SITE INFORMATION AND CORRESPONDENCE_FILE 1

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Marla D. Guensler 15 July 1992 Exxon Company, U.S.A. -'Y Page 2 EA disagrees with Shell's interpretation of the data for the following reasons and would like to point out several items of concern: 1. Shell's tank closure samples contained TPH-g at concentrations up to 4,300 mg/kg and benzene at concentrations up to 10 mg/kg. These concentrations indicate the presence of soils saturated with hydro- carbons. 2. That no petroleum hydrocarbons were found in water samples obtained from monitoring wells MW 1-3 is not surprising, because none of these wells are downgradient of any potential sources, either the former tank field or the product piping and certainly not from the point from where the 4,300 mg/kg sample was collected. Even monitoring well MW4 is not placed directly downgradient of the impacted soils identified during the removal of the former tanks. Additionally, a general lowering of groundwater concentrations is expected to occur as the groundwater elevation continues to rise. Hydrocarbon concentrations decrease as the water level rises because the hydrocarbons are filtered from the water by the "clean' soils that they are coming into contact with and by the influx of water. 3. Regarding the soil hydrocarbon concentrations,EA contacted both Steve Schneider of San Joaquin County Public Health Services and Jacqueline Doub of Aegis Environmental, Inc. in July of 1991 and informed them of the large historical fluctuations in groundwater elevations in the area and that of the first seven monitoring wells installed across the street at the Exxon site only one sample contained TPH-g at concentrations greater than the method detection limit of 10 mg/kg (60 mg/kg) and no BTEX compounds were detected at concentrations greater than the detection limit of 0.05 mg/kg. In spite of this knowledge, the only well to be placed by Shell nominally downgradient of the identified source of hydrocarbons was drilled only to a depth of 40 feet. This means that only about half of the potentially impacted soils have been investigated. 4. The low concentrations of volatile hydrocarbons measured in the soil vapor extracted from well VEW-2 are not indicative of low hydrocarbon concentrations in the soil. According to Shell's "Results Report for Air Monitoring of Extended Pilot Vapor Extraction Test," dated 7 February 1992, soil vapor was extracted only from well VEW-2, except for a 2-hour test performed on well VEW-1. As shown in cross-section A-A' (Figures 1 and 2), the screen and sanded section of Shell's vapor extraction well was placed mostly in the gravel backfill of the former tank field. This causes a serious short-circuiting problem through the porous backfill, and while this would be effective at cleaning up the gravel backfill, it will have only a small influence on