The effect of soil heterogeneity and hysteresis on solute transport: a numerical experiment

Abstract

Heterogeneity and hysteretic flow are known to affect the downward movement of solutes in soils. Numerous studies have examined the effect of heterogeneity on solute transport as well as the effect of hysteresis. In this study we analyse the combined effect of heterogeneity and hysteretic water flow on the downward movement of a conservative solute using numerical simulation. Hysteresis is described by the scaling down model and two Mualem models, his original model II, and his universal model derived from Model II. A non-hysteretic model coinciding with the main wetting and drying curve was used for comparison. Both 1D and 2D simulations were performed in flow domains assumed to be scale heterogeneous, its scale factor being lognormally distributed and second order stationary. The effect of soil texture on the plume's behaviour in combination with hysteresis was analysed by using a fine and coarse sandy material. A 3D finite element model for water flow combined with a particle tracking method for transport was used to perform the simulations. It is shown that hysteresis has a considerable effect on solute transport thereby contradicting earlier findings in literature. The effect on solute transport was different depending on the non-hysteretic curve chosen as a reference. In addition, the effect of hysteresis on solute transport was found to depend strongly on the type of model used. Both Mualem models showed a similar behaviour under the various simulations performed, suggesting the use of the simpler universal model. Differences between the three models were found to be considerable for coarse sandy material. Hysteretic flow conditions may be as important as heterogeneity in retarding solute transport for 1D simulations. The 2D simulations showed that heterogeneity does not necessarily retard the downward movement of the solute plume as compared to the homogeneous flow domain using a non-hysteresis model.