Solute transport through layered soil profiles: Zero and perfect travel time correlation models

In this paper the problem of solute transport through layered soil is modeled with a transfer function, representing the travel time to any depth in the soil as the sum of the travel times through the individual layers. The probability density function (pdf) of this travel time depends on the joint pdfs of the travel times through the individual layers, and hence on the correlation structure as well as the layer properties. Analytic expressions are derived for the flux and resident concentrations in layered soil for two limiting cases of correlation structure, for both convective-dispersive and stochastic-convective model representations of transport within the individual layers. These models are contrasted to the layered convection-dispersion model, which asumes continuous resident fluid concentration at the interface. Calculated concentrations are used to illustrate features of the different models, as well as to suggest experiments that might deduce the appropriate boundary condition to use at the interface between layers of different properties. The physical significance of various boundary condition assumptions is also discussed.