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COMPLETE NATURAL ATTENUATION OF A PCE AND TCE PLUME AFTER SOURCE REMOVAL
Citation:
FERREY, M., C. J. ADAIR, R. G. FORD, R. T. WILKIN, AND J. T. WILSON. COMPLETE NATURAL ATTENUATION OF A PCE AND TCE PLUME AFTER SOURCE REMOVAL. Presented at Fifth International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Monterey, CA, May 22 - 25, 2006.
Impact/Purpose:
To inform the public.
Description:
Disposal of the chlorinated solvents PCE and TCE at the Twin Cities Army Ammunition Plant (TCAAP) resulted in the contamination of groundwater in a shallow, unconsolidated sand aquifer. The resulting plume had moved over 1000 feet from the disposal source area and had impacted private wells adjacent to the TCAAP property. When the contamination was discovered in the late 1980s, the U.S. Army installed a recovery well and a system of containment wells, preventing further downgradient contamination of the aquifer. The presence of cis-DCE indicated that reductive dehalogenation of the parent compounds was occurring. However, the absence of vinyl chloride and ethene, in addition to an unfavorable ground water geochemistry, suggested that the biological reductive dehalogenation was weak and incomplete. Dilution appeared to be the only mechanism for the restoration of the downgradient ground water. However, subsequent detailed ground water modeling showed that dilution alone could not account for the size and distribution of the contaminant plume. To calibrate the transport and fate model to the plume, it was necessary to model the transformation and removal of the chlorinated solvents at a first order rate near 0.6 per year. In microcosm studies done with sediment from another spill site in the same aquifer, the overall rate of removal of TCE was 1.55 ± 0.26 per year. The overall rate of removal in autoclaved controls was 1.65 ± 0.67 per year. The rate of loss from control microcosms that contained sterile water but no sediment was 0.52 ± 0.14 per year. Some abiotic transformation processes removed TCE in the sediment at a rate of 1.0 per year. Cis-DCE or vinyl chloride did not accumulate in the sediment. Magnetite is probably responsible for the abiotic transformation of TCE. The site at TCAAP has higher magnetic susceptibility than the sediment from the other site used for the microcosm study. New microcosm studies constructed with material from the TCAAP are in progress. Results will be presented at the Battelle Conference. Results of the modeling study indicated that if the source of the contamination was removed, concentrations of chlorinated solvents in the plume should collapse to drinking water standards within three years. The contaminant source was completely excavated to the water table in 2003. Groundwater sampling since the excavation indicate that contaminant concentrations are decreasing at a rate consistent with the predictions made by the ground water modeling. The plume essentially disappeared as of summer 2005. The results of this study show that removal of the contaminant source can make a dramatic contribution to the restoration of ground water in a relatively short time period.