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• MW1, which contained free product until August 1992 has not exceeded the primary and secondary <br /> MCLS since December 1993. <br /> MW2 is situated in a clayey facies of the silty sand aquifer beneath this site and has low <br /> transmissivity has not exceeded the primary MCLs for Ethylbenzene since December 1996. <br /> MW3 which contained free product until November 1992 has not exceeded the primary MCLS for <br /> Toluene, Ethylbenzene and Xylenes since March 1993 and the secondary MCLs for Toluene since <br /> September 1993 and for Ethylbenzene since September 1995. <br /> MW8, which is downgradient and offsite, has not exceeded the primary MCLs for Toluene during <br /> this latest sampling. It has never exceeded the primary MCLS for Ethylbenzene and has been below <br /> the primary MCLs for Xylenes since June 1994. <br /> MW 10, which contained free product until November 1992, has not exceeded the primary MCLS for <br /> Benzene since December 1996, for Toluene since March 1994, for Ethylbenzene since March 1993 <br /> 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, MWl 1 and MW12 and maybe <br /> attributed to offsite sources. Confirmations of the MTBE using EPA Method 8260 indicate that <br /> concentrations are below 45 ug/L. MTBE was the only Oxygenant found in this latest sampling and <br /> only in MW 11, at 30 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, 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, see Appendix D. MW8 and MW12 are <br /> purged of approximately 40 and 70 gallons per week respectively. This purged groundwater is <br /> stored on site in the 1000 gallon ConVault and is air stripped prior to discharge to the City of <br /> Stockton's sewer system under permit, see Table 3 and Appendix E. <br /> Vapor extraction has reduced the contaminant Ievels 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 /> 4 R603 Update#50 12197 <br />