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GROUNDWATER MONITORING Depths to groundwater measurements were obtained from all monitor wells associated with this site (MWI, MW2, MW3, MW4, MWS, MW6, MW7, MW8, MW9, MW10, MWI 1, MW12 and MW13) on November 18, 1998 No vacuum influence was measured, the vapor extraction system and the sparging systems have been non-operative for 6 days to allow for representative groundwater samples, see Appendix A — Field Notes and Methods and Procedures Figure 4 represents the groundwater gradient obtained from measurements obtained on September 8, 1998 and shows a groundwater high ridge at MW10 extending towards MWI and MW3 with localized gradient flowing to the east Currently the tops of the well screens are below the top of groundwater (since May 1998) The contact between the bottom of silty clay and the top of the aquifer sand is within the screen interval assuring that the groundwater samples are representative of the aquifer conditions, see Figure 8 The most recent groundwater quarterly sampled all wells (MWI 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 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 November 18,1998 re Figures 6 resents Benzene measurements obtained November 18, 1998 g p Figure 7 represents MTBE measurements obtained November 18, 1998 MTBE at MWI and MWI2 was confirmed using EPA Methods 8260 Table I represents groundwater chemical results showing TPHg, MBTEX and DO Charts have been developed for MWI, MW2, MW3, MW8, MW10 and MW12 that show the continued decline in concentrations of TPHg in the groundwater These charts are located with Table 1 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 Currently only MWI, MW2, MW8 and MW12 exceed the primary MCLS for Benzene in drinking water standards for the State of Calfironia at 6 ug/L, 3ug/L, 2 ug/L and 9 ug/L respectively A RBCA Tier 11 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 MWI, MW8, MW11 and MW12 t Confirmations of the MTBE using EPA Method 8260 indicate that concentrations are below 35 ug/L MTBE was the only Oxygenant found in the December 1997 sampling and only in MWI 1, at i 30 ug/L EXXON removed the product from the underground storage tanks (UST's) removed the 5 R603 Update##54/CLOSURE 3 12198