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r • 09 January 2003 AGE-NC Project No. 95-0114 Page 4of9 The subsurface stratigraphy is composed predominantly of interbedded fine-grained soil, mainly clay, silty clay and silt; the amount of clay in subsurface soil apparently increases off-site towards the northeast(Figure 3).Three continuous clay layers were identified on-site, a 20-to 30-foot-thick layer at approximately 20 feet bsg and two thinner layers, varying from approximately 1 to 10 feet in thickness, at approximately 60 and 80 feet bsg. In general, coarse-grained sediment is limited to thin, Iaterally discontinuous layers of sandy silt, silty sand and sand; the thickest sand lense, of approximately 10-foot thick, is located off-site towards the northeast, at a depth of approximately 120 feet bsg (Figure 3). CPT computer-generated boring logs are included in Appendix A. 3.2. HYDROGEOLOGICAL MODEL Hydrological units maybe defined as significant vertical intervals of sediments 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. i Based on the CPT results, AGE has developed a preliminary site hydrogeologic model comprised of seven informal zones,a vadose(unsaturated)zone and six hydrological units;the model presented is interpretational and will be modified as more data is collected. The seven zones are as follows (depths are approximate and can vary by several feet across the site): Vadose Zone -Extending from surface grade to 25 feet bsg. The vadose zone is predominantly fine grained material from surface grade to 20 feet bsg, with a clay occurring between 20 feet and 40 to 50 feet bsg across portions of the site. Hydrological Units 1, 3 and 5 (HU1, HU3 and HU5) are clay layers characterized by low pore pressure. HU1 is located at the top of the current saturated zone (25 feet bsg) and extends to 40 to 50 feet bsg. HU3 and HU5 are located at depths of approximately 60 and 80 feet bsg,respectively, and vary in thickness between 1 and 10 feet. Hydrological Units 2, 4 and 6 (HU2,HU4 and HU6) are composed dominantly of interbedded silts and clays,with minor layers of silty sand, sand and cemented sand.These intervals are characterized _ by pore pressure variations from low to high pressure,with no laterally consistent pressure patterns. In general, the lower dynamic pore pressures are associated with clay- and sand-rich intervals, whereas higher pressures are mainly associated with clayey silts and "stiff fine-grained" intervals below 100 feet bsg(Figure 3). Advanced GeoEnvironmental,Inc.