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volume of both soil and ground water contamination assumptions have been made <br /> regarding the maximum extent of contaminant migration. <br /> 121 Soil Contaminant Mass Calculation <br /> Following the completion of the remedial soil excavation, soil sample analytical data <br /> were evaluated to determine the mass of gasoline removed from the subsurface. Sidewall <br /> ' soil sample data were also evaluated to estimate the mass of gasoline remaining in the <br /> subsurface at the site. Because averaged values were used, the calculations are believed <br /> to be accurate but necessarily represent a range in terms of percentage of mass removed <br /> versus that remaining. <br /> Based on the results of the remedial excavation soil sampling, the average gasoline value <br /> is 6,128 mg/Kg. Using an excavated soil volume of 1,522 cubic yards, a total gasoline err, (e- <br /> mass <br /> emass of approximately 11,450 Kg, roughly equivalent to 4,000 gallons, are calculated to <br /> have been removed from the subsurface. <br /> Assuming a contaminant thickness of eight feet, and using contaminant contours based on <br /> soil sample analytical data (Figure 3), the total mass of gasoline at the site, prior to <br /> ' remediation was calculated. To perform the calculation, the site was contoured at three <br /> . contaminant levels. Area 1 encompasses a volume of approximately 1,775 cubic yards of <br /> ' soil containing between 100 mg/Kg and 1,000 mg/Kg as gasoline. An, average value of <br /> 500 mg/Kg was assumed for this area. Area 2 encompasses a volume of approximately <br /> 1,350 cubic yards of soil containing between 1,000 mg/Kg and 2,000 mg/Kg as gasoline. <br /> An assumed value of 1,500 mg/Kg was used for this area. Area 3 encompasses a volume <br /> of approximately 1,225 cubic yards of soil containing gasoline levels greater than 2,000 <br /> ' mg/Kg. An average value of 7,600 mg/Kg was used for this area. Using these values, a <br /> total mass of approximately 15,000 Kg as gasoline, roughly 5,300 gallons, are calculated <br /> to have been present at the site. <br /> ' With an initial contaminant mass of 15,000 Kg, and a removed mass of 11,450 Kg, it is <br /> estimated that a mass of 3,550 Kg as gasoline remained at the site immediately following <br /> ' the remedial excavation. This represents a removal of approximately 76% of the total <br /> mass of gasoline. It is concluded, based in part on ground water analytical data, that <br /> additional reductions in mass are occurring via natural attenuation processes. <br /> 55o a <br /> t ' �(400 lad <br /> •' <br /> LO <br /> American <br /> ' American Geological Service, Inc. <br /> CA95DE-029 14 <br />