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Extended Plume Definition:-7500 W I1 th St.,Tracy,CA, Page 17 5.0 MONITORING AND MANAGEMENT OF FLOATING PRODUCT Management of Light Non-aqueous Phase Liquid (LNAPL) ("floating product") was initiated at the time that the underground fuel storage tanks formerly located at the 7500 West Eleventh Street property were removed in December 1998. Subsequently, when floating product was detected in Monitoring Well MW-7 (see Figure 2 for location) in April 2002, the thickness of LNAPL in that well was routinely monitored as part of the scheduled groundwater-quality monitoring. To comply with a directive issued by SJCEHD (San Joaquin County Environmental Health Division 2003), interim measures to remove floating product from the subsurface in the area around Monitoring Well MW-7 were initiated on November 8, 2003. Following is a summary of the history of floating product removal and monitoring at the subject property through completion of the monitoring round conducted in April 2004. J 5.1 Removal of Floating Product Beneath 7500 West Eleventh Street Property When the underground fuel storage tanks previously located on the subject property were exhumed in December 1998, it was found that fuel hydrocarbons had leaked from underground piping beneath the fuel pump islands of the former fueling station. The former locations of the fuel pump islands are shown on Figures 2 and 10. At the same time that the tanks were removed from the site, soil heavily affected by fuel hydrocarbons was excavated from beneath the pump island area and 521.25 tons of that material was disposed off-site at a permitted facility. In addition, some 2,000 gallons of LNAPL and affected groundwater were removed from the subsurface by pumping from Tank Pit No. 1 into a vacuum truck, which was used to transport it to a permitted recycling facility (Dietz Irrigation 1999a). The locations of the pits from which the tanks were extracted are shown on Figure 10. The effectiveness of the soil remediation and floating product removal achieved at the time the tanks were removed from the site in reducing groundwater contamination in the area around the pump islands, where the preponderance of the fuel hydrocarbons affecting the subsurface beneath the 7500 West Eleventh Street property itself were released, can -be assessed by inspection of Table 4. That table includes the concentrations of analytes of concern found in samples of groundwater recovered from Monitoring'Wells MW-3 and MW- 4 over the period from May 11, 2000 to April 30, 2004. As is shown on Figure 2, those monitoring wells are no more than a few feet down the groundwater gradient from the on-site areas where at least several inches of product was found floating on the water table. However, because the floating product had been removed from a wide area of the subsurface when it was pumped out of the tank pit, no LNAPL was detected in those wells when they were first sampled and, since that time, there has been a marked decline in the concentrations of analytes of concern in samples of groundwater recovered from them. 5.2 Interim Measures for Removal of LNAPL from Area around MW-7 Based on an analysis of the changes of concentrations of total petroleum hydrocarbons in Monitoring well MW-7 that are compiled in Table 5 and noting the high concentrations of _ diesel and gasoline detected in the sample recovered from that well on November 28, 2000, SJC