Is transport in porous media always diffusive? A counterexample

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Abstract
For the special case of a stratified porous medium with flow parallel to the bedding it is shown that the transport of solute cannot, in general, be represented by the usual convection‐diffusion equation, even for large time. The necessary conditions for the appearance of a Fickian diffusive process are discussed and compared with previous work done by Gelhar et al. (1979) and Marie et al. (1967). It is shown, however, that when the flow is not exactly parallel to the stratification, diffusive behavior is much more likely to appear. The need for further work on the mechanism of transport in porous media is then emphasized.