Multivariate modeling of Paraopeba River ecotoxicity after the Brumadinho dam collapse

The Brumadinho dam collapse in 2019 released approximately 10 million m 3 of iron-ore tailings, of which about 1.6 million m 3 reached the Paraopeba River. Early post-failure assessments reported acute toxicity in surface waters, but the relative contribution of tailings versus historic basin impacts remained unclear. Here, a combined ecotoxicological monitoring and chemical characterization of surface waters was applied to evaluate toxicological aspects for distinct trophic levels and to identify the main drivers of persistent toxicity at Paraopeba River from June 2022 to July 2024, 3.5 to 5.5 years after the Brumadinho tailings dam collapse, disentangling tailings influence from other anthropogenic sources. We conducted nine sampling campaigns between June 2022 and July 2024, applying standardized acute and chronic toxicity tests with algae ( Raphidocelis subcapitata ), cladocerans ( Ceriodaphnia dubia , C. silvestrii , Daphnia laevis ) and fish larvae ( Danio rerio ) at 17 sites along the Paraopeba River and its tributaries, including upstream non-affected reaches, tributaries without direct contact with tailings, and sites downstream from the Ferro-Carvão stream, the main source of tailings input into the Paraopeba River. Exploratory analyses (PCA and k-means clustering) and logistic generalized linear mixed models (GLMMs) with random effects for sampling point, season and campaign were used to relate toxicity to water composition. The highest frequency of toxicity response for all tested organisms was observed at the Betim River tributary, an urban-impacted site characterized by elevated conductivity and concentrations of fluoride, chloride, sulfate and sodium, rather than in reaches closest to the tailings entry point. Logistic GLMMs converged for R. subcapitata , C. dubia and C. silvestrii . Toxicity to R. subcapitata was positively associated with total aluminum and fluoride, whereas C. silvestrii toxicity was linked to total iron, calcium, conductivity, and sulfate. The C. dubia model showed signs of overfitting and was therefore not considered reliable for inferences. Overall, cladocerans were the most sensitive group, algae showed moderate responses, and fish larvae exhibited low toxicity frequencies. Our results indicate that, three to five years after the dam failure, chronic inputs from urban and industrial sources in tributaries represent an important source of ecological risk to the Paraopeba water, whereas the direct influence of tailings-derived metals on water-column toxicity appears comparatively limited relative to the toxicity conditions documented immediately after the collapse.