In the Vromos Bay area, Southeastern Bulgaria, near the Black Sea coast, drainage waters from flotation tailings ponds and piles of low-grade polymetallic ores are heavily contaminated with radioactive elements (U, Ra, Th) and some toxic heavy metals (mainly Cu, Cd, Pb, Mn). Some of these waters were treated by a natural wetland characterized by a plant community in which Phragmites communis was the dominant species. Different species of Scirpus, Juncus, Carer and Poa as well as some algae were also present. However, Typha latifolia and Typha angustifolia were very rare and were present as single plants in this wetland. The pH of the waters being treated was in the slightly alkaline range (from 7.5 to 8) and flow rate varied from 0.2 - I l/s. The watercourse in the wetland was about 80 m long and water depth varied in the range of about 5 - 30 cm. The water treatment was started in March 1998 and until now (May 1999) the wetland effluents were characterized by pollutant concentrations decreased below the relevant permissible levels for waters intended for use in agricultural and/or industry. The removal of pollutants was due to different processes but the microbial dissimilatory sulphate reduction and biosorption played the main role. Uranium was precipitate mainly as uraninite (UO2), the heavy metals were precipitated mainly as the relevant insoluble sulphide, and the largest portion of radium was absorbed by the organic matter (both viable and dead) and clay minerals present in the wetland. Negative effects of the pollutants on the growth and activity of the indigenous plant and microbial communities were not observed. The efficiency of the water clean up markedly depended on the ambient temperature. However, excellent results were achieved even in the cold winter months (December - February) at temperatures about 0 degrees C.
