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19 February 2002 AGE-NC Project No 95-0109 Page 3 of 6 f r31 STRATIGRAPHY Based on the findings from the CPT soil borings, cross section A-A' was constructed to depict r ' _general llthology beneath the site (Figure 3) Generally, silty sand, sand to gravelly sand were -' encountered at depths between 5 and 20 feet bsg, 30 and 40 feet bsg, 45 and 55 feet bsg, 65 and 100 feet bsg, 120 and 125 feet bsg and 135 and 142 feet bsg Fine grained, sandy silt, clayey silt and ' silt were observed at all other depths CPT computer-generated boring logs 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 ' compnsed of four informal zones, a vadose (unsaturated) zone and three hydrologic units Hydrologic units may be defined as significant vertical intervals of soil having significant lateral extent and having similar hydrologic characteristics(i e hydraulic conductivity,transmissivity,etc ) ' • over its vertical and horizontal extent A `flow unit' may be defined as a hydrologic unit and is r 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 four ' vertical zones identified are as follows (depths are approximate and can vary by several feet across the site) ' The vadose zone from surface grade to approximately 27 feet bsg consists primarily of sand(surface to 15 feet bsg at CPT-3) Fine grained,high pore pressure soil predominates from 27 to 45 feet bsg on-site(15 to 32 feet bsg at CPT-3), and low pore pressure sand, locally cemented sand, increasing ' fine grained material with thin high pore pressure intervals increase toward the south (CPT-3) Q r Hydrologic Unit 1 (HU 1),at the top of the current saturated zone,extends from 55 feet to 64 feet bsg ' on-site and vanes to 69 feet bsg toward CPT-3 The soil consists of moderate to high pore pressure cemented sand, silt, silty sand, and undefined stiff fine grained material, thin low pore pressure r intervals occur south of the site Hydrologic Unit 2 (HU2) extends from 64 to 99 feet bsg on-site and vanes from 69 to 96 feet bsg at CPT-3 The soil consists of gravelly sand and sand on-site(CPT-1 and CPT-2)with increasing silt south of the site (CPT-3), the unit has very low pore pressure on-site with development of intercalated high and low pore pressure intervals at CPT-3 Advanced GeoEnvironmental,Inc r I