SIMULATION OF CHEMICAL-TRANSPORT IN UNSATURATED SOIL

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. © ASCE.