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1.0 BACKGROUND ' 11 Site Description E The site is located at 102 South Wilson Way in Stockton, California as shown on the Site Location Map (Figure 1) The site is flat and at elevation of approximately 18 feet above mean sea level Review of historical data on file at the San Joaquin County Recorders Office indicates that the site was formerly occupied by a gasoline service station from approximately 1930 until 1978 Two 7,500-gallon underground storage tanks (USTs) were located beneath the north central portion of the site, and one 1,000-gallon and one 300-gallon UST were located along the southern property boundary as shown on the Site Plan (Figure 2) The use of the site as a gasoline service station was discontinued in 1978 and the buildings on the property were removed The site is currently vacant with no buildings ' 1.2 Proximity of Municipal Wells A well records search performed by Leedshill-Herkenhoff, Inc (LH) and presented in their Preliminary Investigation Report (dated January 1990) indicates that the California Water Service Company (Cal Water) maintains 14 municipal wells within a one mile radius of the site A map prepared by LH showing municipal well locations is included in Appendix 1 13 Previous Work In December 1978 two 7,500-gallon gasoline USTs were removed from the north central portion of the site, and one 1,000-gallon and one 300-gallon gasoline UST were removed from southern portion of the site After removal of the USTs, the excavations were backfilled to grade and the site was paved Permits for removal of the USTs are on file with the City of Stockton Fire Prevention Bureau (Kennedy/Jenks, 21 October, 1992) In April 1988, Balbi & Chang Associates (Balbi & Chang) conducted and initial site assessment without contacting or obtaining permits from the San Joaquin County Public Health Services-Environmental Health Department (SJCPHS-EHD) Consequently, the initial site assessment had to be repeated The initial site assessment consisted drilling three soil borings (TB-1 through TB-3) TPHg were detected in soil samples collected from a depth of approximately 2 feet below ground surface (bgs) from boring TB-1, located near the area of the former USTs, at a maximum concentration of 195 parts-per-million (ppm) In April 1988, soil in the former UST cavity was overexcavated to a depth of approximately 4 feet bgs and stockpiled onsite The excavation was left open (Kennedy/Jenks, October 21, 1992) r ` In January 1989, Balbi & Chang drilled four soil borings (MW-1, MW-2, MW-3, and B-4) and installed monitoring wells (MW-1 through MW-3) in three of the borings This work was performed to duplicate the work performed in 1988 by Balbi & Chang without the groundzc1roiklczp 1 I r