Environmental risk assessment of potentially toxic elements in Doce River watershed post-mining dam breakdown in Mariana MG-Brazil
The most shocking Brazilian mining disaster in 2015 released about 50 million tons of sludge to the environment from a containment dam. The sludge destroyed cities and villages, dragging everything into the Doce River Basin over 650 km, reaching the Atlantic Ocean. The watershed was contaminated by potentially toxic elements (PTE) such as As, Ag, Al, Cd, Co, Cr, Cu, Mn, Ni, Pb, Sb, and Zn. Although government agencies have evaluated the incident, information is still scarce. Faced with a potential risk, we proposed to assess the environmental risk with a relocated perspective, using geochemical fractions as a starting point for understanding PTE mobilization. Soil and sediment samples were taken at nine sites throughout the Basin. The environmental risk was assessed from the PTE sequential extraction in three fractions: soluble, reducible, and oxidizable, in addition to the pseudo-total concentration. The potential mobile fraction (PMF) showed a considerable mobilization potential of the PTE from soil and sediment samples. Statistical principal component analysis indicated a single source of PTE: the sludge. The risk assessment was mainly dependent on the fractional distribution and the PTE enrichment degree in the affected samples. The fractional distribution contributed largely to Mn, Sb, and Pb mobility, with PMF of 96\%, 81\%, and 100\%, respectively. The mobilization of Cd, Co, Ag, Ni, Pb, Zn, and Cu was predominantly related to the degree of enrichment. The risk evaluation pointed to the disaster magnitude, which deserves attention.