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Section 5 <br /> i Conclusions <br /> 1 <br /> Petroleum-Impacted soil (characterized as diesel and bunker fuel) has been identified at <br />' elevated concentrations in three of the eight soil borings drilled at the UST location TPH <br /> compounds exceeding 1000 ppm were reported in B4, located south of the UST, in B6, located <br />' west of the UST, and in BS, located north of the UST All three borings were located at the <br /> fill end of the tank Based on the distribution of detected hydrocarbons, contaminated soil <br /> ' may have resulted from overfilling of the tank <br /> As shown in Figures 5 and 6, petroleum-contaminated soil appears to be localized at the <br /> immediate vicinity of the tank fill Although TPH compounds were detected in three borings, <br /> additional step out borings all reported no contamination Hydrocarbon contamination was <br /> ' detected only at the 15-foot depth interval, which corresponds to a thin sand sand bed <br /> encountered most borings Samples collected at 20 feet bgs reported non-detectable TPH <br /> concentrations Below the sand layer, finer grained sediments (clay and silt) appear to have <br /> restricted further vertical migration of hydrocarbons Based on the vertical and lateral extent <br /> ' of detected hydrocarbons, Tetra Tech estimates that approximately 100 to 150 yd of <br /> petroleum-impacted soil are present around the bunker fuel UST <br /> 1 <br /> 1 <br /> 1 <br /> TETRA TECH Stockton Developmental Center,Phase It Site Investigation Report Page 15 <br />