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Modeling the Atmospheric Inputs of MTBE to Groundwater Systems

James F. Pankow, Neil R. Thompson, Richard L. Johnson

Department of Environmental Science and Engineering, Oregon Graduate Institute
P. O. Box 91000, Portland, OR 97291-1000, USA
Phone (503) 690-1080, Telecopier (503) 690-1273, pankow@ese.ogi.edu

A numerical transport model was used to calculate the movement of methyl-t-butyl ether (MTBE) and several other volatile organic compounds (VOCs) from the atmosphere downward through the unsaturated zone and into shallow groundwater. Simulations were carried out for periods as long as 10 years to investigate whether a gaseous atmospheric MTBE source at typical ambient concentrations could account for the presence of MTBE in shallow groundwater at the types of low ug/L levels that have been found by National Water Quality Assessment Program currently being conducted by the U.S. Geological Survey. The simulations indicate that downward movement of MTBE to shallow groundwater will be very slow when there is not net downward movement of water through the vadose zone. In contrast, the downward movement of MTBE can be rapid when there is net downward movement of water through the vadose zone. For example, for a vadose zone composed of fine sand, and assuming tens of cm of infiltration, then only a few years will be required for water at a water table that is 5.0 m below ground surface to attain MTBE levels that correspond to saturation with respect to the atmospheric source gaseous concentration. An on/off atmospheric source, as might occur in the seasonal use of MTBE, will lead to concentrations in shallow groundwater that correspond to saturation with the time-average atmospheric source concentration.

Presented:

1996, Society of Environmental Toxicology and Chemistry Symposium, 11th, Washington, D.C., p. 115. (presentation given by Neil Thompson).
1996, U.S. Geological Survey Briefing to Senior Scientists and Managers, Reston, Virginia.
1996, U.S. Geological Survey Planning Meeting on Urban Flow Path Sudies, Denver, Colo.

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