The effects of a tailing dam failure on the sedimentation of the eastern Brazilian inner shelf

Abstract

In the past several decades, anthropogenic changes throughout drainage basins have threatened physical, hydrological and ecological river-to-ocean continuum balances. Dams are one of these anthropogenic influences that can seriously affect sediment fluxes to the ocean. Here, we investigate the formation and dispersion of a high-concentration mud suspension (HCMS) at a river mouth due to a tailing dam failure. In November 2015, the Fundão dam (Mariana-MG, Southeast Brazil) failure released approximately 39.2 million m3 of tailings into the Doce River system. The slurry travelled 660 km down to the river mouth, reaching the Atlantic Ocean. Pre- and Post-dam failure surveys were used to investigate temporal and spatial changes in Suspended Particulate Matter (SPM) concentration, seabed sediment grain size and seabed sediment bulk density along the inner shelf. Results indicate that the combination of the tailing dam failure with an extremely dry seasonal condition led to the formation of a HCMS, altering, at least in a short term, the sedimentary processes along the inner shelf, leading to high sedimentation rates and low seabed sediment bulk density. Changes in particle size defined by an increase in clay content, a significant increase in SPM concentration in the inner shelf and a decrease in seabed sediment bulk density were observed as a result of the dam failure. SPM concentrations were higher near the seabed than at the surface, reaching up to 9000 mg/l. The highest SPM concentration measured during a major flood in January 2014 reached around 100 mg/l, The combination of a HCMS with tailings high clay content resulted in low values of seabed sediment bulk density and the formation of fluid mud along the inner shelf. In the first week after the tailings arrival, a 5 cm thick tailing fluid mud was observed. In this stage of bed formation, bed strength is lower, easing particle reentrainment by waves or currents. Thus, tailings could be advected, spreading the impact along the coast.