Tailings dam failures in Brazil: River contamination, ecosystem recovery, and institutional responses to the Mariana and Brumadinho disasters

Mine tailings are wastes from ore extraction and processing, which are frequently stored in hillslopes behind earth dams. Unfortunately, tailings dams have collapsed in a number of places worldwide, literally dumping the waste into nearby rivers as metal-rich debris and mud. The environmental impact of these accidents is systemic and often severe and long-lasting. This study reviewed recent cases from Brazil (the Mariana accident that affected the Doce River and the Brumadinho accident that affected the Paraopeba River), highlighting the impact of large tailings dams' failures on river water quality and implications derived therefrom. The review showed how the Mariana failure affected the entire course of the Doce River, even impacting the estuarine ecosystem for a decade so far; and how physical barriers such as the Igarapé weir helped delaying the propagation of contamination in the Paraopeba River. Invariably, the contamination of river water was characterized by concentrations of metals (e.g. iron, manganese) and turbidity above the legal thresholds and far exceeding the levels observed before the accidents, which in the Brumadinho case, led to the suspension of drinking water supply to the metropolitan region of Belo Horizonte, affecting 2 million people. Besides, bioavailable phases of metals have accumulated in various trophic levels across the food web, threatening human health. The role of storm events in the exacerbation of water quality degradation and the potential impact of floods on the health of riparian forests were also recognized, because contaminated sediments were resuspended during those events and riverbanks were inundated in the sequel. Moreover, contaminated surface water flowing from urban, industrial, and pastureland areas masked the water quality degradation of Paraopeba River caused by the tailings dams' failures, because this water also contained metals and other toxic elements. On the other hand, inflows of sediments and water from tributary rivers helped dilute the contamination in the main watercourses. The time required to bring the river water quality back to safe levels was assessed in some studies, which pointed to 6–8 years in the Brumadinho case and provided no clear timespan for the Mariana case. Dredging of mine waste from the impacted areas was said to accelerate the recovery of Paraopeba river. The reviewed articles suggested some institutional responses, both systemic and specific to tailings dam accidents. The systemic measures comprised reforestation and implementation of best management practices to prevent water erosion and contaminant transport across the river basins, coupled with integration of accountable water quality monitoring in landscape management plans and policies. The specific measures were mostly related with prevention through the design of promptly implementable emergency action plans, coupled with policies capable of making the mining companies pay the recovery costs in a reasonable timeframe, where the affected population has a voice and participates in the compensation process. Overall, the reviewed papers shed light over key issues related to the impacts of large tailings dam accidents on riverine systems in general, in spite of describing two specific cases, leveraging useful insights to the authorities in charge of water security in emergency scenarios.