THE USE OF STABLE ISOTOPES OF HYDROGEN AND OXYGEN TO TRACE THE SEEPAGE FROM THE TAILINGS STORAGE FACILITIES OF THE MORRO DO OURO MINE, MINAS GERAIS, BRAZIL

The Morro do Ouro mine located near Paracatu, Minas Gerais, and owned by Kinross Gold, is the largest gold mine in Brazil. The mine tailings are stored behind the Santo Antônio dam and the Eustáquio dam, with current storage volumes of 399 and 148 million m³, respectively. Acid mine drainage is suggested by the strong reddish-brown color of the flow from the dam drainage pipes even after treatment with crushed limestone, with the same color found in shallow, hand-dug wells downstream from the tailings dams. The affected downstream population consists of a Quilombola community (descendants of escaped slaves) and a major expansion of the mine involved the displacement of two other Quilombola communities. Two previous studies funded by the City Hall of Paracatu and by Kinross Gold of As in groundwater and surface water were inconclusive. The city-funded study reported on groundwater and surface water upstream from the dams, but only on surface water downstream from the dams, and measured only filtered samples, while the company-funded study did not report the methodology, sample locations, or individual As values. The objectives of this study are (1) to assess the quality of drinking and environmental water downstream of the tailings dams (2) to trace the pathway of seepage from the tailings dams using stable isotopes of hydrogen and oxygen (3) to determine whether any degradation of water quality results from seepage from the tailings dams. In May 2019 filtered and unfiltered water samples were collected in triplicate upstream and downstream from the dams from 10 streams, nine hand-dug wells, seven boreholes, two dam drainage pipes, two springs, two faucets, one wetland, and one water tank. The ICP-MS at Universidade de Brasilia Planaltina was used to measure concentrations of Al, As, Ba, Ca, Cd, Co, Cr, Cu, K, Fe, Mn, Mg, Mo, Na, Ni, P, Pb, Si, Sr, Ti, V, Zn, and Zr. A plot of stable isotopes showed a distinct evaporation line that included the dam drainage pipes, as well as two hand-dug wells, two stream sites and one wetland downstream from the tailings dams, indicating that the downstream sites are receiving water from the tailings ponds. Samples from the downstream hand-dug wells exceeded WHO drinking-water guidelines for As, Ba and Pb, with As concentrations nearly 10 times the WHO guideline. Further results will be reported at the meeting.