GROUNDWATER GEOCHEMISTRY IN THE KOONGARRA ORE DEPOSIT, AUSTRALIA .2. ACTIVITY RATIOS AND MIGRATION MECHANISMS OF URANIUM SERIES RADIONUCLIDES

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 U-238 and alpha-emitting radionuclides in its decay chain, including U-234, Th- 230, Ra-226 and Rn-222, and beta-emitting Pb-210. Disequilibrium betwe en various members of the U-238 decay chain in the Koongarra system ar ises from a combination of factors, including differences in solubilit y, surface affinity, the degree of weathering, diffusion of gaseous Rn -222, alpha-recoil effects and redox processes. Measured groundwater U -234/U-238 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 vario us mechanisms related to the alpha-recoil process. Groundwater concent rations of Th-230, and also Th-230/U-238 ratios were extremely low, in dicating that thorium is immobile in this system. Radium-226 was relat ively immobile in groundwaters of the weathered cone, with lower Ra-22 6/U-238 ratios than deeper groundwaters. This was attributed to co-pre cipitation of radium together with manganese and ferric hydroxides at the base of the weathered zone, and also to the greater abundance of r adium-sorbing minerals in the weathered zone. Large excess concentrati ons of Rn-222 were found in most Koongarra groundwaters, indicating su bstantial loss of Rn-222 from the solid phase despite its short half-l ife. The Pb-210/Rn-222 ratios were relatively constant and it was poss ible to compute an average scavenging residence time for Pb-210 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 ore body. This conclusion is in agreement with the outcome of the geochemi stry study.