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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 liter(µg/L) and 23 µg/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 pg/l-to 1.4 ltg/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 1tg/L to not detected for benzene, and from a high of 2,160 ttg/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 ATR4 shows that both benzene and MtBE have attenuated from a high of 5,800 µg/L to not detected for benzene, and from a high of 8,720 µg/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 in 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 ltg/L to not detected for benzene, and from a high of 220 µg/1-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:1WprocessU5 BP ARCO\020931Trend Analysis Letter.doc