Retardation of mobile radionuclides in granitic rock fractures by matrix diffusion

Transport of iodide and sodium has been studied by means of block fracture and core column experiments to evaluate the simplified radionuclide transport concept. The objectives were to examine the processes causing retention in solute transport, especially matrix diffusion, and to estimate their importance during transport in different scales and flow conditions. Block experiments were performed using a Kuru Grey granite block having a horizontally planar natural fracture. Core columns were constructed from cores drilled orthogonal to the fracture of the granite block. Several tracer tests were performed using uranine, ¹³¹I and ²²Na as tracers at water flow rates 0.7–50 µL min⁻¹. Transport of tracers was modelled by applying the advection–dispersion model based on the generalized Taylor dispersion added with matrix diffusion. Scoping calculations were combined with experiments to test the model concepts. Two different experimental configurations could be modelled applying consistent transport processes and parameters. The processes, advection–dispersion and matrix diffusion, were conceptualized with sufficient accuracy to replicate the experimental results. The effects of matrix diffusion were demonstrated on the slightly sorbing sodium and mobile iodine breakthrough curves.