COMPARAÇÃO DA CONCENTRAÇÃO DE METAIS PESADOS NAS ÁGUAS DO RIO SÃO FRANCISCO EM TRÊS MARIAS, DESDE 1991 ATÉ HOJE, RELACIONANDO A ATUAÇÃO DA CMM-TRÊS MARIAS.

Résumé

The aim was to evaluate some heavy metals contamination level (Zn, Cd, Pb) and water PH on SãoFrancisco river in Tres Marias, between the CEMIG dam and Barra do Retiro Velho, an area underCMM (Companhia Mineira de Metais) influence, comparing concentration values in 1991 and 2003/2006. Sample waters were collected from some places once contaminated by CMM – CompanhiaMineira de Metais, which, for years, since it begins operating in 1969, threw all industrial efluents innatura in São Francisco River. In 1991 Fachoschule München students researched the CMM surroundingarea and found high values of heavy metals on water. Water sampling followed Standard Methods forthe Water and Wastewater (APHA 1992) and Sample waters Collect and Preservation Guide fromCETESB (1988) and were performed in 4 sampling campaigns. Metals contents were analised byICP-AES on IGC/UFMG Research Centre and conductivity and PH were measured in situ by Digimed,DM 20 device. ZN concentraction leval was above legal level on sample points took in waters ofBarreiro Grande brook and São Francisco river, soon after the water returning point, on left bank andon Consciência brook confluence, which value is 25 times higher than the limit established byCONAMA 357. In comparison with sampled points in 1991, we had then much more elevated values,because CMM does not take care about its effluents deposition. ZN, Cd and Pb had high concentrationson CMM influence area, on the river right bank, this means, in points that receive direct effluentsfrom CMM, and more distant points on the same bank, showing that these metals are carried insoluble form, before precipitating with sediment. After Consciência brook, Zn level graduallydiminishes. One can conclude that CMM’s use of a more appropriate effluent treatment led to a levelreduction of these metals on superficial waters, what is indicated by data interpretation. Nevertheless,we must remember that metals on sediments are still present on the area, as it precipitate amongorganic matter, clay minerals and Fe and Mn oxids and hydroxids, and can suffer ressobilization towater form with changing of physical-chemical water conditions – PH changes, for instance.