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25 October 2005 i AGE-NC Project No 02-0926 Page 4 of 7 5 feet bsg to between 42 feet and 48 feet bsg, with lenses of sandy material identified in borings ' MW-3 and CPT-1 at approximately 25 feet and 29 feet,respectively Apparently continuous layers of sand, silty sand, sandy silt, and cemented sand were encountered at 44 feet to 50 feet bsg and 70 feet bsg Stiff, fine-grained material, inter-bedded with silty and clayey sediments and lenses of isandy material,was identified from approximately 60 feet bsg to the total depths of the borings One soil sample, collected at 65 feet in CPT-3, was described as an olive brown, dry clay 3 2 HYDROGEOLOGIC MODEL ' A hydrogeologic,or hydrostratigraphic, unit(HU) is defined as a body of rock or sediment that has hydrogeologic characteristics (i e hydraulic conductivity, pore pressure, etc ) 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 {' • Based on the CPT results and comparison with soil descriptions from borings previously advanced on the site, AGE has developed a preliminary subsurface model of the site soil stratigraphy ' comprised of seven informal zones - a vadose (unsaturated) zone and six hydrologic units The model presented is interpretational and will be modified as more data is collected The eight',ertical zones identified are as follows(depths are approximate and can vary by several feet across the site) ' approximately 43 feet bs the vadose zone is Vadose Zane - Extending from surface grade to app y g, ' composed primarily of clay, silt, silty clay, and clayey silt, with discontinuous lenses of sandy sediments encountered at approximately 25 feet and 29 feet bsg This zone is characterized by low to high pore pressures, local zones of low pore pressure appear to be due to the presence of ' discontinuous lenses of sandy and silty sediments HU1 - This zone extends from approximately 43 feet to between 59 feet and 62 feet bsg An apparently continuous fine sandy layer(consisting of cemented sand and stiff,fine-grained material) is encountered at the top of the zone to depths between 45 feet and 50 feet bsg, underlain by alternating intervals of clayey, silty, and sandy material This unit is characterized by moderate to high pore pressures, local zones of low pore pressure appear to be due to the presence of thin layers of stiff, fine-grained material ' HU2 -HU2 extends from between 59 feet and 62 feet bsg to approximately 70 feet bsg,and consists primarily of clayey silt, silt, and stiff, fine-grained material Pore pressure within this unit is ' Advanced GeoEnvironmental,Inc