HR: 0830h
AN: H21D-0865    [PDF]
TI: The Use of Hydraulic Head and Atmospheric Tritium to Identify Presence of Fractures in Clayey Aquitards: Numerical Analysis
AU: * Farah, E A
EM: efarah@uwaterloo.ca
AF: Department of Earth Sciences, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1 Canada
AU: Parker, B L
AF: Department of Earth Sciences, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1 Canada
AU: Cherry, J A
AF: Department of Earth Sciences, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1 Canada
AB: Surficial clayey aquitards can provide underlying aquifers with strong protection from contamination if vertically connected open fractures are absent. Hence, methods are needed to identify such contaminant pathways. An existing two-dimensional model for steady-state groundwater flow and solute transport (FRACTRAN) was used for cross-sectional simulations to assess the prospects for using field measurements of hydraulic head and atmospheric (i.e. bomb) tritium in surficial aquitards to determine presence and nature of hydraulically connected fractures. Simulations for a 15-m thick horizontal aquitard, with shallow water table and downward groundwater flow, show that field measurements of head and tritium at points appropriately spaced along a horizontal line at the lower part of the aquitard provide unique insight since they offer the highest chance for detecting vertical fractures. Simulations represented sets of predominant vertical and horizontal fractures of uniform aperture (25 æm) and variable length. The simulations focused on fracture-network features assigned based on the literature of field investigations. The horizontal profiles show peaks and troughs for head, and always peaks for tritium concentrations at fracture localities. Use of only head or tritium alone may locate fractures, but may not discover whether each fracture is connected to the ground surface or aquifer top, or both. On the other hand, the coupled patterns of head and tritium can be used to identify fractures more accurately. For example, a head trough and a tritium sharp peak represent a fully penetrating fracture, while a head peak and a rounded-tip tritium peak represent a partially penetrating fracture. Moreover, these two are easily differentiated from an embedded fracture that is represented by a relatively small head trough and a short sharp tritium peak. The method of monitoring along a horizontal line was applied to the conceptual 15-m thick aquitard imitating horizontal groundwater head monitoring and formation sampling events at different spacing intervals. The modeling indicates that, monitoring at 3-m spacing along a line at an elevation positioned deep in the aquitard locates and determines connectivity of most vertical fractures. Moreover, it may be possible from measured tritium distributions in aquitards to estimate the depth of hydraulically active partially penetrating fractures. Thus, horizontal monitoring is a valuable complement to vertical monitoring nests in the study of aquitard integrity.
UR: http://www.science.uwaterloo.ca/earth
DE: 1800 HYDROLOGY
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 4255 Numerical modeling
DE: 4860 Radioactivity and radioisotopes
SC: Hydrology [H]
MN: 2003 Fall Meeting