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MW2 is situated in a clayey facies of the silty sand aquifer beneath this site and has low transmissivity has not exceeded the primary MCLS for Ethylbenzene since December 1996. MW3 which contained free product until November 1992 has not exceeded the primary MCLS for Toluene, Ethylbenzene and Xylenes since March 1993 and the secondary MCLs for Toluene since September 1993 and for Ethylbenzene since September 1995. MW8, which is downgradient and offsite, did not exceed the primary MCLs for Toluene during this latest sampling. It has never exceeded the primary MCLS for Ethylbenzene and has been below the primary MCLs for Xylenes since June 1994 with the exception of the latest testing. MW 10, which contained free product until November 1992, has not exceeded the primary MCLS for Benzene since December 1996, for Toluene since March 1994, for Ethylbenzene since March 1993 and for Xylenes since March 1995. The secondary MCLs for Toluene and Ethylbenzene have not been exceeded since December 1994 and September 1995 respectively. MW12, which is downgradient of 1\4W8, has not exceeded the primary MCLs for Toluene since March 1996 , Ethylbenzene and Xylenes since December 1996 . MTBE has only been found and confirmed in the offsite wells MW8, MW11 and MW12. Confirmations of the MTBE using EFA Method 8260 indicate that concentrations are below 45 ug/L. MTBE was the only Oxygenant found in the December 1997 sampling and only in MW 11, at 30 ug/L. Update of groundwater contaminant plume migration controls i Groundwater plume containment is being provided by vapor extraction of MW1, MW2, MW3 and MW10, by oxygenation of the groundwater by sparging MW2 and MW13 and by weekly purging and venting of MW8 and MW12 by Lawrence Tank Testing. MW8 and MW12 are purged of approximately 40 and 70 gallons per week respectively. This purged groundwater is stored on site in the 1000 gallon ConVault and is air stripped prior to discharge to the City of Stockton's sewer system under permit, see Table 3. Vapor extraction has reduced the contaminant levels in the soil and groundwater. Fluctuating groundwater elevations most likely influence fluctuations in groundwater contaminant concentration levels. Update of Vapor Extraction System Operation The vapor extraction system continued to work through the months March, April, May and June, 1998. Shut downs occurred for quarterly monitoring (March 10 - 11, 1998) and for blower repair (April 13 — May 28, 1998). The system has operated a total of 18952.7 hours (790 days as of July 2, 1998). Vapor extraction total removed hydrocarbons, as gasoline using activated carbon as a control was 223 pounds and direct venting to atmosphere of 11 pounds. For this quarter (April - June, 1998) 6 8603 Update#52/CLOSURE RV 7/98