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to/ <br /> 4.1 Contaminants remaining in the vados zone must not reverse or threaten to reverse the mass <br /> +- reduction rate ofground water pollutants discussed in #4 below. <br /> y Soil sampling investigations conducted as part of the UST removal and the site characterization <br /> investigation, found that soil in the vados zone was impacted in the immediately vicinity of the <br /> former UST. Detectable concentrations of diesel fuel were found vertically from a depth of 16 feet <br /> bgs, the base of the UST over-excavation, to 27 feet bgs, and laterally in an area approximately 25 <br /> `r feet long by 25 feet wide. A diagram illustrating the approximate lateral extent of diesel fuel in the <br /> soil is presented in Figure 4. A diagram illustrating the approximate vertical extent of diesel fuel in <br /> the soil is presented in Figure 6. Based on the known limits of the plume, ATC estimates that <br /> approximately 255 cubic yards of diesel fuel-impacted soil remain in place beneath the former <br /> UST. Diesel fuel was detected at the base of the over-excavation at a maximum concentration of <br /> L. 4,900 milligrams per kilogram (mg/kg), with concentrations attenuating to non-detectable levels at <br /> a lateral distance of 15 feet from the UST. Diesel fuel concentrations in the soil attenuated with <br /> depth, to 490 mg/kg at a depth of 25 feet bgs. Groundwater beneath the UST, found at a depth of <br /> L. approximately 30 feet bgs, was found to have maximum detected concentrations of diesel fuel at <br /> 150 ug/L. Groundwater samples collected from three groundwater monitoring wells clustered <br /> around the UST found diesel fuel at a concentration of 150 ug/L only in the downgradient well <br /> �,. (WM-3) during one quarter of monitoring. All other groundwater samples collected from these <br /> wells for four quarters, found no detected concentration of diesel fuel. During the sampling period <br /> from December 2000 to October 2001, groundwater depths were calculated to fluctuate a maximum <br /> +- of 2.75 feet. Groundwater was found to typically flow to the southwest with a shallow gradient of <br /> approximately 0A04 ft/ft <br /> 'r The UST was removed on March 4, 1999 and the impacted soil was over-excavated to a depth of <br /> 16 feet bgs immediately thereafter. Therefore, the source of the release and the highest <br /> concentrations of diesel fuel in the vados zone were removed. Impacted groundwater at a depth of <br /> 30 feet was found to remain stable over the course of four quarters of groundwater monitoring. <br /> Based on the limited amount of impacted soil in the source area and the shallow gradient of the <br /> groundwater, the impacted groundwater appears to be confined to the area beneath the UST and <br /> immediately southwest of the UST. Therefore, the diesel fuel concentrations in the groundwater are <br /> not expected to increase over time or to migrate off the site. <br /> L. <br /> 4.2 Separate phase product has been removed to the extent practicable. <br /> Separate phase product was not encountered in the groundwater at the site in the Geoprobe boring <br /> investigation or the groundwater monitoring investigation. Therefore, removal of separate phase <br /> product is not a concern. <br /> t <br /> ATC Associates 1nC, 4 Project No,53.75126-0001 <br /> �1 <br />