Evaluation of Pollutant Transport in the Unsaturated Zone

Abstract

The past several decades has seen considerable progress in the conceptual understanding and mathematical description of water flow and solute transport processes in the unsaturated or vadose zone. A variety of analytical and numerical models are now available to predict water and/or solute transfer processes between the soil surface and the groundwater table. The most popular models remain those based on the Richards equation for variably-saturated flow, and the Fickian-based convection-dispersion equation for solute transport. Deterministic solutions of these classical equations likely will continue to be used in the near future for predicting water and solute movement in the unsaturated zone. In this paper we review recent developments in variably-saturated flow and transport research, especially from the point of view of process-based modeling. Among the topics being discussed are single-ion equilibrium transport, physical and chemical nonequilibrium transport, volatilization and degradation, multiple-species solute transport, parameter estimation, and recent advances in numerical modeling. Also briefly discussed are alternative stochastic approaches for evaluating contaminant transport in spatially variable field soils.