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soil of this area. The presence of fat clay from approximately 7 to 10 feet below grade further reduces the potential for vertical migration of residual contaminants. �. 4_2 Groundwater No dissolved petroleum hydrocarbon analytes were detected at any time during groundwater sampling +— from I989 to the present. V 5.0 FATE AND TRANSPORT ANALYSIS 55.11 Analysis of Problem To assess the threat to groundwater beneath the site, an evaluation of the potential migration of the hydrocarbons present in the soil was performed. The Fate and Transport exposure assessment is based y on a chemical's behavior in the environment, the rate and frequency of release, and the media into which it is released. For releases to soil, data on weather patterns, soil conditions and the underlying aquifer were obtained. The seasonal soil compartmental model SESOIL(General Sciences Corp.,June 1995) was utilized to determine the rate of vertical migration (leaching) toward groundwater at the subject site. 5_2 Processes Affecting Fate and Transuort The three general categories of processes affecting subsurface transport of petroleum hydrocarbons are hydrodynamic, abiotic (physiochemical), and biotic. Each of these processes in turn are dependent upon the physical properties of the subsurface and the physiochemical and biological properties of petroleum hydrocarbons. L.. Hydrodynamic processes impact contaminant transport by affecting the rate and direction(s) of groundwater flow(in both the saturated and unsaturated zones) and include advection, dispersion, and preferential flow. These properties are largely controlled by the physical properties of the subsurface in terms of the hydraulic conductivity (permeability) distribution, the driving head distribution (both spatially and temporally), and formation dispersivity. Generally, the contaminant properties do not exert a strong influence on hydrodynamic processes, however, for very low groundwater flow velocities, molecular diffusion (controlled by chemical-specific diffusion) may be significant (Knox and others, 1993). For the petroleum hydrocarbons at the subject site, the abiotic process of sorption is one of the most important in assessing contaminant fate. The physical property of the subsurface which largely controls this process is the organic carbon content of the sediments. The contaminant properties which control the process are solubility and octanol-water partition coefficient(Domenico and Schwartz, 1990). w.-W43681repor \scwil.doc 5 sh*H