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---------------------- Report:Groundwater-quality Monitoring—July 27,2004: 7500 West Eleventh Street,Tracy, CA. Page 12 area of that well that are heavily affected by components of fuel hydrocarbons. If the observed trend continues, it may also be precursor to the return of floating LNAPL in that well, as has been the case in the past. (See Table 2 for the history of contaminant concentrations and floating product thicknesses in that well.) 2.5.2 Analytes of Concern in Groundwater around Fringes of Principal Plume The results of the groundwater-quality monitoring round conducted in July 2004 indicate that the plume perimeter on both its eastern and western sides has remained stable since the monitoring round conducted in April 2004. Trace concentrations of MTBE, at 2.1 and 19 µg/L in the samples recovered on July 27, 2004 from Monitoring Wells MW-2 and MW-5, respectively, when the resolution of the laboratory analytic procedure is considered, were essentially the same. Those wells are located at the western perimeter of the primary plume of affected groundwater. As was the case on April 30, 2004, no detectable concentrations of analytes of concern were present in groundwater recovered from Monitoring Wells MW-15 and MW-17,which are located along the west side of the primary plume. As is also the case for the sample recovered on April 21, 2004, no concentrations of any analytes of concern were detected in the groundwater recovered from Monitoring Well MW-9 on the eastern perimeter of the primary plume, and the concentrations of TPHg and MTBE in the sample recovered from Monitoring Well MW-10 remain at essentially trace levels. Similarly, in Monitoring Well MW-11, which is at the northern down- gradient extremity of the primary plume, the single analyte detected in the sample recovered from that well on July 27, 2004 was MTBE, at a concentration of 16 µg/L, which, while lower, is not significantly different from the concentration of 34 µg/L found in the sample recovered from that well on April 21, 2004. In the absence of any other detectable analytes of concern being present, traces of toluene ay 0.63 µglL and xylene isomers at 1.5 µg/L were detected in the sample of groundwater recovered from Monitoring Well MW-19 recovered on July 26, 2004. This was _ unexpected because that well is located a significant distance to the north of the primary plume of affected groundwater as its areal extent has been interpreted previously (see Figure 3 for current plume geometry) and the groundwater in that well was completely free of any concentrations of analytes of concern when it was last sampled on April 30, - 2004. f' At present, unless the presence or absence of analytes of concern in the groundwater in Monitoring Well MW-19 are confirmed by future rounds of groundwater monitoring, the detections reported by the laboratory for the July 2004 sample are judged to be spurious. This is because, in addition to the historical geochemical data from that area of the site, -i the laboratory reported that when initially analyzed, the sample recovered from that well appeared to contain a low concentration of TPHd; however, when the chromatogram was carefully examined, the laboratory's chemist suspected that the compound apparently present was not a fuel hydrocarbon in the diesel range and appeared to have been an artifact accidentally introduced at the laboratory. When the aliquot was re-analyzed for sic