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URS Mr Harlin Knoll Page 2 San Joaquin County Environmental Health Department March 18, 2004 ATR-1 The contaminant concentrations over time for ATR-1 shows that both benzene and methyl tert-butyl ether (MtBE) have attenuated from a high of 360 micrograms per titer (}rg/L) and 23 pg/L, respectively, to levels below 1 µg/L The groundwater elevation has remained near 37 5 to 38 feet above mean sea level (MSL) (Figure 2) The trend lines shown in Figure 3 for MtBE and benzene concentrations over time show a steadily declining trend in ATR-1 ATR-2 The contaminant concentrations over time for ATR-2 shows that both benzene and MtBE have attenuated from a high of 5 7 µg/L to not detected for benzene, and from a high of 750 }tg/L to 14 }ig/L MtBE during the fourth quarter 2003 results The groundwater elevation has remained near 37 to 38 feet above MSL (Figure 4) The trend lines shown in Figure 5 for MtBE and benzene concentrations over time show a steadily declining trend in ATR-2 ATR-3 The contaminant concentrations over time for ATR-3 shows that both benzene and MtBE have attenuated from a high of 8,850 Rg/L to not detected For benzene, and from a high of 2,160 µg/]L MtBE to not detected during the fourth quarter 2003 results The groundwater elevation has remained near 37 to 38 feet above MSL (Figure 6) The trend lines shown in Figure 7 for MtBE and benzene concentrations over time show a steadily declining trend in ATR-3 ATR-4 The contaminant concentrations over time for ATR-4 shows that both benzene and MtBE have attenuated from a high of 5,800 gg/L to not detected for benzene, and from a high of 8,720 gg/L MtBE to not detected during the fourth quarter 2003 results The groundwater elevation has remained near 37 5 to 38 5 feet above MSL (Figure. 8) The trend lines shown in Figure 9 for MtBE and benzene concentrations over time show a steadily declining trend to ATR-4 ATR-5 The contaminant concentrations over time for ATR-5 shows that both benzene and MtBE have attenuated from a high of 693 pg/L to not detected for benzene, and from a high of 220 gg/L MtBE to 57 µg/L during the fourth quarter 2003 results The groundwater elevation has remained near 37 5 to 38 5 feet above MSL (Figure 10) The trend lines shown in Figure 11 show a increasing then steadily declining contaminant concentration trend for MtBE and a steadily declining contaminant concentration trend for benzene in ATR-5 • K IWprocessV5 BP ARCO\020931Trend Analysis letter doc