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' 12 January 2005 AGE-NC Project No 97-0326 ' Page 6ofII of the UST location, but thins to a thickness of 5 feet in the area of well MW-8 The lower sand layer, at between 135 and 150 feet bsg, exhibits a uniform thickness of approximately 12 feet ' 3 2 HYDROGEOLOGICAL MODEL ' Based on the additional soil information acquired during this investigation, AGE modified the subsurface model of the site soil stratigraphy to include eight informal zones,a vadose(unsaturated) zone and seven hydrogeologic units A hydrogeologic or hydrostratigraphic unit (HU) may be ' defined as a body of rock or sediment that has hydrogeologic characteristics 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' may ' 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 The model presented is interpretational and will be modified as more data are collected, hydraulic 10 data were plezeocone (CPT) measurements The seven 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 35 feet bsg,the depth to the water table The vadose zone is predominantly fine grained(silt and clay),with an approximately 60-foot wide and up to 8- foot thick sand lense at the western half of the site Hydrogeologic Unit 1 (HU 1), at the top of the current saturated zone, extends from 35 feet to ' approximately 70 feet bsg It Is comprised primarily of fine-grained sediment characterized by low to very low pore pressure from 35 to 40 feet bsg, increasing to generally moderate pore pressures from 40 to approximately 60 feet bsg, then decreasing to low to very low pore pressures from 60 to ' 70 feet bsg Hydrogeologic Unit 2 (HU2) extends from 70 feet to approximately 85 feet bsg, consists of a ' laterally-continuous sand layer of apparent variable thickness The zone is characterized by very low to low pore pressures, however, a pore-pressure dissipation test conducted at CPT-4 suggests that the hydraulic conductivity is low in this sand interval (between 2X10-5 and 4X10-7 centimeters per tsecond) Hydrogeologic Unit 3 (HU3) extends from 85 feet to between 95 and 100 feet bsg, it is ' predominantly fine grained except for two thin cemented sand lenses (each approximately 1 foot thick) at the western edge of the survey area This zone is characterized by variable pore pressures, ' Advanced GeoEnvironmental,Inc 1