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i San Joaquin County Environmental Health Department j June 18, 2004 Page 2 In September 1992, a preliminary site assessment was conducted by Canonie Environmental to further evaluate subsurface conditions at the site. This investigation involved drilling a total of nine soil borings. Three of the borings were completed as 4-in6h diameter vapor extraction wells (W-1, W-2, and W-3) and three of the borings were completed as 2-inch diameter vapor-inlet wells (W4, W-5, and W-6). Results appeared to define the extent of soil contamination and indicated that;:j petroleum hydrocarbons exist primarily in the area of the former tanks to a depth of 30 to 35 feet below ground surface (bgs). In December 1993, an additional site assessment was conducted by SECOR to verify the extent of petroleum hydrocarbons in soil at the site. "This investigation involved drilling five sol borings, and installing two vapor extraction/inlet wells (W-7 and W-8). ':During this investigation, unanticipated impacts were found to exist outside of the former tank bed. I In December 1994, an additional site assessment was conducted by SECOR to further evaluate the extent of petroleum hydrocarbons in soil and groundwater',at the site. This investigation involved r 1 drilling five additional soil borings, and installing four groundwater monitor wells (MW-1 through MW- 4). E ,: s , In July 1996, an additional site assessment was conducted by SECOR to further evaluate the extent of subsurface petroleum hydrocarbons on and off the site. This investigation involved drilling six soil borings and installing six groundwater monitor wells (MW-5 through MW-10). i In March 1998, a soil vapor extraction (SVE) feasibility test was conducted by SECOR. TPHg removal rates ranged from 1.1 pounds per day (lbs./day) in W-6 to 1,425 lbs./day in W-3. Based on the flow rates achieved during this test, site lithologyj and data collected during previous investigations, the radius of influence at the site was estimated to be 50 feet. A In September 1998, SECOR submitted a CAP, which recommended SVE with air sparging (AS) for ` remediating petroleum hydrocarbon impacts: beneath the site: Construction of the SVEIAS system was completed at the site in December 1999. The remediation system consisted of seven SVE wells (W-1, W-3 through W-8), two groundwater monitor wells modified for SVE (MW-1 and MW-3), and four AS wells (AS-1 through AS-4) with catalytic oxidation treatment. Initial start-up air discharge compliance testing was completed on December 13, 1999: The San Joaquin County Valley Air Pollution Control District(SJVAPCD)approved an air permit for the site. In September 2003, the SVE/AS system was shut down following the receipt of verbal authorization from the SJCPHD, and has remained off-line through the first quarter 2004. The SJCPHD approved t the non-operation of the system as a result of low soil vapor influent concentrations. Since the SVE/AS system began operation in December 1999, a total of approximately 38,894 pounds of TPHg and 233.2 pounds of benzene have been removed and treated at the site. . F SCOPE OF WORK j The scope of work for post-remediation confirmation soil sampling includes completing three soil borings to confirm the extent and quantity of residual petroleum hydrocarbons in soil at the site. ' Proposed soil boring locations are provided on Figure 3. Trie specific scope of work is discussed below: kh . f • r r