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Report: Groundwater-quality Monitoring—July 14-15,2006, 7500 West Eleventh Street, Tracy, CA. Page 6 1.6 Distribution of Hydrocarbons in the Subsurface The several stages of tank removal, excavation and removal of contaminated soil and groundwater, site characterization and groundwater-quality monitoring that have been conducted at the 7500 West Eleventh Street site since December 1998 have permitted synthesis of historic, geologic, hydrostratigraphic and geo-chemical data. That synthesis has resulted in the following interpretation of the distribution of hydrocarbons in the subsurface. 1.6.1 Primary Plume The site is affected by a primary plume of diesel and gasoline that has affected both soil and groundwater. As is shown on Figures 3 through 9, it emanates from the area where the pump islands were formerly located on the 7500 West Eleventh Street property and extends north-northeast down the groundwater gradient. When it was at its maximum extent, the long axis of the plume measured some 750 ft down the groundwater gradient. The main body of the plume includes groundwater affected by both gasoline and diesel, but ahead of that mass is a fringe of groundwater affected solely by MTBE. Historically, in the interior of the primary plume, LNAPL in the form of free floating product was present beneath the pump islands of the former fueling station at the Navarra Site where it had leaked from piping, and by April 2002, it had migrated down-gradient r as far north as groundwater-quality Monitoring Well MW-7. The locations of the pump islands at the former fueling station and of Monitoring Well MW-7 are both shown on Figure 2. Pumping of floating product and heavily contaminated groundwater from the pits from which the underground tanks were exhumed in December 1998 successfully eliminated floating product from the area around Monitoring Well MW-3. Based on consideration of the hydraulic conductivity and hydrostratigraphy of the subsurface and the subsequent reduction in analytes of concern in Monitoring Well MW-3 and at other locations along the south side of West Eleventh Street, it was estimated that floating product and groundwater containing high concentrations of components of fuel hydrocarbons were removed from the groundwater over a radial distance of some 70 to 100 ft (The San Joaquin Company Inc. 2004, 2002). That assessment has been confirmed by data obtained by analyses of groundwater samples recovered from Monitoring Well MW-13, which was installed in the median of West Eleventh Street in April 2004. The results show that concentrations of petroleum- and diesel-range hydrocarbons are significantly lower than those of similar analytes in Monitoring Well MW-7, which is further down-gradient from the release of hydrocarbons to the subsurface. The floating product that accumulated in Monitoring Well MW-7 over the period April 2002 to October 2003 was eliminated temporarily by a program of purging of LNAPL } from that well in late 2003 and early 2004. However, given the small 2-in. casing sic