Distribuição e registro histórico de metais pesados e assinaturas isotópicas de PB em testemunhos de sedimentos de fundo da Baia do Guajará, Belém-PA

Abstract

tuaries constitute the final destination of a significant part of dissolved and particulate materials that are brought by the rivers toward the ocean. These estuaries also act as the final destination for domestic and industrial effluent, which are most of time discarded in natura. A variety of factors and sources together with the complexity of hydrodynamics and physicochemical conditions of this kind of environment makes the study of metals pollution of estuaries a challenge. The analysis of Pb isotopes is a valuable tool to differentiate anthropogenic from geogenic sources, since the lead dispersed in the environment keeps the isotopic characteristics of the source from which it was derived, once the isotopic compositions of Pb are not affected by physical or chemical processes. The hydrographic system of Belem (PA) belongs to the Guajara estuary. In the eastern margin, is located the Belém city. The west margin is an islander set composed 39 islands. About 30 km of natural drainages divide the city, forming channels that drain into, above all, the Guajara Bay which constitutes the main water reservoir receiving pollutants produced by domestic and economic activities of the city. However, the distribution of pollutants released daily in the bay is difficult to be evaluated, due to the hydrodynamic complexity of the estuary. In this context, the purpose of this study was to combine the determination of metal concentrations with Pb isotope signature to establish a historical record and to distinguish the sources of Pb, whether natural or anthropogenic, in sediments on the west edge of Guajará Bay. Samples were collected using a Russian Peat Borer. A set of seven cores of about 50 cm length was collected and subsequently sliced from 10 to 10 cm, totalizing 31 samples. According to the speed of deposition of approximately 0.7 cm/year, previously determined for the sediments of Guajara bay with geochronology 210Pb, cores collected correspond to a historical record of approximately 70 years. The study included grain size study by centrifugation method, mineralogical identification by X-ray diffraction, geochemical analysis to determine the percentage of organic matter and heavy metal contents by ICP-MS spectrometry and isotopic mass spectrometer thermoionization (TIMS). The grain size and mineralogical study of core in bottom sediments from the west edge of the Guajará bay showed a changing of hydrodynamic regime, which became more energetic, throughout the bay over time. This statement was based on the characterization of sediments, which show predominance of silt and sand fraction with a reduction of the sand fraction with the depth and texture that ranged from sandy to sandy-silty. The mineralogical study no showed change in mineralogical composition of the clays, and kaolinite, illite and smectite are claysminerals predominant. Over time the concentrations of metals vary little, however, have detected a slight tendency to decline in more recent times, probably linked to changes in the hydrodynamic regime. Moreover, there is no evidence in the core of a significant a significant increase in metal concentrations (mean 25.3 to 29.1 mg.Kg-1) with the population increase over the last 70 years. These results indicate that the background of Pb in the bay is significantly higher than in rio Guama (18 mg.Kg-1). The isotopic composition of source 206Pb/207Pb geogenic over the cores ranged between 1.186 and 1.199. The average isotopic signature of 1.193 ± 0.0035 was established as the value of the background to the Guajará bay, this value lower than the average value of 1,196 - 1.20 previously determined for the isotope ratio 206Pb/ 207Pb of Pb geogenic in Belém region.The differences in content and isotopic composition of Pb between this study carried out by Saraiva (2007) and Nascimento (2007) on the west edge of the bay, were explained by the difference of the technique used for sampling. This difference indicated that, despite the population growth accelerated during the last decades the contribution of heavy metals by anthropic action is recent, intensifying over the last 7 years.