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SECOR Ms. Vicky McCartney March 29, 2004 Page 2 SITE BACKGROUND The site is situated in a mixed commercial/residential area on the southwest corner of West Hammer Lane and Kelley Drive in Stockton, California. The current station configuration consists of two 12,000-gallon double-walled gasoline underground storage tanks (USTs), two dispenser islands, associated product lines, and a station building (Figure 1). Historical files indicate that British Petroleum (BP) acquired the site from Mobil in May 1989. The gasoline service station operated under BP ownership until Tosco purchased the property in August 1994. Historical UST permit application information suggests that the site historically operated as a gasoline service station since at least 1978 (EMCON, 1994). PREVIOUS INVESTIGATIONS AND REMEDIAL ACTION In April 1987, KEI observed the removal of one 1,000-gallon waste oil UST on the southwestern portion of the site. One soil sample was collected from the base of the excavation at approximately 10 feet below ground surface (bgs). Laboratory analytical results reported gravimetric waste oil as petroleum oil at 55 parts per million (ppm, EMCON, 1994). Between December 3 and 10, 1992, RESNA .performed a preliminary site assessment for BP by drilling four exploratorysoil borings (B-1 through B-4) to total depths of 31.5 to 36.5 feet bgs and converting them to groundwater monitoring wells MW-1 through MW-4. MW-1 and MW-2 were installed on the southeast and northwest corners, respectively, of the LISTS, MW-3 was installed on the west end of the northern dispenser island and MW-4 was installed between the former waste oil tank and the new waste oil tank. Laboratory analytical results for soil detected maximum concentrations of benzene at 0.1200 ppm (S-21.3-132), toluene at 0.0200 ppm (S-20.0-131), ethylbenzene at 0.0180 ppm (S-21.3-132), and total xylenes at 0.0240 ppm (S-21.3-62). Total petroleum hydrocarbons as gasoline (TPHg) and TPHd were not detected above laboratory detection limits. Laboratory analytical results for groundwater detected maximum concentrations of TPHg at 2,200 parts per billion (ppb, MW-1), benzene at 120 ppb (MW-2), toluene at 15 ppb (MW-1), eihylbenzene at 1.0 ppb (MW-2), and total xylenes at 24 ppb (MW-2). TPHd was not detected above laboratory detection limits (RESNA, 1993). In March 1994, Alisto Engineering (Alisto) conducted a supplemental subsurface investigation by drilling and installing three groundwater monitoring wells (MW-5 through MW-7) to total depths of 31 to 32 feet bgs. MW-5 was installed on the northeast corner of the property, MW-6 was installed in an alley W of. Kelly Drive, and MW-7 was installed on the north side of West Hammer Lane. l Laboratory analysis of soil samples detected maximum concentrations of benzene at 0.13 ppm (MW- 5-16'), toluene at 0.67 ppm (MW-6-16'), and total xylenes at 0.015 ppm (MW-5-16').TPHg and TPHd were not detected above laboratory detection limits (EMCON, 1994). On November 22, 1994, EMCON conducted a supplemental site assessment to establish baseline subsurface conditions during the transfer of ownership of the property from BP to Tosca. The investigation consisted of collecting two grab soil samples (TD-2 and TD-3) below the eastern fuel dispensers at the north and south dispensers, drilling of two soil borings (THP-1, THP-2) to total depths of 26.5 feet bgs, and the drilling and installation of vapor extraction well VW-1 to a total depth of 17 feet bgs. Soil and HydroPunch® groundwater samples were collected and analyzed from borings THP-1 and THP-2. Laboratory analysis of soil samples TD-2 and TD-3 detected maximum 1:1ConocoPhi11ips1111931Work Plans\IRAP&Off-Site Asmt WP Addendum'v 2.doc SECOR International Incorporated