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Groundwater–coastal water interactions influence the fate of inorganic chemicals in nearshore aquifers and their flux to receiving coastal waters. This study evaluates the impact of variable wave conditions on the geochemical changes and distribution of mobile arsenic (As) in a nearshore aquifer. Field measurements in a sandy nearshore aquifer on Lake Erie, Canada, are presented with geochemical changes analyzed over a period of high waves. A numerical model of wave-induced groundwater flow dynamics, validated against field data, is used to provide insight into the physical flow processes underlying the observed geochemical changes. Rapid changes in dissolved As, Fe, Mn, and S demonstrate the importance of reactions as well as dynamic transport in controlling the behavior of reactive species, especially those that are redox sensitive. Field data suggest the presence of sediment traps, which under certain hydrological and geochemical conditions may result in a “hot moment” with episodic release of As. The study provides new insight into factors controlling the fate of reactive species in dynamic coastal environments as required to better predict chemical fluxes to coastal waters. Additionally, it highlights the need to pay particular attention to “hot moments” for reaction and transport caused by storms and waves.

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Metal oxides that form near sediment−water interfaces in marine and riverine settings are known to act as a sediment trap for pollutants of environmental concern (e.g., arsenic and mercury). The occurrence of these pollutant traps near sediment−water interfaces in nearshore lake environments is unclear yet important to understand because they may accumulate pollutants that may be later released as environmental conditions change. This study evaluates the prevalence of pollutant sediment traps in nearshore aquifers adjacent to large lakes and the factors that affect the accumulation and release of pollutants, specifically arsenic. Field data from six sites along the Laurentian Great Lakes indicate widespread enrichment of arsenic in nearshore aquifers with arsenic sequestered to iron oxide phases. Arsenic enrichment at all sites (solid-phase arsenic >2 μg/g) suggests that this is a naturally occurring phenomenon. Arsenic was more mobile in reducing aquifers with elevated dissolved arsenic (up to 60 μg/L) observed, where reducing groundwater mixes with infiltrating oxic lake water. Dissolved arsenic was low (<3 μg/L) in all oxic nearshore aquifers studied despite high solid-phase arsenic concentrations. The findings have broad implications for understanding the widespread accumulation of reactive pollutants in nearshore aquifers and factors that affect their release to large lakes.