Stable carbon isotope fractionation during enhanced in situ bioremediation of trichloroethene

Time-series stable carbon isotope monitoring of volatile organic compounds (VOCs) at the Idaho National Engineering and Environmental Laboratory's (INEEL) field site Test Area North (TAN) was conducted during a pilot study to investigate the treatment potential of using lactate to stimulate in situ biologic reductive dechlorination of trichloroethene (TCE). The isotope ratios of TCE and its biodegradation by products, cis-dichloroethene (c-DCE), trans-dichloroethene (t-DCE), vinyl chloride (VC), and ethene, in groundwater samples collected during the pilot study were preconcentrated with a combination of purge-and-trap and cryogenic techniques in order to allow for reproducible isotopic measurements of the low concentrations of these compounds in the samples (down to 0.04 muM, or 5 ppb, of TCE). Compound-specific stable isotope monitoring of chlorinated solvents clearly differentiated between the effects of groundwater transport, dissolution of DNAPL at the source, and enhanced bioremediation. Isotope data from all wells within the zone of lactate influence exhibited large kinetic isotope effects during the reduction of c-DCE to VC and VC to ethene. Despite these large effects, the carbon isotope ratio of ethene in all these wells reached the carbon isotope ratios of the initial dissolved TCE, confirming the complete conversion of dissolved TCE to ethene. Conversely, the carbon isotope ratios of t-DCE were only marginally affected during the study, indicating that minimal biologic degradation of t-DCE was occurring.