Two independent experts from the United Nations have said that mud released from the burst dam at Samarco's iron ore mine in Brazil contains high levels of toxic heavy metals and other hazardous chemicals.
UN special rapporteurs John Knox and Baskut Tuncak urged the Government of Brazil to take action to protect the environment and health of communities at risk of exposure to toxic chemicals following the tailings dam burst.
The disaster occurred on 5 November when the Fundao dam ruptured at Samarco's Germano mining site near the town of Mariana in Minas Gerais, reportedly killing 17 people.
John Knox and Baskut Tuncak said: "The steps taken by the Brazilian Government, Vale and BHP Billiton to prevent harm were clearly insufficient.
"The government and companies should be doing everything within their power to prevent further harm, including exposure to heavy metals and other toxic chemicals."
Knox added: "The scale of the environmental damage is the equivalent of 20,000 Olympic swimming pools of toxic mud waste contaminating the soil, rivers and water system of an area covering over 850km."
Following the incident, scientists consider that the Doce River is dead and the toxic sludge is slowly moving downstream towards the Abrolhos National Marine Park where it will harm protected forest and habitat.
Responding to the media reports and public commentary, BHP Billiton said in a statement: "The tailings that entered the Rio Doce were comprised of clay and silt material from the washing and processing of earth containing iron ore, which is naturally abundant in the region.
"Based on available data, the tailings are chemically stable. They will not change chemical composition in water and will behave in the environment like normal soils in the catchment."
The National Water Agency (ANA) and Brazilian Geological Service (CPRM) are still collecting, analysing and checking for sediment samples in the Rio Doce.
The sample results indicated that concentrations of metals obtained at these sites do not vary from the results received by CPRM in 2010.
Samarco added that after the incident took place, SGS GEOSOL Laboratórios carried out further tests, which confirm the waste from the Fundão dam is not dangerous to human health.
A joint venture between Brazil-based Vale and Australia-based BHP Billiton, Samarco produces around 30 million tonnes of iron ore per year.