Remediation of Caldas site by monitored natural recovery

Acid mine drainage (AMD) is one of the main environmental issue caused by oxidation of pyrite (FeS2) when exposed to atmospheric conditions during mining. In Brazil, one of the places where AMD occurs is an old uranium mine in Minas Gerais State. The acid waters contain radionuclides and other metals that are precipitated with the addition of lime. Due to the inefficiency of the water treatment plant, part of the precipitate containing the metals overflows from the settling tanks and reach the water reservoir of Indústrias Nucleares do Brasil – INB. The multiplicity of interactions that can occur between the extremely fine particles of the precipitate and the sediments of the reservoir requires a broader approach in order to understand the fixation and/or transport of the metals. In this work the natural recovery approach is reviewed and a chemical and radiochemical characterization of the sediments are presented. Uranium and thorium were determined by spectrophotometry, 226 Ra, 228 Ra and 210 Pb by gamma spectrometry and Zn and Mn by ICP-OES. A panorama of the contamination is shown in different maps by using ArcGis program. Twenty-six sediment samples were collected in the reservoir and a bathymetry survey by sonar was performed to determine the thickness of the sediment layer. Most of the samples have presented activity concentrations of uranium and thorium higher than the background values. Further investigation will assess the potential mobility and bioavailability of contaminants and the role of bacterial sulfate reduction in the immobilization of these contaminants.