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19 July 2004 AGE -NC Project No 95-0121 Page 3 of 9 2 3 HYDROGEOLOGIC UNITS Hydrogeologic or hydrostratigraphic units (HUs) are defined as a body of rock or sediment that has s hydrogeologic characteristics (i e hydraulic conductivity, pore pressure, etc ) that make it distinct from surrounding bodies of rock or sediment (Sanders, 1998), or a part of a body of rock that forms a distinct hydrologic unit with respect to the flow of ground water (Maxey, 1964) A `flow unit' can be defined as a hydrogeologic unit and is characterized by significantly greater hydraulic conductivity values than overlying and underlying units, a flow unit is capable of serving as a preferred migration route, and could potentially allow migration of large contaminant masses over great distances in a dissolved phase 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) ' Although general soil distribution may vary at each boring, AGE interprets the CPT data collected, from soil borings CPT -1 through CPT -4 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) • Vadose Zone - Located between surface grade and 18 feet bsg (depth to ground water), comprised primarily of less permeable silty clay/clayey silt, although stiff fine grained silty sand occurs locally • Hydrologic Unit I (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 ri • 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 Advanced GeoEnvzronmental, Inc