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The three ground water monitoring wells were monitored quarterly for five quarters prior <br /> to November 1991. Quarterly monitoring results reported benzene, toluene, ethylbenzene, <br /> and xylenes (BTEX) concentrations well below California primary Maximum Contaminant <br />' Levels (MCLS) and Taste and Odor Thresholds established by the state of California and <br /> the Federal EPA (Marshack, 1993), with one exception. In August 1991, benzene at a <br />' concentration of 1.3 ppb, was found in a cross-gradient well, MW-2. Detection of this <br /> constituent was not confirmed during any of the four preceding nor three subsequent <br />' quarterly monitoring 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 <br />' water In addition, the PHS/EHD requested an additional investigation of soil and ground <br /> water to determine if a source of petroleum hydrocarbons still existed at the site <br /> In May 1993, ESE drilled two soil borings in the vicinity of boring B-2Y and one monitoring <br /> F0well was installed downgradient and to the west of the former tank excavation This <br /> additional investigation work was documented in an ESE report dated June 11, 1993 <br />' Analyses of soil samples collected during the investigation were reported as nondetectable <br /> for TPH-D and BTEX This confirmed that a source does not exist in the unsaturated soil <br />' zone on site. Analyses of ground water samples from the new monitoring well, MW-4, were <br /> also reported as nondetectable for TPH-D and BTEX <br /> In May 1993, July 1993 and September 1993 ground water monitoring was performed on the <br />' onsite wells. Analysis of ground water from well MW-1 showed detectable concentrations <br /> of TPH-D of 950 micrograms per liter (ug/L) in May and 200 ug/L in July 1993, and <br /> nondetectable concentrations in September 1993. BTEX was nondetectable for the existing <br />' wells for all three of these 1993 events. <br /> Environmental Science <br /> F\5935166\4gmr 93 2 &Enpneermg, Inc <br /> 1 <br />