ARCHIVED REPORTS XR0001662 - Laserfiche WebLink

Home Browse Search

n I 1 Y� 14 September 2004 / . 'AGE-NC Project No 96-0235 " ' Page 4 of 8 1 _ r ` ' 34 STRATIGRAPHY Based on the findings from the CPT soil borings and existing monitoring well'and soil borings,cross sections A-A' through D-D' were constructed to depict general lithology beneath the site(Figures 5 through 8) The lines of section are illustrated on Figure 2 ; s r i ' Dominantly fine-grained soil consisting of silt, clayey silt, silty clay and clay were encountered at depths from the near surface to approximately 210 feet bsg Significant sand units with variable silt to gravel were encountered between 20 feet and 40 feet bsg, 62 feet to 90 feet bsg and 165 feet to 191 feet bsg The shallow sand unit(20 feet to 40 feet bsg)appears to have limited lateral continuity The sand unit encountered at 165 feet bsg appears to have lateral continuity and a documented thickness of greater than 40 feet Lessor, thin, laterally discontinuous sand units were encountered ' at approximately 50 and 100 feet bsg at the southern UST No 3 area CPT computer-generated boring logs are included in Appendix B Geologic cross sections A-A' through D-D' are included as Figures 5 through 8 3 2 HYDROGEOLOGICAL MODEL lie Based on the CPT results, AGE has developed a preliminary subsurface model of the site sediment stratigraphy comprised of'informal zones including a vadose (unsaturated) zone and various hydrogeologic units Hydrogeologic or hydrostratigraphic units (HUs) can be defined as a body of sediment that has hydrogeologic characteristics(I e hydraulic conductivity,pore pressure, etc )that make it distinct from surrounding bodies of sediment(Sanders, 1998), or a part of a body sediment , that forms a distinct hydrologic unit with respect to the flow of ground water(Maxey, 1964) A`flow unit' may be defined as a hydrogeologic 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 _ "' Based on the CPT results,AGE has identified seven informal zones,a vadose(unsaturated)zone and _ six hydrogeologic units-as.follows Vadose Zone - Extending from surface grade to approximately 18 feet bsg, the depth to the water table The vadose zone is predominantly fine-grained material from surface grade to 20 feet bsg The zone is characterized by low pore pressures Hydrogeologic Unit 1 (HU1), at the top of the current saturated zone, extends from 18 feet bsg to Ia maximum depth of 40 feet bsg and is comprised predominantly of clay and fine sand, Interbedded with silt HU1 is characterized by generally low to moderate pore pressures Advanced GeaEnvironmental,Inc v 1 i