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f S E C 0 R Conceptual Site Model Chevron Service Station #9-6171 March 25,2005 Page 35 groundwater contained in the B-Zone Aquifer Is also contaminated, and is separated from the deepest C-Zone Aquifer by a laterally continuous low-permeability aquitard The C- Zone Aquifer is uncontaminated Petroleum hydrocarbons from the former Regal/EXXON stations, the active Shell station and the active Canepa's Carwash may have migrated downgradient to the area beneath the Chevron service station Petroleum hydrocarbons may have also migrated downgradient, through the A-Zone Aquifer, and then been transported to the deeper B-Zone Aquifer when a cracked well casing acted as a conduit or when a gap was encountered in the low permeability aquitard separating the two aquifers There has been no historical need to install a soil vapor extraction system (SVE) on the Chevron property, because concentrations of TPHg, benzene, and MtBE have been encountered in shallow soils where natural attenuation was considered effective remediation However, there is reported documentation of an operational SVE system on the Shell property, an operational SVE system (offline from 2002-2004) on the Canepa's property, and an SVE system on the Regal/EXXON property that was scheduled to be shut down in 2001 pending closure from San Joaquin County Currently, there is an offsite remediation system operating to remediate chlorinated solvents from the A-Zone and 13- Zone Aquifers 92 Secondary Sources Several other gasoline service stations are located within one half mile cross-gradient of the Chevron service station, all of which have documented petroleum hydrocarbon contamination and remediation 9.3 Potential Receptors Approximately sixteen City of Stockton Water Department or the California Water Service Company municipal drinking water wells, at least fifty-five privately owned groundwater monitoring wells, and at least seventeen privately owned domestic wells were identified within a one mile radius of the site Twenty-four wells are located less than one half mile upgradient from the site Thirty-two wells are located less than one half mile downgradient from the site (The EDR Radius Map Report, 2004) Additional receptors include future Ionsite construction workers and businesses 94 Routes of Exposure Potential routes of exposure Include ingestion of soil and groundwater, dermal contact of soil and groundwater, and inhalation of soil vapors Due to the depth of groundwater of approximately 38 62 feet bgs, ingestion and dermal exposure to groundwater Is not a complete pathway No recent chemical analytical data on near surface soils exists Therefore, ingestion of and dermal exposure to near surface soils by future construction activities Is possibly a complete pathway N IFdes\Projects\Chevron\Retaiil#9-6171\Site Conceptual Model\FINAL_CSM doc