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4 0 NATURE AND EXTENT OF CONTAMINATION <br /> This section summarizes information regarding the nature and extent of hydrocarbons at the <br /> Site based on site characterization data obtained, including the corrective actions conducted in 1991 <br /> (Dames & Moore, 1992) and the groundwater monitoring round which was conducted in May, 1994 <br /> (Dames & Moore, 1994) <br /> 4 1 SOIL <br /> According to old site plans, three USTs were located in the vicinity of the maintenance shop <br /> shown in Figure 3 A 10,000-gallon diesel UST and pump island were located on the western side <br /> of the building On the eastern side of the maintenance shop, a 500-gallon UST was used for the <br /> storage of waste lubricating oil and spent solvent At one time a 325-gallon underground gasoline <br /> or diesel tank existed on the eastern side of the building, according to plant personnel, but was <br /> evidently removed, although no records of the removal were kept According to plant personnel, <br /> ' the tanks were at least 30 years old The soil in the vicinity of the 325-gallon tank was tested for <br /> solvents and none were found (Dames & Moore, April 1990) This tank passed leak tests in 1985 <br /> and was removed in 1986 (Dames & Moore, July 1988) <br /> A series of investigations beginning in 1985 by Bechtel National Incorporated (BNI, 1985), <br /> J H Kleinfelder and Associates (JHK, 1987), and Dames & Moore has indicated that hydrocarbon <br /> compounds, indicative of diesel fuel, are the primary constituents present in soils on both sides of <br /> the former maintenance building These investigations indicated that total petroleum hydrocarbon <br /> compounds as diesel (TPH/D) decreased both horizontally and vertically away from these two <br /> apparent source areas with the most extensive lateral migration of diesel fuel occurring between 20 <br /> to 25 feet below the surface <br /> Figures 3 through 8 illustrate the distribution of residual TPH/D in soil to levels less than <br /> 10,000 milligrams per kilogram (mg/kg) TPH/D, following excavation of approximately 2,000 cubic <br /> yards of soil from the two areas in June 1990 (Dames & Moore, 1990) This corrective action <br /> resulted in the removal, treatment, and disposal of approximately 47 percent of the TPH/D at the <br /> site to a Class II landfill Mass balance calculations completed by Dames & Moore (Dames & <br /> Moore, 1994a) indicates that approximately 88 percent of the remaining soils have a concentration <br /> of less than 5,000 mg/kg These calculations were based on the TPH/D concentration contours in <br /> Figures 3 through 8, as described in the August 1994 report Additional volume calculations <br /> completed for this technical evaluation using the same figures These calculations indicate that <br /> remaining soils with TPH/D levels above 1,000 mg/kg amount to approximately 4,900 cubic yards <br /> SAC127 10 5 <br /> DAMES & MOORE <br />