Groundwaters at depths of a km or more make up a substantial fraction of the Earth’s water stores but are tenuously connected to the rest of the hydrologic cycle, having been isolated from the Earth's surface for greater than a million to even a billion years in some regions. These waters contain active microbial communities that may provide important insights into evolution and the potential for life on other planets. This underexplored zone of the Earth’s hydrosphere is key to understanding the distribution and biodiversity of the subsurface biosphere, and for evaluating the impact of anthropogenic activities into the future. Our current understanding of deep groundwater flow systems is poor, particularly in the crystalline rock that makes up the bulk of the continental crust. Historically, permeability-depth relationships have been invoked to explain the long residence times found in these systems but it is unclear whether these low permeabilities would have persisted over geologic time due to the dynamic nature of this property. The negative buoyancy of the highly saline waters found at these depths provides an additional trapping mechanism. Examination of existing data from the Canadian Shield suggests that the world’s oldest groundwaters are currently trapped due to a combination of fluid density and low topographic gradients, regardless of the permeability present.
Grant Ferguson holds a B.Sc. in Honours Geology from the University of Waterloo and a Ph.D. in Civil Engineering from the University of Manitoba. He is a Centennial Enhancement Chair and Associate Professor in the Department of Civil, Environmental and Geological Engineering and School of Environment and Sustainability at the University of Saskatchewan and an Adjunct Associate Professor at the University of Arizona. His research focuses on hydrogeology and hydrogeochemistry of regional groundwaters systems. He was the 2019 recipient of Global Institute for Water Security’s Research Excellence Award and is the past president of the International Association of Hydrogeologists – Canadian National Chapter.