Metais pesados, sedimentos de fundo e peixes no rio Tapajós

Abstract

This study aimed to assess the level of heavy metals, especially mercury, in water, bottom sediment and fishes in the Tapajos river, checking the possible impacts of small-scale gold mining activities on this river environment. The results were compared to some data available in the database of CTHidro sectorial fund - Project AquaRios, held at the aquatic ecoregion Xingu-Tapajos, in order to validate the present data. Water samples were submitted for analysis of concentrations of total mercury (HgT), by cold vapor atomic absorption spectroscopy. To characterize the environment physico-chemical parameters of water samples, such as temperature, pH, conductivity and dissolved oxygen were analyzed . For the analyzes of total mercury in fishes it was also used the technique of cold vapor atomic absorption spectroscopy, and the quantification of methyl mercury (MeHg), followed by the analytical protocol of the National Institute for Minamata, Japan. In the sediment samples, total mercury as methyl mercury were quantified. Other metals were also analyzed for water and sediment samples, such as Fe , Mg, Ca , Cd, Pb, Cu, Cr, Zn, and Al, by using the ICP-OES (Inductively coupled plasma optical spectroscopy) method. The results of total mercury concentration in the samples were lower than 0.5 ng/L. Considering the results for other metals in water, only the Cd, Zn and Al showed concentrations above the values established by CONAMA 357/2005. In the sediment samples HgT concentration ranged from 23.5 to 36.5 ng /g, while for MeHg the range was 15.5 to 24.6 ng/g. For the other metals, concentrations were below the limits prescribed by environmental regulation (CONAMA 357/2005), except the Fe, for which no reference was made to the limit in the regulation. Mercury concentrations in fishes were more significant in carnivorous than in omnivorous feeding habits fishes. The highest concentrations were found in tucunare with HgT levels between 600 and 950 ng/g and 510-600 ng/g MeHg. In other fish species, the concentrations were lower: for (white) pescada, the values were 180-534 ng/g HgT and 230-345 ng/g MeHg ;for the piau to 50-104 ng /g HgT and 25-60 ng /g MeHg, while the concentration in jaraquis were 17-62 ng /g HgT and 7-33 ng /g MeHg. There was no evidence of contamination occurring in biotic and abiotic samples. However, omnivorous feeding habits fishes, due to the lower mercury concentrations, are better suited to the diet of the riverside population, which may reduce the risk of mercury contamination via food.