Geochemical behavior of rare earth elements in mining-affected waters, southwest China

Mining activities have led to significant rare earth elements (REEs) contamination and ecotoxicological risks in aquatic systems. However, the concentration, speciation, and primary controlling factors of REEs in aquatic systems in southwest China have remained unclear. This study investigated the water geochemistry, concentration, speciation, fractionation patterns, and anomalies of REEs in the surface water, shallow groundwater, and deep groundwater within a mining-impacted catchment area in southwest China across different seasons. REE concentrations were in the range of 1.12-73.66 μg/L in surface water, 0.09-96.58 μg/L in shallow groundwater, and 0.09-1.05 μg/L in deep groundwater. Seasonal variations significantly impacted REE concentrations in surface water and shallow groundwater. The variability of environmental parameters, driven by higher temperatures during the wet season, significantly controlled the increases in the proportions of dissolved REE in both surface water and groundwater. The water recharge between surface water and groundwater resulted in analogous REE fractionation patterns and anomalies. Variation in pH, oxidation-reduction potential, iron, manganese and aluminum content in water influenced the fractionation indices of REEs. Cerium and europium anomalies in water samples provided insights into REE sources and associated hydrological dynamics. Our results demonstrate that REE fractionation patterns and anomalies were influenced by water-rock interactions, mining activities and hydrogeological factors, which facilitates a comprehensive understanding of the hydrogeochemistry process in aqueous systems.