There are many impacted sites where aquifer restoration within a reasonable timeframe may be extremely difficult. This is often due to chemical back-diffusion from clayey zones. However, while it is recognized that naturally occurring abiotic and biotic dechlorination reactions in clays can have a substantial beneficial impact on natural attenuation timeframes, there is currently no generally accepted field screening technique to either verify or quantify these reactions. The overall goal of this work was to demonstrate and validate an innovative approach for assessing and quantifying naturally occurring abiotic and biotic dechlorination reactions in low-permeability clays. Such an approach would provide guidance to Remedial Project Managers and regulators with respect to assessing naturally occurring abiotic dechlorination at impacted sites.

This project employed multiple methods and techniques that have been demonstrated in previous laboratory and field projects to attain insight into a cost-effective and easily implementable in situ methodology that can be used to assess the extent to which naturally occurring abiotic and biotic dechlorination processes are occurring within clays (via a first-order rate constant). This in situ approach was utilized at eight sites selected for this project.

Transformation products within clays were below or near analytical detection limits, thus quantification of transformation products within field-collected clay samples could not be used to verify abiotic dechlorination. The use of 1% (v/v) hydrochloric acid extractions and X-ray diffraction for mineral composition provided information needed to estimate trichloroethene abiotic reductive dechlorination in clays. However, the model could not be independently used to verify the laboratory-derived rate constants. A correlation (Pearson r=0.75, p=0.033) was observed between hydroxyl radical generation under oxic conditions in bench-scale microcosms to the hydrogen peroxide concentration measured in situ at the clay-sand interface, suggesting a likely role for oxidative abiotic dechlorination at several of the tested sites. For field sampling and associated soil and groundwater characterization, the estimated cost for evaluation within a typical site is $9,825.

The field-scale and bench-scale sampling employed in this study were readily implementable. However, detection and quantification of abiotic transformation products (e.g., reduced gases such as acetylene, or organic acids) were challenging because these products are readily susceptible to further biotransformation. It also remains unclear how to translate measured hydroxyl radical generation rates measured in laboratory batch soil slurries to approximate first-order dechlorination rate constants near the clay-sand interface in situ. (Project Completion - 2025)

Schaefer, C.E., D. Tran, D. Nguyen, D .E. Latta, and C .J. Werth. 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils. Groundwater Monitoring & Remediation, 45(2): 31-39. doi.org/10.1111/gwmr.12709. 

Kumar, G., A .J. Valocchi, S. Dai, C .Y. Shih, Z. Zheng, C .E. Schaefer, C .J. Werth. 2025. Simultaneous Abiotic Oxidation and Reduction of Trichloroethylene by Reduced Iron Minerals in Low-Permeability Zones. ACS ES&T Water, 5(9): 5672-5684. doi.org/10.1021/acsestwater.5c00672.