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1 Ozone Microsparge System Annual Status Report—Former 76 Station No 6981,Stockton,California September 2 2003 ` MW-16 as 31 ppb This is a decrease of 88% since the system began operation The MtBE concentration in VW-7 was reported as 33 ppb on December 12, 2001,,and <20 ppb on July 22, , 2003 MtBE has not been detected since July 2002 (detection limits have varied from 2 5 ppb to V 100 ppb) in W-7 suggesting that MtBE is no longer present irf VW-7, which represents a greater, than 84% decrease in MtBE Historically, MtBE concentrations from the initial groundwater sampling events (8/21/99) to the most recent sampling event (7/22/03) have decreased by 55% in well MW46 and by over 96% in well VW-7 Additionally, maximum MtBE concentrations have ` decreased from 250 ppb`to 31 ppb m well MW-16, and from 510 ppb to <20 ppb in well VW-7 > i DISCUSSION OF RESULTS r , The ozone treatment appears to be effective at decreasing TPHg and benzene concentrations in groundwater 3 Over the evaluation period, concentrations decreased by,85% (TPHg) and 79% (benzene) m4VW-7 In addition, MtBE no longer appears to be present in VW-7 as it has not been detected since July 2002 TPHg, benzene, and MtBE have all decreased in MW-16 since the system began, however low concentratibns of MtBE have fluctuated-during the reporting period and did not decfease over the previous 7 months The,other onsite wells have demonstrated similar downward trends during the last seven months of µ . treatment Wells MW-1, MW-4 through MW-9, VW-1, VW-3, and PTW-1 have remained below detection limits for TPHg, benzene, and MtBE Well MW-2 has shown a decrease of 90%, 98%, and 61% for TPHg, benzene, and MtBE, respectively, and well MW-11 has shown a greater than 88%decrease for MtBE during the reporting period CONCLUSIONS AND RECOMMENDATIONS The results of the concentration versus time charts show that hydrocarbon concentrations have u generally declined or remained stable with some small increases during the past seven months of remedial system operation The ozone sparge system has been successful in reducing petroleum concentrations in the groundwater at this site This includes TPHg and benzene, which are the compounds of primaryconcern, as well as MtBE y r " The current ozone injection addresses the dissolved impact onsite and also acts -as a barrier to y mitigate any continuing migration of dissolved hydrocarbons offsite Treatment directly offsite is not,feasible due to the proximity of Pacific Avenue, however the offsite well MW-15 will continue to be monitored to determine plume stability _ t The ozone microsparge system will continue to operate for at Ieast the next six months; and monitoring wells MW-16 and VW-7 will continue to be sampled on a monthly basis A ` semi-annual status report will be prepared and submitted after the next six months of operation • 140124 08 _ 5 k s