234U/238U and 230Th/234U activity ratios in the collidal phases of aquifers in lateritic weathered zones

A procedure was developed for comparing solute and colloid phases of groundwaters in contact with uranium ore bodies at Nabarlek and Koongarra in the Alligator Rivers region, Northern Territory, Australia. Single-pass ultrafiltration of large volumes with cut-offs of 18 nm and 1 μm was used. Colloids were composed of Fe and Si species with sorbed U and U daughters. Uranium isotopes were mostly present as soluble species. Thorium was significantly associated with the colloids. The 234U/238U activity ratios (ARs) were similar in solute and colloid phases close to the ore bodies but further down-gradient colloids were generally more depleted of 234U than the solute. The 230Th/234U ARs rose from very low values for both solute and colloid phases close to the ore bodies through several orders of magnitude to much higher values further down-gradient. Colloid 230Th/234U ARs were always significantly greater than solute ARs. Results were consistent with a systematic leaching of U from colloids going down-gradient and very little mobilization of ore body. 230Th relative to U. Ubiquitous complexes 232Th appeared to suppress the solubility of 230Th