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' 06 June 2003 AGE-NC Project No 95-0135 ' Page 4 of 7 - s P ' 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, transmissivity, etc ) over its vertical and horizontal extent A `flow unit' may be defined as a '3 - hydrological unit and is cfiaractenzed by sigmficantly 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) Vadose Zone - Extending from surface grade to approximately 25 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 coarse grain sand unit occurring between 25 feet and 30 feet bsg at CPT3 off-site The zone is characterized by low pore pressures Hydrological Unit 1 HU1 at the to of the c � ), p orient saturated zone, extends from 25 feet to approximately 45 feet bsg and is comprised predominantly of clay,interbedded on the southwest and on the northeast with clayey silt and silt Dominant hydraulic conductivities, based on CPT soil behavior types (SBT), range between 1 x 10") and 1 x 10'meter per second (m/s) for clay-to clayey silt-type soil, but can be as high as 1 x 10-4m/s in laterally discontinuous silts and sands (Table Al in Appendix A) This interval mcharacterized by generally low pore pressures I Hydrological Unit 2 (HU2) extends from 45 feet to approximately 90 feet bsg and is composed dominantly of stiff, fine-grained soil, clayey silt, silt and lesser amounts of clay Hydraulic conductivities for these soil types generally range between 1 x 10-10 and 1 x 10-6 m/s for clays,silts and stiff, fine-grained soils (Table A1),-although a high hydraulic conductivity of between 4x 10' and 1 x 10-3 m/s was determined from a pore pressure dissipation test in CPT-defined, stiff, fine-grained- type soil at 80 feet bsg(CPT-01) HU2 is characterized by varying pore pressures ranging between moderate and high Hydrological Unit 3 (HU3)extends from 90 feet bsg to total depth,HU3 is composed of interbedded stiff, fine-grained soil, silty and gravelly sand, clayey silt and silty clay from 90 feet bsg to approximately 110 feet bsg and sand, silty sand and gravelly sand from 110 feet bsg to total depth Hydraulic conductivities for sand, silty sand and gravelly sand generally range between 1 x 10-5 and I10 m/s(Table Al),hydraulic conductivities ofbetween 1 x 10-4 and 1 x 10-z m/s were determined from pore pressure dissipation tests in CPT-defined sand at between 116 and 118 feet bsg (CPT-01 and CPT-04) HU3 is characterized by generally low pore pressures, and maybe classified as a flow unit I i • f Advanced GeoEnvironmental,inc r x r