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to get a benzene load of 0.057 Kg in Layer 2. Benzene was assumed to have pure phase solubility and biodegradation was not modeled. The receiving aquifer was modeled as having a 150/yr hydraulic conductivity, hydraulic gradient of 0.03 ft/ft, thickness of 5m, FOC of 0.005, bulk density of 1.7 g/cm3 and 10m, 5m and 0.5m dispersivities. The receptor well was placed in the center of the 9m x 9m source. Note that the model inputs were co-determined by myself and Harlin Knoll. SESOIL Model Results Using the above outlined model input parameters, the SESOIL model estimated that the benzene present in the soil column in SB-8 would migrate (leach)to the groundwater beneath the site and result in a groundwater benzene concentration of 3ppb. This model result is higher than past and current benzene results and would indicate that residual benzene contamination in the soil column will not pose a significant threat to the groundwater resource in this area. Based on this review of the SESOIL model results, it is recommended that this site be submitted to the San Joaquin County PHS/EHD for closure. Please contact me at CTN 242-7086 to discuss any questions arising from this memorandum. Sincerely, Curtis A. Peck, R.G. 5337 •,i Attachments: 1) APIDSS Data Requirements Section (C:IAPIDSSISTOCK2.SAV) A) Receptor Point Concentration Table - Benzene B) Concentration in Groundwater over Time Chart - Benzene C) Mass Loading to Groundwater over Time - Benzene D) Carcinogenic Risk- Ingestion of Drinking Water F) SESOIL Output File- Benzene Mass Loading G) SESOIL Output File- Benzene Groundwater Concentration 2) Climatic Data Table 1-1 3) Climatic Data Table 1-7 4) Climatic Data Table 1-8 5) Mass Loading Calculation 6) Site Soil Analytical Data 7) APIDSS SESOIL run with Bioattenuation Activated -C:IAPIDSSISTCK2BIO.SAV