I did not yet see actually any report about tailings composition or water quality, but iron ore or waste is not in itself toxic. The main issues may be the large amount of solids, possibly choking any river life; processing reagents residues, which may be toxic; pH, redox and DO perturbation of water; and ore impurities. Most press reports mention heavy metals, these may be ore impurities but of variable health hazardousness. If any group member could share some data, it would help understanding what is going on. All the major dam accidents (Los Frailes, Baia Mare,) were opportunities for improved knowledge.

I had very much the same thoughts. I would have expected fish kills based on turbidity and possibly low oxygen. The other factors are uncertain, depending on the process. I accept that there may be arsenic (and possibly some other metals) in the tailings but it is probably only a significant problem if redox drops to cause reductive dissolution of the tailings. I find reports of mercury suspect but I suppose nothing is impossible. Also, if the samples were not filtered it is hardly a surprise that there is Al, Fe and Mn in them. Some data would definitely help to bring the truth to light, rather than taking all the dramatic journalism for granted.

According to the company, "the mud is composed of clay and silt material from the washing and processing of earth containing iron ore and based on available data, the tailings are chemically stable, not change chemical composition in water and will behave in the environment like normal soils in the catchment".

Some more news you can find from following link.http://www.mining.com/bhp-questions-un-probe-that-said-samarco-spill-toxic/

regarding to the reagent residues, we should know the process. We know that some processing methods do not require usage of chemicals.

Until one knows exactly what the material is, it is impossible to say who is correct. There is clearly a propaganda war going on and everyone is just expected to swallow what they're told (by both sides). Hopefully the test methods and the results of the chemical testing will be released for peer review. Many of the test methods used to classify waste are only applicable to certain environmental conditions and could easily provide inappropriate answers if used in the wrong environmental context. Presumably down the Rio Doce there are many environmental contexts that might be relevant.