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1 <br /> URS <br /> Mr. Harlin Knoll Page 3 <br /> San Joaquin County Environmental Health Department <br /> March 18, 2004. <br /> ATR-6 <br /> The contaminant concentrations over time for ATR-6 shows that both benzene and MtBE have <br /> attenuated from a high of 75 gg/L to not detected for benzene, and from a high of 56 gg/L to 11 gg/L <br /> MtBE during`the fourth quarter 2003 results. The groundwater elevation has retrained.near 37 to 38 <br /> feet above MSL (Figure 12). The trend lines shown in Figure 13 show a increasing then steadily <br /> declining contaminant concentration trend for MtBE and a steadily declining contaminant concentration <br /> trend for benzene in ATR-6. <br /> ATR-7 <br /> The contaminant concentrations over time for ATR-7 shows that both benzene and MtBE have <br /> attenuated from a high of 10 gg/L to not detected for benzene, and from a high of 580 Vg/L MtBE to not <br /> detected during the fourth quarter 2003 results. The groundwater elevation has remained near 37 to 38 <br /> feet above MSL (Figure 14). The trend lines shown in Figure 15 show a increasing then steadily <br /> declining contaminant concentration trend for MtBE and a steadily declining contaminant concentration <br /> trend for benzene in ATR-7. <br /> ATR-8' <br /> The contaminant concentrations over .time for-ATR-8 shows that both benzene and MtBE have <br /> attenuated from a high of 620 gg/L to not detected for benzene, and from a high of 21 µg/L to 3.1 gg/L <br /> MtBE during the fourth quarter 2003 results. The groundwater elevation has remained near 36 to 38 _ <br /> feet above MSL (Figure 16). The trend lines shown in Figure 17 show a increasing then steadily t <br /> declining contaminant concentration trend for MtBE and a steadily declining contaminant concentration <br /> trend for benzene in ATR-8. <br /> Conclusion <br /> As the trend analyses above demonstrates, contaminant concentrations at the ARCO Station 2093 wells <br /> have either consistently declined over time or have increased then subsequently decreased over time. <br /> Both remediation efforts and natural attenuation have resulted in these reductions of contaminant <br /> concentrations to levels that no longer pose a threat to human health and the environment. For those <br /> wells that stilli have detectable concentrations of contaminants, the trend line demonstrates that these <br /> levels will also decrease to levels that are no longer detectable. Therefore, URS would like to.propose <br /> that this site be considered a candidate for closure and a closure. <br /> I <br /> K:IWprocessU5 BP ARCO\020931Trend Analysis Utter.doc <br />