ARCHIVED REPORTS_XR0003068 - Laserfiche WebLink

Home Browse Search

1 ' 11 November 2004 AGE-NC Project No 99-0624 ' Page 5 of 13 ' having significant lateral extent and having similar hydrogeologic characteristics (i e hydraulic conductivity, transmissivity, etc ) over Its vertical and horizontal extent A `flow unit' may be defined as a hydrogeologic unit characterized by significantly greater hydraulic conductivity values ' than the 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 five 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(estimated depth to the water table) The uppermost 7 feet is silt,underlain by predominantly coarse-grained sediment(sand, cemented sand ' and gravel)from 7 feet to 20 feet bsg Below 20 feet bsg are alternating layers of generally thick,stiff fine-grained sediments(up to 10 feet thick)and thin(less than 4 feet thick)sand,cemented sand,silt and clay The pore pressure in this zone is uniformly low except for thin,high pore pressure intervals ' encountered at cemented sand or at a few clayey silt or stiff fine-grained layers ' Hydrogeologic Unit 1 (HUI) -At the top of the current saturated zone,extends from approximately 55 feet to 65 feet bsg and is comprised primarily of stiff fine-grained sediments (up to 5 feet thick) and thin(less than 4 feet thick) sand, cemented sand, silt and clay The pore pressure is low except ' for moderate pressures encountered at cemented sand or stiff fine-grained layers Hydrogeologic Unit 2(HU2)-Extends from 65 feet to 75 feet bsg and is composed ofpredominately very stiff fine grained silty clay to clayey silt The pore pressure is consistently moderate to high throughout the unit Hydrogeologic Units 3 (HU3) - Extends from 75 feet to 90 feet bsg and is composed of silty sand to poorly graded fine to medium sands The unit has characteristically low poor pressures common in water bearing zones Hydrogeologic Units (HU4) - Extends from 90 feet to total depth of the data and is composed of silty clay and clayey silt The unit has moderate to high poor pressures that may retard the migration Iof contaminants Vertical soil lithology is depicted in cross sectional view in Figures 3 and 4 Advanced GcoEnvironmental,Inc