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Second Quarter 2002 Monitoring Sampling,and Interim Remediation Report 6 <br /> 7-Eleven, Inc,Store#19976, 1399 North Main Street Manteca,CA July 30 2002 <br /> 1 <br /> Interim Remediation <br /> On May 23, 2002, air sparging was conducted, using the horizontal vent wells for vapor extraction and <br /> ' well MW-1A(and sparge well SP-1S/SP-1D) for air injection Pre-sparging extracted vapor levels ranged <br /> from 300 to 600 ppm(v) Vapor levels measured concurrent with sparging indicated that levels in SP-11S <br /> rose from 300 to 750 ppm(v) and then declined throughout the rest of the field event Soil oxygen levels <br /> ' increased over time from 8 3 to 18 2 percent during the 7-hour injection event Field data sheets are <br /> included in Attachment 3 <br /> ' On June 5, 2002, SVE was conducted for a period of 2 and 5 hours, respectively, using the two horizontal <br /> vent wells and various sparge/SVE combinations using different wells Extracted vapor levels ranged up <br /> ' to 800 ppm(v) on June 5, and 875 ppm(v) on June 27 (Attachment 3) <br /> Conclusions and Recommendations <br /> ' Dissolved hydrocarbon/oxygenate levels in wells MW-1 and MW-2 may reflect a continuing ulsato <br /> Y 9 p rY <br /> ' release from the source area of the tank pit Due to equipment unavailability, remediation activities were <br /> not implemented until late May (approximately two months subsequent to second quarter 2002 <br /> monitoring) Current data also suggest that permeability conditions may reduce the efficiency of MTBE <br /> mass removal of sparging in the vertical monitoring wells while conducting SVE using the horizontal vent <br /> wells Air sparging is recommended to continue using well MWA by simultaneous injection of air into the <br /> deeper sections of the well and SVE from the shallower section of the well Air injection into SPA is <br /> ' recommended to occur using the shallow section of the well for injection while extracting vapors from well <br /> MW-2 SVE from the horizontal wells without concurrent air-sparging in any nearby well points is <br /> recommended as a means of reducing vapor-phase MTBE levels in the former tank backfill, which is likely <br /> ' in vapor-flow communication with the existing tank pit backfill Remediation is scheduled to occur every <br /> two weeks pending equipment availability (and installation of a dedicated electric power source at the <br /> ' site) The effect of remediation on dissolved MTBE/hydrocarbon levels will be evaluated in subsequent <br /> reports <br /> 1 <br /> 1 <br /> 19976022 DOC <br />