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Moderate concentrations of dissolved gasoline constituents are found in the groundwater directly below the former underground storage tanks (USTs) at the fuel service station, Building 1138 at the Marine Corps Air Ground Combat Center (MCAGCC) in Twentynine Palms, California. Low concentrations of dissolved constituents were detected in groundwater monitoring wells significantly away from the overlying USTs. The absence of free-phase hydrocarbons in the monitoring wells and of residual hydrocarbon saturation in soil samples collected directly above the water table during monitoring well and soil boring installation at the site fails to indicate that groundwater contamination resulted from free-phase product migration through the vadose zone. The extremely gentle groundwater gradient below the site limits the potential spread of hydrocarbons away from the UST area by advective transport. The low soil moisture content and high permeability, typical of the thick (300 ft) vadose zone below the site, are conducive to vapor transport. Therefore, mass transfer of gasoline-range hydrocarbons from the soil gas to the groundwater was considered to be a likely and significant source of gasoline hydrocarbons in the groundwater. A numerical model, MOFAT (Katyal et al., 1991), was used to evaluate and predict the movement of gasoline from the leaking USTs, the effects of subsequent vapor-phase movement, and partitioning into the groundwater. The results of this model are being used as a hydrocarbon source term in a second numerical model (BIOF&T) to predict the effects of intrinsic remediation on the dissolved hydrocarbons in the groundwater below the site.