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17 October 2003 AGE-NC Project No 02-0964 Page 4 of 8 I31 STRATIGRAPHY Based on the findings from the CPT logs, cross sections A-A' and B-B' were constructed to depict general lrthology beneath the site (Figure 3), CPT computer-generated boring logs are included in Appendix B The site stratigraphy is composed predominantly of fine-grained soil consisting of silt, clayey silt, silty clay and clay,with the exception of five significant sand lenses or layers The sand Intervals are composed of sandy silt, silty sand, sand or gravelly sand of approximately 15-foot thick or less, and are located at depths of approximately 50 feet bsg,between 60 and 75 feet bsg,between 95 and 110 feet bsg,between 120 and 130 feet bsg and between 140 and approximately 160 feet bsg(Figure 3) The sands appeared to be continuous throughout the site, with the exception of the uppermost sand layer, which may be limited laterally to the northernmost portion of the site Measured hydraulic conductivities In the sand-bearing intervals(Figure 3),based on CPT dissipation tests, were typical for sand mixtures 3 2 HYDROGEOLOGICAL MODEL Based on the CPT results, AGE has developed a preliminary subsurface model of the site sediment stratigraphy comprised of nine Informal zones,a vadose(unsaturated)zone and approximately nine hydrogeologtc units Hydrogeologtc or hydrostratlgraphle units(HUs) are defined as a body of rock or sediment that has hydrogeologtc 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 fortes a distinct hydrologic unit with respect to the flow of ground water(Maxey, 1964) A `flow unit' may be defined as a hydrogeologtc 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 The model presented is Interpretational and will be modified as more data Is collected The nine vertical zones Identified are as follows(depths are approximate and can vary by several feet across the site) I Vadose Zone - Extending from surface grade to approximately 35 feet bsg, the depth to the water table The vadose zone Is predominantly fine grained material from surface grade to 25 feet bsg,with a sand unit occurring between 25 feet bsg at CPTI on-site Pore pressures In this unsaturated zone Is characteristically low or at zero IAdvunced GeoEnvironmental,Inc