<p>The environmental assessment of ammonium sulfate-treated rare earth ore mining tailings is crucial to understanding their geomechanical behavior and environmental impact. This study evaluates tailings disposed of using the “dry” stacking methodology, with a focus on permeability, erodibility, turbidity, and ecotoxicity. The experimental program consisted of two phases: (i) Phase I, which involved permeability and erodibility flume tests on intact and deteriorated specimens compacted at 80% and 95% compaction degrees; and (ii) Phase II, which analyzed the tailwater effluents for turbidity, solid content, chemical composition, and ecotoxicity. The results indicate that permeability is predominantly controlled by grain size, with oversize tailings exhibiting higher permeability than undersized materials. Erodibility was higher for fine-grained materials and specimens with lower compaction degrees. The chemical analyses of leachates demonstrated an increase in sulfate, sodium, and ammonia concentrations, particularly in deteriorated samples, suggesting enhanced solubilization under field conditions. The ecotoxicological assessment confirmed toxicity in all samples, with chronic and acute tests revealing adverse effects on aquatic organisms. These findings highlight the necessity of implementing mitigation measures in tailings management to reduce environmental risks associated with rare earth ore mining.</p>