Treatment of acid drainage in a uranium deposit by means of a natural wetland

Abstract Acid drainage waters generated in the uranium deposit G-1, Western Bulgaria, were treated by means of a natural wetland located in the deposit. The waters had a pH in the range of about 2.4 3.9 and contained uranium and radium radionuclides, heavy metals (copper, zinc, cadmium, iron, manganese), arsenic and sulphates in concentrations usually much higher than the relevant permissible levels for waters intended for use in the agriculture and/or industry. The wetland was characterized by abundant and emergent vegetation and a diverse microflora. Typha latifolia, Typha angustifolia and Phragmites australis were the main plant species in the wetland but representatives of the genera Scirpus, Juncus, Eleocharis, Potamogeton, Carex and Poa as well as different algae were also present. The water flow through the wetland varied in the range at about 0.2 1.2 l/s reflecting water residence times in the wetland of about 10 50 hours. An efficient water cleanup took place in the wetland, even during the cold winter months at ambient temperatures close to 0°C. The removal of pollutants was due to different processes but the microbial dissimilatory sulphate reduction and the sorption of pollutants on organic matter (living and dead plant and microbial biomass) and clays present in the wetland played the main role.