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5 ECOR 4.0 EXTENT OF PETROLEUM HYDROCARBON IMPACTS 4.1 Petroleum Hydrocarbons in Soil During UST removal and over -excavation activities in August 1998, soil was found to contain maximum concentrations of TPHg, benzene, and MtBE at 2,300 mglkg, 4.1 mg/kg, and 330 mg/kg, respectively, at the bottom of the excavation. Currently, the maximum concentrations of petroleum hydrocarbons and fuel oxygenates in soil appear to be located in the vicinity of the former USTs, adjacent to groundwater extraction well EX -1 and vapor extraction wells VE -1 through VE -4. The vertical extent of TPHg, BTEX, and MtBE in the vicinity of the former USTs extends to approximately 65 ft bgs as indicated by soil analytical results from EX -1, VE -1 through VE -4, and soil borings S13-2 and S13-3. Trace concentrations of benzene and MtBE were detected at a depth of 72 ft bgs in SB -1. Table 4 summarizes the analytical results from analysis of soil samples collected. 4.2 Estimated Mass of Petroleum Hydrocarbons in Soil In June of 2000 and the second quarter 2001, SECOR performed a calculation to estimate the . total mass of TPHg, benzene, and MtBE remaining in soil. This mass balance calculation was based on historical soil data, provided in Table 2. For this estimate, the following assumptions were made: ♦ The area of impacted soil at the site consists of one contiguous area; ♦ Impacted soil in the area of the former UST extends to a total depth of 65 bgs; and ♦ Soil concentrations in the former UST area are equal to soil data from borings (EX -1, S13-2, S13-3, and VE -1 through VE -4). Analytical data from soil samples collected at the site indicate that typically, the maximum concentration of hydrocarbons is found at approximately 15 ft bgs, and these concentrations decrease with depth. In order to estimate the mass of hydrocarbons in soil, concentrations of petroleum hydrocarbons were assumed to be distributed linearly with respect to depth. The equation used is included as Appendix D. The results of this calculation indicated that the amount of TPHg, benzene, and MtBE remaining in soil were 1,450 lbs, 370 lbs, and 67 lbs, respectively. On August 20, 2003, operation of the SVE system was discontinued following approval by the RWQCB in their letter dated August 18, 2003 and has remained offline. The SVE system had operated since July 2000. The amount of TPHg, benzene and MtBE removed from the subsurface was calculated by using SVE mass removal estimates through the last quarter of operation, 3`d quarter 2003. According to the SVE records, the approximate mass of TPHg, benzene, and MtBE removed was 4,132 lbs, 86 lbs, and 444 lbs, respectively (Tables 5 and 6). This information is summarized in the following table: 95775 NFAR REPORT Final.doc 8 5ECOR International Incorporated w