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Site Background Information: Cherokee Truck Stop AGE-NC Project No. 97-0312 Page 7 of 11 STRATIGRAPHY Based on the findings from the CPT soil borings and existing monitoring well and soil borings,cross section G-G' was constructed to depict general lithology beneath the site.Between surface grade to approximately 60 feet bsg, subsurface soil consisted dominantly of silt, clayey silt, silty clay and clay,separated by a laterally-discontinuous sandy silt/sand lense at approximately 25 feet bsg.Below 60 feet bsg to total depth,the soil consisted dominantly of stiff, fine-grained soil and sand. HYDROGEOLOGICAL MODEL Based on the results of the CPT investigation and previous environmental investigations at the site, AGE has developed a preliminary subsurface model of the sediment stratigraphy.It is comprised of three informal zones: a vadose(unsaturated)zone and two hydrogeologic units.Hydrogeologic or hydrostratigraphic units (HUs) are defined as a body of rock or sediment that has hydrogeologic characteristics(i.e.hydraulic conductivity,pore pressure,etc.)that make it distinct from surrounding bodies of rock or sediment(Sanders, 1998). A hydrogeologic unit may also be a part of a body of rock that forms a distinct hydrogeologic unit with respect to the flow of ground water(Maxey, 1964). A`flow unit'may be defined as a hydrogeologic unit 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 three 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 55 feet bsg,the depth to the water table. The vadose zone is predominantly fine grained material from surface grade to 55 feet bsg,with a small coarse grain sand unit occurring at approximately 25 feet bsg.The zone is characterized by low pore pressures. • Hydrogeologic Unit 1 (HU 1), at the top of the current saturated zone, extends from 55 feet to approximately 60 feet bsg and is comprised predominantly of clay, silty clay and silt, although towards the northeast,lenses of silty sand or stiff,fine-grained soil are common in this unit (Figure 3). This unit is characterized by generally moderate pore pressures of approximately 300 pounds per square inch(psi). • Hydrogeologic Unit 2 (HU2) extends from 60 feet to approximately 110 feet bsg and is composed dominantly of stiff, fine-grained soil,sand, and gravelly sand. Pore pressures Advanced GeoEnvironmen/al,Inc.