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E'. Extended Plume Definition:-7500 W 11th St.,Tracy,CA. Page 24 7.0 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 that have been summarized in Section 3.0 have permitted synthesis of historic, geologic, hydrostrati graphic and geo-chemical data. That synthesis has resulted in the interpretation of the distribution of hydrocarbons in the subsurface that is depicted on Figures 11 through 17. Groundwater elevation and geochemical data from the groundwater-quality monitoring program gathered through April 2004 are summarized in Tables 3 and 4. 7.1 Primary Plume As is shown on Figures 11 through 17, the site is affected by a primary plume of diesel and gasoline that has affected both soil and groundwater. As is shown on Figure 11, it emanates from the area where the pump islands were formerly located on the 7500 West Eleventh Street property and extends north-northeast some 750 ft. down the groundwater gradient. The main body of the plume includes groundwater affected by both gasoline and diesel, but moving ahead of that mass is a fringe of groundwater affected solely by methyl tertiary-butyl Ether(MTBE). 7.1.1 Confirmation of Primary Plume Boundaries Figure 11 shows the primary plume of affected groundwater emanating from the 7500 West Eleventh Street property. Its classic teardrop shape is derived from the groundwater-quality data derived from the April 2004 groundwater-quality monitoring round. With respect to its down-gradient and co-gradient boundaries, the interpretation of the plume shown on that Figure differs little from the earlier interpretation of the plume that was established in April 2002 (The San Joaquin Company Inc. 2002c), after SJC was allowed to install monitoring wells along the east and west sides of Chrisman Road to the north of West Eleventh Street, as was proposed in the original work plan for characterization of the Navarra Site in 1999 (The San Joaquin Company Inc. 1999a). (See Sections 3.1 and 3.2, above.) The complete geometry of the plan area of the primary plume, as first interpreted, was developed from a synthesis of the hydrostratigraphic, geochemical and groundwater contour data developed in 2002. At that time, its ground-gradient front.had not reached Monitoring Well MW-11. However, as can be seen by inspection of Table 4, by July 30, 2003, very low concentrations of gasoline and the fuel oxygenate MTBE had reached that well. At the same time, on its western perimeter, definition of the plume boundary although it had been estimated based on established hydrogeological techniques, it had not been concerned by samples recovered from soil borings or groundwater-quality monitoring wells on that side of the plume. Monitoring Wells MW-15 and MW-17 were installed in April 2004 to confirm location of the western boundary of the plume and Monitoring Well MW-19 was located so as to check that the leading edge of the plume's down-gradient front had not advanced as far north as SJC