Radionuclide Transport by Groundwater Colloids at the Koongarra Uranium Deposit

Abstract

ABSTRACTGroundwater and entrained particles were sampled from several boreholes at the Koongarra uranium deposit in the Northern Territory of Australia. Conventional filtration techniques, hollow fibre ultrafiltration, and stirred-cell ultrafiltration were used to separate dissolved species, large particles (> 1 μm) and colloids (< 1 μm). The colloids and particles included clay minerals, particularly kaolinite, and chlorite, together with fine quartz grains. Iron was present as particle coatings, and in a separate colloidal form. The amount of238U associated with colloids (expressed as a percentage of the total238U which passed through the 1.0 μm filter) ranged up to 6.5%. The corresponding figures for Th were 10-85%. However, the amount of230Th which passed through the 1.0 μm filter was extremely small, and230Th was associated to a much greater extent with larger particles, which are unlikely to be mobile in natural groundwaters. In some fine particle and colloidal fractions, the227Th/230Th activity ratio in the thorium alpha spectrum was unusually high, indicating the presence of substantial quantities of227Ac. This suggested that actinium could be present as a mobile colloid phase. Overall, there was very little colloidal material in these groundwaters, with only iron, uranium, actinium, and thorium showing a significant association with colloids.