Hydrologic controls on the fate of contaminants at the groundwater-lake interface at freshwater beaches

English Abstract:

The role of groundwater-surface water interactions in controlling the delivery of contaminants to surface waters in freshwater beach environments is not well understood. These environments are ubiquitous on large inland lakes and nearshore water quality is often deteriorated due to fecal pollution (e.g., E. coli) and algal fouling. Field data obtained at sandy beaches on the Great Lakes provide important insight into the important influence of dynamic groundwater-lake interactions on the fate of contaminants, including trace elements (arsenic), nutrients, and fecal indicator bacteria (E. coli), in beach groundwater and subsequent fluxes to the lake. The data illustrate the role of wave-induced groundwater recirculations in accumulating E. coli in shallow foreshore beach sand and groundwater, and, in combination with sand erosion, later delivering E. coli from this reservoir back to the lake during periods of intensified wave activity. In addition, wave-induced recirculations across the sediment-water interface, which vary with wave intensity, set up sharp redox and pH gradients in beach groundwater that promote precipitation of iron (hydr)oxides and subsequent sequestration of chemical species including arsenic and phosphate. For instance, elevated dissolved arsenic below the shoreline (up to 56 μg/L) has been observed in beach groundwater at multiple beaches around the Great Lakes suggesting that this arsenic enrichment may be naturally occurring and linked with geochemical conditions set up by wave-induced recirculations. The interacting hydrologic and geochemical processes revealed have important implications for the flux of a wide range of contaminants across the groundwater-lake interface in freshwater coastal environments.