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AGE-NC Project No. 99-0559 Page 10 of 13 SITE STRATIGRAPHY Based on the soil descriptions from CPT and previous soil boring logs, interpretive cross sections were constructed to depict the general hydrogeologic framework beneath the site. The site vadose zone currently extends from the ground surface to 65 feet bsg (water table), and is composed of silt, clayey silt, clay, and 'stiff fine grained' soil, and a gravelly, fine to coarse sand layer between 35 feet and 50 feet. Below the vadose zone, the lithology is composed dominantly of silty fine sand, sandy silt, clayey silt to silty clay and clay. Significant sand intervals were identified at between 90 and 110 feet bsg over the site, a thin discontinuous sand layer was encountered at 150 feet bsg and between 165 and 170 feet bsg. Fine grained, silty clay and clayey silt layers were observed at depths between the surface and 30 to 35 feet bsg, 80 and 90 feet bsg, 105 to 150 feet bsg. Variable lithology including silty fine sand, sandy silt and clayey silt to silty clay were observed at all other depths. HYDROGEOLOGICAL MODEL Hydrological units may be defined as significant vertical intervals of sediments having significant lateral extent and having similar hydrologic characteristics (i.e. hydraulic conductivity, transmissivity, etc.) over its vertical and horizontal extent. A 'flow unit' may be defined as a hydrologic 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. AGE had developed a site hydrogeologic model comprised of six informal zones, a vadose (unsaturated) zone and five hydrological units. However, based on the CPT results and comparison with soil descriptions from soil borings previously advanced on the site, AGE presents a revised hydrologic model as follows (depths are approximate and can vary by several feet across the site): Vadose Zone - the unsaturated zone normally extends from surface grade to current depth to the water table at 65 feet bsg. The vadose zone is predominantly fine grained (silt and clay) from surface grade to 45 feet bsg, with coarse material (sand) at between 50 to 60 feet bsg. Hydrological Unit 1 (HUI ), extends from approximately 60 to 65 feet bsg and on-site is identified in soil samples as mostly silty sand; the CPT logs identified the soil as stiff, fine- grained soil. The unit is currently unsaturated. This unit appears to be coarser westwards, where the CPT logs identified the soil as dominantly sand, silty sand and cemented sand. This unit is characterized by low pore pressure, although a moderately low to moderate Advanced GeoEnvironmental, Inc.