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CAP Addendum:Former Fue 9 Station, 7500 West Eleventh Street, Tracy, Page 18 parameters or system behavior that is to be predicted. However, it can safely be stated that any such sources of error are almost always of much smaller magnitude than those that affect predictions of behavior based on the interactions of a large number of phenomena that produce responses that affect critical aspects of the system but which cannot be measured reliably in practice, if at all. 2.4.1 Previous Successful LNAPL Removal at the Navarra Site Fortunately, at the Navarra Site we have available the results on an earlier stage of the groundwater remediation that was performed at the site of the underground fuel storage tanks that were removed from the 7500 West Eleventh Street property in December 1998. The locations of the tanks that were removed are shown on Figure 4. When four tanks were removed from Tank Pit No. 1, the resulting depression of the groundwater table in the pit induced LNAPL to flow into the pit from the west (Dietz Irrigation 1999). None of the tanks in the pit had leaked, thus they were not the source of the LNAPL. However, as ; other underground infrastructure was being removed from the Site, the source of the LNAPL that had been drawn into Tank Pit No. 1 quickly became evident. It had leaked from a complex arrangement of underground piping located under the pump island area of the former fueling station. That area is also shown on Figure 4. It is some 90 ft. distant from Tank Pit No.l in a co-gradient direction. When the LNAPL was found to have flowed into Tank Pit No. 1,.a vacuum truck was mobilized to the Site and was used to extract the floating product from the surface of the groundwater in the pit. The pumping technique used was to hold the suction end of the vacuum hose close to the water surface so that the LNAPL floating on it and the highly- contaminated groundwater immediately below it could be preferentially extracted. This was done at the expense of the efficiency of the extraction operation, which was affected by the significant volumes of air that entered the hose while the LNAPL was being skimmed away. That inefficiency was tolerated to ensure that the floating product was removed from the pit without causing it to become emulsified in the groundwater, which would have required very large volumes of groundwater to be extracted to achieve the desired effect. Evidence that LNAPL had been removed from a large area of the site was confirmed by the observation that no LNAPL appeared in Monitoring Well MW-3 when it was installed in April 2000. Because, as is shown on Figure 1, that monitoring well is located at the down-gradient edge of the pump island area where the LNAPL originated, it is evident that groundwater in that area would have been affected by floating product prior to the extraction of that material by pumping from Tank Pit No. 1 in 1998. That condition was confirmed when a sample of groundwater was recovered from Monitoring Well MW-3 on May 11, 2000. As is cited in Table 2, that sample contained the relatively high concentrations of 2,800 µg/L of diesel and 11,000 µg/L of gasoline but no LNAPL was detected in that well and effective aerobic processes of natural attenuation have acted to reduce the contaminant load in the groundwater in.that area since that time.