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.
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