The overarching objective of this project is to demonstrate the importance of in situ production of reactive mineral phases for abiotic reduction of chlorinated ethenes (CEs). Previous work in the laboratory under SERDP project ER-2621, has shown that abiotic reduction rates increase dramatically over time due to biogeochemical changes under some reducing conditions. This work has led us to hypothesize that at many field sites, there must be an ongoing formation of reactive phases to sustain abiotic reduction. This hypothesis will be tested using a suite of columns operating under different geochemical conditions. The experimental work has also suggested that sustained abiotic reduction requires sustained formation of those reactive phases (i.e., they are meta-stable), and the laboratory columns will be used ;to test this hypothesis as well. Using data collected from the columns, a decision tool will be developed for assessing the conditions under which abiotic reduction will become rapid and sustained. This tool will provide a more-rigorous approach to interpreting the relationship between site geochemistry and abiotic reduction rates than is currently available.
A set of complimentary laboratory column experiments will be used to assess abiotic TCE degradation under a variety of geochemical conditions. These columns will likely take ~6 months to develop stable biogeochemical conditions. Column reactivity (using a probe/contaminant with appropriate half-life, probably carbon tetrachloride) and column geochemistry will be tracked over time. Once stable conditions have been achieved, experiments with TCE will be conducted to observe degradation rates under: a) steady state, and b) stopped flow conditions. It is anticipated that substantial decreases in reactivity over several months for some column conditions will occur when column flow is stopped. Comparison of the steady state rates under a range of conditions, as well as the loss of reactivity when conditions change, will provide a practical framework for understanding the role of meta-stable phases in sustained abiotic reduction of TCE.
This project will improve the understanding of the potential long-term benefits of abiotic degradation of CEs in contaminated groundwater. This is important because abiotic degradation will likely play a critical role in successful long-term management of complex Department of Defense (DoD) sites. However, reliance on abiotic mechanisms requires a level of confidence in both the conceptual and quantitative understanding of performance that we, as a remediation community, do not yet have. The overall benefits of this project will be to advance the understanding of the factors controlling abiotic degradation of CEs and, as a result, improve confidence in the ability to predict long-term abiotic performance. (Project Completion - 2023).