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

The Montérégie Est regional rock aquifer system covers about 9000 km2 in an area located south-east of Montreal. It includes three watersheds, namely those of the Richelieu and Yamaska Rivers, and the Missisquoi Bay. 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 and faulted, and affected by
low-grade metamorphism. Lithologies and structural contexts are expected to influence groundwater flow of this hydrogeological system. The groundwater resource assessment project in Montérégie Est 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 hydraulic tests from consultant reports, as well as field data on rock fractures. Fieldwork consisted in borehole geophysical logging, especially acoustic televiewer logs and
flowmeter testing, and outcrop observations and measurements. The methodology involves two steps. First, the fracture pattern was characterized using borehole televiewer and outcrop data. Fracture sets were defined within the regional aquifer, which allowed for the interpretation of the various
structural contexts. Preferential orientations for open fractures were identified and then compared to the in-situ stress field. Secondly, hydraulic tests carried out in the study area were compiled, analysed, and compared for each structural context. The drawdown behavior provided information on
the potential fracturing contribution to fluid flow. Hydraulic properties interpreted from the tests helped quantify the aquifer anisotropy induced by dominant open fracture sets in the different structural domains of this study area.

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