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r..� •�„/ .w Me City of Stockton, Fire Station No. 12 A68105.01 August 7, 1995 Page 8 6.1 Vertical Extent of PPC Migration in Soil: During the previous excavation work, PPCs were detected at a depth of 14 feet BSG in the center of the excavation (i.e., soil sample BS-1 in Table 1). The purpose of soil boring B-2 was to explore the soil at depths greater than 14 feet in the same approximate location. PPCs were not detected in soil samples collected from soil boring B-2 at depths greater than 14 feet BSG (Table 2). It appears that the vertical extent of PPC migration does not extend as deep as 15 feet BSG. 6.2 Lateral Extent of PPC Migration in Soil: Based on analytical results of soil samples collected during the previous excavation work from the north and south wall (i.e. soil u samples SW and NW, Table 1) PPC impacted soil was present on the north and south edges of the excavation. The purpose of soil borings B-3 and B-4 was to assess the lateral extent of PPC impacted soil on the north and south of the former excavation location. Based on the analytical results of soil samples collected from soil borings B-3 and B-4 (Table 2) PPC lateral migration does not appear to extend more than two feet beyond the former UST excavation on the north nor more than four feet on the south. 6.3 Volume, Mass, Area, Average Concentration: Based upon the evaluation presented in Section 6.1 above, the impacted soil remaining in the subsurface is assumed to extend vertically no deeper than 15 feet. It was reported to Twining by the City of Stockton, that PPC impacted soils above 14 feet BSG had been removed from the area of the USTs by the previous excavation work and replaced with "clean fill'. The analytical results of the soil sample collected from soil boring B-2 at 10 feet BSG detected PPCs (Table 2). This finding is anomalous with the previously reported data on the depth of the excavation. Evaluation of this anomaly is beyond the scope of this investigation. Calculations presented below to estimate the amount of PPCs in soil are conservatively based on the assumption that no PPC impacted soils were removed. In correspondence dated July 19, 1995, the City of Stockton, Department of Public Works requested that calculations of the volume, mass, area, and average concentration of impacted soil remaining in the subsurface be included in this report. Therefore in order to satisfy the SJCPHS request, certain assumptions are required in order delimit the extent of impacted soil. ` The following assumptions were used to estimate the volume, area, mass, and average concentrations: • No PPC impacted soils were previously removed as indicated by the City of Stockton; • The bottom of the UST was 7 feet BSG; • The source of the PPC release was the from the center of the UST excavation and the bottom of the tank; • PPCs migrated in a radial and symmetrical pattern from the source; 4