Geochronology and elemental characterization (U, ²²⁶Ra, ²¹⁰Pb, Mn, Mo, Ni and Zn) of sediment profiles of Poços de Caldas Plateau - MG, Brazil

The Poços de Caldas Plateau, located in the southwest of Minas Gerais, is the greatest intrusion that occurred in Brazil, formed by processes of volcanism and weathering, which occurred about 60-80 million years ago. These processes were responsible by the formation of zirconium and molybdenum mineralizations and various radioactive anomalies. More recently, the presence in the region of several anthropogenic sources, amongst them, the uranium mining from Industrias Nucleares Brasileiras - Caldas, contributed to the entry of several natural radionuclides in the water bodies of Ribeirão das Antas. The aim of this study was to use sediment profiles and sedimentation rates to reconstruct the insertion of radionuclides and possible contaminants in the Bortolan and Poço Fundo dams over the years. In these profiles, the concentration of the natural radionuclides U, ²²⁶Ra and ²¹⁰Pb, and the Mn, Mo, Ni, Zn and P elements were determined by Gamma Spectrometry, Inductively Coupled Plasma - Atomic Emission Spectrometry and Spectrophotometry. The sedimentation rate and age of the profiles were evaluated using the Constant Rate of Supply. model. It was verified that the use of the Constant Rate of Supply. model to determine the ages of the sediment profiles gave satisfactory results, of the same order of magnitude of the real ages of the Bortolan and Poço Fundo dams, 60 years and 67 years, respectively. The values obtained for the sedimentation rate were higher for the Bortolan dam, due to silting of the dam, urbanization and extensive land occupation for agricultural and industrial activities. The results obtained for the concentration of the analyzed elements showed that Mn, P and Zn presented anthropogenic contributions in the Bortolan dam. The highest concentrations obtained were found in the superficial sections, indicating that the largest contribution occurred more recently, from the year of 1997 until now. The results obtained for U and Ni did not vary along the profiles and are of the same order of magnitude of literature data for sediments of Bortolan dam, indicating that these values can be considered as the base line for the region. In particular, the results of U concentration for the sediments of Bortolan dam were higher than natural background, since the region of Poços de Caldas plateau is characterized by a radioactive anomaly. The Mo did not present significant variation along the profiles of the Bortolan dam and its average concentration is also of the same order of magnitude of the world average value for soil. Therefore, for this element an anthropogenic contribution was not verified. The results obtained for the concentration of the elements analyzed in the Poço Fundo dam profile are of the same order of magnitude as the world average value for soil and lower than the results obtained in the Bortolan dam sediment profile. (author)