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Appendix A - Historical Background AGE-NC Project No 02--0964 Page 4 of 7 • CPT-5 was sampled between 56 and 60 feet bsg,between 104 and 108 feet bsg and between 146 and 150 feet bsg Soil samples were collected from boring CPTI at 55 and 78 feet bsg The computer-generated CPT lithologic logs were integrated with previous subsurface soil descriptions to construct the subsurface geological model The site stratigraphy is composed predominantly of fine-grained soil consisting of silt, clayey silt, silty clay and clay, with the exception of five significant sand lenses or layers The sand intervals are composed of sandy silt, silty sand, sand or gravelly sand of approximately 15-foot thick or less, and are located at depths of approximately 50 feet bsg, between 60 and 75 feet bsg, between 95 and 110 feet bsg, between 120 and 130 feet bsg and between 140 and approximately 160 feet bsg The sands appeared to be continuous throughout the site,with the exception of the uppermost sand layer,which maybe limited Iaterally to the northernmost portion of the site Measured hydraulic conductivities in the sand- bearing intervals, based on CPT dissipation tests, were typical for sand mixtures HYDROGEOLOGICAL MODEL Based on the CPT results,AGE has developed a preliminary subsurface model of the site sediment stratigraphy comprised of nine informal zones, a vadose(unsaturated)zone and approximately nine hydrogeologic units Hydrogeologic or hydrostratigraphic units (HUs) are defined as a body of sediment that has hydrogeologic characteristics that make it distinct from surrounding bodies of sediment or a part of a body of rock that forms a distinct hydrologic unit with respect to the flow of ground water 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 is collected The nine 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 35 feet bsg, the depth to the water table The vadose zone is predominantly fine grained material from surface grade to 25 feet bsg,with a sand unit occurring between 25 feet bsg at CPTI on-site Pore pressures in this unsaturated zone is characteristically low or at zero Hydrologic Units la, 2, 4, 6 and 8 (HU1 a, HU2, HU4, HU6 and HU8) are sand-bearing units characterized by low CPT pore pressures(Appendix B) HU1 a,HU2 and HU9 are dominantly sand units, HU4 and HU6 are composed of inter-bedded layers of sand and sandy silt Hydraulic conductivities,based on CPT soil behavior types(SBT)should range between 1 X10-6 to 1 meters per second(m/s) for sandy silt to gravelly sand, measured hydraulic conductivities, based on CPT pore pressure dissipation tests, were between 1 X 10 5 to 1 X 10-8 m/s Advanced GeoEnvironmentai