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s e e i i r , } 11 November 2004 AGE-NC Project No 02-0962 Page 8 of 14- 4 h 41 STRATIGRAPHY ^ f In general, light brown to orange brown fine-grained soil consisting of clayey silt to silty clay was encountered'from surface grade to 25 feet bsg,characteristically,the material had low plasticity and m w I toughness with a dry to slightly moist moisture content Brownish-gray or brown,moist to saturated, poorly graded sandy silt to fine sand was encountered between 25 and 35 feet bsg A sandy silt to Isilt was encountered at approximately 35 to 40 feet bsg From 40 feet bsg to total depth the stratigraphy is dominated by alternating layers of silty sand or silt and clayey silts to clay layers The layers vary in thickness from approximately 5 to 15 feet A stratigraphic cross section is depicted in Figure 4 The soil boring logs are provided in Appendix A 4 2 HYDROGEOLOGICAL MODEL 4 Based on the CPT results, AGE has developed a preliminary subsurface model of the site sediment stratigraphy comprised of eight informal zones,a vadose(unsaturated)zone and seven hydrogeologic `units Hydrogeologic or hydrostratigraphic units(HUs)are defined as a body ofrock or sediment that 1� has hydrogeologic characteristics(i e hydraulic conductivity,pore pressure,etc )that make it distinct from surrounding bodies of rock or sediment(Sanders, 1998),or apart of a body of rock that farms 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 charactenzed 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 a The model presented is interpretational and will be modified as more data is collected The eight vertical zones are identified as follows (depths are approximate and can vary by several feet across the site) Vadose/First Water Zone-Extending from surface grade to approximately 20 feet bsg Static water is at 20 feet bsg in the monitonng wells, however,the first saturated soil samples were collected at 30 feet bsg Indicating the water table is confined by the overlying fine grained materials The zone is characterized by moderate pore pressures I ,Hydrogeologic Unit 1 (HU1) extends from 20 feet to approximately 30 feet bsg and is comprised predominantly of sandy silt, clayey silt and silt 'Dominant hydraulic conductivities, based on CPT soil behavior types (SBT), range between 1 X10 6 and 1 X 10 8 centimeters per second (cm/s) for silty to clayey type soils This interval is characterized by generally moderate pore pressures i ff I i ' Advanced GeoEnvironmental,Inc 1 r i