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