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Groundwater–surface water interactions in Canada

Canada is a water-rich country. Its rivers and lakes comprise 7% of the global annually renewable surface fresh water supply (Rivera 2014). Although it is very difficult to quantify available groundwater resources, it is estimated that there is more groundwater in Canadian aquifers than surface water in rivers and lakes, similar to what is observed elsewhere (Rivera 2014). In the generally humid and cold Canadian climate, groundwater–surface water interactions are omnipresent (Devito et al. 1996; Langston et al. 2013; Brannen et al. 2015; Foster and Allen 2015), but remain relatively seldom studied. The Canadian Council of Academies (CCA 2009, 185) expert panel on groundwater concluded that a sustainable use of groundwater resources requires that “groundwater and surface water be characterised and managed as an integrated system within the context of the hydrological cycle in a watershed or groundwatershed.” It is only reasonable to expand this recommendation to surface water resources. This has been a growing concern in the last two decades as groundwater and surface water are increasingly considered a single resource (Winter et al. 1998). Using integrated approaches to understanding and managing water resources is increasingly important given the growing demand for water related to development, industry and agriculture (United Nations Educational, Scientific and Cultural Organization [UNESCO] 2015), and the superimposed stress of a changing climate (Intergovernmental Panel on Climate Change [IPCC] 2014). These issues are relevant in a variety of geological and climate conditions around the world, especially in densely developed areas and in dry climates (IPCC 2014). However, the stresses on groundwater–surface water connections should not be overlooked in water-rich countries such as Canada, despite water scarcity not imposing a significant hindrance to development in most regions. It has been reported that some Canadian regions have only limited groundwater and surface-water reservoirs (Cook and Bakker 2012). Moreover, water quality is a growing concern that will be exacerbated in a changing climate (e.g. Crossman et al. 2013), and groundwater-dependent ecosystems are increasingly threatened by human development (e.g. Smerdon et al. 2012). Groundwater–surface water interactions are most often thought of as the connection between aquifers and rivers. In temperate and humid climates, such as those found in Canada, groundwater most often flows into rivers, where it contributes to maintaining river low flows during dry periods, whereas the opposite is more common in dry climates. This groundwater influx provides thermal refugia and supplies nutrients for a variety of plant and animal species (e.g. Kurylyk et al. 2015). Groundwater–wetland and groundwater–lake connections are expressions of similar processes where an aquifer provides water to a shallow surface-water reservoir, or vice versa (e.g. Winter 1999). When water flows from an aquifer to a wetland or a lake, it regulates humidity and temperature conditions which sustain rich ecosystems (e.g. Hoffmann et al. 2009). When water flows from a wetland or lake to an aquifer (typically in drier climates), it contributes to the recharge of underground reservoirs. Submarine groundwater discharge (SGD) is yet another flowpath through which aquifers provide water to marine environments (e.g. Burnett et al. 2003). Salt-water intrusion is the corresponding opposite pathway of this connection (e.g. Ferguson and Gleeson 2012). SGD is extremely important in coastal areas around the world, where a large proportion of the world’s population lives. Considering the importance of groundwater–surface water interactions, relatively little is known about the geological and climate conditions under which these exchanges prevail, how and where they occur, and what affects their stability. More knowledge on this topic is necessary to provide essential information on Canadian water resources, and crucial new data for integrated water resources management. The purpose of this Special Issue is to highlight the variety of issues related to groundwater–surface water interactions, and recent