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Former Discovery Chevrolet March 20, 1998 Tracy,California Problem Asse sment Re ort 6.3.1 Mas Balance of Petroleum Constituents in Groundwater The area of concern for gasoline hydrocarbons in the groundwater is the area shown on the TPHg Isoconcentration Map (Figure 8). The <50 ppb isoconcentration contour generally reflects the area of groundwater impacted by fuel hydrocarbons beneath the site. i Using the area within the <50 ppb isoconcentration contour, and assuming an average impacted aquifer thickness of approximately 5 feet,--equates to approximately 18,320 cubic feet (cf) of saturated soil. Assuming a dissolved plume approximately the size of the TPHg plume shown on Figure 8, an average porosity of 45 percent (0.45) for the predominant soil type (clayey sand) that was impacted by gasoline hydrocarbons, and a conservative assumption that the entire plume contained the highest dissolved concentration of TPHg detected during that sampling event (14,000 ppb or 14 ppm), it can be calculated that this initial plume contains approximately 0.86 gallons of gasoline (VW) based on the following calculation: Vw = (18,320 cubic feet of saturated soil) x (0.45 porosity) _ (8,24) cubic feet of water) x (14.0/1,000,000) x (7.48 gallons per cubic foot) = 0.86 gallons of gasoline in the groundwater beneath the site. In reality, because the entire plume does not contain the highest dissolved concentration of TPHg detected during that sampling event (14,000 ppb or 14 ppm), the volume of gasoline present in the groundwater beneath the site is probably less than 0.3 gallons. Based on the above calculation the amount of hydrocarbons in soil and water=Vs +Vw or --, approximatgly 40 gall'pns of gasoline as TPHg in the subsurface as of February 1998. 3001-12.par l l HORIZON ENVIRONMENTAL INC.