  We propose a nonlinear random walk model to describe the dynamics of dense
contaminant plumes in porous media. A coupling between concentration and
velocity fields is found, so that transport displays non-Fickian features. The
qualitative behavior of the pollutant spatial profiles and moments is explored
with the help of Monte Carlo simulation, within a Continuous Time Random Walk
approach. Model outcomes are then compared with experimental measurements of
variable-density contaminant transport in homogeneous and saturated vertical
columns.
