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09 May 2003 • AGE-NC Project No 99-0559 Page 4 of 7 3 2 HYDROGEOLOGICAL MODEL Hydrological units may be defined as sigmficant vertical intervals of sediments having significant lateral extent and having similar hydrologic characteristics (a e hydraulic conductivity, transmassavaty, 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 preliminary site hydrogeologac 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- Extending from surface grade to current depth to the water table at 60 feet bsg The vadose zone is predominantly fine grained (silt and clay) from surface grade to 45 feet bsg, with • coarse matenal (sand) at between 50 to 60 feet bsg Hydrological Unit 1 (HU 1), at the top of the current saturated zone, extends from approximately 60 I 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 This unit appears to be coarser westwards, where the CPT logs Identified the soil as dominantly sand, silty sand and cemented sand This umt is characterized by I 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 staff, fine-grained soil Hydrological Unit 2(HU2)extends from approximately 65 to 75 feet bsg It as 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 3 extends from 75 to between 85 and 90 feet bsg and Is predominantly � ) g p Y I 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 Hydrological Unit 4(HU4)extends from between 85 and 90 feet to 92 feet to approximately 105 feet bsg and consists primarily of sand, gravelly sand, cemented sand and salty sand Pore pressures an Ithis interval are uniformly low This unit should be characterized as a flow unit Hydrological Unit 5 (HU5)extends from approximately 105 to the total depth penetrated by all CPT borings(approximately between 125 and 160 feet bsg)except the deepest borings CPT-6 and CPT-7 Advanced GemEnvironmental,Inc