During the past decade the application of monitored natural attenuation has become one of the predominant technologies used in the remediation of gasoline spills impacting subsurface soils and groundwater. The success of this method has depended, for the most part, on the biodegradation of those gasoline constituents that dissolve into groundwater and transport with the groundwater most readily.

One of the most mobile components of gasoline formulations during the past 20 years has been methyl tertiary butyl ether (MTBE), which has traditionally been viewed in both the scientific and the regulatory communities as relatively recalcitrant to biodegradation. However, cases of both in and ex situ MTBE biodegradation have recently been documented. In order to better understand and perhaps enhance the process of in situ MTBE biodegradation, a fundamental understanding of where, when and under what hydrogeological conditions MTBE biodegradation occurs is needed. To this end, the northeastern United States offers a wide range of lithologies to study. This study examines which subsurface conditions are most conducive to MTBE biodegradation.

Retail gasoline service stations with leaking underground storage tank (LUST) releases located within a variety of hydrogeological conditions in the northeastern United States were screened for lithology, MTBE concentration in ground water, monitoring well network location, and MTBE concentration trends. Ground water samples were collected from those sites that passed this initial screening phase. MTBE from the samples collected were analyzed using stable carbon isotope ratio analysis (SCIRA) to determine where biodegradation is occurring. Geochemistry in each well was also examined to determine which hydrogeological conditions are most conducive to MTBE biodegradation.

MTBE biodegradation under a variety of subsurface conditions was observed in this study. Loose soil conditions combined with reduced aquifer redox potential exhibited the greatest MTBE biodegradation frequency.

Although statistically significant correlations were not found with respect to MTBE biodegradation and aquifer geochemistry, scatter plots showed notable trends with respect to pH, dissolved oxygen, and methane concentration correlated with MTBE biodegradation. Increased biodegradation frequency in the presence of increasing methane concentrations under varied aquifer redox and lithological conditions was observed.