SEASONAL-VARIATIONS OF HEAVY-METALS IN SEDIMENTS AND AQUATIC MOSSES FROM THE CAVADO RIVER BASIN (PORTUGAL)

Concentrations of Cd, Cr, Cu, Ni, Pb and Zn in surface sediments and a quatic mosses from the Cavado 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 mum) collected at uncontaminated sites a nd concentrations were normalized to the natural levels. The maximum r esulting contamination factors in the aquatic mosses ranged from 6 (Zn ) to 101 (Cr). In the sediments the accumulation rates are lower, betw een 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 sediment s in two different surveys ( 1988 and 1989) were studied by factor ana lysis. Three factors are sufficient to characterize data variance. Fac tors corresponding to plants and sediments show different metal compos ition and provide evidence that industrial effluents and drainage wate rs from a mining area are the main causes of variations of element con centrations. The seasonal variations of metal concentrations both in p lants and sediments are also discussed.