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concrete constructed level with existing pavement A one-inch asphalt overlay was then applied around the cap and feathered over the edge of the existing pavement In June 1991, an 85 foot long product line extending from a former pump island toward the 10,000 gallon diesel tank was removed and inspected by the PHS-EHD Soils from beneath the product line, which had been buried at a depth of 15 feet, were sampled along the trench Following excavation to a depth of five feet all soil samples reported nondetectable TPH/D and BTXE The 150 cubic yards of soil removed along with the product line were treated off site by thermal oxidation The trench was backfilled as specified by the PHS-EHD with 150 cubic yards of pea gravel and eight inches of Class II AB aggregate, then paved with six inches of asphalt ' 4 2 MASS BALANCE OF HYDROCARBON IN SOILS FOLLOWING EXCAVATION ' Figures 10 through 15 illustrate the lateral distribution of TPH/D m soils prior to remedial action based on analytical results of samples collected from soil borings by Bechtel, JHK, and Dames ' & Moore Analytical results are also presented in Tables 3 through 8 The highest concentrations of TPH/D are shown at the east and west ends of the former maintenance shop building at depths of 15 to 20 feet TPH/D concentrations decrease horizontally and vertically away from these two apparent source areas Figures 18 through 21 present the areas of the excavated soils in relation to the impacted areas of soils at depths of 10, 15, 20, and 25 feet bgs A mass balance calculation was completed ' based on the areas of impacted soils presented in Figures 10 through 15 for depths of 10 to 35 feet bgs The method used to complete the mass balance calculation was performed as described in Appendix E The mass of hydrocarbon in soils prior to excavation is estimated by calculating the cross-sectional areas of impacted soils for concentrations exceeding 100, 1,000, 5,000, and 10,000 mg/kg These areas were computed by a computer assisted design program (AUTOCAD) which produced Figures 9 through 21 After areas of each soil layers were computed, areas within each concentration range were converted to concentration and mass of TPH/D as described in Appendix E The mass of the excavation was also calculated by the same method utilizing the excavation plans presented in the Soil Remediation and Groundwater Monitoring Work Plan (Dames &Moore, 1990) SAC116 07 9 AUGUST 1994 DAMES &MOORE