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Stanteec Site History and Previous lWastigations ARCO Service Station No. 2186 January 26, 2009 From August to September 1995, EMCON of Sacramento, California (EMCON) supervised the drilling of six vertical soil borings (B-1, B-3 through B-5, B-7, and B-8) and two 45-degree angle borings (B-2 and B-6). Three borings (B-1, B-4, and B-5) were converted into groundwater monitoring wells (MW-1 through MW-3, respectively). Three borings (B-3, B-7, and B-8) were converted into dual-completion vapor extraction wells (VW-1 through VW-3, respectively). In October 1997, EMCON supervised the drilling of one vertical soil boring (B-9). B-9 was subsequently converted into groundwater monitoring well MW-4. From June through July 1999, two 10,000-gallon and one 8,000-gallon, double-walled, fiberglass gasoline underground storage tanks (USTs), associated dispensers, and piping were removed from the site. Two 20,000-gallon USTs were installed in the same cavity as the former USTs. Pinnacle Environmental Consultants of Sacramento, California conducted compliance soil sampling activities for the tank pull. Analysis of compliance soil samples indicated that gasoline range organics (GRO), benzene, and methyl tert-butyl ether (MTBE) contamination were present in the soil. The excavation areas were backfilled with pea gravel. , In August 1999, IT Group of Roseville, California (IT) conducted a soil and groundwater assessment that included two cone penetrometer test (CPT) borings (CPT-2 and CPT-3) on the eastern edge of the site and one CPT boring (CPT-1) north of the site. Benzene and MTBE were detected in groundwater samples from CPT-2. No petroleum hydrocarbon compounds r were detected in the groundwater samples collected from CPT-1 or CPT-3. Joel In September 2000, Delta Environmental Consultants of Rancho Cordova, California (Delta) conducted a soil vapor extraction (SVE) test at the site. Three separate four-hour tests were conducted. Soil vapors were extracted from wells VW-1A through VW-3A. The tests indicated l that extraction of soil vapors from these wells at nine to ten cubic feet per minute (cfm), with 50 to 100 inches of water column resulted in a radius of influence between 34 and 45 feet. The initial hydrocarbon mass extraction rates were calculated to be between 1.10 and 3.88 pounds of GRO per day. In August 2001, Delta installed three groundwater monitoring wells (MW-5 through MW-7). MW-5 is east of the site on American Street. MW-6 and MW-7 are in Oak Park, across Alpine Avenue, to the northwest and northeast of the site. ��•I 6 In April 2004, one deep groundwater monitoring well (MW-8) was installed in the dominant downgradient direction from the UST cavity (east-northeast) to assess the vertical extent of petroleum hydrocarbon-impacted groundwater beneath the site. One vapor extraction well (VW-4) was installed to complete the remedial well network at the site. One air sparge well (AW-1) was installed for use in conjunction with the vapor wells in the SVE pilot test. Groundwater monitoring and sampling have been conducted at the site since 1995. Groundwater flow is predominately toward the northeast and east. In January and February 2008, three soil vapor probes (VP-1 through VP-3) were installed and sampled in the presence of San Joaquin County Environmental Health Department personnel. 2186 Site History and Previous Investigations.doc