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Report: Groundwater-quality Monitoring—July 14-15,2006, 7500 West Eleventh Street, Tracy,CA. Page 12 2.5 Groundwater Quality in the Near-Surface Unconfined Aquifer `- The results of analyses of samples of groundwater recovered in July 2006 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 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 16m wells in March 2006, the concentrations in the samples recovered in July 2006 were generally higher than those measured in samples recovered from the same wells. The concentration of TPHg in the groundwater in Monitoring Well MW-3 rose from 760 gglL to 3,100 gglL and benzene rose from 150 gg/L to 1,100 µg/L. Those increases are consistent with the trends established by analysis of early data recovered from the site that shows that the concentrations of analytes of concern in groundwater peak during June or July of each year (The San Joaquin Company 2006b). Although, the increase in concentrations of analytes of concern found in wells further down the groundwater gradient were much more modest when the results from the July 2006 monitoring round are compared to the results from the March 2006 sampling event, they also support that annual trend. 2.5.2 Anal es of Concern in Groundwater around Fringes of Principal Plume i, Examination of the results of the analyses (see Table 2) of the groundwater sample recovered from Monitoring Well MW-5 on July 14, 2006 show that MTBE remains at trace concentration in that area on the western fringe of the principal plume of affected groundwater. Further to the north, at Monitoring Well MW-17, which is also on the western fringe of the plume, there also continues to be a trace of MTBE in the groundwater in that well. The samples recovered from Monitoring Wells MW-4, MWFP-2 and MWFP-5 in July 2006 contained only minor traces of a few analytes of concern, which demonstrates that +•• the eastern perimeter of the principal plume of affected groundwater has remained stable since samples were recovered from wells in that area during the March 2006 groundwater monitoring event. At the down-gradient limit of the primary plume of affected groundwater, only a trace concentration of MTBE, at a concentration of 30 µg/l, was detected in the groundwater �- sample recovered on July 14, 2006 from Monitoring Well MW-11, which compares to a concentration of 3.6 µg/l, in the sample recovered from that well in March 2006. However, given the absence of any other analytes of concern in the sample of L- groundwater recovered from that well and the fact that the groundwater in Monitoring sic