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NON-BIOLOGICAL REACTIONS MAY BE AN IMPORTANT MECHANISM FOR NATURAL ATTENUATION OF CHLORINATED SOLVENTS
Citation:
Wilson*, J T. NON-BIOLOGICAL REACTIONS MAY BE AN IMPORTANT MECHANISM FOR NATURAL ATTENUATION OF CHLORINATED SOLVENTS. Presented at Partners in Environmental Technology/Technical Symposium & Workshop, Washington, DC, 12/2-4/2003.
Description:
The role of non-biological transformation of chlorinated solvents was not considered in the U.S. EPA Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water. This deficiency became very apparent when U.S. EPA conducted a beta test of the Protocol on a large TCE plume that originated at the Twin Cities Army Ammunition Plant in Minnesota. Mathematical modeling suggested that a first order rate of removal of cis-DCE and 1,1-DCE near 0.2 per year was necessary to explain the distribution of the existing contamination. The plume was mildly anaerobic. Iron II and Manganese II were present in the ground water, but concentrations of methane were low and sulfide was absent. TCE was readily dechlorinated to cis-DCE and to 1,1-DCE. Although the concentrations of cis-DCE and 1,1-DCE attenuated over time in monitoring wells, and with distance down gradient of the source area, the plume never had appreciable concentrations of vinyl chloride or ethane or ethane. Biological reductive dechlorination could not be used to explain the removal of cis-DCE and 1,1-DCE.
To identify the processes responsible for removal, laboratory microcosms were constructed with sediment from the aquifer near the source of the plume. To distinguish non-biological from biological processes, the sediment in one experimental treatment was heat sterilized. In samples from different depth intervals, the first order rate of removal of cis-DCE varied from 0.31 to 2.31 per year. The rate of removal in sterile water controls was 0.071 per year. No significant difference was observed between rates of removal for sterilized and non-sterilized sediment. For 1,1-DCE, the rate of removal was 1.29 and 1.37 per year in non-sterilized and heat-killed sediments, respectively, while the rate of removal for 1,1-DCE in the container controls was 0.205 per year. The rates of removal in the microcosms are similar to the rates of attenuation in the field. Magnetite was identified in the sediment by X-ray diffraction and optical microscopy. At the concentration of magnetite in the sediment, and the concentration of Iron II in the ground water in the plume, published literature on the rate of non-biological transformation of cis-DCE predicts rates of transformation that are consistent with the rates of removal in the microcosm study.
Many plumes at DoD sites fall into the same pattern as the plume at TCAAP. Although (1) ground water flow is rapid and a large plume develops, and (2) PCE and TCE are readily transformed to cis-DCE, and (3) the plume of cis-DCE also attenuates, (4) the plume of vinyl chloride is small or absent and is contained within the plume of cis-DCE.