Comparison of Two Model Approaches for the Geosphere/Biosphere Interface

This paper presents a comparison between two approaches of modelling radionuclide transport in soil. A scenario within the BIOMOVS study is chosen for the comparison. The scenario treats soil as recipient for contaminated water from the geosphere. The groundwater, containing I-129 and Np-237, reaches the soil below the root zone. This paper compares and discusses the concentration of nuclides in the root zone.One model is based on the compartment theory and is numerically solved by the code BIOPATH. Some input data to the model are obtained from a hydrological model (WATPATH) describing the dynamics of water in the unsaturated zone and saturated zone where important parameters are precipitation and mean air temperature. The BIOPATH approach considers the retention of nuclides in the different reservoirs. The uncertainty in the results due to the uncertainty in the parameter values are determined by the PRISM system.The other model is based on the advection-dispersion equation which is numerically solved by the computer code TRUMP. The model takes into consideration the transport of nuclides with groundwater and percolating precipitation, and transport by dispersion and diffusion. It also considers the retention of radionuclides by sorption to the soil. The model gives the possibility of a detailed description of the flow and distribution of radionuclides (one or two dimensional) in a section of a soil profile as a function of time. Input data to the model is water flow, dispersivity, diffusivity and sorption characteristics in the different parts of the soil profile.