Occurrence of Arsenic in Nearshore Aquifers Adjacent to Large Inland Lakes.

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.