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Damage hidden in the mud

Fine clay with a high metal content in the material released by the dams in Minas Gerais could change the ecological dynamics and sediments at the mouth of the Rio Doce

Rio Doce_DSC_0526Valéria Quaresma/UFES When Valéria Quaresma and Alex Bastos, a couple who are experts in geological oceanography, began studying the sediments of coastal Espírito Santo State near the mouth of the Rio Doce about five years ago, one of their objectives was to develop a baseline for future plans to manage that region, where two of the principal sources of economic support: fishing, and petroleum extraction are in opposition.  The former depends on the health of a coastal ecosystem that can be threatened by accidents that might be caused by the latter.  The duo of professors from the Federal University of Espírito Santo (UFES) could not have imagined that their knowledge would be called upon so soon.  In early November 2015, when dams operated by Samarco, a mining company in Minas Gerais, collapsed, the professors were part of a group of 100 researchers organized by the university.  They immediately prepared to collect comparative samples.  “On November 21, when the plume that descended the Rio Doce reached the mouth of the river, our boat was equipped and ready,” says Quaresma.  The next day her team collected samples of seawater that was tinged orange.

The first account of how the sediments that the river normally carries are behaving is described in an article by Quaresma’s group published in December 2015 in the Brazilian Journal of Geology.  The results show that the finest-grain sediments are being deposited very quickly in a process known as flocculation, when fresh water meets the salinity and different pH of the sea.  Deposition of that sediment, classified as terrigenous mud, occurs principally at depths of at least 10 meters situated to the south of where the Rio Doce discharges, pushed there by a northeast wind—the prevailing wind in that area.  “Then the material can be re-suspended and redistributed northward, depending on the force and direction of the winds and waves,” she explains.  Quaresma has mapped the deposition of those sediments, not only in her work that resulted from the samples collected by her group, but also in data compiled in the survey article published in November 2015 in the Journal of South American Earth Sciences.  Along the coast of the region north of the Rio Doce, larger and less clayey particles predominate.

Although researchers know the region well and plenty of preparations were made to receive the wave of mud that has traveled through part of both Minas Gerais and Espírito Santo states, causing tremendous damage to adjacent cities and the ecology of the river and its surroundings, there has been no immediate assessment of the impact of the debris from the tailings dam.  What was clear from the outset was that this is a terrifying volume of clayey material made of very fine particles that do not readily deposit themselves.  “We are not familiar with the sediment that is in the tailings,” says Quaresma.  “We don’t know how it behaves.”  She plans to follow the course of the mud in a series of future field trips.  “It will take a couple of months before we can see what has happened to the bottom.”

The concern generated by the change in the dynamics of sediment transport goes far beyond its essential role in maintaining coastline stability.  Looking at the samples already collected, researchers are shocked at how muddy the water is.  It makes it hard for them to see their submerged equipment.  Quaresma says that this change in the physical characteristics of the water may completely alter the environment that is necessary to the survival of bottom-dwelling organisms that compose the basis of the marine feed chain: the benthic community.

The fine clay suspension tinged the water at the mouth of the Rio Doce so strongly orange that the collector bottle (at right) is almost invisible when submerged.

Marcos Daniel Leite/UFESThe fine clay suspension tinged the water at the mouth of the Rio Doce so strongly orange that the collector bottle is almost invisible when submergedMarcos Daniel Leite/UFES

In addition to the sediments, researchers are also worried about the chemical content of the mud.  It is no surprise that iron is abundant, because it was the extraction of iron ore that led to the accident.  Quaresma says this can pose a problem because an excess of iron can cause an excessive proliferation of plankton (microscopic beings that float in the column of water) and bring about a significant ecological imbalance.

Chemist Renato Rodrigues Neto, coordinator of the Environmental Geochemistry Laboratory in the Department of Oceanography and Ecology at UFES, heads the group that has been analyzing the elements present in the torrent of debris.  So far, although samples have been examined from only five points at the mouth of the Rio Doce, an important increase in certain metals has already been observed when compared with the situation that prevailed prior to arrival of the mud.  “The presence of vanadium, aluminum, iron, manganese, and chrome has greatly increased,” the researcher says.  He finished his preliminary report with those results at the end of December 2015.  Although horrified by the 50-fold increase in the concentration of iron that his group detected, he says iron is not the element that worries him because it is a naturally-occurring nutrient.

Of greater concern is the greatly increased presence of chrome, an element that can be toxic, depending on how it appears.  “Usually it occurs in its less toxic form,” explains Rodrigues Neto, “but we have not yet done tests to find out what is there right now.”  Such tests are very complicated analyses that will require a partnership with other laboratories.  Also to be done, according to the chemist, is to determine whether the chrome is in a bioavailable form that could be absorbed by organisms.

The seriousness of the accident has led to demands for immediate responses and a rapid organization of researchers assigned to produce them.  Even so, understanding how the environment and the organisms that live in it are going to react and be affected takes time.  Identification of the effects on animals and plants of the region should begin in the coming months.

Scientific articles
BASTOS, A. C. et al. Shelf morphology as an indicator of sedimentary regimes: a synthesis from a mixed siliciclastic-carbonate shelf on the eastern Brazilian marginJournal of South American Earth Sciences.  V. 63, pp. 125-36.  November 2015.
QUARESMA, V. S. et al. Modern sedimentary processes along the Doce River adjacent continental shelfBrazilian Journal of Geology.  V. 45, No. 4, pp. 635-44.  December 2015.