Evaluation of TCE and MTBE in situ Biodegradation: Integrating Stable Isotope, Metabolic Intermediate, and Microbial Lines of Evidence

Compound specific isotope analysis (CSIA) was used to investigate biodegradation of trichloroethene (TCE) and methyl tert-butyl ether (MTBE) at contaminated field sites in Alaska and New York State, respectively. At both sites, geochemical conditions and the presence of metabolic intermediates (cis-1-2-dichloroethene and tert-butyl alcohol [TBA]) suggested the potential for biodegradation of TCE and MTBE, respectively. Given that in both cases these metabolic intermediates could also have been present as cocontaminants in the source zone, CSIA was undertaken to evaluate the possibility of in situ biodegradation. At the TCE-contaminated field site in Alaska, δ¹³C values of TCE in ground water determined in this study showed no evidence of biodegradation (mean δ¹³C of -27.0 ± 1.0[per thousand] for nine wells), and quantitative-polymerase chain reaction analyses of ground water from four wells found no evidence of dechlorinator Dehalococcoides sp. at this site. At the MTBE-contaminated field site in New York, TBA was present in the ground water but was not present in gasoline sampled from underground storage tanks (UST) on-site, suggesting that at this site, TBA was potentially a metabolite of MTBE biodegradation rather than a cocontaminant. However, at all sampling times and locations, δ¹³C and δ²H values of MTBE in ground water were within range of published values for undegraded MTBE in gasoline. While the occurrence of a small extent of in situ MTBE biodegradation cannot be ruled out, the findings suggest that it is more likely that multiple gasoline spills occurred through time, and while present day USTs do not contain TBA as a cocontaminant, gasoline spilled at the site in the past may have. At both contaminated field sites, CSIA, chemical, and microbiological lines of evidence suggest that biodegradation was not a significant attenuation process. The results of these two studies underscore the need for an integrated approach to site assessment that draws on measurements of metabolic intermediates, analysis of stable isotopes, and microbial evidence to give a reliable assessment of in situ biodegradation at contaminated field sites.