The scavenging of uranium from groundwater downgradient of the uranium ore deposit at Koongarra, Australia, has been investigated to provide information about the long-term transport of radionuclides. Rock samples collected from diamond-drill cores were examined mainly using scanning and transmission electron microscopy. Here we focus on the U associated with iron oxides and report on (i) the extent to which U has accumulated in the various types of iron oxides (fissure fillings, clay coatings, and nodules) and (ii) the chemical form of U associated with iron-nodules. The iron nodules have a remarkably large capacity for uranium uptake. The uranium enrichment in the nodules reaches approximately 8 wt %, and their uranium contents are greater than those in the other iron forms, such as fissure fillings and clay coatings. The ability of the iron nodules to enrich uranium (to levels 106 times higher than the groundwater) is greater than those of any other natural materials in the system. Although the initial...