Estudo sedimentológico e geoquímico em sedimentos de fundo na Baía de Guajará-Belém (PA)

Abstract

During the last years, many Brazilian estuaries located close to great cities have been suffered several environmental alterations caused by the accelerated and disordered growth of these cities. These alterations may occur from heavy metals that reach the water bodies being adsorbed by the sediments. In this context, the bottom sediments play an important role, because they can work as indicators of the level of pollution of these environments, becoming still more relevant when anthropic processes are considered. With the objective to evaluate the degree of contamination of the sediments from the Guajará bay, mainly in areas close to the margin of the Onças, jaraquinha, Mirim and Jutuba islands and also close to the Val de Cans channel, sedimentological, mineralogical and chemical analyses were carried out from bottom surface sediments and cores of about 50 cm depth. The identified sedimentary facies in the cores were sandy mud with thinly evenparallel rhythmitic bedding, sandy mud with cross lamination, sandy mud with longitudinal cross bedding and wavy lamination. These structures, particularly the evenparallel rhythmitic bedding and the longitudinal cross bedding, indicate tidal influence and lateral channel migration, respectively. The grain size results of the analyzed bottom surface sediments showed the dominance of sandy silt and silty sand and only one sample has been classified as sand. The mineralogical study defined the presence of quartz, kaolinite, illite, smectite and heavy minerals which include staurolite, tourmaline, rutile, kyanite, zircon, epidote, amphibole (hornblende) and sillimanite, being the most abundant minerals tourmaline, kyanite, staurolite, zircon and rutile. With the exception of epidote and hornblende which suggest direct provenance (first cycle) from metamorphic/igneous rocks, the other minerals seen to be derived from recycled Barreiras and/or Post-Barreiras sediments. The analyses of organic matter and the chemical analyses of the bottom sediments provided the following average concentrations: Organic matter 1,87±0,84%; heavy metals Pb 33,3±4,9 mg.kg-1; Cr 59,7±8,3 mg.kg-1; Co 10,8±2,2 mg.kg-1; Ni 19,3±4,4 mg.kg-1; Zn 73,7±15,1 mg.kg-1; Cu 18±2,8 mg.kg-1; Mn 489,2±166,5 mg.kg-1 and Fe 3,6± 0,6% The chemical results obtained from three cores are not very different from those of the bottom surface sediments, with the exception of Pb and Zn whose mean values are lower in the cores. How one could expect, a significant correlation has been obtained between the metals and Al which stands for the clay minerals. All the concentrations of the analyzed metallic elements in the bottom sediments of the Guajará are lower than their PEL index which represent the concentration above which adverse effects on biota are expected to occur frequently. In the surface sediments all Cr and some of the Ni and Pb values are higher than the TEL index, which represent the concentration below which adverse effects are expected to occur only rarely, whilst in the cores only the Cr and some Ni values surpass the TEL index. Pb, Cr and Ni of the bottom surface sediments suggest anthropogenic influence, but at present their values do not represent any risk for the estuarine environment.