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The results of the mass balance calculation are summarized below MASS BALANCE OF TPH/D IN SOIL TPH/D Initial TPH/D Percent Concentration Mass Mass Removed Removed Remaining Range (pounds) (pounds) M (pounds) ' (mg/kg) > 10,000 8,223 8,223 100 0 5,000-10,000 13,883 10,590 76 3,293 1,000--5,000 25,047 5,247 21 19,800 100-1,000 4,556 163 4 4,393 Total 51,709 24,223 47 27,486 As summarized in this table, approximately 24,000 lbs of TPH/D was removed from the ' vicinity of the two underground tanks This mass of TPH/D u1 soil accounted for roughly 47% of the estimated initial mass Approximately 88% of the remaining impacted soil, by this estimate, had concentrations of TPH/D below 5,000 mg/kg TPH/D The fate and transport modeling presented in the next section will attempt to quantify the potential impact to groundwater of the remaining TPH/D to soil after excavation As described above, the excavated material (approximately 2,050 cubic yards of soil) was removed and transferred to an on-site biotreatment cell Upon excavation and tilling the final volume of soil increased to approximately 2,550 cubic yards, which was treated according to the work plan until concentrations declined enough to meet the requirements for off-site disposal In a Class II Landfill 5.0 FATE AND TRANSPORT MODELING TO ASSESS IMPACTS TO GROUNDWATER A modeling approach was used to simulate the leaching of the indicator chemical naphthalene through the vadose zone to groundwater beneath the site over a period of 25 years The model ' chosen for the leaching simulation was VLEACH, (CH2M Hill, 1990) VLEACH is a one- dimensional finite difference model designed to simulate leaching, volatilization, and sorption of chemicals in the vadose zone The model simulates leaching in a number of polygons, represented by a vertical stack of cells, reaching from the land surface to the water table SAC116 07 10 AUGUST 1994 DAMES &MOORE