Characterization of Uranium and Rare Earth Element Mobility and Attenuation Downstream of Decommissioned Tailings Impoundments at the Bicroft Mine near Bancroft, Ontario
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Attenuation of uranium (U) and rare earth elements (REEs) has been observed in stream and wetland sediments, but the geochemical and mineralogical processes involved in sequestering these elements in natural systems are not well understood. The decommissioned Bicroft Uranium Mine near Bancroft, ON uses a modified stream and wetland system to reduce the concentrations of U and other metals in tailings pond effluent to levels below the Provincial Water Quality Objectives. The Bicroft Mine was operated from 1957 to 1963, and processed low-grade (~0.17 wt% U3O8), disseminated U hosted by pegmatite dykes in amphibolite gneiss, forming 2,284,421 tonnes of tailings, deposited into two tailings impoundments. The Bicroft site has since operated as a passively attenuating stream and wetland remediation system for 55 years, demonstrating the potential longevity and viability for long-term sequestration of U and REEs through natural attenuation. To identify U and REE hosts and their stability in the natural environment, colloids, tailings, stream and wetland sediments were analyzed using various methods, including tangential flow filtration (TFF), ICP-ES/MS, scanning electron microscopy, and synchrotron techniques (bulk and µXANES, µXRF, and µXRD). The results show that Fe- and Mn-oxyhydroxides, goethite, and birnessite are the main mineral hosts for U and REEs in both the colloids and sediments. In addition, detrital grains of U- and REE-bearing minerals were found >200 m downstream in colloids and wetland sediments, showing the potential for long range transport of colloids and particulates in the stream system. Seasonal influences on the stability of trace metals in sediments were observed, and may demonstrate the limitations of passively attenuating remediation systems as a viable method for attenuation. Changes in the redox state of the stream system were observed to influence the attenuation of U and REEs, however, changes in the redox state with depth in the wetland sediments were not observed, and showed no direct influence on the attenuation of U and REEs. The results of this study will help to develop better monitoring strategies for U tailings sites and should reduce the impacts of future U mining operations.