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estimated for groundwater and -air by fate and transport modeling. A more detailed <br /> description of the calculation of exposure point concentrations;for each pathway is <br /> presented below. Fate and transport models were selected `from the American <br /> Petroleum Institutes's Exposure and Risk Assessment Decision Support System software <br /> program (API 1994). <br /> 4.4.3.1 Groundwater <br /> To calculate the potential groundwater concentrations of the four chemicals of concern, <br /> an unsaturated zone (soil) transport model was used to calculate the loading-rates of <br /> chemicals to groundwater. The Jury et al. model (1983) was used to calculate the <br /> leaching of chemical to groundwater and the volatilization emission rate. -The Jury <br /> model is an analytical solution to a differential mass balance equation described below: <br /> SCI./St = DE SZCT/Sz2 - VE SCT/Sdz - µCT <br /> where: r <br /> CT = total soil concentrationm K <br /> ( g/ g) � <br /> DE = effective diffusion coefficient (cm?/sec) I <br /> t = time (days) <br /> VE = effective contaminant.velocity (cm/day) <br /> z = depth measured positive downwards (cm) <br /> µ = first order decay rate constant (1/days) <br /> The mass loading rate of.the chemicals to the water table is calculated using: <br /> M, = VCA + D(8C/bz)A i <br /> II <br /> where: <br /> t <br /> M� = mass loading to water table (grams/day) 1. <br /> V = infiltration rate (cm/day) <br /> A = area of source (cm2) <br /> S:\LDC\YELLO.RPT May 4,1993 4-8 <br />