NUMERICAL-ANALYSIS OF FLOW AND TRANSPORT IN A 3-DIMENSIONAL PARTIALLYSATURATED HETEROGENEOUS SOIL

Numerical simulation of flow and transport of tracer and reactive (sor ptive) solutes was used to investigate solute spreading and breakthrou gh in a realistic, three-dimensional, heterogeneous, partially saturat ed 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 infilt ration with substantial redistribution periods may impart a pseudokine tic behavior to the large-scale sorption and create a three-dimensiona l velocity fluctuation field with significant transverse components. T he 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 transver se spreading, reduces the skewing of the solute breakthrough, and lead s to an approximately Fickian behavior of the transport. Water uptake by the plant roots is shown to increase the variability in the respons e of the flow domain and to produce a drier soil profile, with lower c onductivity and steeper head gradient. Consequently, it reduces the so lute velocity in the vertical direction, smooths its heterogeneity, an d further diminishes the longitudinal spreading of the solute plume an d the skewing of the solute breakthrough. Sorption also reduces the so lute velocity in the vertical direction. For the physically plausible situation in which log retardation factor is negatively correlated wit h log saturated conductivity, sorption contributes to the variability in the solute velocity and increases both the longitudinal spreading o f the solute plume and the skewing of the solute breakthrough.