Dual-porosity modelling of infiltration experiments on fractured granite

The results of dynamic infiltration experiments on core samples of fractured granite from the NAGRA Grimsel Test Site (GTS) are analysed in terms of single and dual porosity models, in which the radionuclides are transported by advection-dispersion through a planar fracture. In the dual porosity model, diffusion into the adjacent altered rock zones (matrix diffusion) is included. In the experiments, a spike of tracer containing non-sorbing (82Br) and slightly sorbing (24Na) radionuclides is introduced into the infiltration fluid. The transit time of fluid through a sample gives the advection velocity for the models and is estimated from the breakthrough of 82Br. The remaining parameters, determined by fitting a solution of the governing equations to the 24Na breakthrough curve, are the longitudinal dispersion length, the sorption retardation factor for the fracture and, in the case of the dual porosity model, the retardation factor and diffusion coefficient for the matrix. They are compared with independent experimental measurements, such as sodium batch sorption experiments on fracture infill material. Computer-code used = RANCHMD, which solves the coupled partial differential equations describing the dual porosity model. 11 refs. 1 fig., 4 tabs