Evaluating Depressional Process of Macropore Flow and Its Impact on Solute Transport

Abstract

The occurrence of macropore flow (MF) has a significant effect on infiltration in the vadose zone. Three heterogeneous column systems were set up in this study to quantify the MF depressional process in unsaturated conditions and its effect on solute transport in various saturated conditions. Series of breakthrough curves were obtained through a miscible displacement experiment with bromide conducted on three parallel undistributed soil columns and a control soil column. Representative mean pore radii (RMPR) were improved to quantitatively separate each soil matric potential region and calculate the effective hydraulic porosity (EHP) for describing the various processes of MF. The resulting two-region model was applied to fit the characteristic parameters of any possible solute transport based on convection–diffusion equations (CDEs). The results show that the transient scenarios of MF could be properly quantified with a modified RMPR (MRMPR) and EHP. With the MF depression, the MRMPR was stable and the number of EHP was sensitive to the MF change before the pore water disconnection at near-saturated conditions. Both the improved RMPR and EHP degraded and the characteristics of two-region flow gradually disappeared because of the exchange between MFs and matric flows. In this course, solute transport was mainly affected by the convective effect of MF. Lateral exchange flows between macropores and matrix pores degraded transport discrepancies when MF transferred to mesoflows at a water head potential of –10 cm. The classic CDE parameters for modeling solute transport along the MF depression process are still effective in every test.