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i 16 May 2002 AGE-NC Project No 99-0657 ' Page 3 of 6 31 STRATIGRAPHY Based on the findings from the CPT soil borings, cross section A-A' was constructed to depict general lithology beneath the site(Figure 3) Sand, cemented sand and silty sand were encountered at depths between 5 feet bsg and 15 feet bsg, 18 feet to 20 feet bsg, 25 feet to 30 feet bsg, 38 feet to 45 feet bsg, 63 feet to 76 feet bsg, 86 feet to 95 feet bsg and 103 feet bsg to 128 feet bsg Sandy silt, silt and clay were observed at all other depths CPT computer-generated boring logs are included in Appendix A ' 32 HYDROGEOLOGICAL MODEL Based on the CPT results, AGE has developed a preliminary subsurface model of the site soil stratigraphy comprised of seven informal zones, a vadose (unsaturated) zone and six 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 EThe model presented is interpretational and will be modified as more data is collected The four 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 32 feet bsg,the depth to the water table The vadose zone is predominantly fine grained material from surface grade to 32 feet bsg, with a coarse grain sand units occurring between 5 feet bsg and 15 feet bsg, 18 feet to 20 feet bsg and 25 feet to 30 feet bsg, across portions of the site The zone is characterized by low pore pressure Hydrological Unit 1 (HU1), at the top of the current saturated zone, extends from 32 feet to 45 feet bsg and is comprised dominantly of stiff fine grained material,with minor sand, silty sand cemented sand intervals The unit is characterized by predominantly high pore pressure Hydrological Unit 2(HU2) extends from 45 feet to approximately 65 feet bsg HU2 is composed of stiff fine grained material, clay and cemented sand HU2 is characterized by low to moderate pore pressure Hydrological Unit 3(HU3)extends from approximately 65 feet bsg to approximately 75 feet bsg and • is predominantly sand and silty sand with low pore pressures Advanced GeoFnvironmental,Inc