Variability in carbon isotopic fractionation during biodegradation of chlorinated ethenes: Implications for field applications

Stable carbon isotopic analysis has the potential to assess biodegradation of chlorinated ethenes. Significant isotopic shifts, which cah be described by Rayleigh. enrichment factors, have been observed for the biodegradation of trichloroethlyene (TCE), cis-dichloroethylene (cDCE), and,viny[ chlorid e (VC). However, until this time, no systematic investigation of isotopic f ractionation during perchloroethylene (PCE) degradation has been undertaken . In addition, there has been no comparison of isotopic fractionation by di fferent microbial consortia, nor has there been a comparison of isotopic fr actionation by consortia generated from the same source; but growing under different conditions. This study characterized carbon isotopic fractionatio n during reductive dechlorination of the chlorinated ethenes, PCE in partic ular, far microbial consortia from two different sources growing under diff erent environmental conditions in order to assess the extent to which diffe rent microbial consortia result in different fractionation factors. Rayleig h enrichment factors of -13.8 parts per thousand, -20.4 parts per thousand, and -22.4 parts per thousand were observed for TCE, cDCE, and VC, respecti vely, for dechlorination by the KB-1 consortium. In contrast, isotopic frac tionation during reductive dechlorination of perchloroethylene (PCE) could not always be approximated by a Rayleigh model. Dechlorination by one conso rtium followed Rayleigh behavior (epsilon = -5.2), while a systematic chang e in the enrichment factor was observed-over the course of PCE degradation by two other consortia. Comparison of all reported enrichment factors for r eductive dechlorination of the chlorinated ethenes shows significant Variat ion between experiments. Despite this variability,:these results demonstrat e that carbon isotopic analysis can provide qualitative evidence of the occ urrence and relative extent of microbial reductive dechlorination of the ch lorinated ethenes.