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Compound Primary MCL Secondary MCL <br /> Benzene 1 0 ug/L none <br /> Toluene 150 ug/L 42 ug/L <br /> Ethylbenzene 700 ug/L 29 ug/L <br /> Xylenes 1750 ug/L 17 ug/L <br /> GROUNDWATER MONITORING <br /> Depths to groundwater measurements were obtained from all monitor wells associated with this site <br /> (MW1, MW2, MW3, MW4, MWS, MW6, MW7, MW8, MW9, MW10, MW11, MW12 and MW13) <br /> on September 8, 1998 No vacuum influence was measured, the vapor extraction system and the <br /> spatging systems have been non-operative for 3 days to allow for representative groundwater <br /> samples, see Appendix A — Field Notes and Methods and Procedures Figure 4 represents the <br /> groundwater gradient obtained from measurements obtained on September 8, 1998 and shows a <br /> groundwater high ridge at MW 10 extending towards MW 1 and MW3 with localized gradient <br /> flowing to the east <br /> The most recent groundwater quarterly sampled all wells (MW1 through MW13) These samples <br /> k\ere analyzed for Total Petroleum Hydrocarbons as gasoline (TPHg), Benzene, Toluene, <br /> Ethylbenzene, Xylenes and Methyl tertiary Butyl Ether (MBTEX) using EPA method 8020, see <br /> Table 1, and Appendix B <br /> The groundwater plume of dissolved gasoline range hydrocarbons has been reduced in size and <br /> concentrations, the following figures display the plume reduction/stability <br /> Fzguie 5A represents TPHg measurements obtained in December 1991 <br /> Figuze 5B represents TPHg measurements obtained September 8,1998 <br /> Figure 6A iepresents Benzene measurements obtained in December 1991 <br /> Figures 6B represents Benzene measurements obtained September 8, 1998 <br /> Table 1 represents groundwater chemical results showing TPHg, MBTEX and DO <br /> Charts have been developed for MW1, MW2, MW3, MW8, MWIO and MWI2 that show the <br /> continued decline in concentrations of TPHg to the groundwater These charts are located with <br /> Table I A computer generated averaging trend line demonstrates the decline of gasoline range <br /> nydrocarbons in the groundwater for all weiis except for Ivl w 12, whicn is currently snowing a stable <br /> trend <br /> Currently only MW2, MW8 and MW12 exceed the primary MCLs for Benzene in drinking water <br /> standards for the State of Calfironia at 2ug/L, 5 ug/L and 5 ug/L respectively A RBCA Tier II <br /> model indicated that benzene in groundwater should be reduced to below 9 9 ug/L to satisfy onsite <br /> commercial use This has been achieved <br /> 5 8603 Update#53/CLOSURE 2 10/98 <br />