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r <br /> U <br /> to assess the maximum potential impact of the site source on the local <br /> f groundwater users, no retardation or degradation was assumed to occur as the <br /> substances were transported to the receptor well.. Additionally, the source at <br /> same concentration across the entire diameter <br /> the site was assumed to have the <br /> i <br /> of the contaminated soil area. This diameter Was taken as 150 feet, the <br /> largest diameter of the soil contamination area. The infiltration rate was <br /> taken as 14 inches per year, which is the average rainfall amount for the <br /> Stockton area. <br /> By modeling leachate transport to this deep well, <br /> there is also the <br /> assumption that the aquifer between the surface and the well intake is <br /> homogeneous and continuous. This assumption may or may not be realistic, but <br /> it is conservative. Additionally, to minimize vertical dilution, the intake <br /> of the well was assumed to be located at 200 feet, the top of the screening <br /> for the receptor wells. The groundwater flow velocity was estimated to be <br /> I <br /> 0.57 cm/day based on an available groundwater contour map and estimates of <br /> aquifer permeabilities and porosities. All of these assumptions are extremely <br /> conservative. In reality, retardation and degradation of contaminants will <br /> occur, and fourteen inches of rain will not infiltrate the area. <br /> Additionally, soil concentrations are not constant across the site, and there <br /> may be no hydraulic continuity between the water table at approximately 40 <br /> feet and the receptor well perforated at 200 feet. <br /> The groundwater transport was calculated using an analytical model (EPA, <br /> 1983) for rapid assessments of .contaminant transport. Because this is a <br /> screening model, the predicted concentrations are conservatively large. This <br /> bias is consistent with the necessary conservative direction of a health <br /> assessment. The modeling results indicate that there is no impact on the <br /> receptor well until after 200 years. Therefore, the impact at 250 years was <br /> used to estimate the impact on the receptor well. <br /> At 250 years, the model <br /> shows concentrations at the receptor well would be 0.61 percent of the on-site <br /> leachate concentration.. <br /> 4-8 <br /> GGA/0179b <br /> Ji <br /> k <br />