SITE INFORMATION AND CORRESPONDENCE

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Marla D. Guensler 15 July 1992 Exxon Company, U.S.A. Page 3 extracting hydrocarbons from the native soils. Soil vapor was extracted from well VEW-1 for a period of 2 hours. The screen of this well is installed mostly in the native soil, but because the screen and sanded section extend up into the backfill, the well will suffer short-circuiting through the backfill of the former tank field (see Figure 2). During extraction of vapors from this well, soil vapor concentrations ranged from 5,500 to 6,500 ppmv. These concentrations would have been higher if short-circuiting had not occurred. Well VEW-1 is located away from the location of the tank closure sample containing 4,300 - mg/kg. Such high concentrations of hydrocarbons being extracted away from the "hot spot",of the tank field indicate that native soils impacted with petroleum hydrocarbons remain below the former tank field and that the test performed mainly on VEW-2'did not address these soils. 5. Shell stated during the meeting that they believe the hydrocarbon concentrations measured in the soil sample,�frorn well MW8 result from UP gradient migration of liquid-phase hydrocarbons from Exxon's former tank field. In the course of Exxon's investigation a total of six wells have been installed along Pershing Avenue - between Exxon's former tank field and well MW8 adjacent to the Shell station. Wells Pl, P2, and IW1 have been drilled and sampled to depths of 43.5, 44.5, and 50 feet, respectively. As Figure 3 shows,,no hydrocarbon concentra- tions in any of the soil samples fromJ these wells approach the 3,800 mg/kg measured in the soil from well MW8 sampled at 28.5 feet. It is for this mason that Exxon does not believe that the hydrocarbons found in well MW8 originate from the Exxon site. 6. Evidence supporting the fact that petroleum hydrocarbons found in monitoring well MW8 did not originate from the Exxon site is found in the groundwater monitoring data, Table I shows the concentrations ()ig/L) of benzene and TPH-g measured in water samples obtained from wells MW 1, MW2, MW4, MW7, and MW8 between July 1987 and May 1991. During this time, the concentrations of benzene and TPH-g in well MW8 were on average always a factor of 10 times greater than the concentrations measured in monitoring wells MW1, MW2, MW4, and MW7. Figures4 and 5 show isoconce'ntration contours of TPH-g and benzene (jig/L) for April 1988 and concentrations for April and September of 1988 and August of 1989. Each of these figures shows that there is a definite decrease in hydrocarbon concentrations between Exxon's former tank field and the concentrations found in well MW8. It is clear from the data that has been accumulated during the investigation of the Exxon site that the hydrocarbons found in monitoring well MW8 are not caused by the hydrocarbons found in Exxon's former tank field. EA believes that more ;investigation by Shell is necessary to