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r Ozone Microsparge System First Semi-Annual Status Report—76 Branded(Former BP)Facility No 11192,Stockton,California S tember 2 2003 DISCUSSION OF RESULTS The ozone treatment appears to be effective at decreasing hydrocarbon concentrations in groundwater Over the evaluation period, MtBE concentrations decreased by approximately 74% in well MW-4, and the most recently reported detection of MtBE (13 ppb on 6/20103) is at the California Department of Health Services maximum contaminant level (MCL) in well MW-3 Although TPHg concentrations were detected in MW-3, the TPHg concentrations appear to be decreasing and appear to consist of degraded gasoline Benzene continues to be below detection limits since remediation started TBA continues to be causing a persistent impact to groundwater quality at the site Although overall concentrations of TBA have decreased in wells-MW-1, MW-3, MW-7, and MW-13, an increase in concentrations was noted in wells MW-4, MW-11 and MW-12 GR believes that the elevated concentrations of TBA in groundwater at the site should start attenuating within the next six months, as the ozone system continues to operate TBA analysis h from select wells will continue on a monthly basis, and the results will be reported in the next semi-annual status report The other monitoring wells at the site have demonstrated similar results after the first six months of groundwater treatment Wells MW-1, MW-7, MW-8, and MW-10 remain not detected for all analytes Analytical results from wells MW-6 and MW-9 indicate an MtBE reduction of over 98% and 86%, respectively, and are currently non detect for all analytes Wells MW-12 and MW-13 were sampled for the first time after being installed after the system was in operation Initially MtBE was detected at concentrations of 220 and 13,000 ppb, respectively, however, during the May 14, 2003 sampling event, the MtBE concentrations we reported as cion detect (<2 0 ppb) in both wells,representing a decrease of over 99% CONCLUSIONS AND RECOMMENDATIONS s The results of the concentration versus time charts show that hydrocarbon concentrations have declined overall during the past six months of remedial system operation The ozone sparging system is demonstrating success in reducing petroleum concentrations in the groundwater at this site This includes MtBE, which is the compound of primary concern Although initial increases in petroleum concentrations have been observed, (MW-5 and MW-11) they have been, and are expected to be short term followed by continuous concentration decreases It is believed that these increases are due to accelerated desorption caused by the aggressive mechanical scrubbing action of the microbubbles Although contaminant concentrations initially increase, overall this process will accelerate the long term remediation of the site r 140245 10 4