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GealogicalTeclucicslnr. Page 9 Sinclair Trucking Project No.: 1030.2 November 1,2005 5,000 mg/Kg. The deep core occupies the space inside and including the 4,000 mg/Kg contour line. ' ® Average concentration was determined by the percentage of the core or ring occupied by a specific contour concentration. o The deep core is 921.6-ft2. ' o 8,000 mg/Kg contour covers 21.9-ft2and represents 1.9% of the core area o 6,000 mg/Kg contour covers 297.2-ft2and represents 25.3% of the core area o 4,000 mg/Kg contour covers 856-ft'and represents 72.8% of the core area ' o Each percentage is multiplied by the corresponding concentration and the sum of the three products is divided by 100 percent to reach the average concentration in a given area. ' • The area between the non-detect and the 4,000 mg/Kg contours is approximately 4,446-ft2 and is called the deep ring. The largest portion of the ring lies between the non-detect and the 2,000 mg/Kg contour lines. To be conservative 1,000 mg/Kg was used as an ' average for the area between non-detect and 2,000 mg/Kg. The deep ring has an average concentration of 1,338 mg/Kg. ' • Average concentration was determined by the percentage of the core or ring occupied by a specific contour concentration. o The deep ring is 4,446-ft2. o 2,000 mg/Kg contour covers 1,501-ft2 and represents 33.8% of the deep ring o NDS 1 mg/Kg contour covers 2,945-ft2 and represents 66.2% of the deep ring o Each percentage is multiplied by the corresponding concentration and the sum of the ' two products is divided by 100 percent to reach the average concentration in a given area. • The thickness of the deep TPH-G contamination is assumed to be 3.5-feet, primarily occupying the depths between 10.5 and 14 feet bgs. • Historical depth-to-groundwater measurements average 11.25 ft bgs. • Mass balance calculations assume the entire 3.5-foot thick units consist of uniform contaminant concentrations. • Contaminated soil removed during soil boring and monitoring well installation activities was not subtracted from the overall mass volume. ' • Natural attenuation was not used in determining remaining mass. 2.5 Remaining Contaminant Mass Estimate Calculations ' Post-excavation calculations suggest that there was 4,506 pounds of TPH-G present in soil before the excavation began. Calculating the remaining TPH-G in soil after the excavation t indicates that as much as 2,645 pounds remain in the soil at the site. The bulk of the contamination found in the mass estimate is around MW-4 corroborating field observations. Bound by limitations by the SWRCB directive, GTI could not remove the balance of the contaminated soil. GTI estimates approximately 1,800 pounds remain in the area surrounding MW-4.