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 (∼2‰) but highly reproducible 13C-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‰ and +2.4‰ for the methanogenic and sulfate-reducing cultures, respectively, corresponding to isotopic enrichment factors (ε) 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‰, 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 (∼2‰) fractionation to biodegradation at complex field sites will prove a challenge.