Simulation of Chemical Transport in Unsaturated Soil

Abstract

A one‐dimensional three‐phase solute‐transport model is developed to simulate the vertical movement of contaminants in the unsaturated region above the water table. The solute transfer across the air‐water, water‐solid, and air‐solid interfaces is modeled using sorption‐equilibrium assumption at the interfaces. The sequential iteration approach has been used to split the transport and equilibrium parts of the governing equations, which are solved using a combined modified Galerkin finite element and Crank‐Nicolson finite difference scheme. The model is used to predict the transient gas‐phase solute‐emission rate to the atmosphere and the liquid‐phase leach rate into the ground water. Two retardation factors were developed for the gas‐ and aqueous‐phase transport in the unsaturated zone. It is shown that for volatile compounds like benzene, neglecting the gas‐phase diffusion in the unsaturated zone can result in the underestimation of their leach rates to the ground water.