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Site Background Information: West Clay Properties Page 7 of 10 SITE HYDROGEOLOGICAL MODEL Based on the CPT results,AGE has developed a preliminary subsurface model of the site sediment stratigrphy comprised of informal zones including a vadose (unsaturated) zone and various hydrogeologic units. Hydrogeologic or hydrostratigraphic units (HUs) can be defined as a body of sediment that has hydrogeologic characteristics(i.e.hydraulic conductivity,pore pressure,etc.)that make it distinct from surrounding bodies of sediment(Sanders, 1998), or a part of a body sediment 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, AGE had previously identified seven informal zones, a vadose (unsaturated) zone and six hydrogeologic units as follows: Vadose Zone - Extending from surface grade to approximately 18 feet bsg, the depth to the water table.The vadose zone is predominantly fine-grained material from surface grade to 20 feet bsg.The zone is characterized by low pore pressures. Hydrogeologic Unit 1 (HUI), at the top of the current saturated zone, extends from 18 feet bsg to a maximum depth of 40 feet bsg and is comprised predominantly of clay and fine sand, interbedded with silt. HU 1 is characterized by generally low to moderate pore pressures. Hydrogeologic Unit 2 (HU2)extends from a minimum depth of 31 feet bsg to a maximum depth of 72 feet bsg and is composed dominantly of stiff, fine-grained soils including silt and clay, interbedded with laterally discontinuous fine sand intervals. HU2 is characterized by generally moderate to high pore pressures. Hydrogeologic Unit 3 (HU3) extends from a minimum depth of 62 feet bsg to a maximum depth of 90 feet bsg; HU3 is composed primarily of laterally continuous silty sand, sand and gravelly sand interbedded with stiff, fine- grained soil,clayey silt,silt and clay.HU3 is characterized by generally low pore pressures,and may be classified as a flow unit.Hydrogeologic Unit 4(HU4) extends from a minimum depth of 83 feet bsg to a maximum depth of 170 feet bsg;HU4 is composed primarily of fine-grained soils including clayey silt and silt interbedded with laterally discontinuous silty sand,sand and gravelly sand.Minor sandy silt and clay intervals were observed at various depths. HU4 is characterized by generally moderate to high pore pressures. Hydrogeologic Unit 5 (HU5) extends from a minimum depth of 166 feet bsg to approximately 191 feet bsg; HU5 is composed primarily of coarse-grained soils including sand and gravelly sand interbedded with minor silt and clayey silt and silt. HU5 is characterized by generally low pore pressures, and may be classified as a flow unit. HU5 was observed in borings CPT-12 and appears to be laterally continuous beneath the site.Hydrogeologic Unit 6 (HU6) was encountered in CPT-03 from approximately 191 feet bsg to approximately 210 feet bsg.The lateral extent of HU6 is not well defined at the site;CPT-01,CPT-02 and CPT-04 were terminated prior to encountering HU6. HU6 is composed primarily of fine-grained soils including clayey silt and silt. HU6 is characterized by generally moderate to high pore pressures. Advanced GeoEnvironmental,Inc.