Carbon isotope fractionation during anaerobic biodegradation of toluene: Implications for intrinsic bioremediation

Carbon isotope fractionation produced by anaerobic biodegradation of toluene was evaluated in laboratory experiments under both methanogenic and sulfate-reducing conditions. A small (similar to 2 parts per thousand) but highly reproducible C-13. enrichment in the residual toluene at advanced stages of microbial transformation was observed in both cultures. The maximum isotopic enrichment observed in the residual toluene was +2.0 parts per thousand and +2.4 parts per thousand for the methanogenic and sulfate-reducing cultures, respectively, corresponding to isotopic enrichment factors (epsilon) Of -0.5 and -0.8. Because the accuracy and reproducibility associated with gas chromatograph-combustion-isotope ratio mass spectrometry (GC/C/IRMS) is +/-0.5 parts per thousand, delineating which of these two terminal electron-accepting processes (TEAP) is responsible for the biodegradation of toluene at field sites will not be possible. However, the potential does exist to use compound-specific isotope analysis (CSIA), in conjunction with other methodologies, as a means of validating advanced stages of intrinsic bioremediation in anaerobic systems. Caution is urged that relating this small (similar to 2 parts per thousand) fractionation to biodegradation at complex field sites will prove a challenge.