URANIUM INFILTRATION FROM A RIVER TO SHALLOW GROUNDWATER

The infiltration of uranium from the polluted River Glatt (Switzerland) into a hydraulically connected saturated aquifer was investigated during a period of almost five years. The uranium concentrations and the U-234/U-238 activity ratios (ARs) were analysed monthly in the water of the river and of four groundwater sampling wells. The speciation of uranium and its relation to other aqueous and solid components were investigated experimentally and in model calculations. The uranium concentration and the ARs in the river varied seasonally with a maximum and a minimum, respectively, in summer. Processes, such as photo-reductive dissolution of iron-oxyhydroxides or the enhanced decomposition of organic matter and calcite in the upstream, eutrophic Lake Greifen may have caused these variations. Laboratory experiments confirmed a possible release of uranium by photochemical reactions. The concentration maximum of uranium was also detected in the shallow groundwater, but with a delay of about six months to the maximum in the river. Under the assumption that the maxima in the river and the groundwater were related, an in situ distribution coefficient (K-D) for uranium of approximate to 7 mL g(-1) can be calculated for this Quaternary gravel aquifer. Uranium was present predominantly as UO2-carbonate complexes, whereas the formation of phosphate and organic complexes (e.g., UO2-humates/EDTA/NTA) was negligible. Particles and colloids played a minor role for the transport of uranium in this aquifer. In the course of this study, we observed an ongoing general decrease of the uranium concentrations and a disappearance of its summerly maxima in the River Glatt. We relate this unexpected effect to a better control of the phosphate concentration in surface waters, which diminished the growth of aquatic biota. This, in turn, led to changes in the redox conditions of lake and river sediments, and herewith to a reduced release of heavy metals, including uranium. The results of this case study can be applied to estimate the migration behavior of other actinide oxyions (e.g., PuO22+/NpO2+) in shallow heterogeneous aquifers, which abound in alpine and pre-alpine locations.