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I ' 01 November 2005 AGE-NC Project No 02-0950 ' Page 7 of 9 ' lateral extent and having similar hydrologic characteristics (i e hydraulic conductivity, transmissivity, etc ) over its vertical and horizontal extents A `flow unit' may be defined as a hydrogeologic unit characterized by significantly greater hydraulic conductivity values than the ' 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 ' The model presented is interpretational and will be modified as more data is collected The six vertical zones identified are as follows(depths are approximate and can vary by several feet across the site) ' Vadose Zone - Extending from surface grade to approximately 18 5 feet bsg (average depth to the ground water table), the vadose zone is generally comprised of silt, clayey silt, and silty clay, with ' thin, discontinuous lenses of cemented sand and stiff, fine-grained material encountered at approximately 12 feet, 15 feet, and 17 feet bsg in the central portion of the site ' Hydrologic Unit 1 (HU i)-At the top of the current saturated zone,HU 1 extends from approximately 18 5 feet bsg to between 23 feet and 25 feet bsg and is comprised primarily of fine-grained sediments (silt, clay, clayey silt, and silty clay) Pore pressure is low throughout the zone ' • Hydrologic Unit 2 HU2 -HU2 extends from between 23 feet and 25 feet to approximately 29 feet rr Y bsg and is composed primarily of fine-to coarse-grained sandy sediments(sand and silty sand),grain ' size appears to increase with depth This unit exhibits low pore pressures and may be considered a flow unit ' Hydrologic Unit 3 (HU3) - Extending from the base of HU2 to approximately 66 feet bsg, HU3 consists of silt, clay, clayey silt, and silty clay exhibiting moderate pore pressures Soil samples ' collected at 30 feet and 35 feet bsg describe a fat, blue-gray clay with high plasticity, consistency, and toughness which may act as a confining layer Additionally, a thin,discontinuos layer of sandy material is described at approximately 34 feet bsg in borings MW-4 and MW-5, and a thin ' (approximately 1 foot to 2 feet thick) layer of sandy sediments and stiff, fine-grained material was encountered at approximately 52 feet bsg in this unit This unit exhibits low to moderate pore pressures Hydrologic Unit 4(HU41-HU4 consists of a layer of silty sand and sand extending from 66 feet bsg to 72 feet bsg This zone is characterized by low pore pressures and may be considered a flow unit Hydrologic Unit 5 (HU 5) - Extending from 72 feet bsg to the total depth of boring CPT-1 (97 feet bsg),the top of HU5 consists of a 4-foot thick layer of silt with alternating layers of clayey material (clay, silty clay, and clayey silt) and stiff, fine-grained material extending to the total depth of the boring This zone is characterized by moderate to high pore pressures Advanced GeoEnvtronmentsl,inc