The Tono uranium mine in Japan, consists of a granite basin infilled with layered marine sedimentary rocks, within which uranium is heterogeneously distributed. This site is being used as a natural analogue to study the migration of uranium. The irregular observed uranium distribution had not satisfactorily been explained by local geochemical properties. This paper examines the hypothesis that the distribution of uranium could be dominated by the accessibility of locations to the migrating uranium, i.e. by the heterogeneous hydrology of the site.
High resolution heterogenous transmissivity fields were generated, conditioned on data from four boreholes through the mine. Steady state Darcy flow calculations and particle tracking simulations of uranium migration have been carried out. The time dependent relative intensity of a set of sources on the boundary of the modelled region were used as parameters to fit the modelled uranium distribution to the observed.
The relative intensities so obtained could be interpreted in a physically reasonable way. Good qualitative agreement was achieved between the observed heterogenous irregular uranium distribution and that fitted. We conclude that this supports the hypothesis, and that detailed knowledge of channelling flows can provide a better understanding of possible radionuclide dispersal.