Numerical analysis of flow and transport in a three-dimensional partially saturated heterogeneous soil

Abstract

Numerical simulation of flow and transport of tracer and reactive (sorptive) solutes was used to investigate solute spreading and breakthrough in a realistic, three-dimensional, heterogeneous, partially saturated soil, along with realistic weather boundary conditions, considering water uptake by plant roots. Results of the analyses suggest that the combination of spatially variable soil properties and periodic infiltration with substantial redistribution periods may impart a pseudokinetic behavior to the large-scale sorption and create a three-dimensional velocity fluctuation field with significant transverse components. The resultant velocity fluctuation field promotes lateral mixing of the solute, smooths out the extremes in solute convection, slows down the longitudinal spreading of the solute plume and increases its transverse spreading, reduces the skewing of the solute breakthrough, and leads to an approximately Fickian behavior of the transport. Water uptake by the plant roots is shown to increase the variability in the response of the flow domain and to produce a drier soil profile, with lower conductivity and steeper head gradient. Consequently, it reduces the solute velocity in the vertical direction, smooths its heterogeneity, and further diminishes the longitudinal spreading of the solute plume and the skewing of the solute breakthrough. Sorption also reduces the solute velocity in the vertical direction. For the physically plausible situation in which log retardation factor is negatively correlated with log saturated conductivity, sorption contributes to the variability in the solute velocity and increases both the longitudinal spreading of the solute plume and the skewing of the solute breakthrough.