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Report;Groundwater-quality Monitoring—October 27, 2003: 7500 West Eleventh Street, Tracy, cA. Page 11 ..� reduction in the concentrations of analytes of concern between July 30 and October 27, 2003 is probably related to the approximate 1-ft. reduction in the elevation of the groundwater table and consequent removal of shallow groundwater from contact with the most heavily-affected zone of subsurface soil. When groundwater in Well MW-7 was first analyzed in May 2000, it was affected by ZAP relatively moderate concentrations of diesel- and gasoline-range petroleum hydrocarbons (at concentrations of 1,300 µg/L and 1,400 pg/L, respectively), and MTBE (at 22 ug/L). However, over the next two years, the concentrations of both diesel and gasoline in samples recovered from that well exhibited a significant upward trend. As the concentration of those analytes increased, MTBE was no longer detected. That is almost certainly due to its presence being obscured in the chromatograms rather than by its w 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 0.30 ft. Since that time, the 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 October 27, 2003, the apparent thickness of floating product in Monitoring Well MW-7 was 0.55 ft. This increase in the thickness of floating product with a falling groundwater table is typical of that seen at similarly affected sites and is caused by the well serving as a sump into which product drains from heavily-affected soils at higher elevations in the section. Concentrations of gasoline were first detected on July 30, 2003 in Monitoring Well MW- 11, but were not present in the sample recovered from that well on October 27. However, the sample contained diesel at a concentration of 85 pg/L, which is slightly higher than the concentration of 69 pg/L of diesel-range hydrocarbons that was first detected in July. Commensurate with the disappearance of detectable concentrations of gasoline in the October 27 sample, MTBE fell to 6.7 pg/L compared to the 13 pg/L present in the sample recovered from that well on July 30. The data obtained from the analysis of the sample of groundwater recovered from Monitoring Well MW-11 is consistent with the conclusions drawn from the July 2003 sampling round: the principal plume of affected groundwater has reached that well and the fringe of MTBE that is advancing ahead of the plume is now down-gradient of that location (The San Joaquin Company Inc. 2003a). 2.4.1.2 Fuel Hydrocarbons and Oxygenates Around Fringes of Principal Plume The sample recovered from Monitoring Well MW-5 on October 27, 2003 contained no �. detectable concentrations of any analytes of concern other than MTBE at a concentration of 2.9 pg/L, which is a slight reduction from the concentration of 4.2 pg/L in the sample recovered from that well on July 30. It is notable that the sample from Monitoring Well MW-5 in October contained no concentration of diesel. Diesel was apparently present at a concentration of 68 pg/L in the sample recovered from that well on July 30. However, that sample was not treated with silica gel according to EPA Method 3630 prior to analysis. It was, therefore, taken to be a false positive for a component of fuel hydrocarbons (The San Joaquin Company Inc. 2003a); the negative result for diesel obtained from the October sample, which was pretreated by EPA Method 3630, confirms `' sic