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and for Xylenes since March 1995 The secondary MCLs for Toluene and Ethylbenzene have not <br /> been exceeded since December 1994 and September 1995 respectively <br /> MW12, which is downgradient of MW8, has not exceeded the primary MCLS for Toluene since <br /> March 1996 and for Ethylbenzene since December 1996 <br /> 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 45 <br /> ug/L <br /> Update of groundwater contaminant plume migration controls <br /> Groundwater plume containment is being provided by vapor extraction of MW1, MW2, MW3 and <br /> MW10 Also the weekly purging and venting of MW8 and MW12 by Lawrence Tank Testing, see <br /> Appendix D MWS is purged of approximately 40 gallons of groundwater per week and MW12 is <br /> purged of approximately 70 gallons of groundwater per week This purged groundwater is stored on <br /> site in the 1000 gallon ConVault and is air stripped prior to discharge to the City of Stockton's sewer <br /> system under permit <br /> Vapor extraction has reduced the contaminant levels in the soil and groundwater Fluctuating <br /> groundwater elevations most likely influence fluctuations in groundwater contaminant concentration <br /> levels <br /> Update of Vapor Extraction System Operation <br /> The vapor extraction system continued to work through the months of July and September 1997 for a <br /> total of 13,744 5 hours (572 days as of October 3, 1997) Vapor extraction total removed <br /> hydrocarbons as gasoline using activated carbon as a control was 223 pounds, averaging 0 39 pounds <br /> per day at an average concentration of 0 208 mg/L, and an average flow rate of 67 cubic feet per <br /> rninute (cfna) On October 3, 1997, after concurrence with Sari Joaquin Vallcy Air D.str�ct, the bur <br /> 110 gallon vapor scrub units were removed from the treatment system and the offgass from the <br /> vapor recovery system was vented directly to the atmosphere, see Table 2 and Appendix C — Field <br /> notes/Chromatograms Previous vapor extraction removals were controlled by Thermal Oxidation <br /> 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 Adsorpnon 223 pounds <br /> Vent to atmosphere 0 pounds <br /> Total Removal 75,072 pounds or approximately 11,585 gallons <br /> 3 <br />