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URS Ms. Lori Duncan Page 2 of 5 San Joaquin County Environmental Health Department August 27, 2004 1 BACKGROUND The site is located at the southwest comer of the intersection of W. Hammer Lane and Brentwood Drive in Stockton, California (Figures 1 and 2). The site formerly operated as an ARCO service station and AM/PM mini-mart. Currently, the site is operating as a Valero retail gasoline station. Current site structures include four underground storage tanks (UST), two pump islands, and a convenience store. Site investigations have been ongoing since October 1990. A total of six groundwater monitoring wells (MW-1 through MW-6), six soil vapor extraction wells (VW-1 , through VW-5S and VW- 5D), and three air sparge wells (AS-1 , AS-2, and AS-3) are installed at the site. Well locations are shown in Figure 2. The investigations indicate that hydrocarbon-impacted soil is limited to the proximity of the UST complex. In February 1998, four gasoline USTs of various capacities and their associated piping were removed and replaced. An SVE/AS system that utilized six SVE wells (VW- 1 through VW-4, VW-5S and VW-5D) and three air sparge wells (AS-1 through AS-3) began operating in September 2000. The system was operated nearly continuously through June 2004. Low influent concentrations prompted the shutdown of the SVE system in June 2004. To date the SVE system has removed approximately 2,000 Is of petroleum hydrocarbons from the subsurface. In addition, 7 gallons (43 . 17 lbs) of free product have been removed from site wells. In July 2004, URS conducted a file review at the Department of Water Resources (DWR) and City of Stockton Public Works Department (CSPWD) for wells within a radius of 2,000 feet from the site and a sensitive receptor survey to identify all potential receptors within a 500-foot radius. DWR records identified 12 domestic wells and 2 public groundwater production wells within a 2,000-foot radius of the site. CSPWD identified one production well within the 2,000-foot radius. The production well is City Well #1 (Well ID #13 in Table 3 and Figure 1), located approximately 950 feet south-southeast of the site. This well produces approximately 800 gallons per minute and is screened at various intervals between 170 and 240 feet bgs. No other municipal production wells were located within the search area by CSPWD. The sensitive receptor survey identified no surface water bodies, child care centers, schools, or wildlife habitats within 500 feet of the site. However, URS did identify two elementary schools within 2,000 feet of the site. Colonial Heights School is located 1 ,000 feet to the northwest, and Carden School is located 1 ,800 feet to the east. Figure 1 shows the locations of the wells and schools. SCOPE OF WORK URS proposes to install two groundwater monitoring wells to investigate the lateral and vertical extent of hydrocarbon-impacted groundwater encountered at the site. One boring will be advanced K:\Wprocess\J5 BP ARC0\00548\2004 well instalIMP 04.doe