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| − | Dispersion of solutes in flowing groundwater results in the spreading of a contaminant plume from highly concentrated areas to less concentrated areas. In many groundwater transport models, solute transport is described by the advection-dispersion-reaction equation. The dispersion coefficient in this equation is the sum of the [[wikipedia:Molecular diffusion | molecular diffusion]] coefficient, the mechanical dispersion coefficient and the macrodispersion effect.
| + | #REDIRECT[[Groundwater Flow and Solute Transport]] |
| − | <div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
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| − | '''Related Article(s):'''
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| − | *[[Advection and Groundwater Flow]]
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| − | *[[Plume Response Modeling]]
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| − | '''CONTRIBUTOR(S):'''
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| − | *[[Dr. Charles Newell, P.E.|Dr. Charles Newell]] and [[Dr. Robert Borden, P.E.|Dr. Robert Borden]]
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| − | '''Key Resource(s):'''
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| − | *[http://hydrogeologistswithoutborders.org/wordpress/1979-english/ Groundwater]<ref name="FandC1979">Freeze, A., and Cherry, J., 1979. Groundwater, Prentice-Hall, Englewood Cliffs, New Jersey, 604 pages. Free download from [http://hydrogeologistswithoutborders.org/wordpress/1979-english/ Hydrogeologists Without Borders].</ref>, Freeze and Cherry, 1979.
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| − | *[https://gw-project.org/books/hydrogeologic-properties-of-earth-materials-and-principles-of-groundwater-flow/ Hydrogeologic Properties of Earth Materials and Principals of Groundwater Flow]<ref name="Woessner2020">Woessner, W.W., and Poeter, E.P., 2020. Properties of Earth Materials and Principals of Groundwater Flow, The Groundwater Project, Guelph, Ontario, 207 pages. Free download from [https://gw-project.org/books/hydrogeologic-properties-of-earth-materials-and-principles-of-groundwater-flow/ The Groundwater Project].</ref>, Woessner and Poeter, 2020.
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| − | ==Molecular Diffusion==
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| − | [[File:Fig1 dispanddiff.JPG|thumbnail|left|400px|Figure 1. Conceptual depiction of diffusion of a dissolved chemical recently placed in a container at Time 1 (left panel) and then distributed throughout the container (right panel) at Time 2.]][[File:Fig2 dispanddiff.JPG|thumbnail|right|400px|Figure 2. Conceptual depiction of mechanical dispersion (adapted from ITRC (2011)<ref name="ITRC2011">ITRC Integrated DNAPL Site Strategy Team, 2011. Integrated DNAPL Site Strategy. Technical/Regulatory Guidance Document, 209 pgs. [//www.enviro.wiki/images/d/d9/ITRC-2011-Integrated_DNAPL.pdf Report pdf]</ref>).]]
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| − | [[wikipedia: Molecular diffusion | Molecular diffusion]] is the result of the thermal motion of individual molecules which causes a flux of dissolved solutes from areas of higher concentration to areas of lower concentration (Figure 1). The diffusion coefficient is a proportionality constant between the molar flux due to molecular diffusion and the concentration gradient and is a function of the temperature and molecular weight. In locations where advective flux is low (clayey aquitards and sedimentary rock), diffusion is often the dominant transport mechanism.
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| − | The diffusive flux ''J'' (M/L<sup>2</sup>/T) in groundwater is calculated using [[wikipedia:Fick's laws of diffusion | Fick’s Law]]:
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| − | {|
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| − | !
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| − | :Equation 1:
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| − | !<big>''J = -D<sub>e</sub> dC/dx''</big>
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| − | |-
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| − | |Where:||
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| − | |-
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| − | :''D<sub>e</sub>''
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| − | |is the effective diffusion coefficient and
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| − | |-
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| − | :''dC/dx''
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| − | |is the concentration gradient.
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| − | |}
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| − | The effective diffusion coefficient for transport through the porous media, ''D<sub>e</sub>, is estimated as:''
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| − | {|
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| − | !
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| − | :Equation 2:
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| − | !<big>''D<sub>e</sub> = D<sub>m</sub> n<sub>e</sub> δ/Τ''</big>
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| − | |-
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| − | |Where:||
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| − | |-
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| − | :''D<sub>m</sub>''
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| − | |is the [[wikipedia:Mass diffusivity | diffusion coefficient]] of the solute in water,
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| − | |-
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| − | :''n<sub>e</sub>''
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| − | |is the effective porosity (dimensionless),
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| − | |-
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| − | :''δ''
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| − | |is the constrictivity (dimensionless) which reflects the restricted motion of particles in narrow pores<ref name="Grathwohl1998">Grathwohl, P., 1998. Diffusion in Natural Porous Media: Contaminant Transport, Sorption/Desorption and Dissolution Kinetics. Kluwer Academic Publishers, Boston. DOI: 10.1007/978-1-4615-5683-1 Available from: [https://link.springer.com/book/10.1007/978-1-4615-5683-1 Springer.com]</ref>, and
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| − | |-
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| − | :''Τ''
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| − | |is the [[wikipedia:Tortuosity | tortuosity]] (dimensionless) which reflects the longer diffusion path in porous media around sediment particles<ref name="Carey2016">Carey, G.R., McBean, E.A. and Feenstra, S., 2016. Estimating Tortuosity Coefficients Based on Hydraulic Conductivity. Groundwater, 54(4), pp.476-487. [https://doi.org/10.1111/gwat.12406 DOI:10.1111/gwat.12406] Available from: [https://ngwa.onlinelibrary.wiley.com/doi/abs/10.1111/gwat.12406 NGWA]</ref>.
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| − | |}
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| − | ''D<sub>m</sub>'' is a function of the temperature, fluid viscosity and molecular weight. Values of ''D<sub>m</sub>'' for common groundwater solutes are shown in Table 1.
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| − | {| class="wikitable" style="float:left; margin-right:20px; text-align:center;"
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| − | |+Table 1. Diffusion Coefficients (''D<sub>m</sub>'') for Common Groundwater Solutes.
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| − | |-
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| − | !Aqueous Diffusion Coefficient
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| − | !Temperature<br /><small>(°C)</small>
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| − | !''D<sub>m</sub>''<br /><small>(cm<sup>2</sup>/s)</small>
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| − | !Reference
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| − | |-
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| − | |Acetone||25|| 1.16x10<sup>-5</sup> ||Cussler 1997
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| − | |-
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| − | |Benzene||20||1.02x10<sup>-5</sup>||Bonoli and Witherspoon 1968
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| − | |-
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| − | |Carbon dioxide||25||1.92x10<sup>-5</sup>||Cussler 1997
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| − | |-
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| − | |Carbon tetrachloride||25||9.55x10<sup>-6</sup>||Yaws 1995
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| − | |-
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| − | |Chloroform||25||1.08x10<sup>-5</sup>||Yaws 1995
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| − | |-
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| − | |Dichloroethene||25||1.12x10<sup>-5</sup>||Yaws 1995
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| − | |-
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| − | |1,4-Dioxane||25||1.02x10<sup>-5</sup>||Yaws 1995
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| − | |-
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| − | |Ethane||25||1.52x10<sup>-5</sup>||Witherspoon and Saraf 1965
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| − | |-
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| − | |Ethylbenzene||20||8.10x10<sup>-6</sup>||Bonoli and Witherspoon 1968
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| − | |-
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| − | |Ethene||25||1.87x10<sup>-5</sup>||Cussler 1997
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| − | |-
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| − | |Helium||25||6.28x10<sup>-5</sup>||Cussler 1997
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| − | |-
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| − | |Hydrogen||25||4.50x10<sup>-5</sup>||Cussler 1997
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| − | |-
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| − | |Methane||25||1.88x10<sup>-5</sup>||Witherspoon and Saraf 1965
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| − | |-
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| − | |Nitrogen||25||1.88x10<sup>-5</sup>||Cussler 1997
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| − | |-
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| − | |Oxygen||25||2.10x10<sup>-5</sup>||Cussler 1997
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| − | |-
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| − | |Perfluorooctanoic acid (PFOA)||20||4.80x10<sup>-6</sup>||Schaefer et al. 2019
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| − | |-
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| − | |Perfluorooctane sulfonic acid (PFOS)||20||5.40x10<sup>-6</sup>||Schaefer et al. 2019
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| − | |-
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| − | |Tetrachloroethene||25||8.99x10<sup>-6</sup>||Yaws 1995
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| − | |-
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| − | |Toluene||20||8.50x10<sup>-6</sup>||Bonoli and Witherspoon 1968
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| − | |-
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| − | |Trichloroethene||25||8.16x10<sup>-6</sup>||Rossi et al. 2015
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| − | |-
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| − | |Vinyl chloride||25||1.34x10<sup>-5</sup>||Yaws 1995
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| − | |}
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| − | ==Mechanical Dispersion==
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| − | Mechanical dispersion (hydrodynamic dispersion) results from groundwater moving at rates both greater and less than the average linear velocity. This is due to: 1) fluids moving faster through the center of the pores than along the edges, 2) fluids traveling shorter pathways and/or splitting or branching to the sides, and 3) fluids traveling faster through larger pores than through smaller pores<ref>Fetter, C.W., 1994. Applied Hydrogeology: Macmillan College Publishing Company. New York New York. ISBN-13:978-0130882394</ref>. Because the invading solute-containing water does not travel at the same velocity everywhere, mixing occurs along flow paths. This mixing is called mechanical dispersion and results in distribution of the solute at the advancing edge of flow. The mixing that occurs in the direction of flow is called longitudinal dispersion. Spreading normal to the direction of flow from splitting and branching out to the sides is called transverse dispersion (Figure 2).
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| − | ==Macrodispersion==
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| − | [[File:NewThinkingAboutDispersion.mp4 |thumbnail|right|500px|Figure 3. Matrix diffusion processes and their effects on plume persistence and attenuation.]]
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| − | Macrodispersion is the name given to the plume spreading caused by large-scale aquifer heterogeneities and associated spatial variations in advective transport velocity. In some groundwater modeling projects, large values of the macrodispersion coefficient are used as an adjustment factor to help match the apparent large-scale spreading of the plume<ref name="ITRC2011" />. However, there is limited theoretical support for using large mechanical dispersion coefficients<ref name="Payne2008">Payne, F.C., Quinnan, J.A. and Potter, S.T., 2008. Remediation hydraulics. CRC Press. [https://www.crcpress.com/Remediation-Hydraulics/Payne-Quinnan-Potter/9780849372490 ISBN:978-1-4200-0684-1]</ref><ref name="Hadley2014"> Hadley, P.W. and Newell, C., 2014. The new potential for understanding groundwater contaminant transport. Groundwater, 52(2), pp.174-186. [http://dx.doi.org/10.1111/gwat.12135 doi:10.1111/gwat.12135]</ref>. In transmissive zones, macrodispersion coefficients are often orders of magnitude greater than molecular diffusion coefficients, leading some to conclude that molecular diffusion can be ignored.
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| − | Recently, an alternate conceptual model for describing large-scale plume spreading in heterogeneous soils has been proposed<ref name="Payne2008" /><ref name="ITRC2011" /><ref name="Hadley2014" />. In this approach, solute transport in the transmissive zones is reasonably well described by the advection-dispersion equation using relatively small dispersion coefficients representing mechanical dispersion. However, overtime, molecular diffusion slowly transports solutes into lower permeability zones (Figure 3). As the transmissive zones are remediated, these solutes slowly diffuse back out, causing a long extended tail to the flushout curve. This process, referred to as [[Matrix Diffusion |matrix diffusion]], is controlled by [[wikipedia: Molecular diffusion | molecular diffusion]] and the presence of geologic heterogeneity with sharp contrasts between transmissive and low permeability media<ref>Sale, T.C., Illangasekare, T., Zimbron, J., Rodriguez, D., Wilkins, B. and Marinelli, F., 2007. AFCEE source zone initiative. Report Prepared for the Air Force Center for Environmental Excellence by Colorado State University and Colorado School of Mines. [//www.enviro.wiki/images/0/08/AFCEE-2007-Sale.pdf Report pdf]</ref> as discussed in the [//www.enviro.wiki/images/8/8a/NewThinkingAboutDispersion.mp4 video] shown in Figure 3.
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| − | <br clear="left" />
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| − | ==References==
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| − | <references />
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| − | ==See Also==
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| − | *[http://iwmi.dhigroup.com/solute_transport/advection.html International Water Management Institute Animations]
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| − | *[http://www2.nau.edu/~doetqp-p/courses/env303a/lec32/lec32.htm NAU Lecture Notes on Advective Transport]
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| − | *[https://www.youtube.com/watch?v=00btLB6u6DY MIT Open CourseWare Solute Transport: Advection with Dispersion Video]
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| − | *[http://cart.grac.org/site-closure-1 Matrix Diffusion Webinar: Technical Challenges and Limitations to Site Closure]
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| − | *[https://www.coursera.org/learn/natural-attenuation-of-groundwater-contaminants/lecture/2R7yh/matrix-diffusion-principles Coursera Matrix Diffusion Online Lecture]
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| − | *[https://www.serdp-estcp.org/Tools-and-Training/Environmental-Restoration/DNAPL-Source-Zones/Chlorinated-Solvents-On-Demand-Video/Module-1 ESTCP Remediation and Matrix Diffusion Webinar]
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| − | *[http://www.gsi-net.com/en/publications/useful-groundwater-resources/colorado-state-matrix-diffusion-video.html Matrix Diffusion Movie]
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| − | *[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]
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| − | *[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-1740 Basic Research Addressing Contaminants in Low Permeability Zones]
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| − | *[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]
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| − | *[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]
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| − | *[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201126 Decision Support System for Matrix Diffusion Modeling]
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| − | *[https://www.coursera.org/learn/natural-attenuation-of-groundwater-contaminants/lecture/2R7yh/matrix-diffusion-principles Online Lecture Course - Matrix Diffusion]
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| − | *[http://www.gsi-net.com/en/publications/useful-groundwater-resources/colorado-state-matrix-diffusion-video.html Matrix Diffusion Video]
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