Soil gas compositions as a tool for understanding acid mine drainage formation and airflow in a uranium waste rock pile

The first site of exploitation and processing of uranium ores in Brazil is located at Plateau of Pocos de Caldas, in Caldas City, Minas Gerais, which is the largest alkaline Complex in South America. The activities of mining and processing of uranium occurred between 1982 to 1995. Fractures and faults with high permeability act as channels to conduct the underground flow of water and gases. They also determine the main features of the surface drainage network. In geological systems, gases can migrate preferentially through fracture zones. These gases can provide information on the conditions that allow its formation, accumulation and migration. During the mine operation, tailing dams and waste rock piles (WRP) were constructed for the disposal of uranium waste. The Waste Rock Pile 4 (WRP4) was built on the valley of Consulta Stream, an area adjacent to the pit mine. An artificial diversion channel for this stream, about 500m long, was built. Currently, the WRP4 generates acid mine drainage (AMD) with pH of approximately 3.5 and uranium concentration of 8.7mg/L. Despite mitigation actions, the discharge of AMD has increased 60% over the past ten years. It has been shown that waste piles may present regions of recharge and discharge of gases, which influence the oxidation pattern within the pile. In this context, soil gas sampling was carried out, at a depth of 40cm, to establish the concentrations of O-2, N-2, H-2, and CO2. The results helped to identify the regions of the pile close to the surface where atmospheric air is likely to be entering the pile and regions where air seems to be discharged, after reacting with the sulfides. Understanding the spatial consumption of O-2 due to sulfide oxidation and patterns of gas concentrations and distribution will help the development of remediation approaches.