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MEMORANDUM August 21, 1996 Richmond, California SESOIL Model Results CREMCO Stockton Drillsite 540 N. Aurora/969 E. Fremont Streets Stockton, California Ms. Cynthia Wong: San Francisco, CA This memorandum addresses your request to perform a SESOIL model run for this site to determine the leachability of residual benzene hydrocarbon contamination from the soil column into the groundwater beneath the site. The model was also run to provide a verification of the current dissolved benzene hydrocarbon contamination seen in the groundwater at the site. The SESOIL model used for this evaluation was part of the American Petroleum Institute Decision Support System (APIDSS) for Exposure and Risk Assessment model. The SESOIL model input parameters were discussed with, directed by and approved by Mr. Harlin Knoll of the San Joaquin County PHS/EHD prior to the final model run in an effort to gain regulatory approval of our methodology and therefore, model results. f The SESOIL model results indicate that under the conservative assumptions used in the model, .I that benzene detected in soil sample SB-8 at 35' (0.052 mg/Kg benzene)would migrate to the groundwater and result in a 3 ppb benzene concentration directly beneath the source at SB-8. Historical groundwater monitoring data between 517191 and 11/17/95 indicates benzene concentrations have been at or below 0.85 ppb. Current groundwater monitoring data for the site (3115196) indicate that the benzene concentrations are at or below 1.0 ppb for the 4 site groundwater monitor wells. The current and historical groundwater concentrations are less than the modeled concentrations and would indicate that the residual soil contamination in S13-8 would not, and does not, pose a significant threat to the groundwater beneath the site. Based on this observation, it is recommended that this site be submitted for closure. SESOIL Model Input Parameters The SESOIL model input parameters are given in the attached APIDSS Data Requirements section (C:IAPIDSSISTOCK2.SAV). The site was modeled for 100 years and contained 3 soil layers. The surface temperature was determined from historical records for the Sacramento area (attached) and evapotranspiration was estimated to be negligible. Historical precipitation data was used (attached) to estimate length of storm season, number of storms and storm duration. The effective porosity of 0.38, bulk density of 1.7 g/cm3 and source dimensions were chosen based on site characteristics. The soil column was divided into 3 layers of 4m, 8m and 0.1 m thickness. The intrinsic permeability's assigned are characteristic of a sandy soil type. The fraction Organic Carbon was estimated at 0.005 and the mass of benzene was confined to the 8m layer based on soil analytical data. The benzene load in the 8m interval was calculated by determining a volume of impacted soil (8m x 9m x 9m, or 648m), multiplying by the bulk density . (1700 Kg/m3), multiplying by the benzene content of SB-8 (0.052 mg/Kg), dividing by 1,000,000