Numerical simulations have been performed for a model for the distribution of radionuclides in the ground water around a deep repository for used nuclear fuel, based on the assumption that different isotopes of the same chemical element A contribute jointly to the chemical potential of A, through two different components of the flux. The corresponding problem consists in a parabolic system strongly coupled, that, in the physically relevant assumption that one of these components is negligible, reduces to a parabolic equation for the total concentration of the element A, possibly coupled with hyperbolic equations for the concentrations of the single isotopes. These simulations evidentiate the qualitative behaviour of the solution in dependence of the diffusion oefficients, showing striking effects that can be observed when one of the component of the flux is much smaller than the other.