ARCHIVED REPORTS_XR0011731 - Laserfiche WebLink

Home Browse Search

' 24 October 2005 AGE-NC Project No 95-0167 Page 6 of 10 logs were Integrated with previous subsurface soil descriptions to construct the subsurface geologic model Ground water flow direction, elevation, and gradient were determined from data collected in the field during the September 2004 quarterly monitoring event The hydrocarbon Impact to soil and ground water was quantified by laboratory data obtained from the soil and ground water samples ' 31 STRATIGRAPHY In general, brown, dry to damp silty clay, clayey silt, and clay was encountered from surface grade ' to approximately 15 feet bsg From 15 feet to approximately 33 feet bsg, sandy material was encountered, grain size increased with depth,and sub-rounded pebbles were present with diameters as large as '/2 inch From 33 feet to 41 feet bsg, clay, silty clay, and clayey silt were primarily encountered Alternating layers of silty and sandy material were identified from approximately 41 feet bsg to the total depth of boring CPT-1 The CPT computer-generated boring log is included in Appendix E Cross sections B-B' and C-C' were constructed to depict the general lithology ' beneath the site (Figures 3 and 4,respectively) ' 3 2 HYDROGEOLOGIC 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 comprised of six informal zones-a vadose(unsaturated)zone and five 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 their vertical and horizontal extents A `flow unit' may be defined as a hydrologic 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 six 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 17 feet bsg, the vadose zone is ' composed primarily of silt and sandy silt underlain by an approximately 5-foot layer of clayey silt, _ silty clay, and clay This zone is characterized by low to moderate pore pressures ' HU1 - HU1 extends from approximately 17 feet to 33 feet bsg and is composed primarily of fine sand with low to moderate to pore pressures in the upper 5 feet of the zone,with sandy silt grading ' Advanced GeoEnwronmental,Inc