Groundwater-lake interactions on water quality in the nearshore zone of the Great Lakes: A regional scale modeling approach

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Canada has about half of the Great Lakes' shoreline. Near-shore areas are especially prone to deterioration of water quality and environmental problems, such as algal blooms and beach closures caused by fecal contamination. Maintaining the health of the Great Lakes brings large economic benefits to Canada: the economic value of recreation-related activities alone has been estimated at more than $2 billion per year. While the importance of groundwater discharge for river flow is well recognized for watersheds around the Great Lakes, direct input of groundwater to the lakes remains poorly constrained, with estimates ranging from a few percent to over 50% of river discharge in some areas. Groundwater discharge further transports nutrients and pollutants from land to the near-shore zone, with potentially severe ecological impacts. Key challenges in determining the role of groundwater discharge are (1) to accurately estimate the lake-ward flow and chemical composition of the groundwater, (2) to account for the biogeochemical transformations taking place in the near-shore mixing zone between ground- and lake-water, and (3) to develop the capability to forecast how land-use changes, water management, climate change and lake water level changes modulate the discharge of groundwater and the associated material fluxes. While the number of studies on submarine groundwater discharge in coastal marine environments is rapidly increasing, much less research is being carried out for large inland water bodies. Because of great length and heterogeneity of the shoreline, and the large fraction of ungauged basins bordering the Great Lakes, estimations of regional groundwater inputs require innovative data assimilation and modeling approaches. The proposed Discovery Grant research will create and use a groundwater flow and reaction model framework to fill the current knowledge gap. The model will be applied to the Canadian side of the Great Lakes, and focus on the land to lake transfers of macronutrients (N, P and Si), which are arguably one of the major anthropogenic pressures on freshwater bodies. Regional groundwater flow model calculations will be coupled to biogeochemical reaction modules for the nutrient elements. Particular attention will be given to the near-shore subsurface reaction zone where groundwater and lake water meet, and the analysis of human-impact scenarios (fertilizer application, coastal development, groundwater extraction, lake level changes). The modeling results will set the stage for assessing the implications of groundwater discharge for water quality and ecosystem health of the Great Lakes and, hence, formulating guidelines for monitoring and water management and governance.