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0 A S S O C I A T E S I N C The closest surface water to the site is McLeod Lake, approximately 1.1 miles to the southwest. Stockton Diverting Canal is approximately 1.5 miles to the northeast. Regional groundwater data indicates groundwater beneath the site at approximately 55 feet below ground surface (bgs) with an easterly gradient toward an apparent pumping depression located cast of Stockton (San Joaquin County Flood Control and Water Conservation District, Spring 1997 data). � 3.0 BACKGROUND ' In April 1987, SEMCO removed and disposed of two 1,000-gallon regular gasoline underground storage tanks (USTs), associated piping, and fuel dispenser from the locations shown on Figure 2. A soil sample was collected from the end of each tank at approximately 17 feet below ground surface. Analytical testing indicated the collected samples contained detectable concentrations of petroleum hydrocarbon constituents. The excavation was subsequently backfilled to the ground surface. Soil sample results are summarized in Table 1. ' Copies of the laboratory report are included in Appendix B. ' On July 24, 1997, to investigate the extent of residual soil contamination at the site, a total of six soil borings were advanced to depths ranging from approximately 16 to 31 feet below ground surface (bgs). Detectable concentrations of total petroleum hydrocarbons as gasoline (TPHg), total petroleum hydrocarbons as diesel (TPHd), and benzene, toluene, ethylbenzene, and xylenes (BTEX) were observed in soil samples collected from the site. The distribution of petroleum constituents beneath the site has been essentially defined and appears to be limited ' laterally to the immediate vicinity of the former tank pit. The highest concentrations were reported between 10 and 20 feet bgs and generally attenuate rapidly with depth. Only low concentrations of toluene (0.064 mg/kg) and xylenes (0.012 mg/kg) were detected at I approximately 30 feet bgs in one boring, the maximum depth explored. Attempts were made to further advance the Geoprobe unit in several of the borings, however probe refusal was encountered at depths ranging from approximately 16 to 30 feet bgs due to the hardness of the ' formation. The results of the subsurface investigation were summarized in Smith Technology's Summary Report, Subsurface Investigation, dated December 22, 1997 (Appendix C). ' In April 1999 at the request of PHS/EHD ATC completed a receptor survey for the subject ' property. ATC reviewed PHS/EHD groundwater well construction logs, Department of Water Resources (DWR) groundwater well construction logs, and conducted a site walk in the vicinity of the site to identify any groundwater wells located within a 2,000-foot radius of the ' site. This survey revealed fourteen wells located within approximately 2,000 feet of the site. The closest of these wells.is located approximately 800 feet upgradient of the project site. The receptor survey findings are summarized in ATC's Well Survey for Andrew and Edith Trust, for the Property at 1112 East Harding Way_ , Stockton, California, dated April 9, 1999 1 (Appendix D). ' w:1443041reponslc1osure.doc 2