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Evaluation of Natural Attenuation: 7500 West Eleventh Slreet, Tracy, CA. Page 22 t% change in the distribution of analytes of concern on its western side. For example, the ' sample recovered from Monitoring Well MW-2 in March 2005 contained 110 µg/L of TPHd, although the water in that well had been free of that analyte since December of 2001. 100 µg/L of TPHd also appeared in the sample recovered from Monitoring Well MW-5 in March 2005, which contaminant had not been detected at that location since July 2003. Of somewhat greater concern was the appearance of 0.51 µg/L of MTBE detected in the sample from Monitoring Well MW-17 on March 4, 2005. Because no concentration of any analyte of concern had been previously detected in groundwater samples from that well and because MTBE was the only analyte detected in the March 4, 2005 sample, it was initially suspected that the result may have been a false positive. However, recovery of a second sample from Monitoring Well MW-17 on March 16, 2005 showed that the MTBE was, in fact, present. The concentration in the duplicate sample was 0.54 µg/L. Because MTBE passes through the subsurface, there is essentially no retardation due to interaction with the soils and our interpretation is that the development of the groundwater mound around Monitoring Well MW-16 (see Figure 2 for groundwater contours) has caused the plume of affected groundwater to expand in a northwesterly direction so that the fringe of MTBE around the central areas of the plume have, by March 2005,reached the location of Monitoring Well MW-17. 6.1.3 AnalytesConcern in Secondary Groundwater Plume As can be seen by inspection of Table 2, theconcentrations of analytes of concern recovered on March 9, 2005 from Monitoring Well MW-12, which monitors groundwater quality in the area of origination of the secondary plume of affected groundwater on the == Navarra Site, remained at either zero or trace concentrations. Only TPHg, at 140 µg/L and Benzene at 1.6 µg/L were detected in the sample. These results extend a pattern of fluctuating concentrations around a generally-declining trend of low concentrations that has been on-going since July 2002. 6.1.4 Groundwater Quality in the Deeper Confined Aquifers As has consistently been the case since groundwater samples were first recovered from those wells in April 2002, with the exception of a slight trace of MTBE that was t apparently present in the sample recovered from Monitoring Well 3A on April 11, 2002, the samples recovered in March 2005 from Monitoring Wells MW-3A, MW-3B and MW-12A were again free of petroleum hydrocarbons. Thus, there is overwhelming evidence that the deeper aquifers, which are separated from the near-surface aquifer and from each other by clay aquitards, are completely free of components of fuel hydrocarbons. 6.2 Absence of Lead Scavengers in Groundwater Although they were not included in the suite of analytes specified by the SJCEHD, SJC believed that it was important to screen the groundwater beneath the 7500 West Eleventh Street site for the lead scavengers 1,2-DCA (also known as ethylene dichloride, EDC) and ethylene dibromide (EDB) during the early stages of the groundwater quality 1 sic '}