Solute diffusion and adsorption in six soils along a soil texture gradient

The Millington-Quirk (MQ) model is widely used to predict solute diffusivity (ratio of diffusion coefficients in soil and free water) in transport and fate models for nutrients and organic contaminants in soil. Recently, we suggested two soil-type dependent (STD) diffusivity models, a simple power function model and a linear impedance factor model, based on the Campbell soil water retention parameter. In this study, soil water retention and Cl- diffusivity as a function of soil water content were measured in six soils taken along a natural clay gradient (11-46% clay) with the objective of testing the STD and MQ diffusivity models. Both new STD models predicted solute diffusivity well for all six soils, while the MQ model poorly predicted diffusivity for the three most clayey soils. Effective diffusivity of a sorbing solute, NH4+, was evaluated by measuring NH4+ sorption isotherms and calculating retardation factors. The data for the six soils showed an almost linear decrease in Cl- and NH4+ effective diffusivity with increasing clay content at a given soil water content. Numerical calculations, based on the measured diffusivity data, of mean diffusion time from a source of constant strength implied that changes in soil texture (11-46% clay) and soil water content (0.2-41.4 cm3 cm-3) had equally large effects on diffusion times. We recommend that one of the suggested STD diffusivity model should be used instead of the MQ model in order to obtain better solute transport and fate simulations.