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1 , rootogicat reckaka 14c. Page 7 Sinclair Trucking Project No.: 1030.2 November 1,2005 © 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. ' o 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. 1 2.3 Excavation An area of approximately 1,360 square feet was opened during the soil excavation activities. Five to six feet of clean overburden was stockpiled near the southeastern portion of the ' property. The remaining contaminated soil was excavated to a depth of approximately 13.5 feet below grade. Approximately 400 cubic yards of contaminated soil was excavated and stockpiled in a staging area in the northeastern portion of the property. ' Figure 2 shows the extent of the excavation in plan view. 2.4 Past-Excavation Mass Estimate Calculations This section discusses the mass estimate calculations generated after the advancement of ten ' (10) GeoProbe borings and two (2) soil borings as well as samples collected during the soil excavation activities and the installation of MW-1 R. A non-detect control point was interpolated between MW-4 and MW-5. The additional sampling points plotted into the ' Surfer 7.00 contour generating software suggested the plume was slightly smaller than the pre-excavation results. The calculations suggest that there were 4,506 pounds of TPH-G in soil before the excavation ' began. Since the highest number of sample points determined the estimate of 4,506 pounds of TPH-G in soil, 4,506 pounds will be considered the mass in soil prior to the excavation activities. The mass balance calculations are summarized in Table 5 of Appendix A. i The following summarizes the data and assumptions used in the post-excavation mass balance estimates: ' vat' n u Fi re 6: Post-Excavation 1 _g_. a zo Shallow • 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 7A0 contour generating software using soil samples collected between 6 and 10.5 feet bgs. • The heaviest contamination is found near the former UST encompassing an area of approximately 53.5-ftp. The area occupies the space inside and including the 6,000