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Former Discovery Chevrolet March 20, 1998 Tr California Problem Assessment Report The thickness of the volume of impacted soil will therefore decrease with lateral distance away from the source. Using the area within the 1,000 ppm isoconcentration contour, and assuming an average impacted soil thickness of approximately 5 feet, an average width of approximately 12 feet, and an average length of approximately 22 feet equates to approximately 1,320 cubic feet (cf) of impacted soil. Assuming an average of 5,320 ppm TPHg from 3 analytical data points within the 1,000 ppm isoconcentration contour, and an average porosity of 50 percent (0.50) for the predominant soil type (clay) that was impacted by gasoline hydrocarbons, the following calculation for estimated volume of gasoline retained in the soil above groundwater (Vs) is: Vs = (1,320 cf) x (0.50 porosity) x (5,32011,000,000 TPHg)x(7.48 gallons per cf). Vs = 26.3 estimated gallons of gasoline retained in the soil above groundwater in the 1,000 ppm isoconcentration contour. Using the area within the 100 ppm isoconcentration contour, and assuming an average impacted soil thickness of approximately 3 feet, an average width of approximately,34 feet, and an average length of approximately, 70 feet equates to approximately 7,140 cubic feet (cf) of impacted soil. Subtracting 60% of the 1,320 cf(the volume of soil from within the 1,000 ppm isoconcentration contour for a thickness of 3 feet) results in a volume of 6,348 cf of soil between the 100 ppm and 1,000 ppm isoconcentration contours. Assuming an average of 554 ppm TPHg from 6 analytical data points within the 100 ppm isoconcentration contour, and an average porosity of 50 percent (0.50) for the predominant soil type (clay) that was impacted by gasoline hydrocarbons, the following calculation for estimated volume of gasoline retained in the soil above groundwater(Vs) is: Vs= (6,348 cf) x (0.50 porosity) x (55411,000,000 TPHg) x (7.48 gallons per cf). Vs = 13.2 estimated gallons of gasoline retained in the soil above groundwater between the 1,000 ppm and 100 ppm isoconcentration contours. 3001-12.par 9 HORIZON ENVIRONMENTAL INC.