Tracing downriver mining contaminants using Sm-Nd and Sr isotope signatures: the case of the iron-rich tailings from the failed Fundão dam at Mariana along the Doce River, southeast Brazil

This work presents TIMS Sm–Nd and Sr isotope data on the fine-grained fraction (< 74 μm) of iron-rich deposits related to the November 5th, 2015 Mariana accident, caused by the failure of the Fundão dam, an iron tailings pond operated by SAMARCO Mining, a Vale and BHP joint venture. The rupture of the Fundão dam caused the gravitational flow of approximately 62 Mi m3 of quartz and iron oxide-rich slurry composed of water, fine sand and silt that flowed into the Gualaxo do Norte and Carmo rivers, and eventually along the whole Doce River, reaching the shoreline in 15 days. The event is considered the most serious environmental disaster, having caused 17 casualties and environmental impacts that persist still today.

River bank sandy sediment samples were collected from 17 localities distributed from 5 to 500 km downriver from the failed dam, including pre- and post-failure river bank sediment. The latter sediment is distinguishable from the former by its distinct brownish color and finer (fine sand + silt) grain size, larger proportion of iron oxide minerals.

In both pre- and post-accident sediments a down river increase in heavy metals content such as Arsenum is observed, probably related to previous contamination from centuries of gold mining in the highlands that source the Rio Doce.

Nd and Sr isotope compositions of the Doce River sediments show indications of mixtures of the following end-member components such as: the juvenile Neoproterozoic magmatic arcs of the Araçuaí orogen and the syn- and post-orogenic granitoid remelting products, the Archean-Paleoproterozoic São Francisco craton TTG associations and the iron-rich slurry tailings from the mined and processed primary Paleoproterozoic banded iron formation, which was massively released into the fluvial system.

In both pre- and post-accident sediments a down river increase in Epsilon Nd and decrease of 87Sr/86Sr ratios are observed, interpreted as contribution from the  bedrock juvenile Neoproterozoic magmatic arc terrane present in the Araçuaí orogenic belt.

Since 2017, Nd-Sr isotope compositions were used to monitor the presence of Rio Doce clastic input into adjacent marine environment, through the seasonal analysis of suspended sediment in seawater. The results show variation from low and high terrigenous input, deriving from complex interactions of onshore pluviosity and offshore wind and current regimes.