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5.0 RESULTS OF CHEMICAL ANALYSIS <br /> • Concentrations of petroleum hydrocarbons were detected only in the soil sample <br /> collected from the capillary fringe interval of B-10 (AW-9) at 11.5 fbg; TPH-G was <br /> detected at 480 parts per million (ppm), TPH-D at 510 ppm, and benzene at 4.4 ppm. <br /> Concentrations of hydrocarbons were not detected in samples collected from Soil <br /> Borings B-11 through B-15. <br /> • During the sampling event performed on April 7, 1992, concentrations of dissolved- <br /> phase petroleum hydrocarbons were detected only in ground water samples collected <br /> from AW-3 and AW-9: TPH-G and benzene were detected only in AW-9 at 72 and <br /> 15 parts per billion (ppb), and TPH-D was detected in AW-3 and AW-9 at 75 and <br /> 65 ppb. <br /> • During the sampling event performed on April 24, 1992, concentrations of dissolved- <br /> phase petroleum hydrocarbons were again detected only in ground water samples <br /> collected from AW-3 and AW-9. Laboratory analysis for TPH-G in AW-3 attributed <br /> 120 ppb to high boiling point hydrocarbons; TPH-D was detected in AW-3 and AW-9 <br /> at 1,400 and 130 ppb; and benzene was detected in AW-3 at 5.5 ppb. The laboratory <br /> report indicated the TPH-G concentration reported for AW-3 did not appear to <br /> reflect the presence of gasoline in the sample. <br /> • Concentrations of petroleum hydrocarbons were not detected in Monitoring Wells <br /> AW-1, AW-2, or AW-4 through AW-8 during either of the sampling events. <br /> 6.0 CONCLUSIONS <br /> Based on review of the results of this and previous studies, Alton Geoscience concludes the <br /> following: <br /> • Analytical results from the soil samples collected from AW-9 indicate the onsite <br /> extent of adsorbed-phase hydrocarbons appears limited to the capillary fringe interval <br /> i between approximately 10 and 12 fbg. <br /> 0 Soil Borings B-11 through B-15 indicate the capillary fringe interval in the vicinity of <br /> AW-6 has not been impacted by petroleum hydrocarbons. Moreover, TPH-D <br /> concentrations detected in AW-6 in 1991 decreased with depth from 180 ppm at 6 fbg <br /> to 5.4 ppm at 11 fbg, suggesting a shallow local source. The reported occurrence in <br /> AW-6 appears unrelated to any identified petroleum releases at the former Mobil <br /> bulk plant. <br /> • The dissolved-phase hydrocarbon plume appears limited to the vicinity of Monitoring <br /> Wells AW-3 and AW-9. <br /> 5 <br />