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I . 07 March 2003 ' AGE-NC Project No 95-0141 Page 6 of 10 ' beneath the site (Figures 3 and 4), the computer generated boring log for CPT-01 is Included in Appendix B 32 HYDROGEOLOGIC UNITS Based on the CPT results,supplemented by soil stratigraphic data,AGE has developed a preliminary subsurface model of the site hydrostratigraphy comprised of six informal zones, a vadose (unsaturated) zone and five hydrological units Hydrological units may be defined as significant vertical intervals of soil having significant lateral extent and having similar hydrologic characteristics (i e hydraulic conductivity,transmissivity,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 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) I Vadose Zone, extending from surface grade to 20 feet bsg, the depth to the water table The vadose zone is predominantly fine grained material from surface grade to 20 feet bsg, and is characterized by low to medium low pore pressures IHydrological U nit 1 (HU 1), a t t he t op o f t he c urrent s aturated zone, e xtends f rom 2 0 f eet t o approximately 45 feet bsg and is comprised dominantly of silt, with apparently laterally discontinuous sand lenses within the upper 10 feet The unit is characterized by a varying range of pore pressures, from generally medium low between 20 and 35 feet bsg, to low between 35 and 40 feet bsg, to medium high between 40 to 45 feet bsg Hydrological Unit 2(HU2)extends from 45 to approximately 75 feet bsg,HU2 is composed of sand, with minor layers of silt and cemented sand The interval is characterized by low pore pressure IHydrological Unit 3 W3)extends from 75 to 113 feet bsg,and is dominantly composed of silt,clay and stiff, fine-grained intervals Unit 3 is characterized by a fluctuating range of pore pressures, varying from low to medium low Hydrological Unit 4(HU4)extends from 113 feet bsg to 117 feet bsg,and is composed of sand The IInterval is characterized by low pore pressures Advanced GeoEnvzronmentai,Inc I