Evaluating Two-Domain Models for Physical Non-Equilibrium Flow and Transport in Three Different Column Setups.

It is widely recognized that preferential solute transport through soil macropores can bypass the soil matrix in the unsaturated zone and cause groundwater pollution. The objectives of this study are: i) to test various conceptual models including Single Porosity Model (SPM), Mobile Immobile Model (MIM), Dual Permeability Model (DPM) for describing physical non-equilibrium in soil columns with different experimental conditions, and ii) to identify data requirements for inverse modeling for DPM. The second objective is in response to difficulties in parameter estimation of transient flow and transport experiments in the unsaturated zone because of ill-posedness of the inverse problem using DPM framework. The study was based on controlled flow and transport experiments using bromide solution (KBr) in soil columns with different macropore geometry and distributions (homogeneous soil, central macropore and multiple macropore columns). The results of the study indicated DPM with first-order water transfer function between macropore and matrix domains to yield best representation of the inflow-outflow data, pressure head profiles, and least sum-of-squares error for all experiments (infiltration, drainage, bromide transport) of the multiple macropore columns. DPM with second-order water transfer function gave results comparable to or worse than with the first-order water transfer function. This was thought to be the effect of using one-dimensional representation of the multiple macropore columns. On the other hand, SPM, MIM and DPM gave comparable results for the central macropore column. This was due to the absence of prominent preferential flow characteristics in the central macropore column.