued monitoring, as needed. With respect to (2), regulatory guidance typically recommends consideration of “multiple lines of evidence” in decision-making<ref name="USEPA2015" /><ref>NJDEP, 2021. Vapor Intrusion Technical Guidance, Version 5.0. New Jersey Department of Environmental Protection, Trenton, NJ. [https://dep.nj.gov/srp/guidance/vapor-intrusion/vig/ Website]&nbsp;&nbsp; [//www.enviro.wiki/images/e/ee/NJDEP2021.pdf  Guidance Document.pdf]</ref>, with typical lines-of-evidence being groundwater, soil gas, sub-slab soil gas, and/or indoor air concentrations.  Of those, soil gas measurements and/or measured short-term indoor air concentrations can be weighted heavily, and therefore decision making might not be completed without them. Effective evaluation of VI risk from sub-slab and/or soil gas measurements would require an unknown building-specific attenuation factor, but there is also uncertainty as to whether or not indoor air data is representative of maximum and/or long-term average indoor concentrations. Indoor air data can be confounded by indoor contaminant sources because the number of samples is typically small, indoor concentrations can vary with time, and because a number of household products can emit the chemicals being measured. When conducting VI pathway assessments in neighborhoods where it is impractical to assess all buildings, the EPA recommends following a “worst first” investigational approach.
  
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While the release at the surface is still active, the LNAPL body can expand until the LNAPL addition rate is equal to the NSZD depletion rate.  However, once the release at the surface is stopped, the expansion will stop relatively quickly, and the LNAPL body will stabilize.  Figure 2a shows a conceptual depiction of this release scenario and Figure 2b shows a sand tank experiment of an LNAPL release.  Because of the buoyancy effects, LNAPL releases that reach the water table will form LNAPL bodies that “like icebergs, are partially above and below the water table”.<ref name="Sale2018"/>
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The limitations of this approach, as practiced, are the following:
  
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==Key Implications of the LNAPL Conceptual Site Model==
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*Decisions are rarely made without indoor air data and generally, seasonal sampling is required, delaying decision making.
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The nature of multi-phase flow processes in porous media (e.g., the interaction of LNAPL, water, and air in the pore spaces of an unconsolidated aquifer) has several important implications for environmental professionals in areas including interpretation of LNAPL thickness in monitoring wells and assessment of the long-term risk associated with LNAPL source zones.  A few of the key implications are described below.
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*The collection of a robust indoor air data set that adequately characterizes long term indoor air concentrations could take years given the typical frequency of data collection and the most common methods of sample collection (e.g., 24-hour samples).  Therefore, indoor air sampling might continue indefinitely at some sites.
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*The “worst first” buildings might not be identified correctly by the logic outlined in USEPA’s 2015 guidance and the most impacted buildings might not even be located over a groundwater plume.  Recent studies have shown [[Vapor Intrusion – Sewers and Utility Tunnels as Preferential Pathways |VI impacts in homes as a result of sewer and other subsurface piping connections]], which are not explicitly considered nor easily characterized through conventional VI pathway assessment<ref> Beckley, L, McHugh, T., 2020. A Conceptual Model for Vapor Intrusion from Groundwater Through Sewer Lines. Science of the Total Environment, 698, Article 134283. [https://doi.org/10.1016/j.scitotenv.2019.134283 doi: 10.1016/j.scitotenv.2019.134283]&nbsp;&nbsp; [//www.enviro.wiki/images/4/4e/BeckleyMcHugh2020.pdf  Open Access Article]</ref><ref name="GuoEtAl2015">Guo, Y., Holton, C., Luo, H., Dahlen, P., Gorder, K., Dettenmaier, E., Johnson, P.C., 2015. Identification of Alternative Vapor Intrusion Pathways Using Controlled Pressure Testing, Soil Gas Monitoring, and Screening Model Calculations. Environmental Science and Technology, 49(22), pp. 13472–13482. [https://doi.org/10.1021/acs.est.5b03564 doi: 10.1021/acs.est.5b03564]</ref><ref name="McHughEtAl2017">McHugh, T., Beckley, L., Sullivan, T., Lutes, C., Truesdale, R., Uppencamp, R., Cosky, B., Zimmerman, J., Schumacher, B., 2017.  Evidence of a Sewer Vapor Transport Pathway at the USEPA Vapor Intrusion Research Duplex.  Science of the Total Environment, pp. 598, 772-779. [https://doi.org/10.1016/j.scitotenv.2017.04.135 doi: 10.1016/j.scitotenv.2017.04.135]&nbsp;&nbsp; [//www.enviro.wiki/images/6/63/McHughEtAl2017.pdf  Open Access Manuscipt]</ref><ref name="McHughBeckley2018">McHugh, T., Beckley, L., 2018. Sewers and Utility Tunnels as Preferential Pathways for Volatile Organic Compound Migration into Buildings: Risk Factors and Investigation Protocol. ESTCP ER-201505, Final Report. [https://serdp-estcp.mil/projects/details/f12abf80-5273-4220-b09a-e239d0188421 Project Website]&nbsp;&nbsp; [//www.enviro.wiki/images/5/55/2018b-McHugh-ER-201505_Conceptual_Model.pdf Final Report.pdf]</ref><ref name="RiisEtAl2010">Riis, C., Hansen, M.H., Nielsen, H.H., Christensen, A.G., Terkelsen, M., 2010. Vapor Intrusion through Sewer Systems: Migration Pathways of Chlorinated Solvents from Groundwater to Indoor Air. Seventh International Conference on Remediation of Chlorinated and Recalcitrant Compounds, May, Monterey, CA. Battelle Memorial Institute. ISBN 978-0-9819730-2-9. [https://www.battelle.org/conferences/battelle-conference-proceedings Website]&nbsp;&nbsp; [//www.enviro.wiki/images/9/95/2010-Riis-Migratioin_pathways_of_Chlorinated_Solvents.pdf  Report.pdf]</ref>.
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*The presumptive remedy for VI mitigation (sub-slab depressurization) may not be effective for all VI scenarios (e.g., those involving vapor migration to indoor spaces via sewer connections).
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The '''VI Diagnosis Toolkit''' components were developed considering these limitations as well as more recent knowledge gained through research, development, and validation projects funded by SERDP and ESTCP.
  
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===Three States of LNAPL===
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==The VI Diagnosis Toolkit Components==
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LNAPL can be found in the subsurface in three different states:
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[[File:DahlenFig1.png|thumb|450px|Figure 1. Vapor intrusion pathway conceptualization considering “alternate VI pathways”, including “pipe flow
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VI” and “sewer VI” pathways<ref name="JohnsonEtAl2020" />.]]
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The primary components of the VI Diagnosis Toolkit and their uses include:
  
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# '''Residual LNAPL''' is trapped and immobile but can undergo composition and phase changes and generate dissolved hydrocarbon plumes in saturated zones and/or vapors in unsaturated zones. The fraction of the total pore space occupied by this discontinuous LNAPL is referred to as the residual saturation, with other phases such as water and air in the remainder of the pore space.
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*'''External VI source strength screening''' to identify buildings most likely to be impacted by VI at levels warranting building-specific testing.
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# '''Mobile LNAPL''' is LNAPL at greater than the residual saturation. Mobile LNAPL can accumulate in a well and is potentially recoverable, but is not migrating (i.e., the LNAPL body is not expanding).
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*'''Indoor air source screening''' to locate and remove indoor air sources that might confound building specific VI pathway assessment.
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# '''Migrating LNAPL''' is LNAPL at greater than the residual concentration which is observed to expand into previously non-impacted locations over time (e.g., LNAPL appears in a monitoring well that had previously been clean).
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*'''Controlled pressurization method (CPM)''' testing to quickly (in a few days or less) measure the worst-case indoor air impact likely to be caused by VI under natural conditions in specific buildings. CPM tests can also be used to identify the presence of indoor air sources and diagnose active VI pathways.
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*'''Passive indoor sampling''' for determining long-term average indoor air concentrations under natural VI conditions and/or for verifying mitigation system effectiveness in buildings that warrant VI mitigation.
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*'''Comprehensive VI conceptual model development and refinement''' to ensure that appropriate monitoring, investigation, and mitigation strategies are being selected (Figure 1).
  
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These three LNAPL states can cause different concerns and in some cases require different remediation goals.  
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Expanded discussions for each of these are given below.
  
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===LNAPL “Apparent Thickness” is a Poor Metric for Risk Management===
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'''External VI source strength screening''' identifies those buildings that warrant more intrusive building-specific assessments, using data collected exterior to the buildings. The use of groundwater and/or soil gas concentration data for building screening has been part of VI pathway assessments for some time and their use is discussed in many regulatory guidance documents. Typically, the measured concentrations are compared to relevant screening levels derived via modeling or empirical analyses from indoor air concentrations of concern.
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[[File:Newell1w2Fig3.png |thumb|left|600px| Figure 3.  Five LNAPL Thickness Scenarios for five different physical settings<ref name="Sale2018"/>.]]
 
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[[File:Newell1w2Fig4.png |thumb|350px| Figure 4.  Apparent LNAPL thickness versus LNAPL transmissivity, showing no correlation<ref name="Hawthorne2015">Hawthorne, J.M., 2015. Nationwide (USA) Statistical Analysis of LNAPL Transmissivity, in: R. Darlington and A.C. Barton (Chairs), Bioremediation and Sustainable Environmental Technologies—2015. Third International Symposium on Bioremediation and Sustainable Environmental Technologies (Miami, FL), page C-017, Battelle Memorial Institute, Columbus, OH.  www.battelle.org/biosymp  [[Media:Hawthorne2015.pdf | Abstract.pdf]]</ref>.]]
 
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LNAPL thickness in monitoring wells is often referred to as the “apparent LNAPL thickness” because at first glance this LNAPL thickness might be expected to be the thickness of LNAPL that is in the formation, but in reality it is not well correlated with the thickness of the LNAPL zone in the subsurface for several reasons.
 
  
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First, different physical settings can produce different LNAPL thicknesses in monitoring wells.  Sale et al. (2018) show five different scenarios that produce very different responses with regard to apparent LNAPL thickness (Figure 3).  Scenario A shows an LNAPL apparent thickness in the monitoring well that is at static equilibrium with LNAPL in an unconfined aquifer.  Scenario B, while also an unconfined aquifer, is comprised of very fine-grained soils that cause the LNAPL thickness in the well to be much higher than in Scenario A.  In Scenario C, the LNAPL has accumulated under a confined unit (likely due to an underground release of LNAPL below the confining unit), and the LNAPL has risen above the groundwater potentiometric surface, leading to a large (and misleading) LNAPL thickness in the monitoring well.  Scenario D, LNAPL in a perched unit, also shows a very different response from the other scenarios.  Scenario E, LNAPL in fractured system, shows that the LNAPL can penetrate below the water table, and that LNAPL thickness in a well is dependent on the pressure from accumulation of LNAPL in the fractures<ref name="Sale2018"/>.
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More recently it has been discovered that VI impacts can occur via sewer and other subsurface piping connections in areas where vapor migration through the soil would not be expected to be significant, and this could also occur in buildings that do not sit over contaminated groundwater<ref name="RiisEtAl2010" /><ref name="GuoEtAl2015" /><ref name="McHughEtAl2017" /><ref name="McHughBeckley2018" />.  
  
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Second, apparent LNAPL thickness is affected by changes in the groundwater surface elevation (or water table). Generally, when groundwater elevations are higher than typical, the LNAPL thickness in monitoring wells will decrease or go to zero because the groundwater will redistribute any mobile LNAPL into what previously was the unsaturated zone. During lower groundwater elevation periods, much more of the LNAPL will occur as a continuous phase near the water table, leading to higher LNAPL thicknesses in wells.
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Therefore, in addition to groundwater and soil gas sampling, external data collection that includes and extends beyond the area of concern should include manhole vapor sampling (e.g., sanitary sewer, storm sewer, land-drain). Video surveys from sanitary sewers, storm sewers, and/or land-drains can also be used to identify areas of groundwater leakage into utility corridors and lateral connections to buildings that are conduits for vapor transport. During these investigations, it is important to recognize that utility corridors can transmit both impacted water and vapors beyond groundwater plume boundaries, so extending investigations into areas adjacent to groundwater plume boundaries is necessary.
  
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Overall, LNAPL thickness measurements are useful for delineating the extent of mobile LNAPL in the saturated zone and can provide useful data for understanding the vertical distribution of LNAPL in the formation<ref name="Hawthorne2011">Hawthorne, J.M., 2011. Diagnostic Gauge Plots—Simple Yet Powerful LCSM Tools. Applied NAPL Science Review (ANSR), 1(2). [http://naplansr.com/diagnostic-gauge-plots-volume-1-issue-2-february-2011/ Website] [[Media:Hawthorne2011.pdf | Report.pdf]]</ref><ref name="Kirkman2013">Kirkman, A.J., Adamski, M., and Hawthorne, M., 2013. Identification and Assessment of Confined and Perched LNAPL Conditions. Groundwater Monitoring and Remediation, 33 (1), pp. 75–86. [https://doi.org/10.1111/j.1745-6592.2012.01412.x  DOI:10.1111/j.1745-6592.2012.01412.x]</ref>. But LNAPL thickness by itself is a very poor indicator of the feasibility of LNAPL recovery<ref name="LNAPL-2">Interstate Technology and Regulatory Council (ITRC), 2009. Evaluating LNAPL Remedial Technologies for Achieving Project Goals. LNAPL-2. ITRC, LNAPLs Team, Washington, DC. www.itrcweb.org  [[Media:ITRC-LNAPL-2.pdf | Report.pdf]]</ref><ref name="Hawthorne2015"/> (see [[NAPL Mobility]]) (Figure 4).  Because there is little correlation between apparent LNAPL thickness and LNAPL mobility, there is also little correlation between apparent thickness and the risk to receptors from the LNAPL.
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Using projected indoor air concentrations from modeling and empirical data analyses, and distance screening approaches, external source screening can identify areas and buildings that can be ruled out, or conversely, those that warrant building-specific testing.  
  
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===Complete LNAPL Remediation Is Very Challenging===
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Demonstration of neighborhood-scale external VI source screening using groundwater, depth, sewer, land drain, and video data is documented in the ER-201501 final report<ref name="JohnsonEtAl2020" />.  
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Sale et al. (2018) described the problems with attaining complete LNAPL remediation this way:
 
  
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<blockquote>''Experience of the last few decades has taught us: 1) our best efforts often leave some LNAPL in place, and 2) the remaining LNAPL often sustains exceedances of drinking water standards in release areas for extended periods. Entrapment of LNAPLs at residual saturations is a primary factor constraining our success. Other challenges include the low solubility of LNAPL, the complexity of the subsurface geologic environment, access limitations associated with surface structures, and concentration goals that are often three to five orders of magnitude less than typical initial concentrations within LNAPL zones.''<ref name="Sale2018"/></blockquote>
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'''Indoor air source screening''' seeks to locate and remove indoor air sources<ref>Doucette, W.J., Hall, A.J., Gorder, K.A., 2010. Emissions of 1,2-Dichloroethane from Holiday Decorations as a Source of Indoor Air Contamination. Ground Water Monitoring and Remediation, 30(1), pp. 67-73. [https://doi.org/10.1111/j.1745-6592.2009.01267.x doi: 10.1111/j.1745-6592.2009.01267.x] </ref> that might confound building specific VI pathway assessment. Visual inspections and written surveys might or might not identify significant indoor air sources, so these should be complemented with use of portable analytical instruments<ref>McHugh, T., Kuder, T., Fiorenza, S., Gorder, K., Dettenmaier, E., Philp, P., 2011. Application of CSIA to Distinguish Between Vapor Intrusion and Indoor Sources of VOCs. Environmental Science and Technology, 45(14), pp. 5952-5958. [https://doi.org/10.1021/es200988d doi: 10.1021/es200988d]</ref><ref name="BeckleyEtAl2014">Beckley, L., Gorder, K., Dettenmaier, E., Rivera-Duarte, I., McHugh, T., 2014. On-Site Gas Chromatography/Mass Spectrometry (GC/MS) Analysis to Streamline Vapor Intrusion Investigations. Environmental Forensics, 15(3), pp. 234–243. [https://doi.org/10.1080/15275922.2014.930941 doi: 10.1080/15275922.2014.930941]</ref>.
  
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In particular, the discontinuous residual LNAPL cannot be removed (or recovered) by pumping, and ''in situ'' remediation is expensive and not completely effective (see [[LNAPL Remediation Technologies]]).  However, many regulatory programs require “LNAPL recovery to the extent practicable.”  The lack of quantitative metrics and the lack of correlation between apparent LNAPL thicknesses and subsurface LNAPL makes this a problematic requirement in many cases and the ITRC (2018) cautions “Thickness or concentration data alone may not provide a sound basis for defining the point at which a cleanup objective is achieved.”<ref name="LNAPL-3"/>  However, Sale et al. (2018) describe metrics such as LNAPL transmissivity, limited/infrequent well thicknesses, decline curve analysis, asymptotic analysis, and comparison to NSZD rates that can be used to determine when LNAPL has been removed the extent practicable<ref name="Sale2018"/>.
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The advantage of portable analytical tools is that they allow practitioners to expeditiously test indoor air concentrations under natural conditions in each room of the building. Concentrations in any room in excess of relevant screening levels trigger more sampling in that room to identify if an indoor source is present in that room. Removal of a suspected source and subsequent room testing can identify if that object or product was the source of the previously measured concentrations.  
  
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===Attenuation Processes are Active and Important===
+
'''Building-specific controlled pressurization method (CPM) testing''' directly measures the worst case indoor air impact, but it can also be used to determine contributing VI pathways and to identify indoor air sources<ref>McHugh, T.E., Beckley, L., Bailey, D., Gorder, K., Dettenmaier, E., Rivera-Duarte, I., Brock, S., MacGregor, I.C., 2012. Evaluation of Vapor Intrusion Using Controlled Building Pressure. Environmental Science and Technology, 46(9), pp. 4792–4799. [https://doi.org/10.1021/es204483g doi: 10.1021/es204483g]</ref><ref name="BeckleyEtAl2014" /><ref name="GuoEtAl2015" /><ref name="HoltonEtAl2015">Holton, C., Guo, Y., Luo, H., Dahlen, P., Gorder, K., Dettenmaier, E., Johnson, P.C., 2015. Long-Term Evaluation of the Controlled Pressure Method for Assessment of the Vapor Intrusion Pathway. Environmental Science and Technology, 49(4), pp. 2091–2098.  [https://doi.org/10.1021/es5052342 doi: 10.1021/es5052342]</ref><ref name="JohnsonEtAl2020" /><ref name="GuoEtAl2020a">Guo, Y., Dahlen, P., Johnson, P.C., 2020a. Development and Validation of a Controlled Pressure Method Test Protocol for Vapor Intrusion Pathway Assessment.  Environmental Science and Technology, 54(12), pp. 7117-7125. [https://dx.doi.org/10.1021/acs.est.0c00811 doi: 10.1021/acs.est.0c00811]</ref>. In CPM testing, blowers/fans installed in a doorway(s) or window(s) are set-up to exhaust indoor air to outdoor, which causes the building to be under pressurized relative to the atmosphere. This induces air movement from the subsurface into the test building via openings in the foundation and/or subsurface piping networks with or without direct connections to indoor air. This is similar to what happens intermittently under natural conditions when wind, indoor-outdoor temperature differences, and/or use of appliances that exhaust air from the structure (e.g. dryer exhaust) create an under-pressurized building condition.  
−
Both LNAPL source zones and their dissolved phase hydrocarbon plumes are attenuated by biodegradation and other attenuation process.  In the source zone, this attenuation is called [[Natural Source Zone Depletion (NSZD)]] (see also [[Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill]]). In the dissolved plume it is called [[Monitored Natural Attenuation (MNA)]] (see also  [[Biodegradation - Hydrocarbons]]).  These processes generally limit the length of dissolved phase hydrocarbon plumes to a few hundred feet<ref name="Newell1998">Newell, C.J., and Connor, J.A., 1998. Characteristics of Dissolved Hydrocarbon Plumes: Results from Four Studies, Version 1.1. American Petroleum Institute, Soil/Groundwater Technical Task Force, Washington, DC. [https://www.enviro.wiki/index.php?title=File:Newell-1998-chararacterization_of_dissolved_Pet._Hydro_Plumes.pdf  Report.pdf]</ref> via processes that have been well known and understood since the mid-1990s.
 
  
−
However, NSZD is “by far, the biggest new idea for LNAPLs in the last decade.”<ref name="Sale2018"/>  Originally, LNAPL bodies were thought to attenuate very slowly via dissolution and volatilization.  In 2006, it was discovered that NSZD rates are orders of magnitude higher than originally thought, largely due to direct biodegradation of LNAPL constituents to methane and carbon dioxide by methanogenic consortiums of naturally occurring bacteria<ref name="Lundegard2006">Lundegard, P.D., and Johnson, P.C., 2006. Source Zone Natural Attenuation at Petroleum Spill Sites—II: Application to a Former Oil Field. Groundwater Monitoring and Remediation. 26(4), pp. 93-106. [https://doi.org/10.1111/j.1745-6592.2006.00115.x  DOI: 10.1111/j.1745-6592.2006.00115.x]</ref><ref name="Garg2017">Garg, S., Newell, C., Kulkarni, P., King, D., Adamson, D.T., Irianni Renno, M., and Sale, T., 2017. Overview of Natural Source Zone Depletion: Processes, Controlling Factors, and Composition Change. Groundwater Monitoring and Remediation, 37(3), pp. 62-81.  [https://doi.org/10.1111/gwmr.12219 DOI:  10.1111/gwmr.12219] [[Media:Garg2017gwmr.12219.pdf | Report.pdf]]</ref>.  NSZD processes play an important role in risk mitigation and the long-term stability of LNAPL bodies<ref name="Mahler2012">
+
The blowers/fans can also be used to blow outdoor air into the building, thereby creating a building over-pressurization condition. A positive pressure difference CPM test suppresses VI pathways; therefore, chemicals detected in indoor air above outdoor air concentrations during this condition are attributed to indoor contaminant sources which facilitates the identification of any such indoor air sources.
−
Mahler, N., Sale, T., and Lyverse, M., 2012. A Mass Balance Approach to Resolving LNAPL Stability. Groundwater, 50(6), pp 861-871.  [https://doi.org/10.1111/j.1745-6584.2012.00949.x DOI: 10.1111/j.1745-6584.2012.00949.x]</ref><ref name="LNAPL-3"/>.
 
  
−
===Risk from LNAPL Source Zones Diminishes Over Time===
+
Data collected during CPM testing, when combined with screening level VI modeling, can be used to identify which VI chemical migration pathways are significant contributors to indoor air impacts<ref name="GuoEtAl2015" />. CPM testing guidelines were developed and validated under ESTCP Project ER-201501<ref name="GuoEtAl2020a" /><ref name="JohnsonEtAl2021" />.
−
At Early Stage LNAPL sites, the expansion of the LNAPL body is a risk that needs to be addressed.  Fortunately, this type of site is relatively rare. For Middle and Late Stage sites, the primary risks are associated with phase changes (dissolution of the LNAPL forming a dissolved plume and volatilization from the LNAPL or dissolved plume forming hydrocarbon vapors).  As described above, MNA can often control the dissolved phase (see [[Monitored Natural Attenuation (MNA) of Fuels]]), while aerobic biodegradation in the unsaturated zone greatly reduces the vapor intrusion risk from hydrocarbon vapors (see [[Vapor Intrusion - Separation Distances from Petroleum Sources]]).
 
  
−
Understanding LNAPL body mobility and stability is important to understand the potential risks posed by LNAPL. The relative magnitude of LNAPL mobility can be determined by measuring the LNAPL transmissivity (see [[NAPL Mobility]]). If the transmissivity is below a threshold level (in the range of 0.1 to 0.8 ft<sup>2</sup>/day) then the LNAPL likely cannot be recovered efficiently by pumping, but above this transmissivity level recovery is feasible<ref name="LNAPL-3"/>.  Michigan’s LNAPL guidance states “if the NAPL has a transmissivity greater than 0.5 ft<sup>2</sup>/day, it is likely that the NAPL can be recovered in a cost-effective and efficient manner unless a demonstration is made to show otherwise.”  Kansas LNAPL guidance requires “recovery of all LNAPL with a transmissivity greater than 0.8 ft<sup>2</sup>/day that can be recovered in an efficient, cost-effective manner.”<ref name="LNAPL-3"/>.  The stability of the entire LNAPL body can be evaluated using statistical tools to determine if migration of LNAPL is occurring<ref name="Hawthorne2013">Hawthorne, J.M., Stone, C.D., Helsel, D., 2013. LNAPL Body Stability Part 2: Daughter Plume Stability via Spatial Moments Analysis. Applied NAPL Science Review (ANSR), 3(5).  [http://naplansr.com/lnapl-body-stability-part-2-daughter-plume-stability-via-spatial-moments-analysis-volume-3-issue-5-september-2013/ Website] [[Media:Hawthorne2013.pdf | Report.pdf]]</ref>.
+
'''Passive samplers''' can be used to measure long term average indoor air concentrations under natural conditions and during VI mitigation system operation. They will provide more confident assessment of long term average concentrations than an infrequent sequence of short term grab samples. Long term average concentrations can also be determined by long term active sampling (e.g., by slowly pulling air through a thermal desorption (TD) tube). However, passive sampling has the advantage that additional equipment and expertise is not required for sampler deployment and recovery.   
  
−
==Overview of Modern LNAPL Conceptual Site Model==
+
Use of passive samplers in indoor air under time-varying concentration conditions was demonstrated and validated by comparing against intensive active sampling in ESTCP Project ER-201501<ref name="JohnsonEtAl2020" /><ref name="GuoEtAl2021">Guo, Y., O’Neill, H., Dahlen, P., and Johnson, P.C.  2021.  Evaluation of Passive Diffusive-Adsorptive Samplers for Use in Assessing Time-Varying Indoor Air Impacts Resulting from Vapor Intrusion. Groundwater Monitoring and Remediation, 42(1), pp. 38-49.  [https://doi.org/10.1111/gwmr.12481 doi: 10.1111/12481]</ref>.  
−
[[File:Newell1w2Fig5.png |thumb|500px| Figure 5.  A higher tier of LNAPL CSM is useful as LNAPL site complexity increases<ref name="LNAPL-3"/>.]]
 
−
The ITRC (2018) describes the typical evolution of an LCSM over the course of the remediation process which can be broken into three separate stages:
 
−
* An ''Initial LCSM'' focuses on identifying the LNAPL concerns, such as a risk to health or safety, any LNAPL migration, LNAPL-specific regulations, and physical or aesthetic impacts.  
 
−
* A ''Remedy Selection LCSM'' supports remedial technology evaluation by characterizing aspects of the LNAPL and site subsurface that may impact remedial technology performance.
 
−
* A ''Design and Performance LCSM'' focuses on presenting the technical information needed to establish remediation objectives, design and implement remedies or control measures, and track progress toward defined remediation endpoints.
 
  
−
One key question when developing an LCSM is “how much data is enough.”  In general, the answer is that the existing data is sufficient for the current stage of the remediation project when it allows the stakeholders to agree on a path forward<ref name="LNAPL-3"/>.  Figure 5 shows that as the level of complexity of a site increases, a higher tier of LCSM is useful to provide enough information for making decisions<ref name="LNAPL-3"/><ref name="ASTM2014a"/>.  The higher tier of information could be higher data density, additional tools for a given line of evidence, or other evaluations.
+
The purpose of maintaining an evergreen '''comprehensive VI conceptual model''' is to ensure that the most complete and up-to-date understanding of the site is informing decisions related to future sampling, data interpretation, and the need for and design of mitigation systems. The VI conceptual model can also serve as an effective communication tool in stakeholder discussions.  
  
−
==LNAPL Concerns, Remediation Goals and Objectives==
+
Use of these tools for residential neighborhoods and in non-residential buildings overlying chlorinated solvent groundwater plumes is documented comprehensively in a series of peer reviewed articles<ref name="JohnsonEtAl2020" /><ref name="JohnsonEtAl2021" /><ref name="JohnsonEtAl2022" /><ref name="GuoEtAl2015" /><ref name="GuoEtAl2020a" /><ref name="GuoEtAl2020b">Guo, Y., Dahlen, P., Johnson, P.C. 2020b. Temporal variability of chlorinated volatile organic compound vapor concentrations in a residential sewer and land drain system overlying a dilute groundwater plume. Science of the Total Environment, 702, Article 134756.  [https://doi.org/10.1016/j.scitotenv.2019.134756 doi: 10.1016/j.scitotenv.2019.134756]&nbsp;&nbsp; [//www.enviro.wiki/images/e/e5/GuoEtAl2020b.pdf  Open Access Manuscript]</ref><ref name="GuoEtAl2021" /><ref name="HoltonEtAl2015" />.
−
Finally, the ITRC (2018) provides a methodology for identifying LNAPL concerns, verifying those concerns, selecting LNAPL remediation goals, and determining LNAPL remediation objectives.  Examples of each of these concepts are provided below:
 
  
−
* '''Potential Concerns:'''  Human or ecological risk concerns, fire or explosivity issues, LNAPL migration, LNAPL-specific regulatory concerns, other concerns such as odors or geotechnical issues.
+
==Summary==
−
* '''Verifying Concerns:'''  Measure LNAPL transmissivity to determine if it is recoverable; measure vertical and horizontal separation distances between buildings and LNAPL bodies to screen for vapor intrusion concerns.
+
In summary, the VI Diagnosis Toolkit provides a set of tools that can lead to quicker, more confident, and more cost effective neighborhood-scale VI pathway and impact assessments. Toolkit components and their use can complement conventional methods for assessing and mitigating the vapor intrusion pathway.
−
* '''Remediation Goals:'''  Reduce mobile LNAPL saturation, abate unacceptable soil concentrations, terminate LNAPL body migration, abate unacceptable constituent concentrations in dissolved and vapor phases.
 
−
* '''Remediation Objectives:'''  Recover LNAPL to the extent practicable based on transmissivity, reduce soil concentrations to below regulatory limits, stop LNAPL migration with a barrier, contain migrating groundwater plume (if present), reduce groundwater and vapor concentration to acceptable levels.
 
−
* '''Remediation Technologies:'''  LNAPL Mass Recovery technologies, LNAPL phase change technologies, LNAPL Mass Control technologies, combinations of technologies.
 
−
 
 
−
Overall, a LNAPL Conceptual Site Model that integrates key site specific information and current technical knowledge about LNAPL sites in general is instrumental to successful site management, where LNAPL concerns drive remediation goals, goals drive remediation objectives, and the objectives form the basis for the selection of remediation technologies.  
 
  
 
==References==
 
==References==
 +
<references />
  
−
<references/>
+
==See Also==
  
−
==See Also==
+
*[https://papers.ssrn.com/sol3/papers.cfm?abstract_id=4000681 Evaluation of Radon and Building Pressure Differences as Environmental Indicators for Vapor Intrusion Assessment]
−
 
+
*[https://pubs.acs.org/doi/10.1021/es4024767 Temporal Variability of Indoor Air Concentrations under Natural Conditions in a House Overlying a Dilute Chlorinated Solvent Groundwater Plume]
 +
*[https://serdp-estcp.mil/projects/details/e0d00662-c333-4560-8ae7-60f20b0e714b Integrated Field-Scale, Lab-Scale, and Modeling Studies for Improving Our Ability to Assess the Groundwater to Indoor Air Pathway at Chlorinated Solvent Impacted Sites]

Latest revision as of 22:25, 18 July 2024

Assessing Vapor Intrusion (VI) Impacts in Neighborhoods with Groundwater Contaminated by Chlorinated Volatile Organic Chemicals (CVOCs)

The VI Diagnosis Toolkit[1] is a set of tools that can be used individually or in combination to assess vapor intrusion (VI) impacts at one or more buildings overlying regional-scale dissolved chlorinated solvent impacted groundwater plumes. The strategic use of these tools can lead to confident and efficient neighborhood-scale VI pathway assessments.

Related Article(s):

Contributor(s):

  • Paul C. Johnson, Ph.D.
  • Paul Dahlen, Ph.D.
  • Yuanming Guo, Ph.D.

Key Resource(s):

  • The VI Diagnosis Toolkit for Assessing Vapor Intrusion Pathways and Impacts in Neighborhoods Overlying Dissolved Chlorinated Solvent Plumes, ESTCP Project ER-201501, Final Report[1]
  • CPM Test Guidelines: Use of Controlled Pressure Method Testing for Vapor Intrusion Pathway Assessment, ESTCP Project ER-201501, Technical Report[2]
  • VI Diagnosis Toolkit User Guide, ESTCP Project ER-201501[3]

Introduction

Most federal, state, and local regulatory guidance for assessing and mitigating the vapor intrusion pathway reflects USEPA’s Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air[4]. The paradigm outlined by that guidance includes: 1) a preliminary and mostly qualitative analysis that looks for site conditions that suggest vapor intrusion might occur (e.g., the presence of vapor-forming chemicals in close proximity to buildings); 2) a multi-step and more detailed quantitative screening analysis that involves site-specific data collection and their comparison to screening levels to identify buildings of potential VI concern; and 3) selection and design of mitigation systems or continued monitoring, as needed. With respect to (2), regulatory guidance typically recommends consideration of “multiple lines of evidence” in decision-making[4][5], with typical lines-of-evidence being groundwater, soil gas, sub-slab soil gas, and/or indoor air concentrations. Of those, soil gas measurements and/or measured short-term indoor air concentrations can be weighted heavily, and therefore decision making might not be completed without them. Effective evaluation of VI risk from sub-slab and/or soil gas measurements would require an unknown building-specific attenuation factor, but there is also uncertainty as to whether or not indoor air data is representative of maximum and/or long-term average indoor concentrations. Indoor air data can be confounded by indoor contaminant sources because the number of samples is typically small, indoor concentrations can vary with time, and because a number of household products can emit the chemicals being measured. When conducting VI pathway assessments in neighborhoods where it is impractical to assess all buildings, the EPA recommends following a “worst first” investigational approach.

The limitations of this approach, as practiced, are the following:

  • Decisions are rarely made without indoor air data and generally, seasonal sampling is required, delaying decision making.
  • The collection of a robust indoor air data set that adequately characterizes long term indoor air concentrations could take years given the typical frequency of data collection and the most common methods of sample collection (e.g., 24-hour samples). Therefore, indoor air sampling might continue indefinitely at some sites.
  • The “worst first” buildings might not be identified correctly by the logic outlined in USEPA’s 2015 guidance and the most impacted buildings might not even be located over a groundwater plume. Recent studies have shown VI impacts in homes as a result of sewer and other subsurface piping connections, which are not explicitly considered nor easily characterized through conventional VI pathway assessment[6][7][8][9][10].
  • The presumptive remedy for VI mitigation (sub-slab depressurization) may not be effective for all VI scenarios (e.g., those involving vapor migration to indoor spaces via sewer connections).

The VI Diagnosis Toolkit components were developed considering these limitations as well as more recent knowledge gained through research, development, and validation projects funded by SERDP and ESTCP.

The VI Diagnosis Toolkit Components

Figure 1. Vapor intrusion pathway conceptualization considering “alternate VI pathways”, including “pipe flow VI” and “sewer VI” pathways[1].

The primary components of the VI Diagnosis Toolkit and their uses include:

  • External VI source strength screening to identify buildings most likely to be impacted by VI at levels warranting building-specific testing.
  • Indoor air source screening to locate and remove indoor air sources that might confound building specific VI pathway assessment.
  • Controlled pressurization method (CPM) testing to quickly (in a few days or less) measure the worst-case indoor air impact likely to be caused by VI under natural conditions in specific buildings. CPM tests can also be used to identify the presence of indoor air sources and diagnose active VI pathways.
  • Passive indoor sampling for determining long-term average indoor air concentrations under natural VI conditions and/or for verifying mitigation system effectiveness in buildings that warrant VI mitigation.
  • Comprehensive VI conceptual model development and refinement to ensure that appropriate monitoring, investigation, and mitigation strategies are being selected (Figure 1).

Expanded discussions for each of these are given below.

External VI source strength screening identifies those buildings that warrant more intrusive building-specific assessments, using data collected exterior to the buildings. The use of groundwater and/or soil gas concentration data for building screening has been part of VI pathway assessments for some time and their use is discussed in many regulatory guidance documents. Typically, the measured concentrations are compared to relevant screening levels derived via modeling or empirical analyses from indoor air concentrations of concern.

More recently it has been discovered that VI impacts can occur via sewer and other subsurface piping connections in areas where vapor migration through the soil would not be expected to be significant, and this could also occur in buildings that do not sit over contaminated groundwater[10][7][8][9].

Therefore, in addition to groundwater and soil gas sampling, external data collection that includes and extends beyond the area of concern should include manhole vapor sampling (e.g., sanitary sewer, storm sewer, land-drain). Video surveys from sanitary sewers, storm sewers, and/or land-drains can also be used to identify areas of groundwater leakage into utility corridors and lateral connections to buildings that are conduits for vapor transport. During these investigations, it is important to recognize that utility corridors can transmit both impacted water and vapors beyond groundwater plume boundaries, so extending investigations into areas adjacent to groundwater plume boundaries is necessary.

Using projected indoor air concentrations from modeling and empirical data analyses, and distance screening approaches, external source screening can identify areas and buildings that can be ruled out, or conversely, those that warrant building-specific testing.

Demonstration of neighborhood-scale external VI source screening using groundwater, depth, sewer, land drain, and video data is documented in the ER-201501 final report[1].

Indoor air source screening seeks to locate and remove indoor air sources[11] that might confound building specific VI pathway assessment. Visual inspections and written surveys might or might not identify significant indoor air sources, so these should be complemented with use of portable analytical instruments[12][13].

The advantage of portable analytical tools is that they allow practitioners to expeditiously test indoor air concentrations under natural conditions in each room of the building. Concentrations in any room in excess of relevant screening levels trigger more sampling in that room to identify if an indoor source is present in that room. Removal of a suspected source and subsequent room testing can identify if that object or product was the source of the previously measured concentrations.

Building-specific controlled pressurization method (CPM) testing directly measures the worst case indoor air impact, but it can also be used to determine contributing VI pathways and to identify indoor air sources[14][13][7][15][1][16]. In CPM testing, blowers/fans installed in a doorway(s) or window(s) are set-up to exhaust indoor air to outdoor, which causes the building to be under pressurized relative to the atmosphere. This induces air movement from the subsurface into the test building via openings in the foundation and/or subsurface piping networks with or without direct connections to indoor air. This is similar to what happens intermittently under natural conditions when wind, indoor-outdoor temperature differences, and/or use of appliances that exhaust air from the structure (e.g. dryer exhaust) create an under-pressurized building condition.

The blowers/fans can also be used to blow outdoor air into the building, thereby creating a building over-pressurization condition. A positive pressure difference CPM test suppresses VI pathways; therefore, chemicals detected in indoor air above outdoor air concentrations during this condition are attributed to indoor contaminant sources which facilitates the identification of any such indoor air sources.

Data collected during CPM testing, when combined with screening level VI modeling, can be used to identify which VI chemical migration pathways are significant contributors to indoor air impacts[7]. CPM testing guidelines were developed and validated under ESTCP Project ER-201501[16][2].

Passive samplers can be used to measure long term average indoor air concentrations under natural conditions and during VI mitigation system operation. They will provide more confident assessment of long term average concentrations than an infrequent sequence of short term grab samples. Long term average concentrations can also be determined by long term active sampling (e.g., by slowly pulling air through a thermal desorption (TD) tube). However, passive sampling has the advantage that additional equipment and expertise is not required for sampler deployment and recovery.

Use of passive samplers in indoor air under time-varying concentration conditions was demonstrated and validated by comparing against intensive active sampling in ESTCP Project ER-201501[1][17].

The purpose of maintaining an evergreen comprehensive VI conceptual model is to ensure that the most complete and up-to-date understanding of the site is informing decisions related to future sampling, data interpretation, and the need for and design of mitigation systems. The VI conceptual model can also serve as an effective communication tool in stakeholder discussions.

Use of these tools for residential neighborhoods and in non-residential buildings overlying chlorinated solvent groundwater plumes is documented comprehensively in a series of peer reviewed articles[1][2][3][7][16][18][17][15].

Summary

In summary, the VI Diagnosis Toolkit provides a set of tools that can lead to quicker, more confident, and more cost effective neighborhood-scale VI pathway and impact assessments. Toolkit components and their use can complement conventional methods for assessing and mitigating the vapor intrusion pathway.

References

  1. ^ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Johnson, P.C., Guo, Y., Dahlen, P., 2020. The VI Diagnosis Toolkit for Assessing Vapor Intrusion Pathways and Mitigating Impacts in Neighborhoods Overlying Dissolved Chlorinated Solvent Plumes. ESTCP Project ER-201501, Final Report. Project Website   Final Report.pdf
  2. ^ 2.0 2.1 2.2 Johnson, P.C., Guo, Y., Dahlen, P., 2021. CPM Test Guidelines: Use of Controlled Pressure Method Testing for Vapor Intrusion Pathway Assessment. ESTCP ER-201501, Technical Report. Project Website   Technical_Report.pdf
  3. ^ 3.0 3.1 Johnson, P.C., Guo, Y., and Dahlen, P., 2022. VI Diagnosis Toolkit User Guide, ESTCP ER-201501, User Guide. Project Website   User_Guide.pdf
  4. ^ 4.0 4.1 USEPA, 2015. OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air. U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, OSWER Publication No. 9200.2-154, 267 pages. USEPA Website   Report.pdf
  5. ^ NJDEP, 2021. Vapor Intrusion Technical Guidance, Version 5.0. New Jersey Department of Environmental Protection, Trenton, NJ. Website   Guidance Document.pdf
  6. ^ Beckley, L, McHugh, T., 2020. A Conceptual Model for Vapor Intrusion from Groundwater Through Sewer Lines. Science of the Total Environment, 698, Article 134283. doi: 10.1016/j.scitotenv.2019.134283   Open Access Article
  7. ^ 7.0 7.1 7.2 7.3 7.4 Guo, Y., Holton, C., Luo, H., Dahlen, P., Gorder, K., Dettenmaier, E., Johnson, P.C., 2015. Identification of Alternative Vapor Intrusion Pathways Using Controlled Pressure Testing, Soil Gas Monitoring, and Screening Model Calculations. Environmental Science and Technology, 49(22), pp. 13472–13482. doi: 10.1021/acs.est.5b03564
  8. ^ 8.0 8.1 McHugh, T., Beckley, L., Sullivan, T., Lutes, C., Truesdale, R., Uppencamp, R., Cosky, B., Zimmerman, J., Schumacher, B., 2017. Evidence of a Sewer Vapor Transport Pathway at the USEPA Vapor Intrusion Research Duplex. Science of the Total Environment, pp. 598, 772-779. doi: 10.1016/j.scitotenv.2017.04.135   Open Access Manuscipt
  9. ^ 9.0 9.1 McHugh, T., Beckley, L., 2018. Sewers and Utility Tunnels as Preferential Pathways for Volatile Organic Compound Migration into Buildings: Risk Factors and Investigation Protocol. ESTCP ER-201505, Final Report. Project Website   Final Report.pdf
  10. ^ 10.0 10.1 Riis, C., Hansen, M.H., Nielsen, H.H., Christensen, A.G., Terkelsen, M., 2010. Vapor Intrusion through Sewer Systems: Migration Pathways of Chlorinated Solvents from Groundwater to Indoor Air. Seventh International Conference on Remediation of Chlorinated and Recalcitrant Compounds, May, Monterey, CA. Battelle Memorial Institute. ISBN 978-0-9819730-2-9. Website   Report.pdf
  11. ^ Doucette, W.J., Hall, A.J., Gorder, K.A., 2010. Emissions of 1,2-Dichloroethane from Holiday Decorations as a Source of Indoor Air Contamination. Ground Water Monitoring and Remediation, 30(1), pp. 67-73. doi: 10.1111/j.1745-6592.2009.01267.x
  12. ^ McHugh, T., Kuder, T., Fiorenza, S., Gorder, K., Dettenmaier, E., Philp, P., 2011. Application of CSIA to Distinguish Between Vapor Intrusion and Indoor Sources of VOCs. Environmental Science and Technology, 45(14), pp. 5952-5958. doi: 10.1021/es200988d
  13. ^ 13.0 13.1 Beckley, L., Gorder, K., Dettenmaier, E., Rivera-Duarte, I., McHugh, T., 2014. On-Site Gas Chromatography/Mass Spectrometry (GC/MS) Analysis to Streamline Vapor Intrusion Investigations. Environmental Forensics, 15(3), pp. 234–243. doi: 10.1080/15275922.2014.930941
  14. ^ McHugh, T.E., Beckley, L., Bailey, D., Gorder, K., Dettenmaier, E., Rivera-Duarte, I., Brock, S., MacGregor, I.C., 2012. Evaluation of Vapor Intrusion Using Controlled Building Pressure. Environmental Science and Technology, 46(9), pp. 4792–4799. doi: 10.1021/es204483g
  15. ^ 15.0 15.1 Holton, C., Guo, Y., Luo, H., Dahlen, P., Gorder, K., Dettenmaier, E., Johnson, P.C., 2015. Long-Term Evaluation of the Controlled Pressure Method for Assessment of the Vapor Intrusion Pathway. Environmental Science and Technology, 49(4), pp. 2091–2098. doi: 10.1021/es5052342
  16. ^ 16.0 16.1 16.2 Guo, Y., Dahlen, P., Johnson, P.C., 2020a. Development and Validation of a Controlled Pressure Method Test Protocol for Vapor Intrusion Pathway Assessment. Environmental Science and Technology, 54(12), pp. 7117-7125. doi: 10.1021/acs.est.0c00811
  17. ^ 17.0 17.1 Guo, Y., O’Neill, H., Dahlen, P., and Johnson, P.C. 2021. Evaluation of Passive Diffusive-Adsorptive Samplers for Use in Assessing Time-Varying Indoor Air Impacts Resulting from Vapor Intrusion. Groundwater Monitoring and Remediation, 42(1), pp. 38-49. doi: 10.1111/12481
  18. ^ Guo, Y., Dahlen, P., Johnson, P.C. 2020b. Temporal variability of chlorinated volatile organic compound vapor concentrations in a residential sewer and land drain system overlying a dilute groundwater plume. Science of the Total Environment, 702, Article 134756. doi: 10.1016/j.scitotenv.2019.134756   Open Access Manuscript

See Also

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