The watershed flux of mercury examined with indicators in the Tucurui reservoir in Para, Brazil

The concentrations of total mercury (Hg) in soils, sediments, sedimentable material and floating plants were studied in order to find out seasonal and areal variations in the flux of mercury in the man-made Tucurui reservoir in Para, northeastern Brazilian Amazon. The rise in Hg concentrations with increasing depth of the soil indicates no elevation in Hg deposition due to the atmospheric Hg emissions from gold mining. Hg content in sediments is dependent mainly on the content of organic matter and is highest on the outskirts of the reservoir. The flux of Hg increased more than fivefold from the dry to the rainy season in the former river Tocantins channel. The concentrations of Hg in floating plants were higher in the roots and submerged leaves than in the shoots and floating leaves. Seasonal variations in Hg levels could be explained by competition with nutrients or growth dilution effects. Properties of the soil, the vegetation type and the natural fluctuation of water might affect the concentration of Hg in the floating plants. Gold mining areas situated in the region of the reservoir are polluting the reservoir mainly via the Tocantins river. Hg is continuously accumulating in the reservoir at the rate of 235 kg year(-1) in suspended solids of the Tocantins river, with great seasonal variation.