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-porosit y 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 velocit ies and levels of soil-water saturation. This model partitions the soil int o two mobile regions that represent the soil matrix and macropores. Theory and methodology are presented to estimate dispersive transport and adsorpti on in each region and diffusive exchange between regions for soil columns s ubjected to steady-state water how. Numerical inversion of the governing tr ansport equations was used in conjunction with non-linear least-squares opt imization to estimate transport parameters for displacement experiments. Po re-water velocity and water content were independently estimated for each r egion using a pair of displacement experiments conducted on the same column but at different degrees of saturation. Results suggest that the fitted ma ss exchange coefficient represents a lumped process resulting from the comb ined effects of intra-aggregate diffusion and local flow variations. We als o conclude that when there is limited interaction between regions, the mass transfer coefficient should be estimated independently. A principal diffic ulty of the application of the dual-porosity model was the non-linear behav ior of the diffusive exchange term at early times after a step change in in let concentration. Another problem was that fitted solutions predicted near ly all adsorption sites to be in equilibrium with solute in the macropore r egion rather than with solute in the matrix region. Despite these difficult ies, the dual-porosity model led to differentiation of transport processes that corresponded to observed structural differences in soil horizons. Publ ished by Elsevier Science B.V.