A multi-tracers approach to quantifying Shield brines mixing and migration from deep to shallow aquifer systems

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Precambrian Shield aquifers, composed of igneous and metamorphic rocks, may represent a vital source of water for population, such as the large parts of Canada. The geochemistry of Shield aquifers is characterized by the presence at depth (> 1000 m) of brines, a groundwater with more than 100,000 mg/L total dissolved solids (TDS; seawater about 35,000 mg/L). Pioneering studies on Shield brines in the 1980's, driven by radioactive waste repository site investigations, used geochemistry to characterize these complex systems. In the Canadian Shield, the largest physiographic region in Canada, brackish groundwater (1000-10,000 mg/L TDS) was identified at shallow depths ( of this program is to develop approaches to trace the origin of brackish groundwater and to model the potential mixing and migration of brines into shallow aquifers. With several underground mines, including one site more than 3000 m deep, this program targets the Abitibi mining region, northwestern Quebec. The short-term objectives are 1) to identify water of various origin and mixing dynamics using innovative isotopic tracers as stable chlorine and bromine isotopes, 2) to define the Pleistocene glacial meltwater by studying water stable isotopes of Barlow-Ojibway clay pore water, and 3) to investigate the role of fractures and water/rock interaction on sources of salinity and groundwater evolution by analyzing fracture flow geochemistry and fracture minerals. Each objective will be addressed through projects of doctorate (2), master (1), and bachelor students (3). They will gain valuable, state of the art training in a field that faces a shortage of professionals in Canada. The results from this research will provide new knowledge to the scientific community and industry (mining, geothermal, nuclear) to mitigate potential impacts of induced migration and mixing of deep brines with shallow groundwater. As mines are getting deeper, the industry faces major challenges with the pumping of saline groundwater, and with the quick deterioration of equipment due to the very corrosive fluids. With Canada fostering the development of the North through sustainable use of natural resources, such a long-term vision to get a comprehensive understanding of the origin and migration of Shield brines is mandatory to assure safe drinking water to northern and First Nations communities, as well as to provide the industry with knowledge to adapt their operations, and to mitigate potential impacts on shallow groundwater resources.