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AGE-NC Project No. 99-0559 Page 10 of 14 Variable lithology including silty fine sand, sandy silt and clayey silt to silty clay were observed at all other depths. CPT computer-generated soil interpretation and boring logs were previously reported. 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 (HU1), 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 pore pressure spike occasionally occurs at between 60 and 65 feet bsg, generally associated with the silty sand or stiff, fine-grained soil. Hydrological Unit 2 (HU2) extends from approximately 65 to 75 feet bsg. It is composed dominantly of clay, clayey silt, sandy silt, silty sand and stiff, fine-grained soil. Pore pressures ranged from low for clays to moderate for stiff, fine grained soil. Hydrological Unit 3 (HU3) extends from 75 to between 85 and 90 feet bsg and is predominantly stiff, fine-grained soil fine grained sediments, with local occurrences of clay, silty clay, silt and sand. Pore pressure is generally low to moderately low, but can peak to high in portions of the stiff, fine-grained intervals. Advanced GeoEnvironmental, Inc.