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

The solubility of uranium in groundwater is very sensitive to changes in re dox conditions. Many secondary (sandstone-type) uranium deposits have been formed when soluble U has precipitated after encountering reducing conditio ns in the subsurface. In the groundwater of the Uitenhage Aquifer (Cape Pro vince, South Africa), U-238-series isotopes were used to assist in studying the history of the reducing barrier. Uranium isotopes were used to determi ne the present position of the barrier. Radium and radon were used to evalu ate the path of migration that the front of the oxygen depletion zone has t aken over the past 10(5) years. During this time the reducing barrier has m oved, leaving in its wake a trail of U in various stages of secular equilib rium with its daughter Th-230. The Ra-226 daughter of Th-230 is not very mo bile. Its growth upon the aquifer wall is reflected in the Rn content of th e water. This in turn, due to the relatively great age of the water, indica tes the extent of the Th-230 ingrowth (from precipitated U) that took place before the barrier migrated. (C) 1999 Elsevier Science B.V. All rights res erved.