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Fourth Quarter 2002 Monitoring,Sampling, and Remediation Progress Report 6 <br /> 7-Eleven Inc Store#19976, 1399 North Main Street Manteca,CA December 24 2002 <br /> ' Remediation Progress <br /> On August 29, 2002, SVE was conducted using the vent segment of dual-nested well SP-1 for a period of <br /> four hours Two hours after the Initiation of SVE, AS was conducted for the remaining two hours of SVE <br /> using the sparge segment of well SP-1 Prior to Initiating the AS, extracted vapor levels ranged up to 800 <br /> ppm[v]at the beginning of SVE and stabilized at 500 ppm[v] one hour Into the event Thirty minutes Into <br /> AS, extracted vapor levels Increased to a maximum concentration of 610 ppm[v] and declined to a low of <br /> 400 ppm[v] by the end of the ASISVE event In addition, DO concentrations showed an increase in <br /> nearby well MW-2 (9 feet away) In response to AS Prior to SVE, DO levels In well MW-2 were observed <br /> at 0 37 mg/L and Increased to a maximum concentration of 7 83 mg/L one hour after Initiation of AS <br /> Other remediation data collected during the quarter are Included in Attachment 3 <br /> i <br />� Periodic mobile treatment events conducted throughout 2002 appear to be successful at reducing <br /> dissolved MTBE levels in source area well MW-2 Other wells previously demonstrating concentrations of <br /> ' more than 100 pg1L MTBE (MW-1, MW-5) have also declined since the second quarter 2002 and/or earlier <br /> sampling events or are remaining stable In concentration As of the fourth quarter 2002, dissolved MTBE <br /> levels at the site ranged between non-detectable and 1,600 pg/L, with only well MW-2 containing more <br /> than 1,000 pg/L In addition, the implementation of AS shows a slight Increase in extracted vapor <br /> concentrations and a significant Increase in DO concentrations In nearby wells, demonstrating good <br /> potential for ongoing stimulation of intrinsic bioremediation by increasing DO availability <br /> Conclusions and Recommendations <br /> ' SVE has been effective In reducing MTBE/hydrocarbon concentrations in the source area vadose zone, <br /> which appears to be successfully reducing dissolved plume concentrations Air and/or oxygen sparging <br /> should possibly be continued through 2003 to further reduce saturated-zone concentrations <br /> 1 <br /> 1 <br /> ' 19976/324 Doc <br />