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FRACTIONATION OF STABLE CARBON ISOTOPES DURING ABIOTIC TRANSFORMATION OF TCE
Citation:
WILSON, J. T., C. J. ADAIR, R. G. FORD, R. T. WILKIN, T. KUDER, R. P. PHILP, AND M. FERREY. FRACTIONATION OF STABLE CARBON ISOTOPES DURING ABIOTIC TRANSFORMATION OF TCE. 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:
At a Superfund Site in Minnesota, ground water is contaminated with trichloroethylene (TCE) with the contaminant plume stretching over five miles long. The ground water is iron and manganese reducing, and the complete absence of dichloroethylene, vinyl chloride, and ethene in the ground water suggests that there is no biological reductive dehalogenation of TCE occurring in the ground water. Microcosms were constructed with ground water sediment collected from the site. One half of the microcosms were heat-killed prior to incubation. Trichloroethylene was added at an initial concentration of 800 µg L-1, and the microcosms were incubated in an anaerobe chamber for 400 days. TCE concentrations declined at similar rates in both the heat-killed and live microcosms. In the heat-killed microcosms, the first order rate of removal of TCE was 1.65 ± 0.673 per year (95% confidence), while the rate in the living microcosms was 1.55 ± 0.263. The rate of disappearance of TCE in container controls constructed with sterile water and no sediment was 0.51 ± 0.136 per year. Cis-dichloroethylene, vinyl chloride, ethene, ethane, and acetylene were not detected in the microcosms. The first order rate of removal that could be attributed to an abiotic transformation process was near one per year. Stable 13C isotope ratio analysis showed an overall enrichment of 13C as the concentrations of TCE declined in the microcosms. The stable carbon isotope ratio of the TCE added to the microcosms was -30.80‰. After 397 days of incubation, the stable carbon isotope ratio in TCE in the sediment microcosms ranged from -28.40‰ to -25.50‰. The isotopic enrichment factor (ε) during abiotic removal of the TCE was -3.2 ± 0.8440 at 95% confidence. The isotopic enrichment factor for carbon isotopes during the abiotic process is at the low end of the range reported in the literature for enrichment factors for biological reductive dechlorination of TCE. Mineralogical analysis of the sediment showed a magnetic susceptibility of 4.4x10-7 kg m-3, indicative of high concentrations of magnetite in the sample. Total iron concentrations were 12,000 mg/kg. It is likely that magnetite in the sediment is responsible for the abiotic transformation of TCE. Often plumes of TCE appear to attenuate with distance from the source. If biological transformations products such as cis-DCE, vinyl chloride, or ethene and ethane do not accumulate in the down gradient portion of the plume, the attenuation in concentration is usually attributed to dilution or dispersion. If the abiotic process examined in the laboratory study removes more than fifty percent of the TCE from a plume of contamination at field scale, it should be possible to document that loss by a shift in the stable carbon isotope ratio in TCE in the down gradient portions of the plume.