Evidence of heterogeneous matrix diffusion in fractured crystalline rock in laboratory migration experiments

The geostatistics of the main properties of transport in crystalline rock at Aspo Hard Rock Laboratory, Sweden have been determined experimentally, using a large number of samples, as a basis for the analysis of the effect of the uncertainty in rock heterogeneity on radionuclide migration in a single fracture. The auto-covariance functions were estimated for the porosity, effective diffusivity and adsorption characteristics, such as partition coefficient, kd and adsorption kinetics. The one-dimensional analytical solutions for the mean values of the temporal moments of the residence time probability density. function (PDF) were derived by using the spectral method. The solutions were applied to the evaluation of migration experiments on granitic drill cores from the Aspo Hard Rock Laboratory. Fractures were created in drill cores by sawing and tritium was used as a non-sorbing tracer in several experiments in each drill core. By systematically varying the flow velocity, u, we could verify the non-linear influence of 1/u on the variance of the residence time PDF according to the theory. The effect of the uncertainty in the heterogeneous rock properties on the solute transport observed in the migration experiments corresponds fairly well to the theoretical effect using the auto-covariance functions.