Natural Analogue Study of the Distribution of Uranium Series Radionuclides between the Colloid and Solute Phases in the Hydrogeological System of the Koongarra Uranium Deposit N.T., Australia

Abstract

There is an urgent need to establish a body of empirical data on the ‘real world’ behaviour of natural colloids in order to model realistically the subsurface transport of radionuclides. The colloid sampling technique used in the present project is based on an ultrafiltration system capable of concentrating particles whose size is greater than 10,000 MW and has an option for fractionating the concentrated colloids into particle size groups. The sampled colloids are not exposed to the atmosphere prior to or during concentration thus ensuring the same redox conditions as in the aquifer at sampling depth. Five boreholes at the Koongarra uranium deposit were sampled jointly by AAEC, Lucas Heights and AERE Harwell personnel during August 1986. Samples of groundwaters, pure aqueous phase, particulate (greater than 1 µm diam. Particle size) and colloid (less than 1 µm diam. particle size) fractions were collected. All samples were taken in duplicate and have been characterised by the laboratories in UK and Australia in terms of their physical, chemical and actinide isotopic composition. Radiometric data are interpreted in terms of the fraction of the radionuclides associated with the colloid concentrates and the isotopic equilibrium between the pure aqueous and colloid phase.