Water quality assessment of the Doce River basin: What are the conditions a decade after the Fundão dam breach?

In November 2015, the Fundão dam breach in Brazil released approximately 44 million cubic meters (Mm 3 ) of tailings into the environment. Roughly 32 Mm 3 were discharged at once, while the remaining material was carried downstream during the 2015/2016 rainy season. The event severely affected water quality throughout the Doce River basin. Containment, remediation efforts, and natural recovery have improved conditions, but the long-term influence remains uncertain. To address this, we analysed publicly available water quality records from July 1997 to March 2025, covering both affected and non-affected sites and representing pre- and post-breach periods. We focused on six key parameters that were consistently monitored across the basin: dissolved aluminium (Al) and iron (Fe), total arsenic (As) and manganese (Mn), total dissolved solids (TDS), and turbidity. Overall, post-breach concentrations of most parameters declined over time, and recent measurements are largely comparable to pre-breach levels. However, TDS and turbidity remained relatively higher in river sections closest to the Fundão dam. Total As and Mn already showed elevated baseline levels before the breach, suggesting that these elements are not reliable indicators of this specific event. Dissolved Al and Fe remained consistently above pre-breach levels across both affected and reference rivers, pointing to broader basin-wide issues, such as natural geology and ongoing human activities, that may act as confounding factors influencing water quality in the Doce River basin.