Stable lead isotopes, trace metals and radionuclides in sediments of the Alligator Rivers Region to assess impacts of uranium mining

摘要

Post-mining environmental protection is the key to successful rehabilitation; especially where runoff and erosion from rehabilitated mine sites can lead to pollution and contamination of the surrounding land and watercourses. Australian guidelines recommend that tailings materials from uranium mining and milling be contained without any detrimental impact on the environment for at least 1000 years.The Alligator Rivers Region (ARR) in Northern Australia includes a large area of Kakadu National Park (KNP), which is world heritage listed for its conservation value. Within the ARR are numerous uranium (U) anomalies and prospects, one current operational U mine and several former U mines.The aim of this study was to develop a sensitive methodology to assess the impacts of U mining on sediments in the ARR. This involved an appraisal of stable lead isotopes, radionuclides and trace metals within sediments and soils, including surface and core samples, to determine the off-site impacts from a spatial and temporal perspective. The sites investigated were Ranger Uranium Mine (currently operating mine site), Nabarlek uranium mine (rehabilitated former mine site), Koongarra Mineral Lease (former exploration activities but not mined) and Ranger Anomaly 2 (identified U orebody within KNP).The study found that there are localised areas on and adjacent to the mine sites and anomalies where soils had elevated levels of trace metals and radionuclides. Lead isotope ratios are highly radiogenic in some samples, indicating the presence of U-rich material. Erosion products with more radiogenic Pb isotope ratios have also deposited in sediments downstream of the former ore bodies.However, there is no indication that the radiogenic erosion products found on the mine sites/anomalies have significantly contaminated sediments further downstream. The monitoring methodology is able to determine very small percentage (~1 per cent) contributions of erosion products equivalent to very low additional mine or orebody derived Pb/U concentrations of less than 0.1 mg kg-1 Pb and 0.05 mg kg-1 U, compared to background concentrations of typically 11 mg kg-1 Pb and 3 mg kg-1 U. This contribution is less than the concentration variability observed within the sediments and would not be discernible without the use of lead isotope ratios.This research project demonstrated that a sediment monitoring approach integrating lead isotope ratios with trace metals and radionuclides can deliver more information than metal and radionuclide concentrations alone, especially in terms of identifying sediment sources and their respective contributions to mining impacts. The isotopic monitoring methodology can be readily reproduced and transferred to other sites and it will ultimately provide better measurement of rehabilitation success.