Comment on “Flow and tracer transport in a single fracture: A stochastic model and its relation to some field observations” by L. Moreno et al.

hereinafter referred to as MT) used a particle-tracking scheme to investigate the physics of solute movement in a variable-aperture planar fracture. 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 MT 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 probability associated with the particle movement in a given direction is proportional to the fluid flux in that direction. The same method was used by Desbarats [1990] to investigate advective transport in aquifers composed of two porous media of different hydraulic conductivities. The node-to-node routing scheme used by MT is a poor model of the physics of advective solute movement in a continuum. In a companion comment [Goode and Shapiro, this issue], we analyze the artificial dispersion introduced by this scheme for the case of uniform flow. Those results are