  A hydrogeological model for the spread of pollution in an aquifer is
considered. The model consists in a convection-diffusion-reaction equation
involving the dispersion tensor which depends nonlinearly of the fluid
velocity. We introduce an explicit flux in the model and use a mixed Finite
Element Method for the discretization. We provide existence, uniqueness and
stability results for the discrete model. A convergence result is obtained for
the semi-discretized in time problem and for the fully discretization.
