SIMULATION OF CONTAMINANT PLUMES WITH LARGE DISPERSIVE CONTRAST - EVALUATION OF ALTERNATING DIRECTION GALERKIN MODELS

The evolution of narrow, sharply defined contaminant plumes, corresponding in shape to those often observed in the field, is examined. It is shown that the transverse dispersion mechanism consistent with such shapes can be as low as of the order of molecular diffusion and that dispersivity contrasts of as much as 4 orders of magnitude are possible. Even at the lowest physically realistic values of transverse dispersion parameters, however, transverse mass spreading is capable of significantly retarding the advance of the plume front. The principal direction and alternating direction Galerkin models together are found to be capable of handling all cases of dispersive contrast including those with dispersivity ratio of infinity. The models are formulated in curvilinear coordinates with certain restrictions on the element deformation. A three-way comparison between principal direction, alternating direction Galerkin, and conventional finite element models is performed with respect to accuracy and efficiency. The effects of longitudinal and transverse numerical dispersion occuring in the various models are examined and related to the discretization.