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w ti,.r City of Stockton, Fire Station No. 12 A68105.01 August 7, 1995 Page 9 • Concentrations of PPCs in the impacted soil are represented by the concentrations detected in the soil sample collected at 10 feet BSG from soil boring B-2; • The mass of soil is equal to 1.5 tons per cubic yard; and(om) • The unit of measure (mg/kg) for the adsorbed phase TPH concentrations is assumed to be equivalent to parts per million (ppm) or 1 x 10-6. Based on the aforelisted assumptions and the findings from soil samples collected in soil borings B-2, B-3, and B-4 the lateral extent of PPC migration is estimated to be within a radius of 9 feet from the center of the former UST. This radius is the approximate distance between soil boring B-2 (where PPCs were detected) and soil boring B-4 (where PPCs were not detected). The vertical extent of PPC migration is estimated to extend between 7 feet BSG (the assumed bottom to the USTs) and 15 feet BSG (the depth of the first non-detect soil sample from soil boring B-2). The average concentrations of PPCs in the volume of impacted soil is estimated to be; 23 mg/kg for TPH-G and 19 mg/kg for TPH-D. Using the aforementioned assumptions, the impacted soil remaining in the subsurface is •� cylindrical in shape with a radius of 9 feet and a height of 8 feet. Given these parameters, the volume of the impacted soil would be 75.4 cubic yards. The plan area of the impacted soil would be 28.3 square yards. The mass of impacted soil would be 113 tons (226,000 lbs). moo One gallon gasoline is equal to 6.5 lbs and one gallon of diesel is equal to 6.8 lbs (Lindeburg, 1992). By multiplying the mass of impacted soil (226,000 lbs) by the adsorbed phase TPH-G (23 ppm or mg/kg) and TPH-D (19 ppm or mg/kg) concentrations previously discussed, the weight and volume of gasoline and diesel present in the impacted soil are estimated as follows: • The mass of gasoline in the impacted soil is estimated to be 5.2 lbs and the volume 0.8 gallons; and .r • The mass of diesel in the impacted soil was estimated to be 4.3 lbs and the volume 0.6 gallons; V It should be noted that laboratory qualifications of adsorbed phase TPH-G and TPH-D are based upon chromatograms which overlap. Therefore the estimated quantity of gasoline and vow diesel in the impacted soil calculated above should be considered a worst-case estimate. 6.4 Potential Environmental Effects: Potential environmental effects were evaluated ` using the "Leaching Potential Analysis for Gasoline and Diesel' from the Leaking Underground Fuel Tank (LUFT) Field Manual (California State Water Resources Board, 1989). This analysis provides recommended maximum allowable concentrations of BTEX, TPH-G, and TPH-D in soil that can be left in place without threatening groundwater. Site