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ENSR AECOM 2.0 Site status Soil and groundwater investigations conducted at the site provide significant information for evaluating the extent and distribution of petroleum hydrocarbon concentrations in soil and groundwater at the site. It is not the intention of this report to document all activities that have ever been performed on the Site; rather, the following sections focus on the site conditions important to evaluation of ongoing remedial activities and potential future activities at the site. 2.1 Site location and description Unocal owned and operated a service station in Stockton, California on the northwestern corner of the intersection of East Miner Avenue and California Street from 1922 through 1992. Former station facilities consisted of a station building, four dispenser islands(up to three operational at any one time),two 1,000- gallon gasoline underground storage tanks (USTs) and one 550-gallon waste oil UST (until 1967)and two 10,000-gallon gasoline USTs and one 280-gallon waste oil UST(from 1967 through 1992). The site is currently a flat-lying vacant lot and the immediate vicinity has been developed for commercial use. Figure 2-1 depicts the location of the site, and Figure 2-2 is a site plan. 2.2 Site history Table 2-1 presents a general chronology of site activities. In summary, during removal of USTs as part of station closure in 1992, petroleum hydrocarbons were detected in soil beneath a product line. Subsequent investigation identified additional hydrocarbons in soil in the vicinity of older former USTs that were removed in 1967. Soil borings and monitoring well installation were performed between 1993 and 2002, and quarterly groundwater monitoring has been performed since 1994. In 2000, an ozone microsparging system was installed to address groundwater impacts and operation commenced. The system was upgraded in 2002, and operation is ongoing. Table 2-1 Summary of previous environmental activities 1922 Union Oil Company of California(Unocal) began service station operations. 1940 Station was rebuilt and expanded; the new station included one service bay, two pump islands, two 1,000-gallon gasoline USTs and one 550-gallon waste oil UST. 1967 USTs were removed and replaced with two 10,000-gallon gasoline USTs and one 280-gallon waste oil UST. New station building with three service bays and three dispenser islands constructed. 1991 Waste oil UST removed. No evidence of impacts reported. Service station ceased operations. 1992 Gasoline USTs removed (GSI, 1992). Excavation pit bottom samples clean; no sidewall samples were collected. Petroleum hydrocarbon detected in soil beneath removed product lines. i1992 Phase I Environmental Site Assessment(ESA)completed (PHR, 1992). 1993 Eight soil borings(U-A through U-H)drilled to maximum depth of 31.5 feet below ground surface (bgs) (GS1, 1993). TPHg detected in area of the former 1,000-gallon USTs that were removed in 1967. J 1994 Three monitoring wells(U-1, U-2, U-3) installed to maximum depth of 60 feet bgs; 5 soil borings (U-I through U-M)drilled to maximum of 50.5 feet bgs(GSI, 1994). Total Petroleum j:\Rem_Eng\Project Files1Pw moo- UNOCAL Portfolio Closure Site 0lio 1187-Stockton on 2-1 January 2606 East Minor<CSM Report\CSM Report_FInaLdoc 41i