Acid waters in a uranium deposit located in western Bulgaria were treated by means of a natural wetland located in the deposit. The waters had a pH in the range of about 5.5.-6.8 and contained uranium, radium, manganese and sulphates as pollutants. The wetland was characterized by an abundant water and emergent vegetation and a diverse microflora. Typha latifolia and Phragmites australis were the main plant species in the wetland but Typha angustifolia and species related to the genera Juncus, Scirpus, Eleocharis, Carex, and Poa as well as different algae were also well present. The watercourse through the wetland covered a distance of about 80 m and the water flow rate varied in the range of about 0.2 - 1.0 1/s.An efficient removal of the pollutants was achieved during the different climatic seasons, even in the cold winter months at water temperatures close to 0°C. The removal was connected with different processes but the microbial dissimilatory sulphate reduction and the sorption of pollutants by the living and dead plant and microbial biomass and by the clay minerals present in the wetland played the main role. However, a significant portion of manganese was removed as a result of the prior bacterial oxidation of Mn~(2+) to Mn~(4+), followed by precipitation as MnO_2. Negative effects of the pollutants on the plant and microbial communities in the wetland were not observed.
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