Hydrochemical assessment of surface water in watersheds near the Uranium Mining and Milling Facilities of Caldas, Brazil

Abstract

Sited in the Pocos de Caldas Plateau, in south-eastern Brazil, the Uranium Mining and Milling Facilities (UMMF) of Caldas are in the process of decommissioning. The main environmental problem in this site is the generation of acid mine drainage (AMD) in the tailings dam, open pit and waste rock piles. The aim of this work is to evaluate, by hydrochemical studies, the influence of acidic effluents from the Caldas UMMF, on the hydrochemistry of surface water along three watersheds: Consulta Brook, Soberbo Creek and Taquari River. Twelve sampling stations were established to carry out the investigation. Two of them were located at effluent retention ponds. Sampling was performed in the rainy and dry seasons of 2011, and the measured parameters were Eh, pH, EC (electrical conductivity), SO42−, HCO3−, Cl−, Na+, K+, Ca2+, Mg2+ and Fe. The results have shown that iron is not a main constituent of the ionic composition of the local AMD. It could also be observed that acidic effluents were being discharged from retention ponds into the watercourses, causing a pronounced increase of the sulfate and calcium concentrations downstream. To trace the AMD migration the (SO42−/HCO3−)/(Na+ + K+/Ca2+) ratio was used. A detailed investigation of the near-surface groundwater is recommended downstream of the effluent ponds sites to evaluate the need for mitigating actions such as constructing hydraulic barriers. The present article shows that using simple techniques, such as Eh–pH and Piper diagrams, it is possible to depict the AMD fluvial migration pattern, which is an important information both to the authority responsible incumbent upon the remediation of the site, and to the affected interested parties who inhabit the impacted region.