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Extended Site Characterization Report: 7500 West Eleventh Street, Tracy, CA. Page 52 s 9.0 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 modem site infrastructure built. I 9.1 Tank and Infrastructure Removal In December 1998, eight underground fuel storage tanks were removed from the subject site. Four were of modem construction and had been registered in compliance with :.� California regulations. The presence of the four others had been unknown until the time 4 ! they were discovered during removal operations for the modern tanks. Due to their being situated in a low-penneability clay soil, they were not a major contributor to the Ieaked petroleum hydrocarbons found to be affecting the subsurface beneath the 7500 West Eleventh Street 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 (amounting 'j to 6,000 linear feet of pipe of various ages and types) beneath the area where the fuel dispensers had been located. f +, 9.2 Primary Plume A plume of hydrocarbons that emanated from the area around the former dispenser island -northehas migrated in an approximately northasterlYdirection across West Eleventh Street and from there down the groundwater gradient, to a point some 750 ft. distant from !_A the 7500 West Eleventh Street property, near the intersection of Carmelo Avenue and Chrisman Road. This primary plume of diesel and gasoline is shown in plan view on Figure 13. It has the classic teardrop shape seen at many sites where fuel hydrocarbons have been released into the subsurface from near-surface sources Freeze and Cherry 1979). €,_11 The most severely affected areas of the subsurface within the primary plume are located along the West Eleventh Street frontage of the subject property, immediately down the hydrogeologic gradient from the former location of the fuel-dispensing island. When the 'r tanks were removed from the site, floating product flowed from that area into the open ` tank pit to its east. When that occurred, 2,000 gallons of floating product and groundwater containing high concentrations of fuel hydrocarbons were immediately -site. In addition, some 520 cubic yards of pumped into vacuum trucks and disposed off soil affected by high concentrations of fuel hydrocarbons were removed by over- excavation from the area around the Iocation of the dispenser pumps. That remediation was highly effective so that, by April 11, 2002, the concentrations of diesel in Well MW- 3 had fallen to 200 �Lg/L, and the concentration of gasoline had fallen to 330 µg/L, with commensurately low concentrations of BTEX compounds and MTBE. sic