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Extended Site Characterization Report: 7500 West Eleventh Street, Tracy, CA Page 32 merges into the primary plume. The sources and characteristics of the two plumes of affected soil and groundwater are further discussed in the Sections that follow. 7.3.1 Sources SJUs conclusions regarding the sources of the primary and secondary plumes of fuel l hydrocarbons affecting the subsurface are based on examination of the tanks removed from the site and the condition of the soil and groundwater exposed in the tank pits and pipe trenches. Those observations are summarized below. 7.3.1.1 Tank Pit No. 1 When the underground storage tanks that had served the fueling station in the years prior to its final closure were removed from Tank Pit No. 1, they were in excellent condition and free of rust. There was no evidence that either the tanks or their adjacent piping had leaked or that there had been any significant release of fuel due to over-filling of the tanks. The location of Tank Pit No 1 is shown on Figure 3. i Four soil samples recovered on December 10, 1998 from the walls of Tank Pit No. 1 at the depth of the surface of the groundwater contained, as can be seen in Table 3, no detectable concentrations of gasoline, although MTBE was present in all four samples at ` very low concentrations. Three of the samples contained low concentrations of one or more of the BTEX compounds. Diesel at a concentration of 2,200 mg/Kg was detected in r.' asoil sample recovered from the west wall at groundwater level in the southwest corner 1: s of the pit. Diesel in the three other samples contained either no detectable concentrations of diesel or very low concentrations of that fuel. The sampling locations referred to above are shown on Figure 4. When the tanks from Tank Pit No. 1 were removed, floating product flowed in from the western wall of the pit. That floating product was skimmed from the water surface until there was no further inflow, and the tank pit was free of floating product. Significantly, although a grab sample of groundwater taken from the pit at that time contained 210,000 }-i µg/L of diesel, it also contained 55,000 µg/L of gasoline, which would be incompatible ' with the fact that no detectable concentrations of gasoline were detected in samples recovered from the walls of the pit at the depth of the water table, unless the source of the floating product that flowed into Tank Pit No. 1 was located at some point remote from and to the west of the pit. _. 7.3.1.2 Tank Pit No. 2 The locations from which soil samples were recovered in the bottom of the tank pit in which the four antiquated and abandoned tanks that had been discovered during Dietz Irrigation's tank removal operations are shown on Figure 6. The results of the sample analyses included in Table 3 show that no MTBE was detected in any of the samples of soil recovered from Tank Pit No. 2. 1 sic