U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12, 1999--Volume 3 of 3--Subsurface Contamination From Point Sources, Water-Resources Investigations Report 99-4018C

Fate of MTBE Relative to Benzene in a Gasoline-Contaminated Aquifer (1993-98)

By James E. Landmeyer, Paul M. Bradley, and Francis H. Chapelle

ABSTRACT

Methyl tert-butyl ether (MTBE) and benzene have been measured since 1993 in a shallow, sandy aquifer contaminated by a mid-1980's release of gasoline containing fuel oxygenates. In wells downgradient of the release area, MTBE was detected before benzene, reflecting a chromatographic-like separation of these compounds in the direction of ground-water flow. Higher concentrations of MTBE and benzene were measured in the deeper sampling ports of multilevel sampling wells located near the release area, and also up to 10 feet (3 meters) below the water-table surface in nested wells located farther from the release area. This distribution of higher concentrations at depth is caused by recharge events that deflect originally horizontal ground-water flowlines. In the laboratory, microcosms containing aquifer material incubated with uniformly labeled ¹⁴C-MTBE under aerobic and anaerobic, Fe(III)-reducing conditions indicated a low but measurable biodegradation potential ¹⁴C-MTBE as ¹⁴CO₂) after a seven-month incubation period. Tert-butyl alcohol (TBA), a proposed microbial-MTBE transformation intermediate, was detected in MTBE-contaminated wells, but TBA was also measured in unsaturated release-area sediments. This suggests that TBA may have been present in the original fuel spilled and not necessarily reflect microbial degradation of MTBE. Combined, these data suggest that milligram per liter to microgram per liter decreases in MTBE concentrations relative to benzene are caused by the natural attenuation processes of dilution and dispersion with less-contaminated ground water in the direction of flow rather than biodegradation at this point-source gasoline release site.