Evaluating the rate of migration of an uranium deposition front within the Uitenhage Aquifer

Abstract

The solubility of uranium in groundwater is very sensitive to changes in redox conditions. Many secondary sandstone-type uranium deposits have been formed when soluble U has precipitated after encountering reducing conditions in the subsurface. In the groundwater of the Uitenhage Aquifer (Cape Province, South Africa), U-238-series isotopes were used to assist in studying the history of the reducing barrier. Uranium isotopes were used to determine the present position of the barrier. Radium and radon were used to evaluate the path of migration that the front of the oxygen depletion zone has taken over the past 10 years. During this time the reducing barrier has moved, leaving in its wake a trail of U in various stages of secular equilibrium with its daughter Th-230. The Ra-226 daughter of Th-230 is not very mobile. Its growth upon the aquifer wall is reflected in the Rn content of the water. This in turn, due to the relatively great age of the water, indicates the extent of the Th-230 in growth from precipitated U that took place before the barrier migrated. Though economic uranium concentrations were not encountered in this study, the investigation serves to demonstrate the importance of investigating not only the present redox conditions as it pertains to U deposition, but the need to understand the past position of the uranium precipitation front. These are revealed by a combined study of the soluble uranium, radium and radon, combined with knowledge of the rate of groundwater flow.