Uranium mobility in the natural environment: Evidence from sedimentary roll-front deposits

Roll-front deposits consist of naturally occurring ore-grade uranium in selected sandstone aquifers throughout the world. The geochemical environment of these roll-front deposits is analogous to the environment of a radioactive waste repository containing redox-sensitive elements during its post-thermal period. The ore deposits are formed by a combination of dissolution, complexation, sorption/precipitation, and mineral formation processes. The uranium, leached from the soil by percolating rainwater, complexes with dissolved carbonate and moves in the oxidizing groundwater at very low concentration (parts per billion - ppb) levels. In this paper data from in-situ leach mining and restoration of roll-front uranium deposits also provide information on the potential mobility of the waste if oxidizing ground water should enter the repository. Uranium solids probably are initially very soluble in carbonate ground water; however, as reducing conditions are re-established through water/rock interactions, the uranium reprecipitate and the amount of uranium in solution again equilibrates with the reduced uranium minerals