In the mining industry, tailings are conventionally stored in artificial ponds that are susceptible to ruptures, as occurred in Southeast Brazil in November 2015. The Fundão failure dumped approximately 40 million m3 of very fine iron-rich tailing material into the Doce River watershed, where part reached the adjacent continental shelf. Part of the spilled material remained suspended in the water column after reaching the ocean. Zeta potential measurements (pH and salt dependence) were conducted to verify the flocculation tendency of suspensions. Surface sediment samples were collected in areas representative of key stages of tailings transport through the river and subsequent deposit on the continental shelf. A surface sediment sample from the continental shelf prior to the failure was used as a control. Previously published data suggested an influence from goethite on the flocculation of samples containing iron ore tailing sludge. Here we show that although a flocculation tendency was observed under influence of salts, the presence and quantity of iron oxyhydroxide were determinant in the flocculation pattern of contaminated sedimentary samples. Only sediment from the continental shelf before the mining dam breach exhibited classical zeta potential behavior dependence on pH and salinity expected in natural sediments containing clay minerals. Sediments contaminated with the iron ore tailing sludge displayed a clear influence of goethite on their zeta potential.@en