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Monitored Natural Attenuation - Transitioning from Active Remedies

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

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Matrix Diffusion

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Monitored Natural Attenuation - Transitioning from Active Remedies

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Biodegradation - Reductive Processes

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

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REMChlor - MD

Admin — Thu, 07 May 2026 17:04:06 GMT

Matrix Diffusion

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Monitored Natural Attenuation - Transitioning from Active Remedies

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Monitored Natural Attenuation (MNA) of Chlorinated Solvents

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

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Chlorinated Solvents

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Monitored Natural Attenuation (MNA)

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Chlorinated Solvents

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Monitored Natural Attenuation (MNA)

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Chlorinated Solvents

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Articles

Debra Tabron — Tue, 28 Apr 2026 21:03:18 GMT

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

Debra Tabron — Tue, 28 Apr 2026 20:57:35 GMT

Dr. Charles Werth

Debra Tabron — Tue, 28 Apr 2026 20:56:54 GMT

Created page with "==Work and Contact Information== EMPLOYER: :University of Texas at Austin :Maseeh Department of Civil, Architectural and Environmental Engineering :301 E. Dean Keeton S..."

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==Work and Contact Information==

EMPLOYER:
:University of Texas at Austin 
:Maseeh Department of Civil, Architectural and Environmental Engineering
:301 E. Dean Keeton St. ECJ 4.200 
:Austin, Texas, 78712

EMAIL: [mailto:werth@utexas.edu werth@utexas.edu]

WEBPAGE: [https://caee.utexas.edu/person/charles-werth/ https://caee.utexas.edu/person/charles-werth/]

==About the Contributor==
Dr. Charles Werth is a Professor in the Department of Civil, Architecture, and Environmental Engineering at the University of Texas at Austin, with over 25 years of academic experience. Dr. Werth’s research and teaching focus on pollutant fate and transport, advanced catalytic technologies for drinking water treatment, and strategies to mitigate the environmental impacts of energy production.

==Article Contributions==
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]

__NOTOC__

[[Category: Contributors|Werth]]

Dani Tran

Debra Tabron — Tue, 28 Apr 2026 20:48:20 GMT

Created page with "==Work and Contact Information== EMPLOYER: :CDM Smith :Bellevue Research and Testing Laboratory :14432 SE Eastgate Way, Suite 100 :Bellevue, WA 98007 EMAIL: [mai..."

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==Work and Contact Information==

EMPLOYER:
:CDM Smith 
:Bellevue Research and Testing Laboratory
:14432 SE Eastgate Way, Suite 100 
:Bellevue, WA 98007

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

==About the Contributor==
Dani Tran is a research scientist with the CDM Smith Bellevue Research and Testing Laboratory. They hold a wide range of experience such as environmental compliance and documentation, execution and project management of both small and larger-scale projects, and maintenance of various analytical instruments (ion chromatography and gas chromatography). Since joining CDM Smith in 2017, they have led and assisted in a multitude of projects through DoD’s ESTCP and SERDP funding.

==Article Contributions==
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]

__NOTOC__

[[Category: Contributors|Tran]]

Polycyclic Aromatic Hydrocarbons (PAHs)

Admin — Tue, 28 Apr 2026 20:19:40 GMT

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

Admin — Tue, 28 Apr 2026 20:19:15 GMT

Dr. Charles Schaefer

Admin — Tue, 28 Apr 2026 20:15:17 GMT

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

Admin — Tue, 28 Apr 2026 20:14:28 GMT

Dr. Stephen Richardson

Admin — Tue, 28 Apr 2026 20:10:32 GMT

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

Jhurley — Tue, 28 Apr 2026 18:55:10 GMT

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

Jhurley — Tue, 28 Apr 2026 18:41:45 GMT

Created page with "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 trichloroeth..."

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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]

User:Jhurley/sandbox

Jhurley — Tue, 28 Apr 2026 18:37:23 GMT

File:TranFig1.png

Jhurley — Tue, 28 Apr 2026 18:32:24 GMT

Jhurley uploaded File:TranFig1.png

New page

User:Jhurley/sandbox

Jhurley — Tue, 28 Apr 2026 18:30:57 GMT

‎Recommended Approach

File:TranFig1.pdf

Jhurley — Tue, 28 Apr 2026 18:26:44 GMT

Jhurley uploaded File:TranFig1.pdf

New page

File:TranFig2.png

Jhurley — Tue, 28 Apr 2026 17:56:25 GMT

Jhurley uploaded File:TranFig2.png

New page

File:SchaeferEtAl2021.pdf

Jhurley — Tue, 28 Apr 2026 17:38:54 GMT

Jhurley uploaded File:SchaeferEtAl2021.pdf

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File:FaltaWang2017.pdf

Jhurley — Tue, 28 Apr 2026 17:38:03 GMT

Jhurley uploaded File:FaltaWang2017.pdf

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@@ Line 349: / Line 349: @@
 |-
 |[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
-|[[Dr. Charles Schaefer|Schaefer, Dr. Charles]]
+|[[Dani Tran|Tran, Dani]]||MNA, natural attenuation, natural attenuation processes, chlorinated solvents
 |-
 |}

← Older revision		Revision as of 17:08, 7 May 2026
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−	'''Contributor(s):'''	+	'''Contributor(s):''' [[Dr. John Wilson]] and [[Dr. David Adamson, P.E.]]
−
−	*[[Dr. John Wilson]]
−	*[[Dr. David Adamson, P.E.]]

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 *[[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]]
 *[[REMChlor - MD]]

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 *[[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):'''

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 *[[REMChlor - MD]]
 *[[Source Zone Modeling]]
 '''Contributor(s):'''
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 *[[Dr. John Wilson]]
 *[[Dr. David Adamson, P.E.]]
 '''Key Resource(s)''':

← Older revision		Revision as of 17:04, 7 May 2026
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	*[[Dispersion and Diffusion]]		*[[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]]		*[[Source Zone Modeling]]
−	*[[Plume Response Modeling]]
−	*[[Matrix Diffusion]]

@@ Line 3: / Line 3: @@
 '''Related Article(s):'''
+*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
 *[[Groundwater Flow and Solute Transport]]
 *[[REMChlor - MD]]
 '''Contributor(s):''' [[Dr. Charles Newell, P.E.|Dr. Charles Newell]] and  [[Dr. Robert Borden, P.E.|Dr. Robert Borden]]

@@ Line 10: / Line 10: @@
 *[[Chemical Reduction (In Situ - ISCR)]]
 *[[Design Tool - Base Addition for ERD]]
-*[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation]]
+*[[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]]

@@ Line 4: / Line 4: @@
 *[[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 Chlorinated Solvents| Monitored Natural Attenuation of Chlorinated Solvents]]
-*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| MNA of Metals and Metalloids]]
+*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| Monitored Natural Attenuation of Metals and Metalloids]]
-*[[Monitored Natural Attenuation (MNA) of Fuels|MNA of Petroleum Hydrocarbons and Fuel Components]]
+*[[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]]

@@ Line 13: / Line 13: @@
 *[[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 (MNA) of Chlorinated Solvents| Monitored Natural Attenuation of Chlorinated Solvents]]
 *[[pH Buffering in Aquifers]]
 *[[Remediation Performance Assessment at Chlorinated Solvent Sites]]

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 |-
 |[[PFAS Destruction by Ultraviolet/Sulfite Treatment]]||[[Dr. Yida Fang |Fang, Yida, Ph.D.]]||PFAS destruction
 |}

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 *[[REMChlor - MD]]
-'''Contributor(s):''' Dani Tran, [[Dr. Charles Schaefer]], Dr. Charles Werth
+'''Contributor(s):''' [[Dani Tran]], [[Dr. Charles Schaefer]], and [[Dr. Charles Werth]]
 '''Key Resource(s):'''

@@ Line 7: / Line 7: @@
 *[[Petroleum Hydrocarbons (PHCs)]]
 *[[Remediation Technologies]]
 '''Contributor(s):''' [[Dr. Stephen Richardson]]
 '''Key Resource(s):'''

← Older revision		Revision as of 20:15, 28 April 2026
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	==Article Contributions==		==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]]		*[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]

@@ Line 10: / Line 10: @@
 *[[REMChlor - MD]]
-'''Contributors:''' Dani Tran, Dr. Charles Schaefer, Dr. Charles Werth
+'''Contributors:''' Dani Tran, [[Dr. Charles Schaefer]], Dr. Charles Werth
 '''Key Resource:'''

@@ Line 10: / Line 10: @@
 ==About the Contributor==
-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.
+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==

@@ Line 1: / Line 1: @@
-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.
+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>
@@ Line 47: / Line 47: @@
 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.
 <br clear="right"/>

@@ Line 14: / Line 14: @@
 '''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 (3)
+*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==
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 ==Recommended Approach==
-[[File: TranFig1.png | thumb | 600 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]&nbsp; [[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]&nbsp; [[Media: SchaeferEtAl2017.pdf | Open Access Manuscript]]</ref>, and open squares from Schaefer ''et al.'', 2025<ref name="SchaeferEtAl2025"/>. ]]
+[[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]&nbsp; [[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]&nbsp; [[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:

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