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Report: Groundwater-quality Monitoring—October 20-21,2005, 7500 West Eleventh Street, Tracy, CA. Page 14 same wells. For example, the concentration of benzene in the groundwater in Monitoring Well MW-3 fell from 1,100 µg(L to 330 µg/L. Further to the north at Monitoring Well MWFP-4, the concentration of THPg fell from 1,300 µg/L to 1,100 µg/L over that time period, while the corresponding reduction in MTBE was from 1,800 gg/L to 780 µg/L. 1 y Those decreases in concentrations of analytes of concern in samples recovered along the principal plume axis in October 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 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 on October 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 October 20, 2005 contained TPHg at a concentration of 100 µg/l, as well as a trace of F . MTBE at a concentration of 0.68 µg& As has been discussed in earlier groundwater monitoring reports (The San Joaquin Company 2006a,b), it is believed that the appearance of those minor concentrations of those analytes in that area is related to the development of the groundwater mound that, as discussed in Section 2.1.1 above, is centered near Monitoring Well MW-16. Changes in the concentrations of analytes of concern detected in the samples recovered from Monitoring Wells MW-4, MWFP-2, MWFP-5 and MW-10 in October 2005 were not significantly changed compared to the results obtained during the June 2005 sampling round. This data indicates that conditions along the eastern perimeter of the principal plume of affected groundwater at the Navarra Site remain stable. At the down-gradient limit of the primary plume of affected groundwater, only a trace concentration of MTBE, at a concentration of 19 teg/L was detected in the groundwater sample recovered on October 20, 2005 from Monitoring Well MW-11. No detectable } : concentrations of any analytes of concern were detected in the groundwater sample recovered from Monitoring Well MW-19. These findings demonstrate that there has been no extension of the down-gradient migration of affected groundwater in the plume. 2.5.3 Anales of Concern in Second Groundwater Plume As can be seen by inspection of Table 2, the concentrations of the majority of analytes of concern detected on October 20, 2005 in Monitoring Well MW-12, which is located in the source area of the secondary plume of affected groundwater on the Navarra Site,were t. i lower than those detected on June 27, 2005, with the concentration of TPHg falling from 820 µg/L to 340µ.g/L over that period. i sic �: j