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' 23 May 2003
<br /> AGE-NC Project No 95-0184
<br /> IPage 4 of 9
<br />' laterally discontinuous units less than 5 feet thick, these layers may be more prevalent and coarser
<br /> (gravelly sand identified at 95 feet) with depth CPT computer-generated boring logs are included
<br /> in Appendix A
<br /> 3 2 HYDROGEOLOGICAL MODEL
<br />' Hydrologic units may be defined as significant vertical intervals of sediments having significant
<br /> lateral extent and having similar hydrologic characteristics (i e hydraulic conductivity,
<br /> I transmissivity, etc ) over its vertical and horizontal extent A `flow unit' may be defined as a
<br /> hydrologic unit and is characterized by significantly greater hydraulic conductivity values than
<br /> overlying and underlying units A flow unit is capable of serving as a preferred migration route, and
<br /> Icould potentially allow migration of large contaminant masses over great distances
<br /> Based on the limited CPT data, AGE has developed a preliminary site hydrogeologic model
<br /> Icomprised of fifteen informal zones, a vadose (unsaturated)zone and fourteen hydrologic units,the
<br /> model presented is interpretational and will be modified as more data is collected The fifteen zones
<br /> lieare as follows (depths are approximate and can vary by several feet across the site)
<br /> Vadose Zone - Extending from surface grade to approximately 30 feet bsg The vadose zone Is
<br /> Ipredominantly fine grained material, with clay content apparently increasing eastwards (Figure 3)
<br /> Hydrologic Units 5, 7,9, 11 and 13 (HU5,HU7 and HU9, HUI I and HU13) are sand-bearing units
<br /> characterized by low CPT pore pressures (Appendix A) HU5 and HU9 are sand units, HU7,HU11
<br /> and HU13 are composed of inter-bedded layers of sand, sandy silt, and stiff, fine-grained soil
<br /> Hydraulic conductivities,based on CPT soil behavior types(SBT),range between 1 X 10 6 to 1 meters
<br /> I per second (m/s) for sandy silt to gravelly sand, and between 1 X 10-9 to I X 10' m/s for stiff, fine-
<br /> grained soil (Appendix A)
<br /> Hydrologic Units 1, 3 and 10(HU1,HU3 and HU10) are composed of clay or inter-bedded silt and
<br /> clay,with minor layers of clayey silt and stiff, fine-grained soil These intervals are characterized by
<br /> low pore pressure in the clay and silt, with variable pore pressure, from low to moderately low, in
<br /> I the stiff, fine-grained soil Hydraulic conductivities range between i X I Wo to 1 X 10'm/s for clay to
<br /> silt, and between 1 X 10 9 to 1 X 10'm/s for stiff, fine-grained soil (Appendix A)
<br /> IHydrologic Units 2, 4, 6, 8, 12 and 14 (HU2, HU4, HU6, HU8, HU12 and HU 14) are composed
<br /> dominantly of,silty clay,clayey silt, stiff, fine-grained soil, and minor cemented sand and clay Pore
<br /> pressures In these intervals tend to be moderate to high, the relatively lower pore pressures are
<br /> associated with the sand and clay, whereas the higher pore pressures are associated with the silty
<br /> . clay/clayey silt Hydraulic conductivities generally range between I X 10-9 to 1 X 10-'m/s,with higher
<br /> Advanced CeoEnvironmental,Inc
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