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Report: Groundwater-quality Monitoring—April 15, 2003: 7500 West Eleventh Street, Tracy, CA. Page II - certainly due to its presence being obscured in the chromatograms rather than by its actual absence. By April 11, 2002, it was evident that there was floating product in Well MW-7 and it was measured to have an apparent thickness of 0.3 ft. Since that time, the I apparent thickness of floating product in that well has fluctuated in the expected manner in response to the rise and fall of the groundwater table. On April 15, 2003, the apparent thickness of floating product in Monitoring Well MW-7 was 0.52 ft. 2.4.1.2 Fuel Oxygenates in Groundwater at Fringes of Principal Plume r: The samples from Wells MW-2 and MW-5, which, as shown on Figure 2, are located at the extreme western perimeter of the plume of affected groundwater, contained only ' traces of MTBE at the ver low concentrations of 3.4 L and 2.5 y µg/ µg/L, respectively. The sample from Monitoring Well MW-10, also located on the perimeter of the plume, but on its eastern edge, also contained low concentrations of the fuel oxygenates MTBE and TBA, at concentrations of 110 µg/L and 30 µg/L, respectively. Similarly, the groundwater sample recovered from Monitoring Well MW-6, which is situated across the street and down the groundwater gradient from the location at which fuel hydrocarbons were discharged to the subsurface beneath the 7500 West Eleventh Street property, contained no detectable concentrations of any analytes of concern other than TBA, at a i ; concentration of 83 µg/L, and MTBE, at a concentration of 350 µg/L. The sample of groundwater recovered from Monitoring Well MW-10, which, as noted above, is located on the eastern edge of the principal plume of affected groundwater, contained gasoline at the low concentration of 81 µg/L. That was the first occurrence of gasoline in water in that well since the sampling round conducted on July 29, 2002, when it was found at a concentration of 60 pig/L. As has been the case in the past, the sample recovered from Monitoring Well MW-10 on April 15, 2003 also contained low concentrations of the fuel oxygenates MTBE and TBA - in this instance, at concentrations of 110 µg/L and 30 µg/L, respectively, I As has been the case since groundwater monitoring began in the area at the down gradient fringe of the plume, the groundwater in Monitoring Well MW-11 contained a trace of MTBE, at a concentration of 6.3 µg/L, on April 15, 2003, but no other analyte of concern was detected. I The low concentrations of fuel oxygenates detected in Monitoring Wells 2, 5, 6 10 and 11 are similar in magnitude to the fluctuating concentrations detected in samples recovered from those wells during earlier rounds of groundwater-quality monitoring. 2.4.1.3 Analytes of Concern in Secondary Groundwater Plume As is also shown on Figure 2, a secondary plume of diesel and gasoline emanates from an area close to the southeastern corner of the truck service bay that it located at the rear of the Casa Mendoza restaurant on the 7500 West Eleventh Street property. Monitoring I: Wells MW-12 and MW-12A are located in this plume. In the sample recovered from Monitoring Well MW-12 on April 15, 2003, the only analytes of concern that were sic f