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11 Report:Groundwater-quality Monitoring—October 27,2004, 7500 West Eleventh Street, Tracy, CA. ,Page tertiary-Butyl alcohol EPA Method 8260B }' Methyl-tertiary butyl ether EPA Method 8260B Di-isopropyl ether EPA Method 8260B Ethyl tertiary-butyl ether EPA Method 8260B Tertiary-amyl methyl ether EPA Method 8260B low 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 the previous sampling rounds. Copies of the laboratory Certificates of Analysis generated from the October 2004 monitoring round are included in Appendix B of this report. 2.5 Groundwater Quality in the Near-Surface Unconfined Aquifer The results of analyses of samples of groundwater recovered on October 27 and 28, 2004 are evaluated below. 2.5.1 Analytes of Concern in Groundwater Along Principal Plume Axis Monitoring Wells MW-3, MW-13, MW-7, MW-14, 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 July 2004, the concentrations in the samples recovered in �. October 2004 generally fluctuated within the range of concentrations that are typical of those that have previously been observed. For example, the concentration of TPHg in the groundwater in Monitoring Well MW-3 fell from 1,800 pg/L to 1,200 gg/L and TPHd +- concentrations in the same well fell from 83 gg/L to 74 pg/L. However, the concentration of TPHg in the sample recovered from Monitoring Well MW-7 on October 28,2004 fell to 14,000 pg/L from the 37,000 pg/L measured in the July 2004 sample. The change in concentration of total petroleum hydrocarbons over the same period was more consistent in the groundwater in Monitoring Well MW-13, with the concentration of both TPHd and TPHg rising by 400 pg/L to 2,500 gg/L in the case of the diesel-range 'M compounds, and by 2,300 gg/L to 11,000 gg/L in the case of gasoline. It is noted that MTBE, at a concentration of 15gg/L,was detected in the October 28, 2004 " - sample from Monitoring Well MW-7. This is the first time that the compound has been detected in that well since May 11, 2000, when it was detected at a concentration of 22 pg/L. This finding is consistent with SJC's interpretation that when the concentration of sic