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 overall goal of this research was to quantify the biotic and abiotic attenuation mechanisms that impact the fate and transport of trichloroethene (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.

Batch studies were performed to evaluate the biogeochemical conditions that promote abiotic TCE reaction under anaerobic and aerobic conditions in natural soils. These included evaluation of TCE reduction and oxidation using pure minerals alone in solution, and in the presence of iron or sulfur reducing bacteria, and evaluation of TCE reduction and oxidation using natural clayey soils known to contain Fe(II) minerals. TCE reaction rates in batch solutions were determined by monitoring TCE decay or reaction products. These were complemented with batch solution and mineral characterization, which involved measurement of dissolved and sorbed Fe(II), mineralogy using X-ray diffraction, mineral surface oxidation state using X-ray photoelectron spectroscopy, and oxidation reduction potential. 

Flow cell studies were performed to determine the contribution of abiotic TCE reactions in the presence of an added electron donor and dechlorinating bacteria. The results were simulated using a newly developed and process-based numerical model. The model was used to assess the relevance of abiotic reaction in the flow cell and in larger systems using abiotic reaction rate constants from the flow cell, as well as from the literature.

Batch study results show that a variety of minerals promote abiotic TCE reduction and oxidation, including Fe(II) carbonates, Fe(II) sulfides, and Fe(II)-containing illite. They also show that TCE reduction rates with iron sulfides are enhanced in the presence of either sulfate or iron-reducing bacteria. TCE reduction rate constants for pure minerals and clayey-soils generally increased with the amount of sorbed Fe(II) determined using a weak-acid extraction, but they were not related to mineral or clay surface areas or magnetic susceptibility. TCE reduction rate constants for pure minerals generally increases with decreasing redox potential of soil-solution slurries. However, this relationship was different for different minerals, indicating both reduction potential and mineralogy are important. Flow cell results show that biological reduction dominated TCE reduction, and abiotic TCE reduction was sustained concurrently at a lower level. Flow cell and larger scale modeling results indicate that in the absence of an added electron donor, abiotic TCE reduction can reduce the TCE back diffusion flux from LPZs by as much as 55%. A simplified method for incorporating reaction and back diffusion from clays was incorporated into reactive transport over three dimensions model.

Results from these studies indicate that abiotic TCE reduction and oxidation are both important natural attenuation pathways for TCE in low permeability clays, and that more reducing conditions favor formation of a suite of Fe(II) minerals that contribute to these reaction pathways.

These results provide a more clear understanding and quantification of reaction mechanisms that contribute to natural attenuation in LPZs, and the corresponding effects of biogeochemical parameters such as grain/pore size, microbial population, mineralogy, redox potential, and mass transfer limitations. This information helps to 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.

Berns, E.C., R.A. Sanford, A.J. Valocchi, T.J. Strathmann, C.E. Schaefer, C.J. Werth. 2019. Contributions of Biotic and Abiotic Pathways to Anaerobic Trichloroethene Transformation in Low Permeability Source Zones. Journal of Contaminant Hydrology, 224:103480. https://doi.org/10.1016/j.jconhyd.2019.04.003

Esfahani, S.G., A.J. Valocchi, and C.J. Werth. 2021. Using MODFLOW and RT3D to Simulate Diffusion and Reaction without Discretizing Low Permeability Zones. Journal of Contaminant Hydrology, 239:103777. https://doi.org/10.1016/j.jconhyd.2021.103777

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.

Schaefer, C.E., P. Ho, E. Berns, and C.J. Werth. 2018. Mechanisms for Abiotic Dechlorination of TCE by Ferrous Minerals under Oxic and Anoxic Conditions in Natural Sediments. Environmental Science & Technology, 52(23):13747-13755.

Schaefer, C.E., P. Ho, E. Berns, and C.J. Werth. 2021. Abiotic Dechlorination in the Presence of Ferrous Minerals, Journal of Contaminant Hydrology,  241:103839.

Shuo, Y. and A.J. Valocchi. 2020. Flux‐corrected Transport with MT3DMS for Positive Solution of Transport with Full‐tensor Dispersion. Groundwater, 58(3):338-348.