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S /�} 1 1 Soil and Groundwater InvestijZation Report ARCO Facility No 2168 Stockton, California Page 2 Seven current and former UST facilities are located in the site vicinity and a laterally extensive co-mingled dissolved petroleum hydrocarbon plume is present encompassing the site In May 1995, EMCON prepared an off-site evaluation report identifying four potential upgradient/offsite sources of contamination impacting ARCO These sources are Texaco Service Station located at 440 West Charter Way, Chevron Service Station located at 508 West Charter Way, Transmission Store located at 515 West Charter Way, and Shell Service Station located at 620 West Charter Way Two non-oil company responsible parties are located south of the Texaco property and a possible additional responsible party (The City of Stockton Corporation Yard) is located south of the Chevron property All of the above responsible parties are located upgradient of the site and have impacted groundwater Underground storage tank (UST) removal activities were conducted in 1986 and 1996 An underground waste oil tank was formerly located in the northeast corner of the site The waste oil tank was reportedly removed on November 18, 1986 Two 6,000-gallon single-walled steel USTs, one 8,000-gallon single-walled steel UST, one 10,000-gallon single-walled fiberglass UST and associated product piping and dispensers were removed . in February 1996 Locations of the former USTs are shown on Figure 2 Following UST removal, the facility was demolished The USTs most recently contained regular, mid- grade and supreme unleaded gasoline Historically the site has also dispensed regular and premium leaded gasoline Approximately 300 cubic yards of hydrocarbon impacted soil was removed from the site during the UST and dispenser removal activities Onsite Soil Conditions Soils encountered during on-site assessment activities consist of silt to depths of approximately 34 to 38 feet below ground surface (bgs) Fine grained sand and silty sand were present to depths of approximately 55 feet bgs Silt is encountered at depths of approximately 55 feet bgs Prior to the investigation outlined in this report, boreholes have not been advanced to depths greater than 56 feet bgs Total petroleum hydrocarbons as gasoline concentrations in the upper 20 feet of soil were generally less that 10 milligrams per kilograms (mg/kg) and benzene concentrations were generally less than 1 mg/kg The detected concentrations and lateral extent of impacts to soil increased at depths of 21 to 40 feet bgs across the entire site with localized TPHG ranging from 130 to 1,600 mg/kg and localized benzene concentrations ranging from 0 51 to 4 5 mg/kg This spatial pattern suggests that the contaminated groundwater originating from upgradient sources has migrated beneath the site i