Sources of sedimentary organic matter and assessment of heavy-metal levels in estuarine sediments after Fundão dam breach

Abstract

Mining tailing is one of many anthropogenic stressors affecting estuarine systems around the world. On the November 5, 2015, the Fundão dam breached, releasing 52 million m3 of iron mining residues affecting Doce River until its estuary, spreading a large plume along the Brazilian coast. The present study characterizes the impact of the mining tailing in these environments and the changes in sedimentary organic matter (SOM) sourced at Doce River estuary (DRE) and adjacent estuaries over time. Three sediment cores and surface sediment samples were collected along the coast of Espírito Santo State. Elemental (C, N) and stable isotope (δ15N and δ13C) fingerprint methods were used to characterize sedimentary organic matter sources. The indexes of sedimentary organic matter stability (recalcitrance index) were quantified by FTIRS technique. Besides, WDXRF analyzes were carried out to statistically determine metal baseline levels and assess the present and past pollution degrees of sediments using pollution indexes (PLI). Results indicate that Doce River estuary significantly provides Fundão dam's tailings (77 % ± 9 %) to the coastal region, with a PLI categorized as moderately polluted to highly polluted (PLI = 3.1), even two years after the dam breach. Estuaries north from Doce River Estuary presented a more significant influence of marine organic sedimentation sources with lower PLI (unpolluted). In contrast, estuaries south from DRE presented greater terrestrial influence with higher PLI values (unpolluted to moderate polluted). Further, changes in SOM stability were noticed mainly at Doce River Estuary, where the recalcitrance index did not show the expected natural pattern. Finally, the findings indicate that Fundão dam failure had a significant impact on Doce River Estuary sediment quality at different moments after Fundão breach, and this ecosystem is expected to continue to suffer from this indefinitely. However, the direct effect of tailings deposition was not observed in the sampled estuaries, PLI shows that ecosystems in the south already suffer from other pollution sources, which increases the environmental risks in a possible tailing deposition scenario.