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Former Discovery Chevrolet March 20, 1998 Tracy.Califomia Problem Assessment Report The calculation for the total estimated volume of gasoline retained in the soil above groundwater (Vs) is 26.3 gallons + 13.2 gallons = 39.5 gallons. In order to remove the 7,668 cf(284 cubic yards) of impacted soil, an additional 16,132 cf (597 cubic yards) of overburden soil would first have to be excavated at the site. The total excavation volume is therefore estimated to be approximately 23,800 cf(8811/2 cubic yards). 6.2 Groundwater Conditions Quarterly groundwater monitoring has been performed at the site by GeoStrategies from March 1995 to December 1997. During this time historical depths to groundwater have ranged between 9.25 and 11.53 feet below the well casing measuring points, and hydrocarbon concentrations in groundwater had been reported primarily in monitoring wells MW-1 and MW-2. The groundwater gradient flow direction has been predominantly toward the north-northwest (typical groundwater contour map shown in Appendix C). Analytical tests of the groundwater for MTBE and other oxygenated compounds have reported none of these compounds present (Table 2 - Historical Groundwater Data in Appendix B). 6.3 Petroleum Mdrocarbons in Groundwater Analytical results for the groundwater samples collected on February 19, 1998 from the 10 Geoprobe borings and the December 18, 1997 quarterly groundwater sampling of monitoring wells MW-1 through MW-5 were used to construct a THPg groundwater isoconcentration map (Figure 8) and a benzene isoconcentration map (Figure 9). The current isoconcentration maps indicated that the extent of hydrocarbon impact to the shallow groundwater beneath the site has been adequately defined. The only oxygenated fuel additive compound detected in the groundwater samples was MTBE. The laboratory analytical results from borings GP-3 through GP-8 indicated MTBE concentrations ranging from 0.53 ppb to 6.4 ppb, well below the interim action level of 35 ppb (Department of Health Services, November 10, 1994, Summary of California Drinking Water Standards). 3001-12.par 10 HORIZON ENVIRONMENTAL INC.