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Stantec Site History and Previous Investigations Former ARCO Service Station No. 595 May 18, 2009 In September 1999, product lines and dispensers were upgraded and replaced in the same trench locations. Twenty-one soil samples were collected from beneath the product lines, vapor lines, dispensers, and other product lines previously abandoned-in-place. Total petroleum hydrocarbons as gasoline (TPHg) were detected at concentrations ranging between 1.94 milligrams per kilogram (mg/kg) and 11,300 mg/kg. Benzene was reported at concentrations ranging between 0.0316 mg/kg and 48.3 mg/kg. Methyl tert-butyl ether (MTBE) was reported at concentrations ranging between 0.110 mg/kg and 6.47 mg/kg. Tert-butyl alcohol (TBA), di- isopropyl ether, ethyl tert-butyl ether, tert-amyl methyl ether, 1,2-dichloroethane (1,2-DCA), and 1,2-dibromoethane were not detected in the samples. Approximately 189 tons of soil generated during the removal of the underground facilities and the trenching for new piping were disposed of at BFI Landfill in Livermore, California. In June 2000, four groundwater monitoring wells (MW-1 through MW-4) and three soil borings (B-1 through B-3) were completed at the site. The subsurface assessment confirmed that petroleum hydrocarbons had impacted the soil and groundwater beneath the site. In July 2001, three additional groundwater monitoring wells (MW-5 through MW-7) and two deep soil borings (PSB-1 and PSB-2) were drilled to further delineate the lateral and vertical extent of petroleum hydrocarbons in soil and groundwater beneath and downgradient of the site, and in the vicinity of an on-site domestic water supply well (WSW). With the exception of minor concentrations of benzene, toluene, and xylenes in the 15-foot, 25-foot, and 55-foot samples from the boring for MW-5, petroleum hydrocarbons and fuel oxygenates were not detected in any soil samples collected during the assessment. With the exception of minor concentrations of benzene, toluene, ethylbenzene, and xylenes (collectively BTEX), constituents in boring PSB- 2 at 95 feet (ft) below ground surface (bgs), concentrations of TPHg and BTEX were not detected in discrete-depth groundwater samples and groundwater monitoring well samples. MTBE and 1,2-DCA were reported in discrete-depth groundwater samples from boring PSB-1 (75 ft and 90 ft bgs), discrete-depth groundwater samples collected from boring PSB-2 (64 ft and 90 ft bgs), and in wells MW-5 and MW-7 at concentrations up to 94 parts per billion (ppb) (MW-7, August 2001) and 23 ppb (MW-7, August 2001), respectively. Approximately ten cubic yards of soil were generated during fieldwork activities and disposed of at Forward Landfill in Manteca, California. The historical groundwater gradient direction within the shallow zone at the site has generally been to the northwest at gradients ranging from 0.0004 to 0.01 ft per foot. The historical groundwater gradient direction within the deep zone at the site has ranged from the northeast to the southwest at gradients ranging from 0.0007 to 0.11 ft per foot. Documented in SECOR's Site Assessment Report dated October 2, 2000, a WSW survey was performed to identify the location of permitted WSW within a half-mile radius of the site. The water well survey consisted of a review of California Department of Water Resources (DWR) files for the area. Additionally, SECOR contacted the City of Stockton and the County of San Joaquin Department of Public Works (CSJDPW), which are the local water purveyors. According to the City of Stockton and the CSJDPW, no municipal wells owned and operated by these agencies are located within the half-mile search radius. However, a total of five WSW were identified within the search area, one of which is an active domestic well located on the southwest corner of the Atlantic Richfield Company property. 595 Site History and Previous Investigations.doc 1