Carbon Isotope Fractionation during Microbial Dechlorination of Trichloroethene,<i>cis</i>-1,2-Dichloroethene, and Vinyl Chloride: Implications for Assessment of Natural Attenuation

Carbon isotope fractionation during dechlorination of chlorinated ethenes was investigated using a methanogenic microbial enrichment culture. Subcultures were amended with trichloroethene (TCE), <i>cis</i>-1,2-dichloroethene (<i>cis</i>-DCE), and vinyl chloride (VC), respectively. Carbon isotope ratios and concentrations of reactants and of all dechlorination products were monitored during two experiments. All dechlorination steps were accompanied by significant isotope fractionation. The isotope ratios of the reactants were described with a Rayleigh type model, and the following enrichment factors (∈P/R) were obtained: -6.6 and -2.5‰ for dechlorination of TCE, -14.1 and -16.1‰ for dechlorination of <i>cis</i>-DCE, and-26.6 and-21.5‰ for dechlorination of VC. Isotope and mass balances suggested that ethene (ETH) was degraded. In additional experiments with ETH as reactant, ETH became enriched in ¹³C as its concentration decreased indicating the cultures were capable of degrading ETH. The average value for the enrichment factor obtained for the degradation of ETH was -3.0‰. The large carbon isotope fractionation observed in this study confirms that carbon isotope ratios are a sensitive tool for monitoring dechlorination of chlorinated ethenes to nontoxic end products.