An observation of rapid mobilisation of uranium and its decay products

Uranium series radionuclides were determined in a sample collected from a rainwater puddle at the Ranger Uranium Mine (Northern Territory, Australia). The 1 l sample contained 0.25 g of uranium with a ²³⁴U/²³⁸U activity ratio of 1.07. The activities of the ²³⁸U daughter products mobilised by the rainwater were 3.8 Bq for ²³⁰Th and 66 Bq for ²²⁶Ra. These values are 0.1% and 2.2%, respectively, of the activities required for secular equilibrium with 0.25 g uranium. The activities of two daughters of ²³⁵U were 12.5 Bq for ²²⁷Ac and between 18.7 and 32.8 Bq for ²²⁷Th, values that represent 9.0% and between 13 and 24%, respectively, of the activity required for secular equilibrium with 0.25 g uranium. The results indicate that ²³⁴U, ²²⁷Th and ²²⁷Ac were the most readily mobilised daughter radionuclides of the uranium and actinium series. If ²²⁷Th and ²²⁷Ac are generally mobile, this could account for the frequent observations of ²²³Ra in Australian ground waters in concentrations higher than would be expected for an isotope with a half-life of 11.3 days.