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June 21, 1993 Working To Restore Nature ARCO Service Station No 2130 percentages for well MS-2 ranged from 85 to 87 percent for two of the three test phases In all instances, TPHg concentrations increased in well MS-3 during each test phase by 1,800, 5,500, and 6,800 ppb as compared to baseline. Benzene reductions of 36, 71, and 54 percent were identified in well MS-1 during each test phase, while benzene removal percentages for well MS-2 ranged from 66 to 97 percent for i two of the three test phases Benzene concentrations increased in well MS-3 during each test phase by 1,240, 1,042, and 720 ppb as compared to baseline The increases of TPHg and benzene exhibited in well MS-3 may be due to ground-water elevation changes resulting in the mass transfer of gasoline from unsaturated soil to ground water Moderate increases t')) in DO content were observed for some samples, however, no trends were observed It appears that the sparging durations were not long enough to significantly increase the DO content within the study zone Soil Gas Baseline TPHg vapor concentrations measured prior to sparging ranged from 3,400 (well AS-1) to 74,000 (well MS-3) milligrams per cubic meter with the higher concentrations located in the vicinity of the existing underground tanks During sparging, the measured changes in gas concentration as a function of location varied widely Concentrations of TPHg vapor at wells AS-1 and MS-1 consistently increased (Table 10) while measured values at well MS-2 and locations further away from well MS-2 decreased CONCLUSIONS ] Evaluation of dissolved TPHg reductions during sparging indicate that observed influence is limited to a minimum distance of approximately 10 feet from the sparge well Reductions •` of dissolved TPHg were not observed in well MS-3 (located 19 5 feet from AS-1) at any 30003-8 11 I I•