Abstract
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, runoff from the waste rock dump accumulates in a retention pond (RP4). This water is permitted to discharge to the nearby Magela Creek once minimum flow of 5 m 3/sec is reached and following filling of the pond. The discharge proceeds via a channel, experimental wetland and a backflow billabong (Djalkmarra Billabong) which acts as a natural wetland filter and flows out to Magela Creek. This study examines monitoring data for water releases over 3 wet seasons. I wet season with no release and 4 dry seasons. The monitoring data comprised electrical conductivity (EC). pH, Na, K. Ca, Mg. HCO − 3 SO 2− 4, Cl − and U (total or filtered, < 0.45 μm). Some ICP-MS scans of trace elements were also undertaken 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 from solution, from about 50 ppb down to < 1 ppb. Soluble salts may remain 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) suggests that cationic forms of U, such as (UO 2) 3 (OH) 5, predominate, favouring adsorption on to the humic-rich sediments of the natural wetland. The application of this principle enables U to be removed efficiently from waste water and to be contained within the mine lease.
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