Elucidating the hydrochemistry and REE evolution of surface water and groundwater affected by acid mine drainage

The impact of pyrite mining on water quality is a global concern. This study investigates the impact of acid mine drainage (AMD) from an abandoned pyrite mine in the Qinling Mountains on surface and groundwater hydrochemistry and rare earth elements (REEs) evolution. A total of 54 water samples were collected in 2021, of which the Muzi River downstream of the mining area was repeated three times in three sampling periods. Hydrogeochemical methods and stable isotope techniques were used to analyze the impacts of AMD. Results showed that tailing water in comparison to groundwater and surface waters exhibits low pH with high concentrations of SO₄^2-, potentially toxic elements (PTEs), and REEs, and is characterized by normalized middle REE (MREE) enrichment. Groundwater is less influenced by AMD and shows HCO₃-Ca and HCO₃-Ca·Na types. AMD contaminates surface water to different degrees. Surface water received SO₄^2- input from AMD, exhibited SO₄-Ca, SO₄·HCO₃-Ca, and HCO₃·SO₄-Ca types within the mining area, and evolved from HCO₃·SO₄-Ca to HCO₃-Ca downstream as AMD influence diminishes. High concentrations of PTEs and REEs are presented in AMD and seepage near the slag heap, and decreased rapidly along the flow path, while SO₄^2- migrated over longer distances. The water in the study area primarily originates from atmospheric precipitation, with close relation among surface water, groundwater, and tailing water. Water-rock interactions and pyrite oxidation governed the hydrochemical composition, with sulfide oxidation facilitated the carbonatite-water reaction, which alleviated sulfide oxidation-induced acidification. The concentrations of PTEs are regulated by adsorption and precipitation, carbonate buffering, and dilution along the flow path. REEs are mainly controlled by pH, inorganic complexation, and secondary mineral adsorption. As the pH changes from acidic to neutral or weakly alkaline, REEs shift from sulfate-complex dominated to carbonate-complex dominated. These insights contribute to a better understanding of AMD impacts on surface and groundwater, providing a basis for the rational management of AMD.