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L 4'1 A fat clay exists from approximately 7 feet to 9 feet below grade in the area of interest (according to soil boring SB2). This very low permeability layer directly below the impacted area acts as a natural barrier to contaminant migration. The SESOIL model conservatively ignores this natural barrier. The infiltration model incorporated in SESOIL does not consider the effect of urbanization. �.. The contaminants in question are under a sidewalk and a building. These structures will greatly limit the infiltration of any precipitation. The effect of urbanization has been conservatively ignored. In reality, very little infiltration will be available to drive the 'r contaminants deeper. Volatilization at the surface is set to zero to model how the overlying concrete blocks contaminant vapors from escaping. This assumption of zero volatilization further increases the conservativeness of the model. 5.3.4 Soil Properties The USDA Soil Conservation Service identifies these soils as "Jacktone Clay" (USDA, 1992) with soil organic matter of 2-5% in the top 22 inches. Therefore, there is reason to believe that these soils contain relatively high amounts of organic carbon. Fine grained soils tend to .. contain more organic carbon than coarse grained soils. Organic carbon content also tends to rti decrease with depth. For layer No. 1, the surface layer, the organic carbon content of 1.16% was conservatively calculated from the lower most range (2-5%) of organic matter reported by the Soil Conservation Service (for a depth range of 0 to 22"). Because site specific organic information is unavailable with depth, average percent organic matter are specified based on soil texture after Dragun, 1988. The mean percent organic matter in silty clay is not reported but both clay and sandy clay are reported to average 0.38% organic matter. The use of average values is probably conservative because the Soil Conservation Service information indicates that the soils near the surface contain high amounts of organic carbon. Therefore, 0.2 % (= 0.38%11.724) is considered a conservative estimate of the subsurface organic carbon content (i.e., less than one fifth of the minimum reported 0 to 22" depth value). i The dry bulk density of the soil is estimated to be 2,500 lbs/cy which is equal to 1.5 gm/cc (94 lbs/cf). The numerical results are not sensitive to the range of possible bulk density values. w:19436fteports\se il.doo 7 SW TH