Estimating parameters for a dual-porosity model to describe non-equilibrium, reactive transport in a fine-textured soil

Several models have recently been proposed to describe solute transport in two or more mobile regions, yet there have been relatively few attempts to calibrate these models for a particular soil. In this study, a dual-porosity approach is used to describe the steady-state reactive transport of a Br− tracer through a fine-textured Ultisol over a range of pore-water velocities and levels of soil-water saturation. This model partitions the soil into two mobile regions that represent the soil matrix and macropores. Theory and methodology are presented to estimate dispersive transport and adsorption in each region and diffusive exchange between regions for soil columns subjected to steady-state water flow. Numerical inversion of the governing transport equations was used in conjunction with non-linear least-squares optimization to estimate transport parameters for displacement experiments. Pore-water velocity and water content were independently estimated for each region using a pair of displacement experiments conducted on the same column but at different degrees of saturation. Results suggest that the fitted mass exchange coefficient represents a lumped process resulting from the combined effects of intra-aggregate diffusion and local flow variations. We also conclude that when there is limited interaction between regions, the mass transfer coefficient should be estimated independently. A principal difficulty of the application of the dual-porosity model was the non-linear behavior of the diffusive exchange term at early times after a step change in inlet concentration. Another problem was that fitted solutions predicted nearly all adsorption sites to be in equilibrium with solute in the macropore region rather than with solute in the matrix region. Despite these difficulties, the dual-porosity model led to differentiation of transport processes that corresponded to observed structural differences in soil horizons.