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Avaliação da qualidade dos sedimentos de fundo do Rio Murucupi, Barcarena-PA.
(Universidade Federal do Pará, 2011-07-15) REIS, Rafael Melo dos; CORRÊA, José Augusto Martins; http://lattes.cnpq.br/6527800269860568
The development of industrial and urban centres in Barcarena often causes substantial changes in the bodies of water of surrounding areas, primarily in water quality, as a result of inputs of potentially toxic trace elements, as shown the studies of Pereira et. al. (2007), Víglio (2008), Porto (2009) e Pereira (2009). In addition, major environmental accidents has been recorded, mainly related to the disruption and overflow of accumulation dams of industrial waste. In this context, in 2003 and 2009, the river basin Murucupi, wich incudes the urban centres of Vila dos Cabanos and Laranjal, was contaminated by rejects from the beneficiation process bauxite, known as “red mud”. The objetive of this study was to assess preliminarily the bed sediment quality of Murucupi river and distribution of major elements (Al2O3, Fe2O3, TiO2 e Na2O) and trace (Zr, V, Cr, Th, Hf, Ga, La, Sc, Pb, Nb, As, Cd, Cu, Hg, Ni e Zn), enriched in red mud, along the river. For this, triplicate cores were collected with 50 cm deep in bed sediments, in seven ponits along the Murucupí river, in the 5,3 km stretch between the access bridge to the Vila dos Cabanos (M-1 point) and the river mouth (M-7 point). Each sediment core collected in the field was subdivided into five depth intervals in the laboratory: 0-10 cm (A interval), 10-20 cm (B interval), 20-30 cm (C interval), 30-40 cm (D interval) e 40-50 cm (E interval). The triplicate samples from each respective depth interval were then thoroughly homogenised, and later, size, mineralogical and chemical analyses were performed in each sample. The results showed that the sediments are primarily composed of quartz, mica group of minerals, priority muscovite, and albite, in addition to clay – kaolinite, smectite and ilite. The superficial concentrations of the major elements P2O5 and S, and of the trace elements V, Cr, Ga, Pb, Hg and Zn, were higher in M-1 point with a strong decrease toward to mouth, indicating contamination by discharges of urban runoff and domestic sewage from the Vila dos Cabanos and Laranjal. In other hand, the superficial concentrations of S, As and Pb, were high in M-7 point, indicating contributions of these elements from the Arrozal stream, possibily arising from the port of Barcarena and surrounding areas, brought to the mouth of the Murucupi river through tidal movements. The concentrations of these trace elements into beb sediments were much smaller than values measured by Braga (2007) in red mud. This fact is associated to methodological differences used na chemical analysis and to thickness of the sample. However, the statistical correlations performed among Al and Cr, V, Ga and Th, and among Mg, K and Hf, La, Sc and Zr, as well as among Mg, K and the "silt + clay" particle size fraction over the depth, demonstrate that exist a "geochemical signature" of the events of contamination by red mud, recorded in A interval of sediment cores collected in the Murucupi river. The results obtained by the distribution of elements with depth showed that, in general, the major elements Al2O3, MgO and K2O are associated with “silt + clay” particle size fraction, and that the variations in the concentrations of the trace elements, mainly reflect environmental conditions more favorable for mobilization or persistence of these elements over the depth. With regard to the sediments quality, the concentrations mensured for each element were compared with sediment quality indices, known as Sediment Quality Guidelines – SQG, developed by Canada and adopted by Brazil through CONAMA resulution nº 344/2004, and with the indices “consensus-based”, developed by Macdonalds et al. (2000), and also, with reference values of background in the region. Some places, the bed sediments of Murucupi river showed concentrations above ISQG and below PEL for the following elements: 1) Cr in M-1 point, A (38,4 mg.Kg-1) and E (43,1 mg.Kg-1) intervals; 2) Ni in M-6 point, E (18,9 mg.Kg-1) interval, and in point B-1, A (18,6 mg.Kg-1), B (18,3 mg.Kg-1), C (18,4 mg.Kg-1) and E (18,3 mg.Kg-1) intervals; 3) As in M-1 point, C (5,9 mg.Kg-1) interval; and 4) Hg in M-1 point, E (0,199 mg.Kg-1) interval, and in M-2 point, E (0,233 mg.Kg-1) interval. These results indicating that, in these sites, adverses effects to benthic organisms may occur due the high concentrations of Cr, Ni, As and Hg in beb sediments, wich entails additional studies of bioavailability in sediments and toxicity in benthic organisms. Highlights that the elements Pb, Cu, Ni, As, Hg and Zn are enriched over the referece value of regional background at all points sampled in Murucupi river, being that the Cr enrichment also presented in relation to the concentration mensured by Nascimento (2007) in bed sediments of Guamá river. and Pb enrichment only at specific points, related to the hydrodynamics of the river. The results obtained for the Ni raised two hypotheses: 1) in the basins of the Murucupi river and of the reference stream, may exist difusal or eventual sources of contamination for the Ni; or 2) the Ni can be naturally enriched in the region, since the statistical correlations among Mg and Ni in the sediment cores of Murucupi river were good or excellent in nearly all depth intervals. In general, the results obtained showed that the contamination in sediments of Murucupi river is much more associated with difusal sources of pollution, probably from urban runoff and discarges of domestic sewage, than with the environmental accidents involving red mud.