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Mr Michael Infurna, SJCEHD June 17,2005 Site Investigation Work Plan Palisades Station No 835, Stockton, CA Page 4 (mg/Kg) and 3 5 mg/Kg, respectively, at a compliance soil sample collected beneath the former UST complex Dissolved TPHG and benzene were reported at maximum concentrations of 4,000 micrograms per liter (µg/L) and 1,700 µg/L, respectively (well MW-3, December 1992) MTBE analysis was not performed at this time MTBE analysis in groundwater was initiated during the fourth quarter 1996 MTBE impact to groundwater was reported in samples collected from wells MW-1 through MW-4, MW-6, and MW-8 between the fourth quarter 1996 and the third quarter 1997, at a maximum concentration of 220 µg/L MTBE concentrations in groundwater have generally increased with time MTBE has been reported at a maximum concentration of 54,000 µg/L (well VEW-3, December 2002) TBA and tertiary amyl methyl ether (TAME) have also been reported in groundwater beneath the site TPHG, benzene, MTBE, and TBA concentrations reported during the fourth quarter 2004 groundwater sampling event are illustrated on Figure 6 Figure 7 through Figure 11 illustrate historical concentrations for select petroleum hydrocarbons, and groundwater elevations versus time for select wells TBA has been reported at a maximum concentration of 11,800 µg/L at well MW-7 Given the location of well MW-7 (immediately downgradient of the 7-11 service station property), it appears possible that the impact observed at this well may have originated from an off-site source Potential Sensitive Receptors A Caltrans irrigation well is situated immediately south of Country Club Boulevard, between the northbound Interstate 5 off-ramp and Plymouth Road (see Figure 4) The environmental consultant representing the former Shell service station (Cambria) is currently sampling this well Cambria personnel also identified five domestic water supply wells, located at 2452, 2454, 2456, 2468, and 2472 Country Club Boulevard The approximate locations of these domestic wells are included on Figure 4 At the time of the most recent sampling event, only the domestic well located at 2452 Country Club Boulevard was sampled ETIC completed a door-to-door reconnaissance of the area surrounding the site in May 2001, to identify potential off-site sensitive receptors to groundwater impact beneath the site A domestic well was identified at 2740 Bristol Avenue, approximately 900 feet northwest of the site SJCEHD personnel identified an imgation well at the Safeway facility, located at 2720 Country Club Boulevard SJCEHD personnel have previously recommended, in a letter • K\USA Gasoline\0835\Work Plans\pahsades835 workplan 46-17-05 doc 5TRAT'V5