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.