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b— Report:Groundwater-quality Monitoring—March 24-31,2006, 7500 West Eleventh Street, Tracy, CA Page 12 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 L- March 2006 monitoring round are included in Appendix C of this report. 2.5 Groundwater Quality in the Near-Surface Unconfined Aquifer The results of analyses of samples of groundwater recovered in March 2006 are evaluated below. 2.5.1 Anal es of Concern in Groundwater Along Principal Plume Axis Monitoring Wells MW-3, MW-13, MW-7, MWFP-4 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 December 2005, the concentrations in the samples recovered in March 2006 were generally lower than those measured in samples recovered from the same wells. For example, the concentration of benzene in the groundwater in Monitoring Well MW-3 fell from 330 gg/L to 150 gg/L, and TPHg in MW-13 fell from 1,500 gg/L to 390 gg/L, with a commensurate decline in the concentration of benzene in the water in that well from 180 gg/L to 120 gg/L, although, further to the north, MTBE, the only analyte of concern • detected in the sample recovered from Monitoring Well MW-18 on March 30, 2006, rose to 64 4g/L, compared to 41 gg/L of that analyte on December 20, 2005. The variations in concentrations of analytes of concern in samples recovered along the principal plume axis in March 2006 compared to those from the same wells recovered in December 2005 extend a pattern of seasonal increase and decrease in the concentrations of fuel hydrocarbons that are related to groundwater flowing into and ebbing away from t.., the zone of affected soil in response to the seasonal increases in regional agricultural irrigation that cause temporal concentrations of analytes of concern to reach a seasonal maximum during the summer months. 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 MTBE, the only analyte F of concern detected in those wells in March 2006, remained at trace concentrations that �— were essentially unchanged from concentrations measured in samples recovered from those wells in December 2005. �~ The sample recovered from Monitoring Well MW-17 on March 30, 2006 contained a v trace of MTBE at a concentration of 0.70 gg/L. As has been discussed in earlier `' SJC