Estudo dos metais pesados na Amazônia: estudo de caso nas cidades de novo Aripuanã, Barcelos e Manaus no estado do Amazonas

Abstract

Concentrations of heavy metals, as cádmium (Cd), Lead (Pb) and mercury (Hg) were observed in samples of sediment of the bottom of narrow channels and rivers, fishes and human hair, collected in three municipalities of Amazonas State in Brazil: Manaus, Barcelos and Novo Aripuanã which present different features with regard to the use and occupation of the soil. Among the municipalities, Manaus presents the largest demographic densities and industrialization, generating consequently more degradation of the quality of the waters, what stresses the necessity of frequent studies in this system. With the purpose to investigate the substances of heavy metals in the sediments of the bottom of the narrow channels and rivers, in the tucunarés and in human hair, it was carried out a survey of references about the Hg in fishes and hair contaminated from Minamata in 1950. From 268 samples, 110 were of sediments of the bottom of narrow channels and river, 84 of tucunarés and 74 of human hair. The greatest quantities of samples presented substances of trace metals below of what is allowed by the World Health Organization, with the exception of Hg that in the samples of the sediment of the bottom of the Negro river in front of Manaus city indicated 6,8 µg/g. The samples of the sediment of the bottom were surveyed every two months during the year of 2008. The fishes were examined in the year of 2009 in two periods: dry season and rainy season and the hair in the first semester of 2010. The concentrations of Al, Ag, B, Ba, Ca, Cd, Cr, Cu, Fe, La, Mg, Mo, Pb, Sc, Ti, V, Y, W and Zn were determined by atomic absorption spectrometry with plasma ICP simultaneous and those Hg by atomic absorption spectrometry with cold vapor. The sediment of the bottom of 20cm, 40cm and 60cm of Novo Aripuanã river was analyzed by diffractommetry of x-ray, being found the minerals quartz, kaolinite, rutile and montmorillonite. The results obtained indicated low concentration of metals Cd and Pb, as well in the dissolved forms as in the particulate. Nevertheless such concentrations were greater in the municipality of Barcelos. In the sediment of the bottom, the greatest concentrations of Pb were observed in the Negro river, though this element occurred in small quantities in the exchangeable fraction of the sediment (>1%). On the other hand the greatest concentrations of Pb in tucunarés borboleta were detected in the Negro river. In the exchangeable fractions of the sediments, the Cd occurred in low proportions (up to 5%), mainly in the Negro river (Basin of the low Negro river) and Madeira river. The survey also revealed that there is correlation between Al and Sc in the sediments of the bottom and that in the tucunarés of 60cm the substances of Hg were greater than in the tucunarés of 40cm which always low. Nevertheless no significant tendency of spatial or temporal distribution of Hg occurred in the samples. Therefore it is believed that the highest substances registered for some samples resulted from non-punctual diffused sources, as atmospherical deposition and sources originating from the forest. In the smallest tucunarés, the concentrations of Cd and Pb were always lower than the limits of detection (0,005 and 0,08 µg.g-1 dry weight respectively). On the other hand, tucunarés with more than 3kg collected near the city of Barcelos (Rio Negro Basin) presented high concentrations of Hg (average: 0,60 +/- 0,34µg/g humid weight) and methylmercury (average: 0,50 +/- 0,2µg/g humid weight). The last one represented on average 92% of the total mercury determined in the muscles of fish. Such concentrations were smaller than the maximum allowed by Brazilian legislation for predatory fish (1 μg/g) humid weight and, probably, resulted form the high substances of methylmercury produced in the environment, due to the processes of production and discharge of mercury from the vegetation and the flooded soil. The results obtained in human hair in the municipality of Novo Aripuanã differed according to the age and the type of feeding (0,002 to 0,01 μg/g from 10 to 14 years of age; from 0,020 to 0,033 μg/g with the age from 15 to 18 years) and the greatest occurrence was from the age 19 to 24 years (0,066 to 0,096 μg/g). Consequently, it was concluded that the tucunarés of Aripuanã river were not contaminated. The Negro river Basin presented quantities relatively elevated of Hg an Pb with regard to other basins of all the elements investigated. On the other hand, concentrations of some trace metals were observed, they continue to increase perhaps influenced by the neighboring vegetation or even by the river and small channels and because of this fact they must be evaluated periodically, whereas others are decreasing and another class is stable in the sediments of the bottom of small channels and rivers in Manaus city.