GROUNDWATER QUALITY AT BEACHES OF THE GREAT LAKES: CHARACTERIZATION AND PROCESSES

Groundwater discharge as a pathway for delivering nutrients and contaminants can significantly impact on the ecological health of the Great Lakes. Here, we present the spatial and temporal distributions of water quality parameters beneath four beaches of Lake Huron (Mountain View and Balm) and Lake Ontario (Marie-Curtis and Burlington) in Canada. The locations of the beaches range from relatively pristine rural to heavily human-impacted urban settings. To quantitatively assess the transport of nutrients from the groundwater to the lake, at each site, groundwater was sampled along a transect perpendicular to the shoreline, with additional pore water sampling on both sides of the transect to determine alongshore variability. Sampling was carried out in early-, mid- and late-summer of 2013. This study focuses on analyses of measured distributions of groundwater nutrients (C, N, P, S and Si) to delineate the relative roles of landward and lake inputs, physical mixing processes and biogeochemical reactions. End-member mixing analysis was used to determine groundwater-lake water connectivity and evaluate physical and geochemical processes based on conservative mixing of the different water sources. The results show that groundwater nitrate originating from septic tanks was effectively attenuated by denitrification in the nearshore and lake water is a source of sulfate to nearshore groundwater where it is partly consumed by bacterial sulfate reduction.