Plumes in kinetic transport: how the simple random walk can be too   simple
  We consider a discrete time particle model for kinetic transport on the two dimensional integer lattice. The particle can move due to advection in the $x$-direction and due to dispersion. This happens when the particle is free, but it can also be adsorbed and then does not move. When the dispersion of the particle is modeled by simple random walk, strange phenomena occur. In the second half of the paper, we resolve these problems and give expressions for the shape of the plume consisting of many particles. 
Michel Dekking, Derong Kong, and Annegreet van Opbroek
math.PR