Characterization of groundwater discharge to Nottawasaga Bay, Lake Huron with hydraulic and 222Rn measurements

Groundwater discharge to the Great Lakes is not well quantified although it may be an important non-point contributor of pollutants to nearshore waters. Poor quantification is in part due to the lack of established field methods for large inland waters, particularly at the regional-scale. This paper evaluates the suitability of radon-222 (222Rn) and local-scale hydraulic gradient methods for quantifying lacustrine groundwater discharge (LGD) into eastern Nottawasaga Bay, Lake Huron, as well as the hydrogeological controls on the observed spatial variability in discharge. A regional-scale 222Rn survey was first conducted over 17 km of shoreline revealing a potential LGD hotspot area that may be linked to a sand-gravel tunnel valley aquifer system. A 222Rn mass balance model was applied to estimate groundwater discharge rates (m3 d− 1 m− 1 of shoreline) over the study area with estimates compared to those calculated based on horizontal and vertical hydraulic gradient measurements. Regional-scale electrical resistivity tomography survey results were consistent with the LGD patterns, illustrating the importance of the lake bed lithology. Overall the cumulative groundwater discharge to eastern Nottawasaga Bay is estimated to be 5-13% of the annual mean discharge from the Notttawasaga River. As pollutant concentrations can be elevated in surficial aquifers compared to tributaries, the contribution of LGD to pollutant loading may be higher than represented by this percentage. Robust approaches to estimate LGD to the Great Lakes including identification of discharge hotspots are needed to predict and effectively manage pollutant inputs to the lakes from groundwater.