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SMTH <br /> vertical concentration distribution in the impacted thickness is also assumed constant. Using <br /> the highest concentration found in a one dimensional model is more conservative than a more <br /> representative three dimensional model. <br /> Soil Profile <br /> The impacted zone is estimated to extend from a depth of 3 to 8 feet. The soils beneath the <br /> site are comprised of clay and silt from the surface to a depth of approximately 28 feet, then <br /> silt and silty sand from approximately 28 feet to the water table. The subsurface has been <br /> explored to a depth of 55 feet. During the period of investigation, the depth to groundwater <br /> has ranged from 38 feet to 47 feet below grade. SESOIL permits a maximum of 4 layers in <br /> the computer model. Therefore, the following layers are specified: <br /> Laver No. Thickness Description <br /> 1 3' Clean surface soil <br /> 2 5' Impacted soil layer <br /> 3 20' Silt and Clay <br /> 4 10' Silt and Sand <br /> A fat clay exists from approximately 7 feet to 9 feet below grade in the area of interest <br /> (according to soil boring S132). This very low permeability layer directly below the impacted <br /> area acts as a natural barrier to contaminant migration. The SESOIL model conservatively <br /> ignores this natural barrier. <br /> The infiltration model incorporated in SESOIL does not consider the effect of urbanization. <br /> The contaminants in question are under a sidewalk and a building. These structures will <br /> greatly limit the infiltration of any precipitation. The effect of urbanization has been <br /> conservatively ignored. In reality, very little infiltration will be available to drive the <br /> contaminants deeper. <br /> Volatilization at the surface is set to zero to model how the pavement blocks contaminant <br /> vapors from escaping. This assumption of zero volatilization further increases the <br /> conservativeness of the model. <br /> Soil Properties <br /> The USDA Soil Conservation Service identifies these soils as "Jacktone Clay" (USDA, 1992) <br /> with soil organic matter of 2-5% in the top 22 inches. Therefore, there is reason to believe <br /> that these soils contain relatively high amounts of organic carbon. Fine grained soils tend to <br /> contain more organic carbon than coarse grained soils. Organic carbon content also tends to <br /> decrease with depth. <br /> NA150114\mode1\param.doc 2 <br />