Water Quality Impacts of Legacy Ion-Adsorption Rare Earth Element Mining and Processing.

In situ leaching of ion-adsorption rare earth deposits (IAREDs) in southern China creates evolving, multi-contaminant legacies that differ markedly across the mining lifecycle. This study examines the spatiotemporal hydrogeochemical patterns in a representative watershed, spanning active mining areas, abandoned areas undergoing natural recovery, and downstream residential areas receiving lime-treated effluent. In active mining areas, the injection of MgSO4 lixiviant rapidly mobilizes rare earth elements (REEs) and potentially toxic elements (PTEs, e.g., Cd and Pb) mainly through cation exchange and sulfate complexation, with colloidal transport substantially enhancing REE mobility during the wet season. In abandoned areas, years after mining ceased, sustained low pH from nitrification drives a slower, seasonally pulsed release of legacy contaminants: REEs are primarily released through pH-dependent dissolution in wet periods, while Pb shows prolonged persistence linked to gradual dissolution of residual phases. Downstream, lime-based treatment effectively removes most REEs and many PTEs via co-precipitation, yet it fails to sequester mobile thallium (Tl), resulting in a more focused but persistent contamination signature. Compared to previous IARED hydrogeochemical studies that mainly describe active-phase leaching or single contaminants, this work provides new evidence of lifecycle-dependent contaminant behavior, including the previously under-recognized importance of colloidal transport in active areas, nitrification-driven legacy release in abandoned areas, and post-treatment thallium persistence, and offers a conceptual basis for zone-specific monitoring and risk management in similar watersheds.