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Work Plan for Additional Assessment S E C O R - - ARCO Service Station No. 548 Stockton, California Page 2 of 6 In 1992, RESNA conducted an SVE test at the site. The test concluded that the site soil could be mitigated by SVE, with each well having a radius of influence of 30 feet. In 1993, two additional monitoring wells (MW-5 and MW-6) and one SVE well (VW-4) were installed . Soil samples did not contain detectible amounts of TPHg. One sample from B-15 (near the dispensers) had concentrations of benzene reported at 0. 024 mg/kg. During the drilling of boring B-18 , a product line was accidentally fractured, resulting in the release of product into the soil. VW-4 was installed adjacent to B- 18, for future mitigation of the released product. In 1994, EMCON conducted a site evaluation for prioritization of remediation efforts at ARCO sites in San Joaquin County. EMCON calculated approximately 383 pounds (lbs) of total petroleum hydrocarbons at the site (URS , 2005) . In February 1998, four gasoline USTs of various capacities and their associated piping were removed . Soil samples indicated that soil was impacted with GRO and benzene in the area of the USTs. In October 1998, a dual completion SVE well (VW-5S and VW-5D) and three air sparge (AS) wells (AS-1 , AS-2, and AS-3) were installed. Soil samples collected at this time confirmed the presence of hydrocarbon impacted soils in the vicinity of the USTs. Maximum concentrations of GRO and benzene were 7 , 830 mg/kg and 11 . 5 mg/kg, respectively. An SVE/AS system utilizing six SVE wells (VW- 1 through VW-4, VW-5S and VW-5D) and three air sparge wells (AS-1 through AS-3) began operation in September 2000. The system was operated nearly continuously until June 2004. Low influent concentrations prompted the shutdown of the SVE system in June 2004. The SVE system removed approximately 2 , 100 lbs of petroleum hydrocarbons from the subsurface. In addition, seven gallons (43. 17 lbs) of free product were removed from site wells during this time. In December 2004, two groundwater monitoring wells (MW-7 and MW-8) were installed to monitor the lateral and vertical extent of groundwater impacts at the site. MW-7 is located off site and down gradient of the former USTs and MW-8 was installed in the vicinity of the former USTs to monitor deep groundwater. TPHg , benzene, toluene, ethylbenzene, and xylenes were reported in soil samples from MW-8. No detections were found in MW-7. Currently all groundwater monitoring , vapor, and air sparge wells are sampled quarterly. Well construction details for all site wells are presented in Table 1 . Historical groundwater analytical and gradient information are shown in Tables 2 through 4. SENSITIVE RECEPTORS A sensitive receptor survey was performed by SECOR in March 2006. According to the Environmental Data Resources Inc (EDR) database sensitive receptor survey report, there are twenty nine (29) water wells located within a one mile radius of the site (EDR, March 23, 2006) . Two of these wells are located within a quarter mile of the site (SECOR, 2006). According to the Geotracker database (www. geotracker. swrcb, ca.gov) there are four water supply wells located in the vicinity of the site at an unknown distance. Two wells are owned by the City of Stockton and the other two wells are owned by the County of San Joaquin , In July 2004, URS Corporation (URS) conducted a file review at the Department of Water Resources (DWR) and City of Stockton Public Works Department (CSPWD) to identify 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 two public groundwater production wells within a 2 , 000400t radius of the site. CSPWD identified one active production well within the 2, 000-foot radius. The production well is City