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The contact between the bottom of silty clay and the top of the aqwfer sand is within the screen interval assuring that the groundwater samples are representative of the aquifer conditions The most recent groundwater quarterly sampled all wells (MW 1 through MW 13) These samples were analyzed for Total Petroleum Hydrocarbons as gasoline (TPHg), Benzene, Toluene, Ethylbenzene, Xylenes and Methyl tertiary Butyl Ether (MBTEX) using EPA method 8020, see Table 1, and Appendix B Also field measurements were obtained for Total Disolved Oxygen, Sulfate, Nitrate, Ferrous Iron and Oxygenation Reduction Potential (ORP) The groundwater plume of dissolved gasoline range hydrocarbons has been reduced in size and concentrations The following figures display the plume Figure 5 represents TPHg measurements obtained January 11, 1999 Figures 6 represents Benzene measurements obtained January 11, 1999 Table I represents groundwater chemical results showing TPHg, MBTEX and DO Charts have been developed for MWl, MW2, MW3, MWS, MW10 and MW12 that show the continued decline in concentrations of TPHg in the groundwater These charts are located with Table l A computer generated averaging trend line demonstrates the decline of gasoline range hydrocarbons in the groundwater for all wells except for MW12, which is currently showing a stable trend Currentlyonly MW8 and MW 12 exceed th y e primary MCLS for Benzene in drinking water standards for the State of Calfironia at 7 ug/L A RBCA Tier II model indicated that benzene in groundwater should be reduced to below 9 9 ug/L to satisfy on-site commercial use This has been achieved MTBE has only been found and confirmed in the wells MW 1, MW8, MW 11 and MW12 Confirmations of the MTBE using EPA 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 Groundwater plume containment was being provided by vapor extraction of MW 1, MW2, MW3 and MW 10, by oxygenation of the groundwater by air sparging MW2 and MW13 The vapor extraction system and sparging system was turned off December 19, 1998 Weekly purging and venting of MWS and MW12 by Lawrence Tank Testing was terminated on December 14, 1998 MW8 and MW12 were 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 stepped prior to discharge to the City of Stockton's sewer system under permit, see Sewer Discharge Table 4 2 R603 Update#55 01/99