Fracture systems controls on fluid flow in the regional sedimentary rock aquifer system of Montérégie Est, southern Québec, Canada

Pierre Ladevèze, Marc Laurencelle, René Lefebvre, Alain Rouleau, Heather Crow, Christine Rivard

Dans les comptes rendus d’articles de la conférence: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Regional Aquifer Characterization II

ABSTRACT: The Montérégie Est regional rock aquifer system covers about 9,000 km2 in an area located south-east of Montreal. It includes two watersheds, namely those of the Richelieu and Yamaska Rivers. At the regional scale, groundwater is flowing from the Appalachian Uplands through the St. Lawrence Lowlands. The sedimentary rocks in Montérégie Est are fractured and their structural properties are quite diverse. In the St. Lawrence Lowlands, rocks are slightly deformed, while rocks in the Appalachians are folded, faulted, and affected by low-grade metamorphism. Lithologies and structural contexts are expected to influence groundwater flow of this hydrogeological system. In Montérégie Est, fractured-rock aquifers are a prime target for water supply in comparison with granular aquifers. Almost 75% of the municipal wells are exploiting the regional fractured-rock aquifer. In those wells, investigations made by consultants show that most of the yield was provided by the fracture network. A regional groundwater resource assessment project in Montérégie Est was supported by a program of the Quebec Environment Ministry. The project includes several interrelated studies, one of which is the characterization of the controls exerted by geological structures and fracture networks on groundwater flow. Data collected for this specific study include existing hydraulic tests documented in hydrogeological consultant reports, as well as newly acquired field data on rock fractures. Fieldwork consisted in borehole geophysical logging, especially acoustic televiewer logs, providing images of borehole walls allowing the identification of fractures (on 30 wells distributed over the whole study area), and fracture observations and measurements on outcrops and quarry walls (on 19 locations). The methodology of the study involved two steps. First, fracture patterns were characterized using borehole televiewer and outcrop data. Then, those patterns were compared to hydraulic data to provide a conceptual hydrogeologic framework of the effect of fractures on groundwater flow through the regional fractured-rock aquifer. Two distinct fracture patterns were identified within the structural domains of the St. Lawrence Lowlands and Appalachian Uplands. Within those two structural domains, orthogonal fracture systems were observed. Outcrop measurements have clarified this pattern with the measurement of vertical fractures that cannot be sampled by vertical boreholes. Identifying fracture sets striking parallel to the actual average maximum horizontal stress direction showed that there are probably some fracture sets which can be held open. Using borehole logs, fracture density with depth was also assessed. Most of wells deeper than 20 m show a decrease in fracture density within the first 100 m of depth below the rock subcrop. For some wells, the televiewer accuracy allowed the identification of open fractures interpreted as potential pathways for fluid flow. According to those measurements, this decreasing trend of open fracture density with depth is not always valid. Data from hydraulic testing and various borehole logging carried out by consultants in the study area were compiled, analyzed, and compared with the fracture patterns. The drawdown behavior provided information on the potential contribution of fracturing to groundwater flow. Hydraulic properties interpreted from the hydraulic tests helped quantify the aquifer anisotropy induced by dominant open fracture sets in the different structural domains of Montérégie Est. Those comparisons also provided indications as to the applicability, or not, of an equivalent porous medium for fluid flow in the regional rock aquifer system. This study also provided information regarding the trends with depth of fracture patterns and hydraulic properties.

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