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Mr Nuel Henderson,Jr, SJCPHS July 1, 2003 Remedial Action Plan ARCO Station No 6100 Page 6 The depths chosen for TPHG iso-concentration contours are 0-25 feet bgs, 25-35 feet bgs, 35-45 feet bgs, 45-55 feet bgs, 55-65 feet bgs, 65-75 feet bgs, 75-85 feet bgs, 85-95 feet bgs, 95-110 feet bgs, and greater than 110 feet bgs The depths chosen for benzene iso-concentration contours are 0-25 feet bgs, 25-35 feet bgs, 35-45 feet bgs, 45-55 feet bgs, 55-65 feet bgs, 65-75 feet bgs, and greater than 75 feet bgs In the upper 45 feet bgs, the petroleum hydrocarbon impact appears to be limited to site boundaries Petroleum hydrocarbons detected between the depths of 45 and 75 feet bgs in Sand 1 (see figures 9, 10 and 11) appear to follow the southeastern dip of Sand 1 Figure 12 represents the silt strata between Sand 1 and Sand 2 Figures 13 and 14 present petroleum hydrocarbon concentrations detected in the dipping Sand 2, and Figure 15 presents the petroleum hydrocarbon impact at depths greater than 110 feet bgs The majority of residual petroleum hydrocarbon impact to soil appears to be concentrated between 10 and 45 feet bgs The highest TPHG and benzene concentrations observed during tank removal and excavation (November 1990) were at 7,400 milligrams per kilogram (mg/Kg) and 230 mg/Kg, respectively, in soil samples collected from the base of the excavation at approximately 25 feet bgs The highest TPHG and benzene concentrations observed during piping and dispenser upgrade activities were observed in soil samples collected during the November 1990 upgrade at 5,700 mg/Kg and 115 mg/Kg (approximately 6 feet bgs), respectively During the product piping and dispenser upgrade conducted in January and February 2002, the highest TPHG and benzene concentrations were observed at 2,400 mg/Kg and 8 9 mg/Kg (approximately 2 5 feet bgs) During the site assessment studies conducted to date, the highest TPHG and benzene concentrations were observed in soil samples collected at approximately 30 feet bgs from soil boring B-8 (VW-1) at 28,000 mg/Kg and 120 mg/Kg, respectively The highest concentrations of TPHG (10,000 mg/Kg) and benzene (140 mg/Kg) detected in an off- site boring were observed in the soil sample collected from B-26 (VW-17) at approximately 65 feet bgs Sail samples collected during site assessment studies prior to August 2000 were not analyzed for methyl tertiary butyl ether (MTBE) However, in the subsequent site assessment studies conducted to date, MTBE was below laboratory detection limits in all the soil samples except at B-33, where MTBE was detected at 0 12 mg/Kg (at 63 feet bgs),dust above laboratory detection limit of 0 10 mg/Kg Groundwater Groundwater beneath the site is perched, and therefore, the distribution of petroleum hydrocarbons is not definable As discussed in the previous section, and detailed in the P IBp-Arco far UR5161001Reports\Arco 6100 RAP doe