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immediately adjacent to sample#218, or in MW-4,the mass of contaminant in Layer 2 <br /> for both Scenarios was assumed to be 0 Kg for BTE and TPH-g. <br /> 4) SESOIL Chemical Specific Parameters <br /> The specific physical parameters for benzene,toluene, ethylbenzene, xylene and <br /> naphthalene (surrogate for TPH-g)were input into the model. Note that there was no <br /> Unsaturated Zone Degradation Rate Constant applied in the SESOIL model. This is a <br /> very conservative model assumption. <br /> SESOIL Model Results <br /> The SESOIL model output results, based on the above referenced data input parameters,for <br /> Receptor Point concentration in Groundwater for both the Practical and Conservative Scenarios <br /> indicate that no BTEX or Naphthalene (TPH-g surrogate)will migrate to the groundwater located <br /> 45 feet below the residually contaminated soils at this site. Note that conservative assumptions <br /> were made throughout the modeling procedure. <br /> The SESOIL model results for the Receptor Point Concentrations in Soil for both Scenarios are <br /> included to demonstrate that the uppermost surface of Layer 1 in both Scenarios has residual <br /> concentrations of contaminants remaining after the leaching of the contaminants. SESOIL <br /> assumes that the contamination is spread evenly throughout the soil column and that there is no <br /> loss of mass during the leaching process. Note that these residual concentrations are less than <br /> the input concentrations given in 3A)and 3C) above. This calculated residual contamination is <br /> used to derive exposure risks due to dermal exposure in the APIDSS model. <br /> The Data for Risk Assessment section is included for informational purposes only as the modeled <br /> results do not indicate a threat to human health based on the risk values generated by these <br /> modeled scenarios for dermal exposure to site soils. Because none of the modeled contaminants <br /> would reach groundwater, it is not possible to derive health risk values for this exposure pathway. <br /> Conclusion <br /> Based on the SESOIL model results of site specific soil contamination for two contaminant mass <br /> scenarios,the remaining soil contamination does not pose a threat to the groundwater located 45 <br /> feet beneath the site. It is recommended that the San Joaquin County PHSIEHD Technical <br /> Review Committee grant Chevron closure at this site. <br /> Please contact me at 510-242-7086 with questions or comments regarding this memorandum. <br /> Sincerely, �� D GF 0 <br /> aF0 <br /> c�\5 A�-LENA�0 <br /> r <br /> Curtis A. Peck, R. G. W U 0 <br /> Lead Hydrogeologist No. 5337 <br /> cc: JM Randall <br /> TE Buscheck (P <br /> JN Stambolis �rFOp CA1-��� <br /> U Kelmser <br /> Attachments: <br /> 1) SESOIL Model-APIDSS Data RequirementwTables- Practical Scenario-#9-7780.sav <br /> 2) SESOIL Model -APIDSS Data Requirements/Tables-Conservative Scenario-#9-7780A.sav <br />