Geogenic controls and anthropogenic inputs on the seasonal hydrogeochemical variability of the Paraopeba River basin,...

This study investigates the combined effects of geology, land use and land cover, hydrological seasonality, and disturbance history on the hydrogeochemistry of surface waters in the Paraopeba River Basin (PRB), located in Minas Gerais State, southeastern Brazil, and forming part of the São Francisco River Basin, a region characterized by high lithological complexity and intense anthropogenic pressure. A total of 264 sites were sampled during the dry season (2022) and 262 during the rainy season (2023), allowing an integrated assessment of spatial and temporal variability. Hydrochemical facies are predominantly mixed bicarbonate, with water chemistry mainly controlled by silicate weathering. Localized deviations toward Na, Cl and SO₄^2- enriched compositions are observed in urban and industrialized areas, reflecting anthropogenic contributions that are more pronounced during the dry season. In the confluence of the Paraopeba River with the Maranhão River it is observed a major hydrogeochemical control, resulting increased concentrations of Mn, Ba, SO₄^2-, Cl^-, NO₃^-, and P downstream in the Paraopeba River, with persistent influence even under seasonal dilution conditions. The effects of the tailings related to the reservoir failure in Brumadinho, a municipality in central Minas Gerais, were evaluated; however, the analyzed dataset does not show a consistent downstream geochemical signature attributable to tailings remobilization. Current variability is predominantly controlled by diffuse and point-source inputs associated with land use. Geochemical reference values for Al, Fe, Mn, Ni, Ba, and Sr were established using the mMAD, TIF, and UTL methods within four integrated geospatial classes. The UTL method showed greater consistency in defining thresholds, approximating upper percentiles (P95-P98) or maximum values. Seasonal effects are more pronounced for Fe and Al, whereas Mn shows greater mobility in the dissolved phase. Overall, hydrogeochemical variability is primarily controlled by geology, modulated by hydrological seasonality, and intensified by anthropogenic pressure.