A six-year ecotoxicological assessment of the Doce river and coastal marine areas impacted by the Fundão tailings dam failure, Brazil

Abstract

Encompassing six years, 10 sampling campaigns, and more than 800 toxicity tests utilizing a range of organisms from different trophic levels as bioindicators, this study offers a comprehensive ecotoxicological assessment of the environmental impacts resulting from the Fundão dam rupture—one of Brazil's most significant environmental disasters. We employed a novel approach that integrates acute and chronic laboratory toxicity tests to establish a toxicity index classifying samples into five categories: non-toxic, slightly toxic, moderately toxic, toxic, and highly toxic, based on a toxicity value calculated by considering diverse organisms' responses. This evaluation, conducted between 2018 and 2023, spanned both dry and rainy seasons, assessing water and sediment conditions across the Doce River and the adjacent marine region near its mouth. Our findings reveal that, even years after the dam failure, the iron-enriched mine tailings continue to exert a significant impact on the water and sediment of the Doce River and its adjacent marine areas, with the most severe effects observed near the river's mouth and towards the North. Notably, sediment samples consistently exhibited higher toxicity values than water samples across sites and seasons, highlighting the role of sediments as reservoirs for contaminants. The most pronounced toxicological impacts were detected during the initial years following the disaster, with a gradual, though cautious, decrease in toxicity observed in recent campaigns. Emphasis is placed on the sensitivity of microcrustaceans, copepods, and sea urchins, which consistently demonstrated the highest sensitivity to contaminants, making them reliable bioindicators for ongoing monitoring efforts. While recent campaigns identified a gradual decrease in the toxicity index of the environmental samples, especially in the marine environment, this trend must be interpreted with caution. Continuous monitoring is essential to confirm sustained recovery, detect potential recontamination, and identify long-term sub-lethal effects that could impede the full restoration of the affected ecosystems.