Environmental risk assessment following Brazilian mining dam collapse: A diffusion approach

The 2015 collapse of an iron-ore sludge containment dam in southeastern Brazil had devastating impacts on the Doce River Basin (DRB). Although some reports address environmental risks associated with the sludge-water interface, few comprehensive studies evaluate the sludge-soil interaction, particularly its diffusive behavior, which is crucial for risk assessment. This study investigates the impact of sludge on the mobility of potentially toxic elements (PTE) in soil. For this purpose, soil samples were subjected to diffusion tests using the planar-source method over 7 days, with sludge as the PTE head. Diffusion coefficients for Pb, Ni, Cr, and As were found to be around 10^-12 m² s^-1 for porous media, indicating the potential of sludge-soil migration. While Ni and Pb showed a better fit to the one-diffusion profile, As and Cr fit the double Gaussian model, highlighting the complexity of the system. Therefore, the environmental risk assessment must employ diverse approaches, with diffusion as one of the parameters required for evaluation. Herein, the 3D-surface short-term tests allowed for projecting the contamination front on long-term time scales under a worst-case scenario. Our findings provide new insights into assessing the risks posed by PTE from the dam collapse on soil ecosystems and offer guidance for mitigating their environmental impact, highlighting the soil system. In addition, for the first time, the real diffusion coefficients for PTE in DRB were determined, along with the double-Gaussian diffusion profiles for As and Cr.