Diagnóstico da qualidade ambiental das bacias do rio Mau e Caima : estudo da dinâmica dos processos naturais e antrópicos e definição de zonas vulneráveis

Abstract

O estudo geoquímico e biológico realizado na bacia hidrográfica do rio Caima permitiu a identificação e a avaliação dos impactos ambientais nesta bacia hidrográfica com particular ênfase nas áreas envolventes de antigas áreas mineiras. Nestas áreas foram detectadas concentrações elevadas em Pb, Zn, Cd, Cu e Fe nos sedimentos de corrente e em Pb e Zn nas águas de superfície (especialmente no período do Outono). Uma avaliação completa da qualidade do meio aquático foi estabelecida a partir da associação de índices geoquímicos (Índice de Contaminação-IC) com índices biológicos (Índice de Poluossensibilidade Específica-IPS e o Índice Biológico Diatómico-IBD).

The main purpose of the geochemical and biological study was the identification and assessment of the possible environmental impacts in the hydrographical basin of Caima and Mau rivers, with special emphasis on the areas surrounding old mines of Pb, Cd and Zn. Data from stream sediment samples (minus 80 mesh fraction) were used to undertake a preliminary delimitation of the extent of contamination from Coval da Mó, Palhal and Braçal mines. The chemical data, particularly Pb, Cd and Zn concentrations, reveal that significant environmental contamination occurs in the vicinity of the abandoned mines. The presence of tailing deposits is identified as the main source of Pb, Cd and Zn in the area. Extremely high Pb (3303 mg kg-1) and Cd (18.5 mg kg-1) concentrations were observed in stream sediments collected in the Corga do Coval da Mó and Mau streams as a result of the surface runoff. A decreasing metal gradient was observed from Corga do Coval da Mó stream down to the Caima river. In order to study the effects of anthropogenic contaminants and other environmental variables on the taxonomic composition, structure and morphological changes of benthic diatom communities within a former mining area (Coval da Mó), 20 sites were selected in the Caima and Mau river basins. At the mining area, in the Corga do Coval da Mó stream, diatoms were extremely rare. Two km downstream low abundance and diversity of diatom taxa indicated a stressed environment. There, especially, Fragilaria capucina var. rumpens but also Fragilaria crotonensis and Achnanthidium minutissimum showed abnormal valves which may be related to the high levels of metals in this area. Six km downstream in Fílvida mainstream, an increase in species richness and diversity was registered while the relative percentage of teratologies was lower. Available data on water chemistry suggest that the diatom community patterns are mostly justified by the metal contamination, because nutrients and organic matter revealed low concentrations, and acidification did not occur. The levels obtained for heavy metal concentration in sediments (in particular Pb, Zn, Cd, Cu and Fe) and those obtained for superficial waters (Pb and Zn only in autumn), showed the existence of contamination in the surroundings of industrial and abandoned mining areas. A more reliable interpretation of the collected data and a more complete assessment of the water quality were made possible by the association of geochemical indices (CI – Contamination Index) and biological indices SPI - Specific Pollution Sensitivity and BDI – Biological Diatom Index).