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

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

Methyl tert-butyl ether (MTBE) and benzene have been measured since 1993 in a shallow, sandy aquifer contaminated by a late-1980's release of gasoline. 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. 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. A pdf file is available for download (1.1Mb).

The transport of MTBE was simulated using two numerical modeling approaches. One approach was based on varying the first-order biodegradation rate constant in space to account for zones of strongly anaerobic ground water within the plume and adjacent oxygenated water outside the plume boundary. A copy of U.S. Geological Survey Factsheet 086-99 is available in pdf format.

Another approach was based on varying the electron accepting process to account for redox zonation, and to determine the effect of the various redox processes on MTBE fate.The model developed for this evaluation, SEAM3D, was done by researchers at Virginia Tech. Mark Widdowson is a faculty member at Virginia Tech.

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