Desbarats  used a particle-tracking scheme to investigate the physics of three-dimensional solute transport in aquifers composed of two porous media of different hydraulic conductivities. The spatially heterogeneous fluid velocity was assumed to be the only mechanism of solute movement; local or pore scale dispersion and molecular diffusion were assumed to be negligible. The particle-tracking scheme used by Desbarats consisted of routing particles from node to node in a finite difference grid. In this scheme, the direction of an individual particle is randomly selected and the prob- ability associated with the particle movement in a given direction is proportional to the fluid flux in that direction. The same method was used by Moreno et al.  to investigate advective transport in a variable-aperture planar fracture.