Rare Earth Elements Concentration Patterns in Surface Waters of Lakes Ontario, Erie, and Huron (North America)

Abstract

The North American Great Lakes constitute a distinctive hydrological system comprising five interconnected lakes (Superior, Michigan, Huron, Erie, and Ontario) that together represent one of the planet's most significant freshwater reserves. Extensive environmental surveillance by federal, state, and provincial governments targets major water quality parameters such as temperature, pH, total dissolved solids, electrical conductivity, and dissolved oxygen, as well as concentrations of nutrients and major ions. However, trace element concentrations are more scarcely measured, and the comparatively little available data on trace element concentrations in the Great Lakes is typically older, discontinuous, or focused on historically contaminated areas. Consequently, the myriad of processes and sources involved in the distribution patterns of trace elements is poorly studied, and there remains a lack of understanding the natural baselines for these elements, including for the Rare Earth Elements (REE). The REE play a crucial role in various technological applications, including electronics, renewable energy technologies, and other high-tech industries. Because of their increasingly applications, REE are currently a significant concern, particularly in mining and industrialized areas, due to their enduring toxicity, radioactive properties, and the potential for bioaccumulation. To understand the REE distribution pattern in the North American Great Lakes, we assessed REE concentrations in >70 surface water samples from Lakes Huron, Erie, and Ontario. The concentrations of dissolved REE, filtered at <0.22 µm, exhibited significant spatial heterogeneity across the lakes, with higher ΣREE values in Lake Huron (0.065±0.082 μg/L, n=27, 2022) than in Lake Erie (0.041±0.033 μg/L, n=14, 2021 and 2022) and Lake Ontario (0.033±0.041 μg/L, n=27, 2021 and 2022). Interestingly, there was no consistent upstream-to-downstream increase in dissolved REE concentrations within the basin, but dissolved REE levels decreased nearshore-to-offshore across all lakes. Enrichment of light REE over heavy REE, particularly in samples closer to the shore, was suggestive of riverine inputs and aqueous speciation modeling indicated strong control of speciation (hydrochemistry) on REE dynamics. Finally, we employed normalization and pattern-filling to assess REE enrichments in lake surface waters. Anomalies for Gadolinium (Gd), exceeding 20%, on average, across the lakes, were notably higher than for other REE but exhibited significant spatial variability, with enrichment observed especially in proximity to urban centers and in Lake Ontario. This research contributes valuable baseline data, enhancing our understanding of the dynamics of Rare Earth Elements in the Great Lakes and providing a foundation for further studies worldwide.