THE ROLE OF BIOGEOCHEMICAL PROCESSES IN MINIMIZING URANIUM DISPERSIONFROM A MINE SITE

The Ranger Uranium Mine located in the Alligator Rivers Region of the Northern Territory lies in the tropical zone and has an annual wet-dry monsoonal climate. Following the commencement of the wet season, runo ff from the waste rock dump accumulates in a retention pond (RP4). Thi s water is permitted to discharge to the nearby Magela Creek once mini mum flow of 5 m(3)/sec is reached and following filling of the pond. T he discharge proceeds via a channel, experimental wetland and a backfl ow billabong (Djalkmarra Billabong) which acts as a natural wetland fi lter and flows out to Magela Creek. This study examines monitoring dat a for water releases over 3 wet seasons, 1 wet season with no release and 4 dry seasons. The monitoring data comprised electrical conductivi ty (EC), pH, Na, K, Ca, Mg, HCO3-, SO42-, Cl- and U (total or filtered , < 0.45 mu m). Some ICP-MS scans of trace elements were also undertak en with particular reference being made to Re and U. Specific features of the sequence of water accumulation, release and reconstitution of Djalkmarra Billabong are able to show that U is effectively removed fr om solution, from about 50 ppb down to < 1 ppb. Soluble salts may rema in in the water column and are removed by dilution following discharge to Magela Creek. Sediment levels show no increase in U concentration with time. The pH of the billabong water during releases (6.0-6.6) sug gests that cationic forms of U, such as (UO2)(3) (OH)(5+) predominate, favouring adsorption on to the humic-rich sediments of the natural we tland. The application of this principle enables U to be removed effic iently from waste water and to be contained within the mine lease.