The mobility of radium-226 and trace metals in pre-oxidized subaqueous uranium mill tailings |
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| Institution: | 1. Department of Geology, University of Oviedo, C/ Arias de Velasco, s/n, 33005 Oviedo, Spain;2. Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany;3. Institute of Energy and Climate Research (IEK-6) – Nuclear Waste Management and Reactor Safety, Research Centre Jülich GmbH, 52425 Jülich, Germany;1. School of the Environment, Trent University, 1600 West Bank Drive, Peterborough, ON K9L 0G2, Canada;2. Biology Department, University of Ottawa, 30 Marie Curie, Ottawa, ON K1N 6N5, Canada;3. Water Quality Center, Trent University, 1600 West Bank Drive, Peterborough, ON K9L 0G2, Canada |
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| Abstract: | The exchange of 226Ra and trace metals across the tailings-water interface and the mechanisms governing their mobility were assessed via sub-centimetre resolution profiling of dissolved constituents across the tailings–water interface in Cell 14 of the Quirke Waste Management Area at Rio Algom's Quirke Mine, near Elliot Lake, Ontario, Canada. Shallow zones (<1.5 m water depth) are characterized by sparse filamentous vegetation, well-mixed water columns and fully oxygenated bottom waters. Profiles of dissolved O2, Fe and Mn indicate that the tailings deposits in these areas are sub-oxic below tailings depths of ～3 cm. These zones exhibit minor remobilization of Ra in the upper 5 cm of the tailings deposit; 226Ra fluxes at these sites are relatively small, and contribute negligibly to the water column activity of 226Ra. The shallow areas also exhibit minor remobilization of Ni, As, Mo and U. The release of these elements to the water cover is, however, limited by scavenging mechanisms in the interfacial oxic horizons. The presence of thick vegetation (Chara sp.) in the deeper areas (>2 m water depth) fosters stagnant bottom waters and permits the development of anoxia above the benthic boundary. These anoxic tailings are characterized by substantial remobilization of 226Ra, resulting in a relatively large flux of 226Ra from the tailings to the water column. The strong correlation between the porewater profiles of 226Ra and Ba (r2=0.99), as well as solubility calculations, indicate that the mobility of Ra is controlled by saturation with respect to a poorly ordered and/or impure barite phase (Ra,Ba)SO4]. In the anoxic zones, severe undersaturation with respect to barite is sustained by microbial SO4 reduction. Flux calculations suggest that the increase in 226Ra activity in the water cover since 1995 (from <0.5 to 2.5 Bq l?1) can be attributed to an increase in the spatial distribution of anoxic bottom waters caused by increased density of benthic flora. The anoxic, vegetated areas also exhibit minor remobilization with respect to dissolved As, Ni and Zn. The removal of trace metals in the anoxic bottom waters appears to be limited by the availability of free sulphide. Collectively, the data demonstrate that while the water cover over the U mill tailings minimizes sulphide oxidation and metal mobility, anoxic conditions which have developed in deeper areas have led to increased mobility of 226Ra. |
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