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t . Ozone Microsparge System First Semi-Annual Status Report—75 Branded(Former BP)Facility No 11192, Stockton,California Scptember 2 2003 OBSERVATIONS s TPHg Concentrations Figures 3 and 4 illustrate the TPHg concentrations from quarterly and monthly sampling events of November 2002 through July 2003 The TPHg concentration reported in MW-3 and MW-4 on November 15, 2002, prior to the startup of the system were<12,000 parts per billion (ppb) and 150 ppb, respectively On July 22, 2003, the TPHg concentration in MW-3 was reported at<5,000 ppb Due to the elevated detection limits, it is not possible to determine the percent reduction of TPHg in. MW-3 Since November 15, 2002, the laboratory has not reported TPHg concentrations in MW-4, except for the most recent event of July 22, 2003, where the concentrations was reported at 55 ppb The laboratory reported that the November 15, 2002 concentration of 150 ppb had a discrete peak at MtBE, and based on MtBE concentrations in MW-4, it is likely that the reported TPHg concentration was a result of MtBE interference if the 11/15/02 result does truly represent gasoline, then the TPHg concentration in MW-4 has decreased by approximately 57% since the system began operation Benzene Concentrations The benzene concentration reported in MW-3 and MWE-4 on November 15, 2002 was <120 and <0 50 ppb, respectively Benzene has remained at non-detectable concentrations for the entire period covered by this report MtBE Concentrations Figures 3 and 4 illustrate the MtBE concentrations from monthly and quarterly sampling events of November 2002 through July 2003 The MtBE concentrations in MW-3 and MW-4 on November 15, 2002 were reported at <500 and 150 ppb, respectively The last sampling results, on July 22, 2003, reported the MtBE concentration in MW-3 as <200 ppb, however, a percent decrease cannot be determined based on elevated laboratory detection limits In well MW-4, MtBE has declined from 150 ppb to 39 ppb, a reduction of 74% TSA Concentrations Figures 3 and 4 illustrate the TBA concentrations from monthly and quarterly sampling events of November 2002 through July 2003 TBA has historically been detected in well MW-3 TBA is a known daughter product of MtBE degradation Although concentrations of TBA have fluctuated in well MW-3, an overall decline in concentrations has been observed since November 15, 2002 TBA has decreased by 39% since November 15, 2002 140245 10 3