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Groundwater Assessment Report ARCO Facility No 2186 Stockton,Califomia Page 2 SITE BACKGROUND In August and September 1995, EMCON drilled six vertical soil borings (B-1, B-3 through B-5, B-7, and B-8) and two 45° angle borings (B-2 and B-6) Three of the bonngs (B-1, B-4, and B-5) were converted to groundwater monitoring wells (MW-1 through MW-3), and three of the borings (B-3, B-7 and B-8) were converted to dual completion vapor extraction wells (VW-1 through VW-3) In October 1996, EMCON drilled boring B-9, which was converted to groundwater monitoring well MW-4 In June 1999, the former gasoline USTs and associated dispensers and piping were replaced The former UST complex consisted of two 10,000-gallon and one 5,000-gallon single-wall fiberglass tank The 8,000-gallon UST formerly stored super unleaded gasoline and the two 10,000-gallon USTs formerly stored regular unleaded gasoline The gasoline UST fill ports were located at the northern ends of the tanks Fiberglass piping located at the northern ends of the USTs proceeded along the western edge of the site and connected the USTs to two dispenser islands located north of the station building In August 1999, IT conducted a soil and groundwater assessment consisting of two onsite CPT borings (CPT-2 and CPT-3) located at the eastern edge of the site, and one offsite CPT boring (CPT-1) located to the north of the site Benzene and methyl tertiary butyl ether (MTBE) were detected in water samples collected from CPT-2 Delta conducted a soil vapor extraction test at the site in September, 2000 Three separate 4-hour tests were conducted Soil vapors were extracted from wells VW-lA, VW-2A, and VW-3A The tests indicated that extraction of soil vapors from these wells at 9 to 10 cubic feet per minute, 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 TPHG per day WELL SEARCH A water supply well owned by the City of Stockton was identified approximately 1,100 feet west of the site in 1998 (Pinnacle, Workplan to Further Assess Hydrocarbon- Impacted Soil and Groundwater, April 15, 1999) This well was reportedly situated in Oak Park IT performed a second well search in June 2000 (Delta, Work Plan for Additional Groundwater Assessment and Feasibility Testing, July 14, 2000) The IT data identified two agricultural wells 400 feet northwest and 900 feet northeast of the site, and a third well approximately 1,800 feet northwest of the site P\ARCO\2186\REPORTS\2186report doc