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Upgradient(1999) concluded the following from the subsurface investigation: <br /> (1). Drilling in and around the UST excavation indicated that fuel leakage from the fueling <br /> facility was limited to the fuel lines immediately around the excavation. The boring <br /> through the excavation (B1) indicated that diesel penetrated less than I0 feet below the <br /> former excavation floor, to a maximum depth of 20 to 25 feet bgs. Soil borings within 5 <br /> feet of the lateral margins of the excavation reportedly encountered no contaminated soil <br /> whatsoever(Appendix 4). This indicated that lateral leakage from the UST or dispenser <br /> did not occur to any appreciable extent. <br /> (2) Upgradient reported that the depth to groundwater in the Stockton area in general was <br /> generally near its historically highest level (1999), and was reported to be between 40 <br /> and 45 feet bgs at a site less than 2000 feet to the north. Based on long-term groundwater <br /> measurements by the Flood Control Section of the San Joaquin County Public Works i <br /> Department, Upgradient concluded that it was unlikely that groundwater had been <br /> shallower than 35 feet at the site (Upgradient, 1999). Upgradient concluded that it was <br /> therefore not likely that the saturated zone had been impacted by petroleum hydrocarbons. <br /> (3) Upgradient reported that the release had been stopped and the source(s) of hydrocarbons <br /> in the vadose zone had been removed or,permanently contained. The UST and dispenser <br /> had been removed and the residual hydrocarbons were restricted to the silt tied that <br /> underlies the site to a depth of more than 25 feet. Silt beds typically have fairly low <br /> permeability, and diesel has limited mobility in soil. Hence, the contaminants were <br /> effectively contained (Upgradient, 1999). <br /> (4) Upgradient explained under itehn 3 above, that it was unlikely that groundwater had been <br /> or will ever become.,impacted because the soil at the site is generally silty and there is at <br /> least a 15-foot buffer zone of clean soil between the estimated high groundwater level and <br /> the residual soil petroleum hydrocarbon mass. <br /> EnvironmentallSTETARSCR09102008.wpd 9 <br />