INTRAVAL phase 2, test case 8. Alligator Rivers Natural Analogue - Modelling of uranium transport in the weathered zone at Koongarra (Australia). Progress report

Abstract

A study of uranium transport in the Koongarra site of Alligator Rivers Uranium deposit (Australia) is carried out. The analysis of the solid phase uranium concentration measured at various depths provides a useful picture of the dispersion process. Results of this analysis seem to support the hypothesis that groundwater movement and dispersion of uranium has mainly taken place in the transition zone of weathering. As the rock has become fully weathered and turned into clayey material, the groundwater movement and uranium dispersion has basically stopped. From the dispersion patterns it can be concluded that either the direction of groundwater flow did not change significantly during the past few million years, or other environmental factors played a more important role. Comparing uranium solid phase and solution data, there appears to be a linear relationship between uranium in the accessible solid and the liquid phase. This suggest that it might be a good assumption to use the distribution coefficient between these phases, Pa, to include sorption in modelling. In the preliminary step of modelling, a mean flow velocity vector, constant in time and space has been used to simulate the dispersion fan. The movement of the weathering transition zone has been neglected. The calculated contour lines seem to be in qualitative agreement with the measured ones. Study of Koongarra site will assist to identify various processes which affect dispersion of radionuclides over long periods of time. Insight will be obtained into the ways of including complicated geochemical processes in a simplified manner in a transport model. Simulation of the dispersion fan measured at various depths in Koongarra with Metropol-4 will increase the model confidence.<br>