Field‐scale transport of interacting solutes through the unsaturated zone: 1. Analysis of the spatial variability of the transport properties

Abstract

The effect of physicochemical interactions between the soil solution and the soil matrix on the spatial variability of soil properties pertinent to the transport of mixed Na/Ca−Cl salts in the unsaturated zone was analyzed. These properties were the soil hydraulic conductivity and the soil water retention functions and the retardation and the elution factors which account for Na/Ca exchange and chloride exclusion. On the local scale, effects of the soil solution concentration and composition (in terms of the chloride concentration C and the sodium adsorption ratio SAR, respectively) on these soil properties were derived using a theoretical approach which combined the mixed‐ion diffuse double layer theory, the structure of the clay particles, the soil's pore size distribution, and hydrodynamic principles. On the field scale the effect of the soil solution C and SAR on these soil properties was analyzed by coupling the theoretical approach with measured spatial distributions of the soil hydraulic properties at a reference “inert” state as well as of the soil cation exchange capacity and the soil specific surface area. The effect of the soil solution‐soil matrix interactions on the spatial variability of the soil hydraulic properties and the retardation and the elution factors was quantified in terms of mean values and coefficients of variation CV, expressed as functions of the soil solution C and SAR, and the degree of effective saturation Θ. Results of the analyses suggested that the spatial variability (relative to the inherent field variability in the inert reference state) of both the hydraulic conductivity K and the water content θ increased as both the SAR and Θ increased and as C decreased but decreased as C was further decreased (when C < ≈ 15 meq L−1). The mean value of K and θ relative to the inert reference state increased and decreased, respectively, as C increased and as both SAR and Θ decreased. The relative variability of both the retardation factor for Na, RfNa, and the elution factor for Cl, Eƒ, increased as the SAR increased and as both C and Θ decreased. The mean value of RfNa increased and the mean value of Cƒ decreased as both C and Θ decreased and as the SAR increased.