Performance assessment studies of models for water flow and radionuclide transport in vegetated soils using lysimeter data

Increasingly the burial of nuclear waste in deep underground repositories i s being regarded as a safe long-term solution for disposal. However, to sup port this safety assessment models of the associated risks are required. An important component of these models is the upward migration of radionuclid es from a contaminated water table into arable and pasture crops. A five-ye ar experiment to investigate the processes which control these transfers ha s been undertaken at Imperial College. Selected data from this experiment w ere made available to participants of the BIOMOVS II programme in order to allow them to perform blind hydrological and radionuclide transport simulat ions: The results show the importance of correctly characterising the soil hydrology and indicate that model conceptualisations derived from surface c ontamination studies may not adequately capture the various processes which influence the upward movement of radionuclides in the vadose zone. These i nclude not only the water movement, but also chemical and biological proces ses. Finally, the difficulty and importance of a priori parameter selection is highlighted. (C) 1998 Elsevier Science Ltd. All rights reserved.