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Report: Groundwater-quality Monitoring-December 20-21,2005, 7500 West Eleventh Street, Tracy, CSI. Page 13 Tertiary-amyl methyl ether EPA Method 8260B STL is certified to perform the specified analyses by the California Department of Health Services (DHS). The results of the analyses of samples of groundwater are presented in Table 2, which also includes the results of analyses of groundwater samples recovered from all previous sampling rounds. Copies of the laboratory Certificates of Analysis generated from the December 2005 monitoring round are included in Appendix B of this report. L 2.5 Groundwater Quality in the Near-Surface Unconfined Aquifer The results of analyses of samples of groundwater recovered in December 2005 are evaluated below. 2.5.1 Analytes of Concern in Groundwater Along Principal Plume Axis �. Monitoring Wells MW-3, MW-13, MW-7, MWFP-4, MW-16 and MW-18, are, as is shown on Figure 3, arrayed along the approximate long axis of the primary plume of affected groundwater. As can be seen by inspection of Table 2, compared to the concentrations of analytes of concern that were found in the groundwater samples recovered from those wells in October 2005, the concentrations in the samples recovered in December 2005 were, in most cases, lower than those measured in samples recovered from the same wells. For example, the concentration of TPHd in the groundwater in Monitoring Well MW-3 fell from 280 p.g/L to 120 �ig/L and the concentration of MTBE fell from 1,600 µg/L to 1,200 [g/L. The concentration of TPHg in Monitoring Well MW- 13 fell from 3,300 µg/L to 1,500 µg/L. Although TPHg at a concentration of 420 µg/L appeared in Monitoring Well MWFP-4 when that analyte had been undetectable in that well in October 2005, as was the case most of the analytes of concern in samples recovered from the other wells along the long axis of the primary plume, the concentration of MTBE in the groundwater in that well fell. 2.5.2 Analytes of Concern in Groundwater around Fringes of Principal Plume Examination of the results of the analyses (see Table 2) of the groundwater samples }.. recovered from Monitoring Wells MW-2 and MW-5 show that on December 20, 2005, the western fringe of the principal plume of affected groundwater remained close to the locations of those wells. However, the sample recovered from Monitoring Well MW-17 on December 20, 2005 contained no detectable concentrations of any of the analytes of concern, which indicates that the local westerly expansion of the fringe of the plume of affected groundwater that was first observed on March 4, 2005 has receded from that area by December 2005. The concentrations of analytes of concern detected in the samples recovered from 6- Monitoring Wells MW-4, MWFP-2, MWFP-5 and MW-10 in December 2005 were not significantly changed compared to the results obtained during the October 2005 sampling sic