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i <br /> ���MA <br /> December 16, 1992 Working To Restore Nature <br /> Unocal Service Station No 6981, Stockton, California <br /> i <br /> Dissolved Hydrocarbons and Selected Chemicals in Ground Water <br /> Results of laboratory analyses of ground-water samples collected from wells MW-6 through <br /> MW-8 in September 1992, and wells MW-9 through MW-13 in August 1992 (Table 7) were <br /> used to construct isopleth maps depicting the concentrations of dissolved TPHg and benzene <br /> in ground water beneath and near the site (Plates 14 and 15 respectively) As depicted on <br /> 1 Plate 14, the highest concentration of TPHg after the pump test sampling was detected in <br /> well MW-7 (550 parts per billion [ppbj), downgradient of the dispenser islands and former <br /> tanks Elevated levels of TPHg (65 ppb) were also detected in the sample from well MW-6 <br /> Concentrations of TPHg and benzene were below laboratory detection limits in samples <br /> 1 <br /> from wells MW-9, through MW-13 <br /> As shown on Plate 15, concentrations of benzene were also highest in downgradient well <br /> 1 • MW-7 (61 ppb) Benzene concentrations ranged from 2 8 to 19 ppb in samples from wells <br /> MW-6, and MW-8 Benzene was not detected in wells MW-9 through MW-13 Results of <br /> analyses are shown in Table 7 <br /> 1 <br /> As ground-water levels have lowered in this area, the concentrations of TPHg and benzene <br /> have decreased in ground-water Initial concentrations of TPHg of 13,000 and 860 ppb from <br /> samples collected from MW-7 and MW-8 in March of 1992 have decreased to 550 and <50 <br /> 1 ppb respectively in the September 1992 sampling A similar decrease in benzene is also <br /> shown (Table 7) These decreases in hydrocarbon concentrations occurred during an <br /> approximate 4 5 foot decrease in the ground-water elevation (Table 8) The hydrocarbons <br /> 1 have probably been trapped in the soil pores or adsorbed to the soil particles <br /> Nitrates and total and fecal cohform bacteria were detected in ground-water samples <br /> collected from monitoring wells during our investigation Specific wells sampled, sample <br /> 0 dates, and concentrations are shown in Table 9 <br /> 3$032-10 13 <br />