Modeling uranium transport in Koongarra, Australia: The effect of a movingweathering zone

Natural analogues are an important source of long-term data and may be view ed as naturally occurring experiments that often include processes. phenome na, and scenarios that are important to nuclear waste disposal safety asses sment studies. The Koongarra uranium deposit in the Alligator Rivers region of Australia is one of the best-studied natural analogue sites. The deposi t has been subjected to chemical weathering over several million years, dur ing which many climatological, hydrological, and geological changes have ta ken place, resulting in the mobilization and spreading of uranium. Secondar y uranium mineralization and dispersed uranium are present from the surface down to the base of th weathering zone, some 25 in deep. In this work, a s imple uranium transport model is presented and sensitivity analyses are con ducted,for key model parameters. Analyses of field and laboratory data show that three la,vers can be distinguished in the Koongarra area: (1) a top l ayer that is fully weathered, (2) an intermediate layer that is partially w eathered (the weathering zone), and (3) a lower layer that is unweathered. The weathering one has been moving downward as the weathering process proce eds. Groundwater velocities are found to be largest in the weathering zone. Transport of uranium is believed to take place primarily in this zone. It appears that changes in the direction of groundwater flow have not had a si gnificant effect on the uranium dispersion pattern. The solid-phase uranium data show that the uranium concentration does not significantly change wit h depth within the fully weathered zone. This implies that uranium transpor t has stopped in these layers. A two-dimensional vertically integrated mode l for transport of uranium in the weathering, zone has been developed. Simu lations with a velocity field constant in time and space have been carried out, taking into account the downward movement of this zone and the dissolu tion of uranium in the orebody. The latter has been modelled by a nonequili brium relationship. In these simulations, pseudo-stead by state uranium dis tributions are computed. The main conclusion drawn from this study is that the movement of the weathering zone and the nonequilibrium dissolution of u ranium in the orebody play van important role in the transport of uranium. Despite the fact that the model is a gross simplification of what has actua lly happened in the past two million years, a reasonable fit of calculated and observed uranium distributions was obtained with acceptable values for the model parameters.