A semi-analytical model for solute transport in layered dual-porosity media

摘要

The vulnerability of groundwater from chemical leaching through soil is a concern at some locations. Because measurements are laborious, time-consuming, and expensive, simulation models are frequently used to assess leaching risks. But the significance of simulated solute movement through a layered soil is questionable if vertical homogeneity of physical soil properties has been assumed. In the present study, a semi-analytical model for solute leaching in soils is presented. The model is relatively simple, but it does account for soil layers having different physical properties. The model includes the mobile-immobile model (MIM) to describe one-dimensional (1-D) nonequilibrium, transient solute transport under steady-state flow conditions. The MIM is rewritten as a second-order differential equation and solved by a numerical scheme. Differing from fully analytical or fully numerical solutions, the new approach solves the differential equation numerically with respect to time and analytically with respect to distance. Numerical experiments for a single layered soil profile show that the semi-analytical solution (SA-MIM) is numerically stable for a wide range of parameter values. The accuracy of SA-MIM predictions is comparable to that of analytical solutions. Numerical experiments for a multilayered profile indicate that the model correctly predicts effluent curves from finite layered soil profiles under steady-state flow conditions. The SA-MIM simulations with typical parameter values suggest that neglecting vertical heterogeneity of flow paths in a layered soil can lead to inaccurate prediction of soil-solute leaching. The quality of predictions is generally improved if parameter estimates for the different soil layers are considered. However, the mobile-immobile-parameter estimates obtained in a number of previous studies may not be transferable to a field situation that is characterized by a slow and steady flow of water. Further field experiments to determine mobile-immobile parameters under such conditions are desirable.