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{ <br /> ESTIMATE OF REMAINING CONTAMINANT MASS <br /> As required by Appendix B (RWQCB, 1996), an <br /> ' estimate of the remaining contaminant mass in the <br /> soil and groundwater is to be presented in the <br /> closure report As discussed above, detectable <br /> hydrocarbons are no longer present in the <br /> ' groundwater and therefore this section estimates <br /> the contaminant mass remaining in the impacted <br /> soil As shown on Plate 6, the shape of TPHg <br /> plume is an ellipse with a long axis of 65 feet and <br /> a short axis of 35 feet The plan view area is <br /> approximately 15,000 square feet As shown in <br /> Plate 4, the thickness vanes over the length of <br /> what is assumed to be lenticular or lensoidal in <br /> cross-section An average thickness of 10 feet is <br /> assumed, this yields a volume of 15,000 cubic feet <br /> or 555 cubic yards Assuming a weight of 3,000 <br /> pounds per cubic yard, this totals approximately <br /> ' 1,665,000 pounds or 760,000 kilograms of <br /> impacted soil Assuming an average concentration <br /> of 500 mg/kg,this yields a mass of approximately <br /> 380 kg of TPHg, or approximately 135 gallons of <br /> gasoline (assumes a specific gravity of 0 75 for <br /> gasoline) <br /> 1 <br /> 1 <br /> 1 <br />' L130000%387371WP\38737N19 Harding Lawson Associates 19 <br />