Synthesis review on groundwater discharge to surface water in the Great Lakes Basin

Groundwater in the Great Lakes Basin (GLB) serves as a reservoir of approximately 4000 to 5500 km³ of water and is a significant source of water to the Great Lakes. Indirect groundwater inflow from tributaries of the Great Lakes may account for 5–25% of the total water inflow to the Great Lakes and in Lake Michigan it is estimated that groundwater directly contributes 2–2.5% of the total water inflow. Despite these estimates, there is great uncertainty with respect to the impact of groundwater on surface water in the GLB. In terms of water quantity, groundwater discharge is spatially and temporally variable from the reach to the basin scale. Reach scale preferential flow pathways in the sub-surface play an important role in delivering groundwater to surface water bodies, however their identification is difficult a priori with existing data and their impact at watershed to basin scale is unknown. This variability also results in difficulty determining the location and contribution of groundwater to both point and non-point source surface water contamination. With increasing human population in the GLB and the hydrological changes brought on by continued human development and climate change, sound management of water resources will require a better understanding of groundwater surface–water interactions as heterogeneous phenomena both spatially and temporally. This review provides a summary of the scientific knowledge and gaps on groundwater–surface water interactions in the GLB, along with a discussion on future research directions.