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URS • Mr Harlin Knoll Page 3 San Joaquin County Environmental Health Department March 18, 2004 ATR-6 The contaminant concentrations over time for ATR-6 shows that both benzene and MtBE have 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 MtBE during the fourth quarter 2003 results The groundwater elevation has remained near 37 to 38 feet above MSL (Figure 12) The trend lines shown in Figure 13 show a increasing then steadily declining contaminant concentration trend for MtBE and a steadily declining contaminant concentration trend for benzene in ATR-6 ATR-7 The contaminant concentrations over time for ATR-7 shows that both benzene and MtBE have attenuated from a high of 10 gg/L to not detected for benzene, and from a high of 580 gg/L MtBE to not detected during the fourth quarter 2003 results The groundwater elevation has remained near 37 to 38 feet above MSL (Figure 14) The trend lines shown in Figure 15 show a increasing then steadily declining contaminant concentration trend for MtBE and a steadily declining contaminant concentration trend for benzene in ATR-7 ATR-8 The contaminant concentrations over time for ATR-8 shows that both benzene and MtBE have attenuated from a high of 620 pg/]L to not detected for benzene, and from a high of 21 gg/L to 3 1 gg/L MtBE during the fourth quarter 2003 results The groundwater elevation has remained near 36 to 38 feet above MSL (Figure 16) The trend lines shown in Figure 17 show a increasing then steadily declining contaminant concentration trend for MtBE and a steadily declining contaminant concentration trend for benzene in ATR-8 Conclusion As the trend analyses above demonstrates, contaminant concentrations at the ARCO Station 2093 wells have either consistently declined over time or have increased then subsequently decreased over time Both remediation efforts and natural attenuation have resulted in these reductions of contaminant concentrations to levels that no longer pose a threat to human health and the environment For those wells that still have detectable concentrations of contaminants, the trend line demonstrates that these levels will also decrease to levels that are no longer detectable Therefore, URS would like to propose that this site be considered a candidate for closure and a closure • K 1Wprocessll5 BP ARCQ1020931Trend Analysis Letter doc