HR: 14:40h
AN: H52B-05    [PDF]
TI: Representing Regional-Scale Fracturing in Groundwater Studies: Field Data to Conceptual Model
AU: * Allen, D M
EM: dallen@sfu.ca
AF: Simon Fraser University, Department of Earth Sciences 8888 University Drive, Burnaby, BC V5A 1S6 Canada
AU: Mackie, D C
EM: dmackie@srk.com
AF: SRK Consulting Ltd., Suite 800, 1066 West Hastings St., Vancouver, BC V6E 3X2 Canada
AB: Determining the sustainability of regional aquifers requires understanding of the controlling geologic architecture, hydrologic conditions, and water use information. In fractured aquifers consideration must be given to the nature of the fracture network at all scales, which creates a hierarchy of secondary permeability. Site-scale investigations typically provide detailed data that, while important at a local scale, may not fully represent the heterogeneity imparted by large-scale structural elements such as fracture zones, faults, and folds. Groundwater resources on the southern Gulf Islands in southwestern British Columbia are derived from fractured sedimentary bedrock aquifers. The islands consist of a thick sequence of alternating sandstone- and mudstone-dominant formations, which have been folded and extensively fractured during several tectonic events. A fracture-mapping program was undertaken to identify and quantify fracture distribution around faults and folds and to qualitatively analyze lithology-related fracture variation. Results indicate that tectonically-related fracturing is heterogeneously distributed and that fractures concentrations are highest around individual fault and joint zones. Fracturing is pervasive in the mudstone-dominant formations and fine-grained interbeds within sandstone-dominant formations, and therefore, the mudstone-dominant formations may act as the primary aquifers on the islands. In contrast, the sandstone-dominant formations have low primary porosity, and unless fractured, are considered relatively impermeable. Pumping test data show hydraulic responses to be strongly influenced by linear flow, particularly in wells situated adjacent to or close by fracture zones and faults, which themselves could be identified by surface geophysical surveys, geologic investigations, and on air photos. At the site scale, borehole flow meter experiments, examined in conjunction both with borehole video logs and the standard suite of borehole geophysical logs, indicate that fluid flow (both fresh and saline) is primarily through fine-grained interbeds or through discrete narrow vertical fractures in the sandstones. Major ion chemistry indicates that cation exchange, whereby calcium exchanges for sodium, is dominant throughout the islands, providing further evidence that the groundwater flows predominantly through fine-grained units. A conceptual model is presented which attempts to integrate the observed scale-related fracturing, and which may be applicable for representing regional-scale fracturing for groundwater studies.
DE: 1829 Groundwater hydrology
SC: Hydrology [H]
MN: 2003 Fall Meeting