Simulation of the migration in fracutred rock by a model based on capillary tubes

In this paper the ability of a model based upon capillary tubes to reproduce the hydrodynamic dispersion in connection with flow of contaminated groundwater in fractured rock is investigated. It is assumed that the cross sectional areas are circular and that the flow is laminar. The molecular diffusion is neglected as well as the impact of variations in velocity over the cross-sectional area. It is assumed that the cross sectional areas in an ensemble of tubes belong to a gamma distribution. The velocity differences between tubes having different cross sectional area cause hydrodynamic dispersion. The model is applied to field tracer experiments at two sites. It is shown that the mean size of the cavities is smaller at a large depth and that the distribution is more narrow. The parameter in the gamma distribution has to be given different values to reproduce the breakthrough course at the different sites. It is also pointed out that there is no general relation between conductivity of a porous medium and its porosity without consideration of the pore size distribution. (au)