While it has become recognized that naturally occurring abiotic and biotic dechlorination reactions in clays can have a substantial beneficial impact on natural attenuation time frames in contaminated aquifers, there is currently no generally accepted field screening technique to either verify or quantify these reactions. Thus, the overall goal of this project is to demonstrate and validate an innovative approach for assessing and quantifying naturally-occurring abiotic and biotic dechlorination reactions in low-permeability clays. Specific objectives are to:

1. Demonstrate that clay ferrous mineral content can be used to verify and quantify abiotic dechlorination in clays, thereby serving as a simple screening tool;
2. Demonstrate that compound specific isotopic analysis (CSIA) for carbon can be used to both verify and estimate overall dechlorination rates in clays;
3. Determine the extent to which dechlorination transformation products generated in situ within the clay matrix can be used as a line of evidence for naturally occurring dechlorination reactions; and
4. Develop guidelines and disseminate results of this demonstration to regulators and Remedial Program Managers (RPMs).

The overall technical approach is to use 3 parallel field screening techniques to not only serve as lines of evidence for trichloroethene (TCE) dechlorination in clays, but also to provide a screening level estimate of the overall (biotic + abiotic) dechlorination rate constant. These field screening techniques will be applied at up to 10 TCE-impacted sites where contaminant uptake in clay is likely sustaining groundwater plumes. For each site, the ability of the field screening techniques to both identify and quantify dechlorination in the clay will be validated by performing bench-scale batch tests on collected clay; these bench-scale tests (developed in part through previous and ongoing SERDP research efforts) serve as a means to more rigorously determine dechlorination rate constants in clays, but with much greater effort than the field screening methods.

The first field screening technique involves measurement of the clay ferrous mineral content using the 1,10-phenanthroline method, which has been shown to correlate with abiotic dechlorination activity in clay and rock matrices. Determination of the ferrous mineral content will provide an estimate of the abiotic dechlorination rate constant for TCE. The second field screening technique involves identifying and quantifying the expected abiotic and biotic TCE dechlorination transformation products, including lesser chlorinated ethenes, reduced gases, and organic acids. The latter are expected oxidative transformation products under aerobic conditions that can be generated either abiotically or biotically. Coupled diffusion and reaction modeling can then be applied to provide an estimate of the dechlorination rate constant. The third field screening technique involves use of CSIA (carbon) for chlorinated ethenes. Evaluation of the isotopic mass balance in the clay will be compared to the isotopic mass balance in the adjacent high permeability zone to verify dechlorination in the clay, and to provide an estimate of the overall dechlorination rate constant.

This demonstration is expected to provide the Department of Defense (DoD) with a valuable data set that is expected to validate the use of readily applied screening tools for verifying and quantifying biotic and abiotic dechlorination in clays. Such information will serve as a critical component in conceptual site models, and will help RPMs make the case for full or partial monitored natural attenuation remedies, when appropriate. (Anticipated Project Completion - 2024)