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1.0 INTRODUCTION This Preliminary Site Conceptual Model (PSCM) has been prepared by Thrifty Oil Co (Thrifty) ' to provide a detailed description of the extent of delineation of the soil and groundwater plume beneath the above-referenced site and surrounding area This PSCM was prepared based on a detailed review and compilation of data from historical and current assessment, remedial, and 1 monitoring activities conducted at the site The PSCM was requested by the San Joaquin County Environmental Health Department (SJCEHD) in a telephone conversation between Thrifty and the EHD on June 27, 2003, and was confirmed in a facsimile sent by Thrifty to the EHD dated ' June 27, 2003 The purpose of this PSCM is to construct a strategic framework for the effective and efficient ' implementation of environmental assessment and remediation activities associated with the petroleum hydrocarbon release beneath the site This strategic framework, which will be periodically updated as new data is collected, ultimately provides a comprehensive representation of the background of the release, the distribution of the release, sensitive receptors that may be potentially impacted by the release, and the potential effects of local geologic/hydrogeologic conditions on the distribution of the release ' 2.0 SITE DESCRIPTION Former Thrifty Oil Co Station#171 is located on the southeast corner of North Wilson Way, and East Harding Way in Stockton, California(Figure 1) This site is generally square and ' approximately 11,500 square feet in size Currently, the site maintains an existing kiosk structure in the central portion, three-associated pump islands dust south, also east and west of the structure, and three underground gasoline storage tanks east of the kiosk The three tanks each have the capacity of 10,000 gallons and store regular unleaded, unleaded plus, and premium unleaded gasoline The site is generally covered with asphalt, with concrete tank pad, pump islands, and concrete driveways located along both streets (Figure 2) ' 3.0 PREVIOUS SITE ASSESSMENT ACTIVITIES ' Previous site assessment and remedial work has been conducted at the site from 1987 to the present Tabulated soil and groundwater analytical data and boring logs from the assessment and monitoring activities are presented in Appendixes A, B, and C, respectively, and summarized ' below In December 1987, Hydrotech Consultants, Inc performed the initial site investigation at the ' site The investigation consisted of advancing four borings (B-1 through B-4) to depths of 20 feet below ground surface (bgs) adjacent to the underground storage tanks (USTs) Detectable concentrations of total petroleum hydrocarbons (TPH) were found only in the soil sample ' collected from boring B-4 at 10 feet bgs at 1,983 milligrams per kilogram (mg/Kg)