Role of groundwater-lake interactions on the behavior of chemicals in a nearshore aquifer
Abstract
It is well recognized that groundwater-surface water interactions play an important role in controlling the flux of groundwater-derived inorganic chemicals to surface waters. While these interactions have been well-studied in marine, and river and stream settings, the influence of these interactions on chemical fluxes to large lakes is poorly understood. Nearshore aquifers adjacent to inland coastal waters, such as the Great Lakes, are characterized by dynamic flow patterns and the mixing of groundwater and lake water, which have distinct chemical compositions. This sets up an important reaction zone in the nearshore aquifer that may modify the fate of groundwater-derived chemicals discharging to the lake. The overall thesis objective was to evaluate the interacting hydrologic and geochemical processes near the groundwater-lake interface and their impact on the accumulation, transformation and mobility of inorganic pollutants (arsenic and nutrients).
Field investigations combined with numerical modelling illustrate that transient forcing, in particular varying wave conditions and lake water levels, may influence the sequestration and mobility of inorganic chemicals in the reaction zone near the groundwater-lake interface. Field data further indicate that nearshore aquifers on the Great Lakes are naturally enriched with arsenic. Arsenic enrichment may be due to continual adsorption of arsenic onto metal (mostly iron) oxides that precipitate below the groundwater-lake interface as lake water with trace arsenic concentrations recirculates across the interface. While changes in lake water levels, and redox and pH conditions may trigger the release of sequestered arsenic to nearshore waters it depends on the geochemical conditions in the aquifer. Finally, geochemical processes in the reaction zone near the groundwater-lake interface are shown to affect nutrient fate close to the interface and should be considered in estimating nutrient fluxes to large lakes. Temporal variability in the geochemistry in the reaction zone due to seasonal changes in dissolved organic carbon availability was shown to cause variable nitrogen flux to nearshore waters. While this thesis focuses on arsenic and nutrients, the findings are relevant for understanding the fate and transport of a wide range of reactive groundwater pollutants and their flux to large lakes. The thesis findings are needed to (i) better quantify the role of groundwater as a pathway for delivering pollutants to large lakes, (ii) improve future predictions of chemical cycling in nearshore lake environments, and (ii) inform lake water quality management programs.