Use of Compound-Specific Stable Carbon Isotope Analyses To Demonstrate Anaerobic Biodegradation of MTBE in Groundwater at a Gasoline Release Site

Currently it is unclear if natural attenuation is an appropriate remedial approach for groundwater impacted by methyl tertiary butyl ether (MTBE). Site-characterization data at most gasoline release sites are adequate to evaluate attenuation in MTBE concentrations over time or distance. But, demonstrating natural biodegradation of MTBE requires laboratory microcosm studies, which could be expensive and time-consuming. Recently, compound-specific carbon isotope ratio analyses (13C/12C expressed in δ13C notation) have been used to demonstrate aerobic biodegradation of MTBE in laboratory incubations. This study explored the potential of this approach to distinguish MTBE biodegradation from other abiotic processes in an anaerobic groundwater plume that showed extensive decrease in MTBE concentrations. To our knowledge, this is the first study to use δ13C of MTBE data in groundwater and laboratory microcosms to demonstrate anaerobic biodegradation of MTBE. The δ13C of MTBE in monitoring wells increased by up to 31‰ (−25.5‰ to +5.5‰) along with a 40-fold decrease in MTBE concentrations. Anaerobic incubations in laboratory microcosms indicated up to 20-fold reduction in MTBE concentrations with a corresponding increase in δ13C of MTBE of up to 33.4 ‰ (−28.7‰ to +4.7‰) in live microcosms. Little enrichment was observed in autoclaved controls. These results demonstrate that anaerobic biodegradation was the dominant natural attenuation mechanism for MTBE at this site. The estimated isotopic enrichment factors (εfield = −8.10‰ and εlab = −9.16‰) were considerably larger than the range (−1.4‰ to −2.4‰) previously reported for aerobic biodegradation of MTBE in laboratory incubations. These observations strongly suggest that δ13C of MTBE could be potentially useful as an “indicator” of in-situ MTBE biodegradation.