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Gold tailings dams from the Witwatersrand Basin usually contain elevated amounts of heavy metals and
radionuclides. Uranium, in the form of uraninite (UO2) and brannerite (UTi2O6), is normally associated with
gold-bearing ores in the basin. As a result of acid mine drainage (AMD), uranium is released into
groundwater and fluvial systems. Its transport, retardation and immobilization depend strongly on the
uranium species and prevailing geochemical conditions.
This study was aimed at the quantitative assessment of the distribution of uranium and the modeling of its
geochemical speciation. Geochemical analyses of tailings, water and sediment in areas of previous mining
were performed. The results indicate that there is active leaching of uranium from the tailings, transport of
soluble uranium species through water systems, with subsequent deposition of insoluble uranium species in
sediments of fluvial systems. Analysis of tailings material indicated that mobilization and transportation of
uranium from the tailings resulted in its decoupling from its progeny which remained largely unaffected by
the weathering effects. Mobilization occurs as uranium is oxidized to the U(VI) state which dominates
aqueous chemistry. The U(VI) is reduced to U(IV) which is immobile and is subsequently deposited in the
wetland sediments downstream from the primary acid mine drainage. Geochemical modeling of uranium
speciation revealed the two most influential hydrogeochemical facies in uranium mobility, namely a
sulphate-dominated AMD system; and lime-neutralized carbonate-dominated system. In both cases the
uranium was shown to be soluble throughout a very wide pH regime, thus yielding important information for
risk assessment considerations.
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