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
Mass Transport of Methyl Tert-Butyl Ether (MTBE) Across The Water Table and
Significance for Natural-Attenuation Remediation at a Gasoline-Spill Site in
Beaufort, South Carolina
by Matthew A. Lahvis, Ronald J. Baker, and Arthur L. Baehr
ABSTRACT
Mass transport of methyl tert-butyl ether (MTBE) across the water table coupled
with diffusion through the unsaturated zone is a possible mechanism for natural
attenuation of MTBE in ground water. Field and laboratory investigations were
conducted to assess the significance of this pathway for natural-attenuation
remediation. At a gasoline-spill site in Beaufort, S.C., estimates of the volatilization
rate of MTBE downgradient from the spill ranged from 0.007 to 0.012 g m-2 yr-1
depending on the assumed rates of ground-water infiltration and MTBE biodegradation
in the unsaturated zone. The volatilization rates were determined by model calibration
to MTBE concentrations in the unsaturated zone. Results of solute-transport
modeling indicate that volatilization has only a minor effect on migration of
MTBE in ground water downgradient from the gasoline spill. Volatilization rates
downgradient from a spill were compared to rates near a spill in laboratory
column experiments. Results of these experiments indicate that the capillary
zone is a significant barrier to MTBE volatilization. Rates of volatilization
can range over several orders of magnitude depending on whether the source is
above or below the capillary zone. This large difference implies that volatilization
in the spill area may have a significant effect on the migration of MTBE in
ground water.
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