Modelling contaminant transport of soil and landfills: A comparison study of three numerical codes

Abstract. The negligibly radioactive materials from a control area can be fed into further material cycles, to an incineration plant or disposed according to the type of clearance after clearance procedure, which is described in the German Radiation Protection Ordinance (Deutscher Bundestag, 2024b). The clearance values of the Radiation Protection Ordinance are based on the 10 µSv concept that limits the additional radiation exposure for the population or worker. In the past, analytical models were used to calculate radionuclide concentrations for groundwater path dose estimation. In this work, numerical groundwater models are used to simulate the distribution of radionuclides and provide spatial information on concentrations, which can be used as input data for dose estimation.

The transport processes of advection, diffusion, dispersion, sorption and decay are implemented in the simulation codes “distributed density-driven flow (d³f++)” (Fein and Schneider, 1999; Fein, 2004), “Dune for Multiphase flow and transport (DuMu^x )” (Flemisch et al., 2011; Koch et al., 2021) and “Simulation of Processes in Groundwater (SPRING)” (delta h, 2024). To compare the codes and their implemented transport processes, a simple 2D column and a generic 2D landfill body were modelled. The study demonstrated a good agreement between the three computational codes, thereby strengthening trust in numerical modelling for future applications in dose estimation. The differences in the concentration breakthrough curves can be attributed to the differences in the implementation of initial conditions (IC) for saturation, the influence of dispersion in the different codes and the upwind methods, highlighting the sensitivity to these parameters and numerical solvers. Although minor discrepancies emerge in the results, the study demonstrates that the concentrations and the times of maximum concentration are largely comparable.