Using satellite images and Nd-Sr isotopes in suspended sediments in the adjacency of Doce River (APA Costa das Algas and Esquecidos Reef) to monitoring the presence of sediments from the 2015 Fundão dam colapse and assess potential contamination zones.

In November 2015, the collapse of the Fundão mining tailings dam resulted in the release of approximately 50 million cubic meters of iron oxide and quartz-rich slurry into the Doce River, reaching the ocean. Preview work¹ using ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotope ratios of marine suspended sediment samples collected between 2016 and 2020 from a network of sediment traps indicated the presence of Doce River signatures at Abrolhos bank, ~200 kilometers North from the doce river mouth. Recent data have shown that the isotopic signals in Abrolhos are currently close to the pre-dam colapse condition, indicating a reduction in contamination. However, there is great concern and uncertainty regarding the persistence of contamination in the areas adjacent to the mouth of the Doce River. This study aims to address this issue through the monitoring of the isotopic signal of the material from the Fundão dam using ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotope ratios in two regions adjacent to the mouth of the Doce River and of great socio-environmental interest: the APA Costa das Algas (south of the mouth of the Doce River) and the Esquecidos reef (north of the mouth of the Doce River). Sediment traps were installed in these two regions near and away from the coast to monitor the extent of the isotopic signal. We employed Mixture models based on isotopic ratios to estimate the contribution of material from the Fundão dam in the sediment, thereby evaluating its contamination. In parallel, we monitored meteorological and oceanographic parameters, as well as the spatial distribution of sediment plumes, using remote sensing images (applying a newly developed regional algorithm for suspended particulate matter/sediments). The spatial distribution of sediments was overlaid with shipping routes and humpback whale routes in an attempt to identify possible contamination risks zones. Our results, although temporally restricted as the monitoring was conducted from September 4, 2022, to October 6, 2023, showed a strong component from the Doce River over the APA Costa das Algas region (south of the Doce River), with isotopic data for Sr and Nd within the values corresponding to the isotopic domain of the Doce River mouth. Such contribution varied from 77% to 93% during the rainy season. During the period between July and August 2023, when the SSE component of surface winds intensifies, this contribution drops to 65%. For the region of the Esquecidos Reefs, closer to the coast, the relative contributions varied from 77% to 100%. For the more offshore region, all isotopic signatures showed mixing fractions, depicting a similar contribution of the signal from the Doce River mouth and the São Matheus River. Satellite monitoring of suspended particulate matter concentrations also made it possible to determine the extent of sediment plumes along the coast. The analysis of the mean annual extent of the coastal sediment plume together with the marine traffic data (from: https://www.marinetraffic.com) showed that there is a considerable density of ship routes passing through the Doce River. The possibility that pollutants can be transported from Rio Doce to Abrolhos through the hull of ships or ballast water is something that should be investigated and taken into account in coastal management. Furthermore, marine mammals, such as humpback whales, move along the coast to Abrolhos and can be contaminated on the way as they pass through the sediment plumes. Monitoring the isotopic signal of the Fundão dam material in the sediments and the extent of sediment plumes through satellite algorithms has proven to be a valuable tool to assess possible impacts and determine a coastal management plan for the region.