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CTW&gk,1(7"utfeslnc. Page 5 Sinclair Trucking Project No.: 1030.2 November 1,2005 2.2 Pre-Excavation Mass Estimate Calculations ' In response to the September 2, 2005 SICEHD letter, GTI has prepared mass estimate calculations for both pre- and post-excavation soil analytical results. This section discusses the mass estimate calculations generated after the advancement of ten (10) GeoProbe borings and two (2) soil borings prior to soil excavation activities as well as the installation of MW-5- The calculations suggest that there were 5,485 pounds of TPH-G in soil prior to the February 2005 soil excavation activities. tThe pre-excavation mass balance calculations are summarized in Table 4 of Appendix A. ' The following summarizes the data and assumptions used in the pre-excavation mass balance estimates: Fieure.4: Pre-Excavation Shallow a TPH-G contours were generated using soil data gathered from shallow 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 6 and 10.5 feet bgs. a The heaviest contamination is found near the former UST encompassing an area of approximately 140-ft'. The area occupies the space inside and including the 6,000 mg/Kg contour line. This area, called the core, has an average concentration of 7,196 rng/Kg. a Average concentration was determined by the percentage of the core or ring occupied by ' a specific contour concentration. o The core is 140-ft2. o 12,000 mg/Kg contour covers 3-ftp and represents 2.1% of the core area ' o 10,000 mg/Kg contour covers 17.2-ft2and represents 12.3% of the core area o 8,000 mg/Kg contour covers 40.4-ft2 and represents 28.9% of the core area o 6,000 mg/Kg contour covers 79.4-ft2and represents 56.7% of the core area ' o Each percentage is multiplied by the corresponding concentration and the sure of the four products is divided by 100 percent to reach the average concentration in a given ' area. a The area between the non-detect and the 6,000 mg/Kg contours is approximately 1,580-ft2 and is called the ring. The largest portion of the ring lies between the non-detect and the 2,000 mg/Kg contour lines and is assumed to have an average concentration of 1,000 ' mg/Kg. The ring has an average concentration of 1,49.9 mg/Kg. a Average concentration was determined by the percentage of the core or ring occupied by ' a specific contour concentration. o The ring is 1,580-ft2. - o 4,000 mg/Kg contour covers 145-ft'and represents 9.2% of the ring area o 2,000 mg/Kg contour covers 352-ft'and represents 22.3% of the ring area o ND<1 mg/Kg contour covers 1,083-ft"and represents 68.5% of the ring area