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Ozone N icrosparge System Pilot Test Status Report, Former Unocal Service Station No.0187, Stockton,CA <br /> November 19,2003 <br /> Recently, however, benzene concentrations decreased to below baseline concentrations between <br /> March and September 2003. On September 18, 2003, benzene concentration in well NP-2-60 <br /> was 64 ppb. <br /> • In well NP-2-116,benzene concentrations remained near the baseline concentrations during the <br /> duration of the test period,as shown on Graph 6 <br /> • Baseline benzene concentration in well U-3 was 515 ppb. After approximately six months of <br /> system operation, the benzene concentration decreased to non-detect. For the remainder of the <br /> test period, benzene concentrations in well U-3 were at or near the reporting limit, as shown on <br /> Graph 7. <br /> • Benzene concentrations in well U-10 decreased from 57.9 ppb at the test baseline to be at or near <br /> the reporting limit, as shown on Graph 8. <br /> i <br /> Discussion of Results <br /> A summary of the results of the ozone microsparge pilot test is as follows: <br /> • Sample results from wells NP-1-22, NP-1-65, NP-1-111, NP-2-22, NP-2-60, U-3, and U-10 <br /> indicate that ozone microsparge treatment has been effective at decreasing petroleum <br /> hydrocarbon concentrations in groundwater near these wells. <br /> • Hydrocarbon concentrations in well NP-2-116 have remained at baseline concentrations. <br /> CONCLUSIONS AND RECOMMENDATIONS <br /> The ozone microsparging system has been effective in reducing dissolved hydrocarbon concentrations in <br /> the groundwater at this site. GR recommends that operation of the treatment system be continued and <br /> further groundwater sampling performed to confirm the system's effectiveness. GR also recommends an <br /> additional system status report be prepared at the beginning of the second quarter 2004. <br /> 240042.11-3 6 <br />