EFFECT OF GROUNDWATER-LAKE INTERACTIONS ON THE DISTRIBUTION OF ARSENIC IN FRESHWATER BEACHES

Groundwater discharge may be an important pathway for delivering contaminants to the Great Lakes. Complex and dynamic groundwater flow and geochemical processes occurring close to the groundwater-lake interface strongly control the transport and fate of heavy metals in the near-shore aquifer. Groundwater samples collected at two nearby freshwater beaches on Lake Erie from August 2011 – August 2012 reveal elevated concentrations of heavy metals, particularly arsenic (> 30 µg/L), in the groundwater directly below the shoreline. The shoreline location is also characterized by the highest groundwater discharge rates. The distributions of dissolved iron, manganese and sulphate coupled with sediment analysis suggest that the arsenic concentrations are strongly controlled by iron and manganese redox cycling. High arsenic concentrations are present at both beach sites despite distinctively different landward groundwater chemistry. Numerical groundwater simulations show that wave action may lead to significant recirculation of lake water through the nearshore sediments and this is likely to significantly influence the redox processes and subsequent arsenic mobility. The near-shore groundwater flow direction at the sites also reverse depending on the seasonal lake and groundwater levels and the specific wave conditions. If the elevated arsenic concentrations in the groundwater at this site are naturally-occurring, they may also be elevated at other sandy shorelines along the Great Lakes. This may have important implications for the Great Lake chemical budgets as changing lake hydraulic conditions may cause the release and subsequent discharge of arsenic and other chemical species.