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_ a 02 January 2007 AGE-NC Project No. 95-0121 Page 5 of 10 Based on CPT data results and comparison with soil descriptions from borings previously advanced on the site, AGE has developed a preliminary subsurface model of the site soil stratigraphy comprised of a vadose (unsaturated) zone and five hydrogeologic units (Figure 3). i Although general soil distribution may vary at each boring, AGE interprets the CPT data collected from soil borings CPT- 1 through CPT4 to represent the general hydrogeologic units beneath the site. The model presented is interpretational and will be modified as more data is collected. The vadose zone and five vertical zones identified are as follows (depths are approximate and can vary by several feet across the site): • Padose Zone - Located between surface grade and 18 feet bsg (depth to groundwater); comprised primarily of less permeable silty clay/clayey silt, although stiff fine grained silty sand occurs locally. • Hydrologic Unit 1 (HUI) - Located between 18 feet (depth to ground water) and 25 feet bsg, characterized by variable medium pore pressures; comprised primarily of less permeable, semi-confining to confining, silty clay/clayey silt. • Hydrologic Unit 2 (HU2/First Flow Unit) - Located between 25 feet and 32 feet bsg. Generally characterized by low to medium pore pressures and comprised primarily of permeable, silty sand and fine to medium sand. • Hydrologic Unit 3 (HU3) - Located between 32 feet and 40 feet bsg. Generally characterized by variable low to medium pore pressures and comprised primarily of less permeable, semi- confining to confining, stiff fine grained silt and clay. • Hydrologic Unit 4 (HU4/Second Flow Unit) - Located between 40 and 53 feet bsg. Generally characterized by low and medium pore pressures and comprised primarily ofpermeable silty sand and fine sand. • Hydrologic Unit 5 (HUS) - Located between 53 and 100 feet bsg. Generally characterized by low to high pore pressures and comprised primarily of less permeable, semi-confining to confining, stiff fine grained, silt, clay and localized silty sand. This unit may be generally subdivided with additional investigation and monitoring, based on the laterally discontinuous sand layers encountered. I