Vertical transport and fate of low-water-solubility chemicals in unsaturated soils

Abstract

Adsorption, dispersion and changes in moisture content control the rate of transport of solutes in an unsaturated soil column. A one-dimensional mathematical model has been developed to examine the simultaneous transport of heat, moisture and chemical mass in unsaturated soils. Chemical concentration is assumed to be low. The field equation for chemical transport is strongly coupled to the field equations for moisture content and temperature while the converse is not true. Vertical temperature, moisture content and chemical concentration profiles were determined using the iterative Thomas algorithm procedure on implicit backward difference approximations to the defining set of nonlinear partial differential equations. For a specified set of hydraulic properties, initial distributions of chemicals in the soil column and external weather conditions, chemical concentration profiles for 100-days simulations illustrated the influence of the combined effects of adsorption and dispersion on chemical transport in unsaturated soils. Numerical dispersion was minimized by keeping nodal spacing small in the vicinity of steep gradients of moisture content, temperature and chemical concentration. Numerical dispersion occurred more frequently in the solution for chemical transport, particularly when dispersivities were low. For this situation the criterion for nodal spacing was Δz < 2 a L, where a L was dispersivity.