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

Carbon isotope fractionation produced by anaerobic biodegradation of toluen e was evaluated in laboratory experiments under both methanogenic and sulfa te-reducing conditions. A small (similar to 2 parts per thousand) but highl y reproducible C-13. enrichment in the residual toluene at advanced stages of microbial transformation was observed in both cultures. The maximum isot opic enrichment observed in the residual toluene was +2.0 parts per thousan d and +2.4 parts per thousand for the methanogenic and sulfate-reducing cul tures, 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) i s +/-0.5 parts per thousand, delineating which of these two terminal electr on-accepting processes (TEAP) is responsible for the biodegradation of tolu ene 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 biore mediation 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.