In estuarine systems, trace metal sources are both natural and anthropogenic. It
is often difficult to distinguish between these two sources due to highly variable
concentration in the water column. Many studies have shown the usefulness of
sediments in understanding the anthropogenic contribution to trace burdens in estuarine systems. Located within a very important industrial region, near Belém city in the Guajará estuary, the Barcarena River receives discharges of urban and industrial runoff. The objective of this study is to study Barcarena River bed sediments to assess the anthropogenic chemical inputs into the river system. Fifteen sampling locations were chosen along the river channel. Bed river sediments samples were collected, and analyzed for their chemical and mineralogical composition. Data revealed that the clay mineral identified was composed mainly of smectite, kaolinite and Illit. The trace elements and heavy metals analysis showed the following average concentrations and standard deviation: Pb (32.54 mg.Kg-1, σ = ± 5.6), Cr (104.33 mg.Kg-1, σ = ± 7.98), Ni
(34.27 mg.Kg-1, σ = ± 4.48), Cu (34.71 mg.Kg-1, σ = ± 4.38), V (116.33 mg. kg-1
, σ = ± 42.37), Nb (19.86 mg.Kg-1, σ = ± 3.83), Zr (122.31 mg.Kg-1 σ = ± 21.33). The highest concentration of the studied elements was found near São Francisco urban center, probably due to the presence of a harbor and unloading of solid waste and sewage in the region. Concentrations of trace elements in the sediment were a function a particle size distribution and organic content. The element concentration of Pb, Cr, Cu and Nb in sediments of Barcarena river when compared with sediments of Guajará Bay (Saraiva, 2007), Guamá river (Nascimento, 2007) and bauxite ore (Braga, 2007) show higher values than the two first but lower values than bauxite. The finger print diagrams with heavy metals versus Al suggest the possibility of physical mixing between the bauxite and sediment of the river Barcarena. The elements Pb and Cu had shown concentrations higher than TEL values (Threshold Effect Level), 35 mg.Kg-1 and 35.7 mg.Kg-1 respectively. Therefore, the bottom sediments probably are causing adverse effects to benthic organisms. The elements Cr and Ni, shown concentration values that exceeded the TEL index (Cr = 37.3 mg.Kg-1 and Ni =18 mg.Kg-1) and PEL index, (Cr = 90 mg.Kg-1 and Ni = 36 mg.Kg-1. A value above the PEL index suggests that toxic effects on aquatic biota are frequent. Currently, the introduced metals are deposited near the source area and are mostly associated with the sediment, implying that the river bed sediment acts mainly as a sink, rather than a pool. The accumulated and enriched toxic trace elements can pose a potential pollution of river water and consequently the aquatic biota.
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