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►- Work Plan for Refined Plume Definition and Management of Floating Product-7500 W 11th 3t., Tracy, CA. Page 10 ,.. 4.7 Summary of Completed Characterization and Monitoring Programs 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 interpretation of the distribution of hydrocarbons in the subsurface that is depicted on Figures 4, 5, 6 and 7. Groundwater elevation and geochemical data from the groundwater-quality monitoring program gathered through October 2003 are summarized in �- Tables 1 and 2. Detailed results of the site characterization and groundwater-quality monitoring programs that have been completed to date have been presented in a series of engineering reports prepared by SJC (The San Joaquin Company 2003a, 2003b, 2003c, �- 2002a 2002b, 2002c, 2002d, 2001d, 2001e, 2001f, 20018,). As is shown on Figures 4, 5, 6 and 7, the site is affected by a primary plume of diesel and *^ gasoline that has affected both soil and groundwater. As is shown on Figure 4, 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). A secondary plume of diesel and gasoline, which is also shown on Figure 4, emanates from an area to the rear of the Casa Mendoza restaurant on the 7500 West Eleventh Street property. Although no physical evidence of an underground or above ground storage tank has been found there, it appears that, at some time in the past, there was a source of fuel hydrocarbons located in the vicinity of Monitoring Wells MW-12 and MW-12A (see Figure Adohydrocarbons for locations). When the 6,000 linear feet of underground piping was removed from the site in December 1998, a disconnected pipeline was found that ran from the general area of those monitoring wells toward the West Eleventh Street frontage of the property where the underground storage tanks that were removed at that time were located. As is also shown on Figure 4, underground fuel tanks were formerly located on the property at 24195 Chrisman Road, which adjoins the 7500 West Eleventh Street site to the south. They had contained diesel but were removed in December 1998. Although no additional investigation of the 24195 Chrisman Road site has been performed, SJC believes that because samples recovered from the bottom of the tank pit at the time of removal contained only low concentrations of diesel (Dietz Irrigation 1999b), it is likely that any contribution made by diesel migrating from the 24195 Chrisman Road site to the subject property would have had little, if any, deleterious affect on the environmental condition of the subsurface beneath the subject property. Finally, there are dispersed, low concentrations of a variety of hydrocarbons affecting wide areas of shallow soil beneath the 7500 West Eleventh Street property and nearby land. The presence of these hydrocarbons can be attributed to minor leaks and spills from vehicles using unpaved areas of the former service station and being parked on the shoulders of the sic