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11 February 2002 AGE-NC Project No 99-0559 Page 6 of 10 beneath the site (Figure 3 and 4) Cross sections A-A' and B-B' were previously presented in Preliminary Investigation &Evaluation Report, dated 14 December 2000 ' The site vadose zone currently extends from the ground surface to 55 feet bsg(water table), and is composed of silt, clayey silt, clay, and `stiff fine grained' soil, and a gravelly, fine to coarse sand layer between 35 feet and 50 feet A significant sand interval also occurs between 90 and 105 feet ' bsg over the site and a thin discontinuous sand layer was encountered at 150 feet bsg Fine grained, silty clay and clayey silt layers were observed at depths between the surface and 30 to 35 feet bsg, 80 and 90 feet bsg, 105 to 150 feet bsg Variable Iithology Including silty fine sand, sandy silt and ' clayey silt to silty clay were observed at all other depths CPT computer-generated boring Iogs are Included in Appendix A ' 3 2 HYDROGEOLOGICAL MODEL ' Based on the CPT results and comparison with soil descriptions from borings previously advanced on the site, AGE has developed a preliminary subsurface model of the site soil stratigraphy ' comprised of six informal zones, a vadose (unsaturated) zone and five hydrological units Hydrological units may be defined as significant vertical intervals of soil having significant lateral extent and having similar hydrologic characteristics(I e hydraulic conductivity,transmissivlty,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 The model presented is interpretational and will be modified as more data is collected The six I 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 55 feet bsg,the depth to the water table The vadose zone is predominantly fine grained (silt and clay) from surface grade to 25 feet bsg, with coarse material (sand) occurring locally Sand predominates from 30 to 45 feet bsg with fine grained soil occurring locally From 45 to 55 feet bsg, the soil is predominantly fine grained, locally sandy, and has localized high pore pressure intervals Hydrological Unit 1 (HU1), at the top of the current saturated zone, extends from 55 feet to 62 feet bsg and is comprised primarily of sand and characterized by very low to low pore pressure The interval appears to contain more fine grained material and low to moderate pore pressure toward the south (CPT3) Advanced GeoEnvironmentai,Inc