Evaluating the surface water quality after the rupture of an iron mining tailings.

The aim of this article is to assess whether the collapse of the Fundão tailings dam still impacts the surface water quality of the Doce River in the state of Minas Gerais (Brazil). The records of 15 water quality parameters were evaluated, comparing data from before and after the accident. Nine sampling points were selected, six of which corresponded to water intakes for the water supply systems. The records showed that after the collapse of the Fundão Dam, there was an increase in the percentage of non-conforming values for dissolved Fe, dissolved Al, true color, turbidity, and total suspended solids, but a reduction in total Mn and total Pb, based on the Normative Consideration COPAM/2008. The time series analysis showed no trends, with Levene's test indicating variance changes only for dissolved Fe, total dissolved solids, and dissolved Al. Mann-Whitney tests revealed no median changes in key parameters after the Fundão Dam collapse. For the seven trace metals evaluated (As, Cd, Cr, Pb, Hg, Ni and Zn), the Mann-Whitney and Pettitt tests allow us to infer that there was a change in the median concentrations and an abrupt change in the data series after the dam rupture. It is not possible to associate any plausible cause for the changes observed in some tests and monitoring stations in the records prior to the accident. For records after the accident, the prospect of the Doce River's recovery still lacks greater statistical robustness, emphasizing the relevance of maintenance of water quality monitoring stations in the Doce River channel.