Solute Transport in Dual-Permeability Porous Media

In a well known approach to model transport in dual-permeability or dual-porosity media, the pore space is partitioned into two flow domains such as inter- and intra-aggregate pore space in aggregated soils or fractures and a rock matrix. A variety of simplifications have been made: I. simple advection-dispersion equation (ADE) with one effective flow domain, II. mobile-immobile model (MIM) with water flow only in the mobile region and solute exchange between the mobile and immobile regions, III. dual-advection dispersion equation (DADE) with different flow but equal dispersivity in both regions and solute transfer between the regions, and IV. stream-tube model (STM) with flow according to the ADE in both domains but no solute exchange between them. A benchmark numerical model was used to assess the simplifying assumptions. Use of the DADE typically leads to the smallest error while acceptable results may be obtained for the MIM for a relatively small velocity in the "slower" domain and the STM for small values of the transfer parameter α. The ADE yielded the poorest result but can be used for higher values of the rate parameter associated with Damköhler number greater than 1. Breakthrough curve data for different depths are presented for an Andisol with pronounced "bimodal" behavior. Except for the ADE, all models did a reasonable job to describe the data but especially for MIM and STM there was considerable variation in parameter values between depths. The best description was provided by the benchmark model.