Avaliação da geocronologia e composição química de sedimentos do Reservatório de Sobradinho, Bahia

Abstract

The economy of the Sobradinho region, located in Bahia, and its adjacent cities is heavily influenced by agricultural activity. The predominant cultivation of onions in these areas requires careful irrigation management, which can alter the characteristics of the soil and the surrounding environment. In this context, the present study aims to identify changes in the chemical composition of sediment in the Sobradinho reservoir and assess the environmental impact on sediments, resulting from agricultural practices and urban development in the region. Qualitative and quantitative analysis of sediment samples was conducted, followed by their dating, carried out in the CRCN-NE laboratory using the analytical techniques of Energy-Dispersive X-ray Fluorescence Spectroscopy (EDXRF) and Gas Proportional Flow Counter (CPFG). Initially, sediment cores with a length of 1 meter were collected, which were subdivided into fractions of 3 cm depth for a more detailed analysis. The Energy-Dispersive X-ray Fluorescence Spectroscopy (EDXRF) instrument was calibrated with voltages of 15 kV and 50 kV for the quantification and identification of the elements Al, Fe, Ni, Pb, Si, Ti, and Zn. Sediment dating was performed using the radioactive isotope 210Pb, whose decay to 210Bi is accompanied by the emission of beta particles, with the amount of radioactive activity measured at each depth. This approach allowed the identification of variations in the concentrations of chemical elements over time, with the most significant changes observed in the levels of nickel and silicon, which showed changes between the years 2010 and 2019. These variations can be attributed to various chemical and environmental phenomena, such as the formation of complexes between metals, natural erosion processes caused by wind and rain, and non-natural impacts arising from agricultural practices in the region. The intensive cultivation of onions and grapes, together with the growing agricultural industrialization and the use of nitrogen and phosphorus-rich fertilizers, contributed to the eutrophication process and the reduction of oxygen availability in the water. Additionally, hypoxic conditions were exacerbated by the increase in water temperature, influenced by environmental factors such as the El Niño phenomenon, which began in 2010, and by the emission of pollutants resulting from the modernization of the region and the surrounding cities of Sobradinho. The results of the analysis reveal an average reduction of 23.44% in the levels of iron (Fe) in sediment over the period from 2011 to 2016. Furthermore, there was a significant increase in concentrations of titanium (Ti) and silicon (Si) in 2016, with average variations of 738.25 mg/kg and 15,000.05 mg/kg, respectively. The analysis also indicates an inverse correlation between the concentrations of nickel (Ni) and silicon (Si) over time, suggesting complex interactions between these elements.