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Extended Site Characterization Report: 7500 West Eleventh Street, Tracy, CA. Page 31 of affected groundwater emanating from the 7500 West Eleventh Street site. .:. l 7.2.5 24195 Chrisman Road As shown on Figure 12, two underground fuel tanks were formerly located on the property at 24195 Chrisman Road, which adjoins the 7500 West Eleventh Street site to the south. Both of the 1,000-gal. tanks, which had contained diesel and had previously been used to support agricultural operations, were removedin December 1998. Soil samples recovered from the bottom of the pit contained diesel at concentrations up to 370 -� mg/Kg and barely detectable traces of the BTEX compounds. That information was =a included in a report filed with the SJCEHD on January 13, 1999 (Dietz Irrigation 1999b). Because no additional investigation of this site has been performed, it is uncertain whether any releases to the soil or groundwater that occurred there have commingled with releases of fuel hydrocarbons on the 7500 West Eleventh Street site. The tanks were located behind a retaining wall separating the two properties. On the north side of that wall, the ground surface on the subject property is raised several feet above the ground surface on the 24195 Chrisman Road site. That topography, together with the relatively low concentration of diesel found in soil in the bottom of the tank pits (370 mg/Kg) compared to concentrations of up to 2,290 mg/Kg found in shallow soil in the area around Wells' MW-12 and MW-12A on the 7500 West Eleventh Street site, make it unlikely that any contribution made by diesel migrating from the 24195 Chrisman Road site to the subject property would be of any more than minor significance. 7.3 Plumes Emanating from the 7500 West Eleventh Street Site To develop a rational interpretation of the subsurface conditions in the area of the 7500 West Eleventh Street site, one must synthesize the geological, hydrogeologicaI and geochemical data gathered during the first stage (2000) of the site characterization and the extended program (2002) described herein, together with other information obtained <' during the course of the program and by Dietz irrigation at the time the underground storage tanks and piping were removed (1998). Following is SJC's interpretation of the sources by which, and the distribution and lateral and vertical extent to which, the subsurface beneath the subject property and adjacent areas has been affected by petroleum hydrocarbons. This interpretation is shown on Figures 13, 14, 15 and 16. Figure 13 shows the affected areas of the subsurface in plan view, and Figures 14, 15 and 16 illustrate the distribution of petroleum hydrocarbons along sections A-A', B-B' and C- C', the locations of which are also shown on Figure 13. As can be seen on Figure 13, there is a primary plume of affected groundwater emanating from the area where the fuel dispenser pumps were formerly located on the 7500 West ' Eleventh Street property. This plume extends some 750 ft. north-northeast down the -' groundwater gradient and reaches an area near the intersection of Carmelo Avenue with Chrisman Road. A secondary plume had its source near the southeastern corner of the truck service bay located at the rear of the Casa Mendoza restaurant. Its down-gradient extent is limited to some 130 ft. or, if greater, its leading edge is obscured where it � ' SJC