Introducing a transition domain for describing the solute exchange between macropores/fractures and matrix in dual-permeability system

Abstract

The dual-permeability model (DPM) is highly efficient for describing the solute transport with multiple flow paths in macroporous/fractured media. However, an accurate and efficient description of the solute exchange process between the macropores/fractures and matrix in the dual-permeability system still remains a challenge. Therefore, a new approach, the dual-permeability model with transition domain (DPMTD), is proposed under saturated conditions. DPMTD characterizes the boundary layer by dividing the part of the matrix blocks in contact with the macropores (or fractures) into the transition domain. This new method takes into account the effect of concentration distribution in the matrix on the solute exchange process without upgrading the dimension of equations. The expressions of the first-order mass transfer coefficient, which is used to describe the interdomain solute exchange in the DPMTD, for four defined aggregate geometries are obtained. A sand column experiment containing an artificial macropore is used to evaluate the performance of the DPMTD and DPM. The simulation results of the DPMTD and DPM reveal that the DPMTD captures the bimodal transport (especially the first peak) more effectively because it calculates the rapid solute exchange in the early time and the slow solute exchange in the later stage more accurately. Subsequently, we studied the effect of η, which indicates the volume proportion of the boundary layer to the dual-permeability media in the DPMTD, on results. In short, this study emphasizes the importance of the boundary layer, and provides a new efficient approach to describe solute exchange process between macropores (or fractures) and matrix in dual-permeability system.