ABSTRACT

The concentrations of uranium series radionuclides in groundwater were determined to investigate the migration behavior of radionuclides in the Koongarra ore deposit. Particular attention was given to ²³⁸U and alpha-emitting radionuclides in its decay chain, including ²³⁴U, ²³⁰Th, ²²⁶Ra, and ²²²Rn, and beta-emitting ²¹⁰Pb. Disequilibrium between various members of the ²³⁸U decay chain in the Koongarra system arises from a combination of factors, including differences in solubility, surface affinity, the degree of weathering, diffusion of gaseous ²²²Rn, alpha-recoil effects and redox processes. Measured groundwater ²³⁴U/²³⁸U activity ratios were below unity in the surficial weathered zone (shallower than about 20 m depth), and greater than unity in the deeper unweathered zone (>30 m depth). These were attributed to various mechanisms related to the alpha-recoil process. Groundwater concentrations of ²³⁰Th, and also ²³⁰Th/²³⁸U ratios were extremely low, indicating that thorium is immobile in this system. Radium-226 was relatively immobile in groundwaters of the weathered zone, with lower ²²⁶Ra/²³⁸U ratios than deeper groundwaters. This was attributed to co-precipitation of radium together with manganese and ferric hydroxides at the base of the weathered zone, and also to the greater abundance of radium-sorbing minerals in the weathered zone. Large excess concentrations of ²²²Rn were found in most Koongarra groundwaters, indicating substantial loss of ²²²Rn from the solid phase despite its short half-life. The ²¹⁰Pb/²²²Rn ratios were relatively constant and it was possible to compute an average scavenging residence time for ²¹⁰Pb in the groundwater of about 6 days using a simple box model. The patterns of dispersion of uranium series radionuclides in Koongarra groundwaters also suggest that present-day migration is toward the south of the orebody. This conclusion is in agreement with the outcome of the geochemistry study.