Seasonal variations of heavy metals in sediments and aquatic mosses from the Cávado river basin (Portugal)

Abstract

Concentrations of Cd, Cr, Cu, Ni, Pb and Zn in surface sediments and aquatic mosses from the Cávado river basin were determined in order to evaluate the overall metal contamination and trace the main pollution sources. The natural background levels were calculated for both plants and sediments (fraction < 63 μm) collected at uncontaminated sites and concentrations were normalized to the natural levels. The maximum resulting contamination factors in the aquatic mosses ranged from 6 (Zn) to 101 (Cr). In the sediments the accumulation rates are lower, between 3 (Zn) and 18 (Pb). The degree of contamination was also evaluated by calculating a metal pollution index and the more polluted reaches were identified. Metal concentration variations in plants and sediments in two different surveys (1988 and 1989) were studied by factor analysis. Three factors are sufficient to characterize data variance. Factors corresponding to plants and sediments show different metal composition and provide evidence that industrial effluents and drainage waters from a mining area are the main causes of variations of element concentrations. The seasonal variations of metal concentrations both in plants and sediments are also discussed.