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1 I CW(Ogkof Techaics lnr. Page 8 Sinclair Trucking Project No.: 1030.2 November 1,2005 mg/Kg contour line. This area, called the core, has an average concentration of 7,108 mg/Kg. ' • Average concentration was determined by the percentage of the core or ring occupied by a specific contour concentration. o Example: The core is 53.5-ft2. ' o 12,000 mg/Kg contour covers 0.5-ft2and represents 1% of the area o 10,000 mg/Kg contour covers 6.4-ft2 and represents 11.9% of the area o 8,000 mg/Kg contour covers 15.3-ft2 and represents 28.6% of the area ' o 6,000 mg/Kg contour covers 31.3-ft2and represents 58.5% of the area o Each percentage is multiplied by the corresponding concentration and the sum of the four products is divided by 100 percent to reach the average concentration in a given ' area. • The area between the non-detect and the 6,000 mg/Kg contours is approximately 1,784-ftZ and is called the 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 ring has an average concentration of 1,327 mg/Kg. • Average concentration was determined by the percentage of the core or ring occupied by a specific contour concentration. o Example: The ring is 1,783.5-ft2. o 4,000 mg/Kg contour covers 124.2-ft2and represents 7% of the area o 2,000 mg/Kg contour covers 208.9-ftZ and represents 11.7% of the area o ND<1 mg/Kg contour covers 1,450.4-ft2and represents 81.3% of the 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 thickness of the shallow TPH-G contamination is assumed to be 4.5-feet, primarily occupies the depths between 6 and 10.5 feet bgs. • Historical depth-to-groundwater measurements average 11.25 ft bgs. • Mass balance calculations assume the entire 4.5-foot thick units consist of uniform contaminant concentrations. ' • UST and septic tank volumes are small and not subtracted from the overall mass volume. • 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. Fijzure 7: Post-Excavation Dee ' • TPH-G contours were generated using soil data gathered from deep soil samples collected prior to the soil excavation activities. The contours were generated by Surfer 7.0® contour generating software using soil samples collected between 10.5 and 1,4 feet bgs. ' • The heaviest contamination is found in bi-lobal contours near the southern end of the former UST and surrounding MW-4. This bi-lobal contamination encompasses an area of approximately 922-ft2. This area, called the deep core, has an average concentration of