Carbon and Chlorine Isotope Fractionation During Microbial Degradation of Tetra- and Trichloroethene

Two-dimensional compound-specific isotope analysis (2D-CSIA), combining stable carbon and chlorine isotopes, holds potential for monitoring of natural attenuation of chlorinated ethenes (CEs), in contaminated soil and groundwater. However, interpretation of 2D-CSIA data sets is challenged by a shortage of experimental Cl isotope enrichment factors. Here, isotope enrichments factors for C and Cl (i.e., epsilon(C) and epsilon(Cl)) were determined for biodegradation of tetrachloroethene (PCE) and trichloroethene (TCE) using microbial enrichment cultures from a heavily CE-contaminated aquifer. The obtained values were epsilon(C) = -5.6 +/- 0.7 parts per thousand (95% CI) and epsilon(Cl) = -2.0 +/- 0.5 parts per thousand for PCE degradation and epsilon(C) = -8.8 +/- 0.2 parts per thousand and epsilon(Cl) = -3.5 +/- 0.5 parts per thousand for TCE degradation. Combining the values for both epsilon(C) and epsilon(Cl) yielded mechanism-diagnostic epsilon(Cl)/epsilon(C) ratios of 0.35 +/- 0.11 and 0.37 +/- 0.11 degradation of PCE and TCE, respectively. Application of the obtained epsilon(C) and epsilon(Cl) values to a previously investigated field site gave similar estimates for the fraction of degraded contaminant as in the previous study, but with a reduced uncertainty in assessment of the natural attenuation. Furthermore, 16S rRNA gene clone library analyses were performed on three samples from the PCE degradation experiments. A species closely related to Desulfitobacterium aromaticivorans UKTL dominated the reductive dechlorination process. This study contributes to the development of 2D-CSIA as a tool for evaluating remediation strategies of CEs at contaminated sites.