FATE OF MTBE RELATIVE TO BENZENE IN A GASOLINE-CONTAMINATED AQUIFER (1993-98)

Methyl tert-butyl ether (MTBE) and benzene have been measured since 19 93 in a shallow, sandy aquifer contaminated by a mid-1980s release of gasoline containing fuel oxygenates. In wells downgradient of the rele ase area, MTBE was detected before benzene, reflecting a chromatograph ic-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 re lease area, and also up to 10 feet (3 m) below the water table surface in nested wells located farther from the release area. This distribut ion of higher concentrations at depth is caused by recharge events tha t deflect originally horizontal ground water flowlines. In the laborat ory, microcosms containing aquifer material incubated with uniformly l abeled C-14-MTBE under aerobic and anaerobic, Fe(III)-reducing conditi ons indicated a low but measurable biodegradation potential(<3% C-14-M TBE as (CO2)-C-14) after a seven-month incubation period. Tert-butyl a lcohol (TBA), a proposed microbial-MTBE transformation intermediate, w as detected in MTBE-contaminated wells, but TEA was also measured in u nsaturated release area sediments. This suggests that TEA may have bee n present in the original fuel spilled and does not necessarily reflec t microbial degradation of MTBE. Combined, these data suggest that mil ligram per liter to microgram per liter decreases in MTBE concentratio ns 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 gas oline release site.