抄録
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.
