Numerical analysis of the nonsteady transport of interacting solutes through unsaturated soil: 1. Homogeneous systems

Transient one‐dimensional vertical transport of mixed Na/Ca solutions in a saturated‐unsaturated homogeneous soil during infiltration was studied using a modified version of the transport model of Bresler (1973 a ), taking into account physicochemical interactions between the soil solution and the soil matrix expressed in terms of changes in the hydraulic conductivity and retentivity functions, anion exclusion, and cation exchange. These latter, in turn, were derived from theoretical considerations based on the mixed ion diffuse double‐layer theory, the structure of the clay particles, the pore size distribution of the soil, and hydrodynamic principles. The solute transport was analyzed for three soils of different textures and for different sets of boundary and initial conditions. Results of the analyses suggested that the effect of soil‐matrix‐soil‐solution interactions on the transport of water and solutes may be significant and generally increases as the wetting zone soil water content and soil solution sodium adsorption ratio increase, as its solute concentration decreases, as the clay fraction of the soil increases; and as the soil texture becomes finer. For a given soil the magnitude of these interactions and their effect on the transport process are affected by the surface boundary conditions and the initial conditions. For a given set of boundary and initial conditions the retardation of the water and solute movement due to the soil‐solution‐soil‐matrix interactions, relative to a reference inert state, depends on the soil texture. The implication of this finding with respect to solute transport in spatially variable fields is discussed briefly.