Stable carbon isotope evidence for intrinsic bioremediation of tetrachloroethene and trichloroethene at area 6, Dover Air Force Base

Area 6 at Dover Air Force Base (Dover, DE)has been the location of an in-depth study by the RTDF(Remediation Technologies Development Forum Bioremediation of Chlorinated Solvents Action Team) to evaluate the effectiveness of natural attenuation of chlorinated ethene contamination in groundwater. Compound-specific stable carbon isotope measurements for dissolved PCE and TCE in wells distributed throughout the anaerobic portion of the plume confirm that stable carbon isotope values are isotopically enriched in C-13 consistent with the effects of intrinsic biodegradation. During anaerobic microbial reductive dechlorination of chlorinated hydrocarbons, the light(C-12) versus heavy isotope (C-13) bonds are preferentially degraded, resulting in isotopic enrichment of the residual contaminant in C-13. To our knowledge, this study is the first to provide definitive evidence for reductive dechlorination of chlorinated hydrocarbons at a field site based on the delta C-13 Values of the primary contaminants spilled at the Site PCE and TCE. For TCE downgradient wells show delta C-13 values as enriched as -18.0 parts per thousand as compared to delta C-13 values for TCE in the source zone of -25.0 to -26.0 parts per thousand. The most enriched delta C-13 value on the site was observed at well 236, which also contains the highest concentrations of cis-DCE, VC, and ethene,the daughter products of reductive dechlorination. Stable carbon isotope signatures are used to quantify the relative extent of biodegradation between zones of the contaminant plume. On the basis of this approach, it is estimated that TCE in downgradient well 236 is more than 40% biodegraded relative to TCE in the proposed source area.