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MTBE has only been found and confirmed in the offsite wells MW8, MW11 and MW12 <br /> Confirmations of the MTBE using EPA Method 8260 indicate that concentrations are below 35 <br /> ug/L MTBE was the only Oxygenant found in the December 1997 sampling and only in MWi 1, at <br /> 30 ug/L <br /> Update of groundwater contaminant plume migration controls <br /> Groundwater plume containment is being provided by vapor extraction of MW1, MW2, MW-3) and <br /> MW10, by oxygenation of the groundwater by sparging MW2 and MW13 and by weekly purging <br /> and venting of MW8 and MW12 by Lawrence Tank Testing MW8 and MW12 are purged of <br /> approximately 40 and 70 gallons per week respectively This purged groundwater is stored on site in <br /> the 1000 gallon ConVault and is air stripped prior to discharge to the City of Stockton's sewer <br /> system under permit, see Sewer Discharge Reports — Appendix C <br /> Vapor extiaction has reduced the contaminant levels in the soil and groundwater and air sparging of <br /> MW2 and MW13 have enhanced biodegradation and reduced concentrations in wells MW2, MW8 <br /> and MW12 <br /> Update of Vapor Extraction System Operation <br /> The vapor extraction system continued to work through the months July, August and September <br /> 1998 Shutdowns occuired for quarterly monitoring (September 5 - 12, 1998) The system has <br /> operated a total of 20805 hours (867 days as of September 28, 1998) Vapor extraction total removed <br /> hydrocarbons, as gasoline using activated carbon as a control was 223 pounds and direct venting to <br /> atmosphere of 18 pounds For this quarter (July - September 1998) the VES averaged 0 1 pounds <br /> pet day removal at an average concentration of 0 013 mg/L, and an average flow rate of 60 cubic <br /> feet per minute (efm), see Table 2 On October 3, 1997, after concurrence with San Joaquin Valley <br /> Air District, the four 110 gallon vapor scrub units were removed from the treatment system and the <br /> offgass fiom the vapor recovery system was vented directly to the atmosphere, see Appendix D — <br /> Field notes and Chromatograms Previous vapor extraction removals were controlled by Thermal <br /> Oxidation and removed 74,849 pounds of gasoline range hydrocarbons <br /> Totals of gasoline range hydrocarbons removed by the vapor extraction system are as follows <br /> Thermal Oxidation destruction 74,849 pounds <br /> Carbon Adsorption 223 pounds <br /> Vent to atmosphere 18 pounds <br /> Total Removal 75,090 pounds or approximately 12,014 gallons <br /> 6 R603 Update#53/CLOSURE 2 10198 <br />