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Because the SESOIL model allows for four distinct soil layers, the maximum number was <br /> used to represent the Santa Fe Railway Company site. The following presents the four <br /> layers that were chosen to conservatively represent the site <br /> (A) Silty-clay from 0 to 6 feet bgs. <br /> (B) Silty-clay from 6 to 15 feet bgs <br /> (C) Silty-clay from 15 to 40 feet bgs. <br /> (D) Silty-clay from 40 to 55 feet bgs <br /> Groundwater was assumed to be present at 55 feet bgs in the modeling scenario The <br /> designated divisions for the soil column were made based on the observed chemical <br /> concentration transitions. Although petroleum hydrocarbons were not detected in Layers <br /> A and D, petroleum hydrocarbons were detected in Layers B and C The maximum <br /> detected chemical concentration in each layer was used as the total uniform concentration <br /> within that layer This conservative approach increases the total mass that is actually <br /> present in the layer This approach preserves the maximum vertical migration potential of <br /> any high concentration within the layer The total mass of aged diesel fuel was calculated <br /> based on the maximum concentration within the layer <br /> The loading of mass within each layer in the SESOIL model was performed in the following <br /> manner <br /> (1) The total mass of petroleum hydrocarbons which was present in any layer was <br /> determined based on the maximum concentration within that layer <br /> (2) The total mass was then loaded in the top portion of the layer immediately <br /> below the layer that the mass was determined in Step 1 Therefore, the total <br /> mass determined in layer B was loaded in the top portion of layer C <br /> Likewise, the total mass determined in layer C was loaded in the top portion <br /> of layer D <br /> FIGURE 4-2 presents a comparison of site conditions with the loading of mass within each <br /> layer of the SESOIL model <br /> G 10600 IMAWAFE FNSL\kuW 26 93 4-8 Cb mRakO tM <br />