HR: 0800h
AN: H41B-0412    [Abstracts]
TI: Fracture Partitioning and its Controls on the Distribution of Transmissivity in Sedimentary-Rock Aquifers, Chateauguay River Basin, Province of Quebec
AU: * Morin, R H
EM: rhmorin@usgs.gov
AF: U.S. Geological Survey, MS 403, Denver Federal Center, Denver, CO 80225 United States
AU: Godin, R
EM: rejean_godin@uqac.ca
AF: Université du Québec à Chicoutimi, 555, boulevard de l'Université, Chicoutimi, PQ G7H 2B1 Canada
AU: Nastev, M
EM: mnastev@nrcan.gc.ca
AF: Geological Survey of Canada, 490, rue de la Couronne, Québec, PQ G1K 9A9 Canada
AU: Rouleau, A
EM: Alain_Rouleau@uqac.ca
AF: Université du Québec à Chicoutimi, 555, boulevard de l'Université, Chicoutimi, PQ G7H 2B1 Canada
AU: Lamontagne, C
EM: charles.lamontagne@mddep.gouv.qc.ca
AF: Ministère du Développement durable, Environnement et Parcs, 675, boulevard René Lévesque Est, Québec, PQ G1R 5V7 Canada
AB: The Chateauguay River Basin in the Province of Québec is underlain by sedimentary rocks of Lower Paleozoic age. These gently dipping sandstones and dolostones are interspersed with orthogonal sets of subhorizontal bedding-plane partings and subvertical joints. Geophysical logs collected from 13 boreholes were used to characterize the three primary rock units: (1) relatively soft arkosic sandstones, (2) well-cemented quartz-rich sandstones, and (3) dolostones with dolomitic sandstones. Analysis of the resistivity, natural gamma, and compressional-velocity logs as well as examination of borehole images recorded with an acoustic televiewer identify a fracture partitioning scheme unique to each formation, where joint spacing and bed thickness are related to lithologic properties. This finding is further corroborated by fracture measurements at outcrops and quarries. Results of hydraulic tests in these boreholes using flowmeter and straddle-packer techniques confirm that ground-water flow is predominantly controlled by distinct fractures that have transmissivities several orders of magnitude greater than those of the competent rock matrix. Moreover, there appears to be a correlation between the spatial distribution of these permeable features and the rock unit. In the softer sandstones, beds are more massive and joints are sparse. On average, only one permeable zone associated with a particular bedding-plane parting is identified for every 30 meters of depth, although it may extend laterally for several kilometers. The hard sandstones have thinner beds and more closely spaced joints. Permeable zones are intercepted more frequently (approximately 5 per every 30 meters of depth), but hydraulic communication appears to be more limited laterally. Finally, dolostones have even thinner beds and more closely spaced joints; the vertical distribution of permeable zones resembles that of the softer sandstones but no extensive horizontal connection between boreholes has been observed. Thus, the three primary rock units in this study area each exhibit distinctive fracture partitioning patterns. These fracture networks produce bulk transmissivities that are similar for all formations. However, the spatial distribution of this transmissivity is markedly different for each rock unit.
DE: 1829 Groundwater hydrology
DE: 1835 Hydrogeophysics
DE: 1895 Instruments and techniques: monitoring
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
SC: Hydrology [H]
MN: Fall Meeting 2005