Abstract
Groundwater flow above deep geological repositories in salt domes may lead to the transport of radionuclides into the biosphere. To mitigate this risk, groundwater age is used as an exclusion criterion for repository site selection, and groundwater life expectancy is an established measure for radionuclide travel times. Complexities arise in computing age since groundwater flow above salt domes is highly density-dependent due to the presence of brines. Groundwater flow and solute transport are therefore strongly coupled and are also affected by mixing processes, including diffusion and mechanical dispersion, which are aquifer-specific and highly uncertain. Numerical simulations have been carried out to address this uncertainty for 2D topography-driven and density-dependent groundwater flow above salt domes. Simulation results show that the components of longitudinal and transverse dispersion have a strong influence on the density-dependent flow system and therefore, along with diffusion, significantly affect groundwater age and life expectancy. Underestimation of the associated parameters may lead to overestimation of life expectancy and the critical overestimation of repository safety. Selecting appropriate parameter values and consideration of their uncertainty for mixing processes is therefore critical when modeling life expectancy in the safety assessment of repository sites.