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�.4. Report:Groundwater-quality Monitoring—January 26,2004: 7500 West Eleventh Street, Tracy, CA, Page 10 In the case of Monitoring Well MW-4, the sample recovered contained gasoline at a .. concentration of 600 µg/L but no diesel was detected. The only other analyte of concern ' detected in that sample was MTBE at 630 µg/L. Both of those concentrations are moderately higher than the respective concentration of 130 µg/L and 190 j.g/L that were detected in the sample recovered from that well on October 27, 2003. The results of the analyses of the samples recovered from Monitoring Wells MW-3 and MW-4 on January 26, 2004 are consistent with the previously recorded pattern in which increases and decreases have occurred around a steadily declining trend that has been L ongoing since groundwater affected by floating product was pumped from the large excavations opened on the 7500 West Eleventh Street site at the time the underground ' storage tanks were removed. As noted previously, a program of periodic purging of LNAPL from Monitoring Well MW-7 was initiated on November 8, 2003. By January 26, 2004, that program had AMP reduced LNAPL in the monitoring well to no more than trace. Accordingly, it was decided that it would be technically appropriate to purge Monitoring Well MW-7 and recover a sample for analysis. The results of the analysis detected the presence of TPH(d) at 420 µg/L, TPH(g) at 1,800 µg/L, benzene at 57 µg/L, toluene at 33 gg/L, ethyl benzene at 57 pg/L and total xylene isomers at 220 µg/L. No fuel oxygenates were detected. However, SJC remains concerned that, given the historical presence of floating product in �- the well and the probable limited extent of the zone that is influenced by the floating product purging program, the cited concentrations of analytes of concern do not necessarily reflect the concentrations of the analytes of concern beyond a radius of a few feet from Monitoring Well MW-7. Concentrations of gasoline were first detected on July 30, 2003 in Monitoring Well MW- �' 11, but were not present in the samples recovered from that well in October 2003 nor on January 26, 2004. Though the October 2003 sample contained diesel at a concentration of 85 µg/L, which was slightly higher than the concentration of 69 µg/l, of diesel-range i" hydrocarbons first detected in July, no diesel was detected in the sample recovered on January 26, 2004. MTBE rose from 6.7 µg/L in October 2003 to 16.0 µg/L in January 2004, which is similar to the 13 gg/L present in the sample recovered from that well on July 30, 2003. The data obtained from the analyses performed on the sample of groundwater recovered from Monitoring Well MW-11 on January 26, 2004 (and during preceding sampling rounds) indicating an absence of any component of fuel hydrocarbon other than MTBE is consistent with SJC's interpretation that the principal plume of affected groundwater may have advanced to that location. The plume front is represented by low concentrations of MTBE, an analyte that is known to be little retarded as it migrates through subsurface soils and that is not known to be bioremediated by naturally- occurring biota. All 2.4.1.2 Fuel Hydrocarbons and Oxygenates Around Fringes of Principal Plume The sample recovered from Monitoring Well MW-5 on January 26, 2004 contained no detectable concentrations of any analytes of concern other than MTBE at a concentration sic