A case study on the Uranium tailings dam of Pocos de Caldas uranium mining and milling site

This Annex describes the geochemical processes controlling the mobilisation of heavy metals and radionuclides in the tailings dam of the Pocos de Caldas Uranium Mining and Milling Facility. It was shown that residual pyrite oxidation causes the production of acid drainage that leaches metals and radionuclides from the solid phase. The remediation scheme, application of dry cover on the tailings, was focused on the reduction of oxygen diffusion into the tailings and radon exhalation from the material Three principal final covers designs were taken from the scientific literature aiming to obtain the most adequate performance for the studied situation The designs studied included: (i) a compacted clay liner (CCL); (ii) a composite liner (CL); and (iii) a capillary barrier (CB). Relevant processes investigated were: (i) saturated hydraulic flow; (ii) unsaturated hydraulic flow (only for the capillary barrier); and (iii) radon exhalation to atmosphere. The computer models utilized for the analyses were: (i) the program Hydrologic Evaluation of Landfill Performance (HELP); (ii) the program SEEP/W; and (iii) the program RADON. (author)