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

Transport of Methyl Tert-Butyl Ether (MTBE) and Hydrocarbons to Ground Water from Gasoline Spills in the Unsaturated Zone

By Matthew A. Lahvis and Arthur L. Baehr

ABSTRACT

At point-source gasoline-release sites, methyl tert-butyl either (MTBE), a fuel oxygenate, is commonly found in ground water at concentrations that exceed those of benzene, toluene, ethylbenzene, and xylenes (BTEX). A mathematical model is developed to explain such occurrences. Type curves are developed to predict benzene and MTBE transport across the water table and to evaluate the relative importance of the advection diffusion and biodegradation pathways. In general, the mass flux of benzene is several orders of magnitude less than that of MTBE, if biodegradation of MTBE is assumed to be negligible. Consequently, ground-water plumes of MTBE could potentially extend several orders of magnitude farther downgradient than BTEX plumes at a gasoline-spill site. Depth to the water table has the greatest effect on the selective removal of hydrocarbons in the unsaturated zone.