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Site Characterization Report: 7500 West Eleventh Street, Tracy, Cil. Page 32 ''.. 6.0 CONCLUSIONS A fueling station operated on the property at 7500 West Eleventh Street in Tracy, California from circa 1930 to late 1998. During that period, the station was reconstructed several times and elements of the older construction remained present in the subsurface when more modern site infrastructure built. In December 1998, eight underground fuel storage tanks were removed from the subject `r site. Four were of modern construction and had been registered in compliance with California regulations. The presence of the four others had been unknown until the time they were discovered during removal operations for the modern tanks. Although these older tanks contained product and were in very poor condition, it appeared that, due to their being situated in a low-permeability clay soil, they were not a major contributor to the leaked petroleum hydrocarbons found to be affecting the subsurface beneath the property. Investigations performed at the time the tanks and fuel pump islands were removed from the site revealed that the preponderance of fuel that entered the subsurface had leaked from a dense and highly-complex array of piping beneath the area where the fuel t ! dispensers had been located. A plume of hydrocarbons that emanated from that area has migrated in an approximately north-northeasterly direction down the groundwater gradient and across West Eleventh Street. This primary plume of diesel and gasoline is shown in plan view on Figure 13. Groundwater immediately down gradient from the source of the leakage beneath the dispenser pump islands is affected by concentrations of diesel and gasoline up to 14,000 pg/L and 11,000 pg/L, respectively. MTBE was detected in the groundwater at that location at a concentration of 10,000 pg/L. Further down gradient and across West - >; Eleventh Street, the highest concentrations of diesel and gasoline detected in the groundwater are considerable lower, at 1,300 pg/L and 1,400 µg/L, respectively, with the corresponding concentration of MTBE at 150 pg/L. In addition to the primary plume of diesel and gasoline in the subsurface, the shallow soils in essentially all of the areas investigated by the site characterization program contained low concentrations of a variety of petroleum hydrocarbons the presence of which can be attributed to minor leaks and spills that occurred over the more than 70 years during which the fueling station operated. However, as is also shown on Figure 13, a second distinct plume of diesel and gasoline was also identified in the subsurface beneath the site. That plume appears to have originated at some unknown source located near the southeast corner of the truck service bay that is located to the rear of the Casa Mendoza restaurant. Groundwater near the apparent source of that second plume is affected by diesel at a concentration of 25,200 µg/L and gasoline at a concentration of 11,000 pg/L. When considering the current environmental condition of the subject site, it is important to note that a volume of affected soil amounting to some 520 cubic yards, as measured in sic i