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May 4, 1993 <br /> Unocal Station No 6981, Stockton, California Working To Restore Nature <br /> 4.4.2 Dissolved Hydrocarbons and Selected Chemicals in Ground Water <br /> Results of laboratory analyses of groundwater samples collected from wells MW-6 through <br /> MW-8 in September 1992, and wells MW-9 through MW-13 in August 1992 (Table 6) 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 11 and 12 respectively) As depicted on <br /> Plate 11, the highest concentration of TPHg after the pump test sampling was detected in <br /> well MW-7 (550 parts per billion [ppb]), 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 were below laboratory detection limits in samples from wells MW-8 <br /> r <br /> through MW-13 <br /> Data collected by GeoStrategies during the first quarter ground water monitoring event was <br />�. also used to construct isopleth maps depicting the concentrations of dissolved TPHg and <br /> benzene in ground water beneath and near the site (Plates 13 and 14 respectively) These <br /> plumes show similar trends in shape and extent as the plumes depicted on Plates 11 and 12 <br /> Higher concentrations of TPHg and benzene were reported in the ground water samples <br /> i <br /> collected from MW-6 and MW-7 during the first quarter 1993 <br /> As shown on Plate 14, concentrations of benzene were also highest in downgradient well <br /> MW-7 (1000 ppb) Benzene concentrations during the first quarter 1993 sampling ranged <br /> from 3 6 to 1,000 ppb in samples from wells MW-6, and MW-7 respectively Benzene was <br /> not detected in wells MW-8 through MW-13 Results of analyses are shown in Table 6 <br /> i <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 /> ppb respectively in the September 1992 sampling A similar decrease in benzene <br /> concentrations is also shown (Table 6) These decreases in hydrocarbon concentrations <br /> 38032-13 8 <br /> I <br />