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C = concentration at the water table (mg/L) <br /> D = hydrodynamic dispersion coefficient (cm2 <br /> /sec) <br /> SC/Sz = concentration gradient at the water table <br /> The Jury model assumes linear, equilibrium partitioning between vapor, liquid, and <br /> adsorbed chemical phases. The model also assumes uniform soil properties consisting <br /> of a constant soil water content, bulk density, porosity, liquid water flux, and a constant <br /> organic.: carbon fraction. <br /> Data requirements for the Jury model are summarized in Table 4-2. Parameters were <br /> selected that would increase the leaching rate and groundwater loading rate so as not <br /> to underestimate potential groundwater concentrations. Biodegradation rates were not <br /> used in the fate and transport modeling. For the residential receptors the infiltration <br /> rate through the soil column was increased to 20 cm/year which assumes that 66 percent <br /> of the precipitation infiltrates through the subsurface. This is a conservative value <br /> overestimating infiltration since the soil at the site has poor infiltration properties <br /> (USDA 1951). For the occupational receptors it was assumed that 2 cm per year would <br /> infiltrate through the 6 to 8-inches of paving due to potential cracks. The majority of <br /> the exposure scenarios were calculated using the site measured depth to groundwater of <br /> 84 feet bgs. The receptor with the highest risk, RME child resident was also evaluated w <br /> with the reported seasonal high groundwater depth of 65 feet bgs. <br /> Groundwater saturated zone modeling was conducted using AT123D, developed by Yeh <br /> (1981). AT123D is a screening level analytical model for estimating the transport of -- <br /> dissolved chemicals in a groundwater system. AT123D solves the differential mass <br /> balance (advective-dispersive) equation that describes contaminant fate and transport in <br /> the saturated zone. Groundwater flow is assumed to be uniform, one-dimensional, <br /> steady and uniform in the downgradient direction. Processes simulated by AT123D <br /> include: <br /> Three-dimensional dispersion (longitudinal, lateral, and vertical). <br /> One dimensional uniform advection. <br /> Linear, reversible equilibrium adsorption. <br /> SALDMYELLO.Rn May 4,199$ 4-9 <br />