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• S ECOR _ • --- Work Plan for Monitoring Well Installation July 31, 2007 Former ARCO Service Station No. 4932 Stockton, California Page 3 boring for well MW-6 (8.2 mg/kg) suggested that additional investigation for vertical off-site delineation was warranted. Based on groundwater analytical results, SECOR concluded that the lateral extent of GRO and benzene had been defined by wells MW-5 through MW-8; however the presence of MTBE in groundwater samples collected from well MW-6 and depth- discrete groundwater samples from boring SB-2 confirmed that further lateral and vertical delineation of the dissolved MTBE plume was required within these areas. SECOR further concluded that the presence of GRO and benzene in upgradient well MW-9 suggested that an additional source(s) of petroleum hydrocarbons might be present in the area. Results of this investigation are documented in SECOR's Additional Site Assessment Report, dated November 9, 2001. In 2001, SECOR conducted a door-to-door survey of businesses and residences in the area to identify any water supply wells within a 500-foot radius of the site. A questionnaire was supplied to those businesses or residences that could not be contacted first hand. Based on review of verbal and written survey results returned to SECOR, there were no water supply wells confirmed within a 500-foot radius of the site. According to the SJCEHD, in a letter dated November 21, 2000, two water supply wells were documented in the vicinity of the site. One water supply well was located in a field approximately '/z mile east of the site at the Stockton State Hospital and the other water supply well was located approximately 2,000 feet northwest of the site at the intersection of Wyandotte and Commerce Streets. Results of the well survey are included in SECOR's report titled, Additional Site Assessment Report, dated November 9, 2001. Also in 2001, SECOR performed a utility survey to evaluate whether nearby off-site utilities or other underground improvements may act as potential conduits for migration of petroleum hydrocarbons. Since depth to water at the site is approximately 27 feet bgs and no underground improvements or utilities were identified near this depth, it is unlikely that any subsurface utilities or other unknown underground improvements are preferential migration pathways for petroleum hydrocarbon-impacted groundwater. SECOR concluded that higher potential exists for vapor-phase contaminant migration through these areas (SECOR, Additional Site Assessment Report, dated November 9, 2001). As a result of the discovery of LPH in wells MW-4 and SVE-1 during the Fourth Quarter 2001 sampling event on July 20, 2001, weekly interim groundwater remediation was initiated on October 29, 2001, which consisted of the removal of LPH and petroleum hydrocarbon-impacted groundwater from the wells with a bailer. Interim GWE continued for approximately six weeks until measurable LPH was no longer present. Between October 29, 2001 and December 17, 2001, approximately 10 to 15 gallons of LPH had been removed from the wells. As a result of the continued presence of elevated concentrations of petroleum hydrocarbons and MTBE in on- site wells, interim groundwater extraction (GWE) was reinitiated on March 18, 2002 utilizing a vacuum truck. Between March 18 and August 2, 2002, approximately 23,350 gallons of LPH and petroleum hydrocarbon-impacted groundwater had been removed from beneath the site. On March 14 and 15, 2002, West Hazmat installed one deep groundwater monitoring well (DMW-1) and one GWE well (EW-1) at the site. DMW-1 and EW-1 were completed at 81 feet bgs and 44 feet bgs, respectively. Stabilized groundwater levels in wells DMW-1 and EW-1 were measured at 29 and 28 feet bgs, respectively. Soils collected between approximately 30 feet and 69.5 feet bgs from the boring for deep well DMW-1 and all samples collected between 15 feet and 47.5 feet bgs from the boring for EW-1 reported detectable concentrations of TPHg 4932 WP for Mw Installation 2007.doc SECOR International Incorporated