Biogeochemical Processes that Control Natural Attenuation of Trichloroethylene in Low Permeability Zones

Dr. Charles Werth | University of Texas at Austin

ER-2530

Objective

ER-2530 Project Graphic 1

Photo of flow cell with overlaid illustration of trichloroethylene (TCE) reaction pathways in high and low permeability zones

The most challenging hazardous waste sites for groundwater remediation are those with dense non-aqueous phase liquid (DNAPL) sources in highly heterogeneous aquifers. At Department of Defense (DoD) sites with a history of contamination, DNAPL has slowly dissolved from trapped ganglia or pools resulting in groundwater plumes with high levels of contamination. Dissolved contaminants in high permeability zones (HPZs) within these plumes can diffuse over long time periods into low permeability zones (LPZs) such as rock matrices, clay aquitards and lenses, i.e., “matrix diffusion.” When contaminant mass in the HPZ is depleted through natural attenuation or active remediation, back-diffusion of contaminants from LPZs into groundwater flowing through HPZs can occur for decades.

The objectives of this project are to quantify the biotic and abiotic attenuation mechanisms that impact the fate of trichloroethylene (TCE) within and at the boundaries of LPZs comprised of clays and silts, and to incorporate these processes into a computationally efficient model that can be used to directly address key questions regarding natural attenuation time scales and cleanup at TCE-impacted sites. The team hypothesizes that a suite of naturally occurring biotic and abiotic mechanisms may serve to significantly attenuate diffusive flux of TCE and TCE degradation products from LPZs over long time periods, resulting in decreased impacts (both magnitude and timeframe) of back-diffusion on groundwater quality. These mechanisms may occur within the LPZ itself, or at the LPZ-HPZ interface. The team also hypothesizes that biogeochemical conditions (e.g., reducing, aerobic, Fe minerals, organic carbon) will impact these naturally occurring mechanisms.

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Technical Approach

This project will elucidate the critical physical, chemical, and biological processes that control natural attenuation of chlorinated ethenes in LPZs, and result in the development of a computationally efficient model that can be used to assess the impact of natural attenuation mechanisms on back diffusion from LPZs. In Task 1 core samples will be collected at two DoD field sites where back diffusion is suspected of prolonging site cleanup. The team will sample both LPZs and HPZs, and characterize contaminant concentrations, mineralogy, aqueous species, and microbial populations to help interpret batch and flow cell experimental results. In Task 2, natural attenuation reaction mechanisms will be evaluated in well-mixed batch studies, where mass transfer limitations are minimized. The team will use 14C-TCE and stable isotope probing to quantify reaction mechanisms and pathways, and will use this information to help guide flow cell experiments and numerical model development. In Tasks 3 and 4, the team will evaluate natural attenuation mechanisms in flow cells, where diffusion controls mass transport in LPZs. The effects of pore size, mineralogy, microbiology, and electron shuttling will be evaluated along the redox gradient in LPZs. As necessary, radio-labeled TCE and stable isotope probing will help quantify the effects of diffusion limitations on coupled reaction mechanisms, and LPZ high resolution sectioning and analysis of spatially distributed mineralogy, microbiology, and contaminant concentrations will contribute to this forensic effort. In Task 5, the team will develop a computationally efficient numeral model to simulate flow cell experiments, and use this to probe important reaction mechanisms in the field.

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Benefits

The results will provide a more clear understanding and quantification of reaction mechanisms that contribute to natural attenuation in LPZs, and the corresponding effects of biogeochemical parameters, i.e., grain/pore size, microbial population, mineralogy, redox potential, and mass transfer limitations. For example, a clear understanding of the role of oxidation versus reductive transformation processes for TCE degradation is lacking, which will impact TCE fate and potentially any cleanup technology selected for partial or full-site remediation. The project will develop a computationally efficient and accessible model that will allow scientists and engineers to interpret controlling mechanisms at field sites, and provide recommendations for what data is needed to parameterize the model, determine the controlling mechanisms, and predict plume longevity under different site management scenarios. These outcomes will help improve management of chlorinated solvent plumes and refine tools for selecting how and where remediation resources should be allocated, and provide for improved justification of monitored natural attenuation (MNA) due to better insight into the naturally occurring degradation mechanisms present in LPZs. (Anticipated Project Completion - 2019)

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Publications

Schaefer, C.E., P. Ho, C. Gurr, E. Berns, and C. Werth. 2017. Abiotic Dechlorination of Chlorinated Ethenes in Natural Clayey Soils: Impacts of Mineralogy and Temperature. Journal of Contaminant Hydrology, 206:10-17.

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Points of Contact

Principal Investigator

Dr. Charles Werth

University of Texas at Austin

Phone: 512-232-1626

Program Manager

Environmental Restoration

SERDP and ESTCP

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