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' 15 June 1999 <br /> AGE-NC Project No 98-0450 <br /> ' Page 5 of 27 <br /> ' 26 STRATIGRAPHY <br /> Silty clay was generally encountered from a depth of approximately 5 feet to 30 feet or 40 feet bsg <br /> across the site Light colored sand was encountered at a depth of 30 feet bsg in the former dispenser <br /> area and at 40 feet bsg in the former UST area The sand body thickness increased as viewed from <br /> south to north under the former UST <br /> ' Silty clay was encountered at a depth of 45 feet bsg in the dispenser area(boring SB7) and silty clay <br /> was not encountered until a depth of 55 feet bsg under the former UST excavation Silty clay was <br /> ' only encountered in the soil samples collected north of the site Sol] borings are depicted in <br /> Appendix A Based upon soil samples submitted for geotechnical analysis, fine sand encountered <br /> below approximately 30 to 35 feet bsg has a permeability of 2XI0 5 cm/sec and an average porosity <br /> of 40% The total percentage of organic material calculated from the soil sample analyzed was 1 3% <br /> (Appendix B) <br /> A stratigraphic cross sections of soils at the site with analytical results of soil samples is depicted <br /> in Figure 3 <br /> 3.0. ASSESSMENT OF HYDROCARBON-IMPACTS IN SOIL AND GROUND WATER <br /> 3 1 ON-SITE EXTENT OF HYDROCARBON IMPACT IN SOIL <br /> ' Petroleum hydrocarbon impacts appear to extend south of the former UST area and beneath the <br /> former fuel dispenser island The orientation of cross-section lines A-A' across the site is presented <br /> ' on Figure 2 The subsurface soil profile and the inferred extent of hydrocarbon-Impacted soil are <br /> illustrated on cross-sections A-A' (Figure 3) As shown on the cross sections, concentrations of <br /> TPH-g, and BTEX were detected for soil samples collected from approximately 15 to 45 feet bsg <br /> ' The mayor portion of the petroleum hydrocarbon impact appears limited to the silty clay unit ranging <br /> from approximately 15 to 40 feet bsg under the former USTs and 15 to 25 feet bsg below the former <br /> ' dispensers The degree of hydrocarbon impact vertically decrease by within the clay layer ranging <br /> from approximately 15 to 30 feet bsg Soil samples collected from within the water-bearing zone at <br /> 40 to 55 feet bsg, below the former USTs, had a minor hydrocarbon Impact While soil at depths of <br /> ' 60 and 65 feet bsg had no detectable hydrocarbon impact = _ <br /> Based on the inferred vertical and lateral extent of on-site petroleum hydrocarbon impact, it is <br /> ' estimated that approximately 2,000 cubic yards of impacted soil is present on-site <br /> Adianced GeoEn%ironmental,Inc <br /> 1 <br />