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San Joaquin County, Local Oversight Program December 10, 2001 Page 3 Benzene was found at concentrations exceeding the maximum contaminant level(drinking water standard) in two of the monitoring wells - at a concentrations ranging from 1 6 µg/L in MW-8 and 3 5 pg/L in MW-4 For the sample from MW-5 the detection limit was raised above the MCL and <5 0 µg/L was reported See Table 3 and Figure 3 The other aromatic hydrocarbon compounds, toluene, ethylbenzene, and total xylene isomers, were only detected in ME-58 groundwater, as indicated in Table 3 MtBE was found in six of the monitoring wells as indicated in Table 3 and Figure 3. Three of the morutormg wells (MW-4, MW-5, and MW-8) were with concentrations above the maximum contaminant level(primary drinking water standard) of 13 pg/L Three of the monitoring wells (MW-1, MW-6, and MW-7 were reported with below the secondary drinking water standard of 5 0 µg/L_ The other U S EPA Method 8260B Fuel Oxygenates were not detected above their respective detection limits in groundwater samples as indicated in Table 3 Ethylene dibromnde and 1,2- Dnchloroethane were below their respective detection limits 4 0 MONITORING WELLS MW-5, MW-8, AND MW-4 Monitoring wells MW-5 and MW-8 were installed in September 1995 Monitoring of the MtBE concentration in the monitoring wells began in February 1999 For the first four momtornng events(2/23/99, 6/27199, 8/31/99, and 213/00) MtBE was not detected (<1 0 gg/L) in water from MW-5 and was consistently low in MW-8 (6 1, 2 0, 8 0, and <1.0 µg/L). There was a sampling hiatus during the summer of 2000 When sampling resumed in Fall 200 MW-5 was reported with 140 gg/L on 11/19/00 followed by 81 µg/L on 2/23101, 250 µg/L on 5/19101, 200 µg/L on 8/18/01, and 210 µg/L on 11/17/01. For these same sampling events adjacent MW-8 was reported with 19, 40, 58, 54, and 78 gg/L MW-5 and MW-8 are cross to downgradnent of the fuel pump island Conversely, MW-4, contiguous with the tank excavation on the downgradnent side, was first reported with 530 µg/L on 2/23/99 followed by 160 µg/L on 6/27/99, 240 pg/L on 8/31/99, and <1 0 µg/L on 2/3/00 The more recent events have shown 91, 53, 32, 32, and 17 pg/L The following schedule will be observed for groundwater monitoring during 2001 First Quarter 2002 Sampled Mid 2 weeks of February Report by April 12 Second Quarter 2002 Sampled Mid 2 weeks of May Report by July 11 H20GEOL AGRMNDWATERCONSULTANCY