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1 <br />' Analytical results for soil samples collected from soil borings MW-1, MW-2, and MW-3 (later <br /> converted to ground water monitoring wells) and B-2Y indicated concentrations of TPH-D less <br />' than 4 mg/Kg <br />' The three ground water monitoring wells were monitored quarterly for four quarters prior to <br /> November 1991(ESE, 1991a, ESE, 1991b, ESE, 1991c, ESE, 19914) Quarterly monitoring <br /> results reported benzene, toluene, ethylbenzene, and xylenes (BTEX) concentrations well below <br />' California primary Maximum Contaminant Levels (MCLS) and Taste and Odor Thresholds <br /> established by the state of California and the Federal EPA (Marshack, 1993), with one <br /> exception In August 1991, benzene at a concentration of 1 3 micrograms per liter (µg/L) or <br /> parts per billion(ppb), was found in a cross-gradient well, MW-2 Detection of this constituent <br /> was not confirmed during any of the four preceding nor three subsequent quarterly monitoring <br /> events <br />' ESE requested site closure from the PHS/EHD on two separate occasions Site closure was <br /> denied because of the detectable concentrations of petroleum hydrocarbons in the ground water <br />' In addition, the PHS/EHD requested an additional investigation of soil and ground water to <br /> determine if a source of petroleum hydrocarbons still existed at the site <br />' In May 1993, ESE drilled one soil boring in the vicinity of boring B-2Y and one monitoring <br /> well was installed downgradient and to the west of the former tank excavation This additional <br /> investigation work was documented in an ESE report dated June 11, 1993 (ESE, 1993a) <br />'0 Analyses of soil samples collected dunng the investigation were reported as nondetectable for <br /> TPH-D and BTEX This confirmed that a source does not exist in the unsaturated soil zone on <br /> site Analyses of ground water samples from the new monitoring well, MW-4, also reported <br /> nondetectable concentrations of TPH-D and BTEX <br /> In May 1993, July 1993 and September 1993 ground water monitoring was performed on the <br /> onsite wells (FSE, 1993a, ESE, 1993b, ESE, 1993c) Analysis of ground water from the <br /> downgradfent well MW-1 showed detectable concentrations of TPH-D of 950 micrograms per <br /> liter (µglL) in May 1993, 200 µg/L in July 1993, and nondetectable concentrations in September <br /> 1993 BTEX was nondetectable for the existing wells for all three of these 1993 events <br /> 1 <br />' 1r 164351861CIASURE RPT 3 Environmental Science&Engineering,Inc <br />