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Page 7 GeologicalTeckraicslnc. g City of Escalon L*74- Project No.: 750.2 May 16,2005 L2003 Mass Estimate Still being liberal, recalculating the original mass using a 20-foot thick zone`of heavy contamination suggests that, prior to vapor extraction activities, there was approximately 1,248 gallons of TPH-G present in the soil at COE. Analytical results from soil borings advanced during the October 2003 soil investigation suggest that the contaminant r concentrations have been greatly reduced. The October 2003 soil samples suggest that soil vapor extraction and natural attenuation have Lbeen effective at the site. Vapor extraction remedial activities have removed approximately 1,013 gallons of TPH-G from the site as discussed in Section 3.7. If we assume that 10% of the original estimate has undergone natural biodegradation, this would account for an Ladditional 124.8 gallons of TPH-G removed. These calculations suggest a total of 1,138 gallons of TPH-G has been removed and approximately 110 gallons remain in the soil. The calculation suggests approximately 90% of contaminant mass has been removed since site Linvestigation activities began in 1996. r r-� 2005 Mass Estimate After several years of remedial activities at the COE site, the revised contaminant mass estimated calculations show approximately 56 gallons of TPH-G remain in the soil (Table 8). Figure 8 shows the soil contamination plume and a "mass balance" line of section. Figure 9 shows a cross sectional view of TPH-G contamination remaining at the site using the following information and assumptions: C • The plume is divided into three contamination zones. • Each zone is assumed to be roughly cylindrical in shape. • Concentrations in the zones are an average of samples collected from those zones. • Area is determined b ma view (Figure 8 . Y P ( g ) • Thickness is based on soil analytical data and shown in Figure 9. L • Assumes October 2003 contamination concentrations have been reduced by 50%. • Assumes soil contamination detected between 1996 and 1999 has been reduced by 75%. 0 Contamination has not impacted groundwater.. • 100 cubic yards of soil removed from the plume area during tank and dispenser removal activities. Actually a total of 270 cubic yards were removed but only 100 cubic yards was removed from the area of the plume footprint (Figures 7 & 8). The following items likely reduce the mass of contaminants at the site. However, contaminant mass estimate calculations do not include: C--� • Aeration of intermediate to deep subsurface by staggered well extraction methods. r • Aeration of surface and shallow subsurface during trenching activities. L