ARCHIVED REPORTS XR0011753 - Laserfiche WebLink

Home Browse Search

Report. Groundwater-quality Monitoring—October 27,2004, 7500 West Eleventh Street, Tracy,CA. Page 12 I diesel and gasoline compounds in the groundwater in that well began to rise to elevated concentrations prior to November 2000, the subsequent lack of MTBE in samples from the well is attributed to masking of its presence in chromatograms by the presence of high concentrations of other petroleum compounds rather than by a physical absence of MTBE , h in the groundwater(The San Joaquin Company, 2002d, 2004a). k 1 f A significant finding from the analyses of samples recovered from the wells arrayed along the long axis of the primary plume of affected groundwater is that no detectable concentrations of any analytes of concern were detected in the groundwater samples recovered from Monitoring Wells MW-11 and MW-19 on October 27, 2004. Combined with the presence of the single detectable analyte, TPHg, at the very low concentration of 51 µg/L in the sample recovered from well MW-18, these data indicate that the plume of affected groundwater is currently stable and that its northern extremity had retreated some 150 ft southward compared to the case at the time of earlier monitoring rounds it when, although at extremely low concentrations, traces of components of fuel hydrocarbons were detected as far north as Monitoring Well MW-11. 2.5.2 Analytes of Concern in Groundwater around Fringes of Principal Plume The results of the groundwater-quality monitoring round conducted in October 2004 indicate that the plume perimeter, on both its eastern and western sides, has remained substantially stable since the monitoring round conducted in July 2004. Trace concentrations of MTBE, at 1.2 Ag/L, and TBA, at 190 gg/L, were detected in the sample recovered on October 27, 2004 from Monitoring Well MW-2, with 76 gg/L of total xylene isomers and 1.1 µg/L of MTBE being the only analytes detected in Monitoring ' Well MW-5. Both of those wells are located at the western perimeter of the primary I. Plume of affected groundwater. Samples recovered from Monitoring Wells MW-15 and MW-17, which are located beyond the western limit of the primary plume, showed that groundwater at those locations remained free of any analytes of concern. As has been the case since April 21, 2004, no traces of any analytes of concern were detected in the groundwater recovered from Monitoring Well MW-9 on the eastern perimeter of the primary plume, and the sample recovered from Monitoring Well MW- 10, on the same side of the plume contained only MTBE, at the trace concentration of 22 µg/L• As was noted in Section 2.5.1 above, the northern limit of the plume of affected groundwater retreated southward in the period between July 27, 2004 and October 28, 2004. This is evidenced j d ced by the lack of any detectable concentrations of any analytes of concern the samples recovered on the latter date'from Monitoring Wells MW-11 and MW-19. See Figure 3 for current plume geometry. 2.5.3 Analytes_of Concern in Secondary Groundwater Plume 'I N As can be seen by inspection of Table 2, the concentrations of analytes of concern in ; Monitoring Well MW-12, which monitors groundwater quality in the area of origination I . sic