Impacto do rompimento de uma barragem de rejeitos de minério de ferro sobre a qualidade das águas superficiais. Estudo de caso: bacia do Rio Doce

Abstract

Due to the great impact associated to the failure of a mine tailings dam and due to the fact that there are few studies analyzing the resulting impact, this work has great relevance for analyzing the impact of the dam failure in the surface water quality and the ability to recover of the waterbodies directly affected. As a study object, the quality of surface water was analyzed before and after the rupture of an iron ore tailings dam in the State of Minas Gerais on November 5, 2015. The development of this study consists on the use of the water quality data from IGAM in twoperiods: between October 2008 and October 2015, characterizing the pre-failure period; and between November 2015 and March 2017, characterizing the post-failure period. Water quality analyzis were carried out for the period (i) above and (ii) after the rupture in the impactedstretches; and (iii) a comparison between the both periods. The following multivariate statistical techniques were used: Cluster Analysis (CA), Principal Component Analysis (PCA), Factor Analysis (FA) and non-parametric Kruskal-Wallis (KW) analyzes, followed by Dunn's multiple comparisons and the non-parametric Mann-Kendall (MK) trend test. As a result, it was found that even before the failure, the studied watercourses have already been strongly impacted by economic activities and the discharge of domestic sewage into the rivers. Above all, the stretch of Belo Oriente-MG that has continued to be one of the most impacted even after the disruption, especially due to the confluence of the Piracicaba river upstream of this municipality. After the disruption, it is observed that the highest concentrations were measured immediately after the passage of the tailing wave by the monitoring station. However, the chromium, mercury and cadmium metals were again monitored below the LD one or two days after the passage of the tailing wave; in addition, all the parameters in almost all the stations presented downward trend with the time, except the dissolved oxygen that, as expected, presented tendency of increase with the time. The parameters that did not correlate with the time and either present a downward trend with the time were monitored by the station located in Aimorés-MG, which, being the last municipality from Minas Gerais in the route of the tailing wave, it was the site where the wave presented a lower speed and lower volume. The less impacted stretch after the failure is downstream, between Resplendor-MG and Aimorés-MG; and the most impacted stretch is upstream, between Barra Mansa-MG and Belo Oriente-MG. Therefore, the river sections in analyzes showed a tendency to recover the quality of the surface water with the time and that the greater the distance between the point of the dam failure and the point of monitoring, thelower is the impact on the surface water quality