Determination and Modeling of Solute Diffusion Tortuosity Factors in Unsaturated Soils.

Solute diffusion flux in soil is described by Fick’s law along with a tortuosity factor to account for the tortuous and reduced diffusive pathway blocked by soil particles. Predictive models based on empirical or conceptual relationships with other more commonly measured soil attributes have been proposed to replace the time-consuming and multifarious laboratory measurements. However, these models have not been systematically tested and evaluated with soils of different textures under comparable conditions. This study determined solute diffusion coefficients and calculated tortuosity factors of a sand, a sandy clay loam, and a clay at various degrees of water saturation, and used the experimental data to test the predictive capabilities of these models. All the test models can fit the experimental data reasonably well as evidenced by low root mean square errors (RMSEs). When their proposed parameter values were used, the widely accepted Millington and Quirk tortuosity model resulted in highest RMSEs for all three test soils. In terms of model efficiency, the tortuosity factors of the sand and sandy clay loam soils are best represented by a quadratic function of volumetric soil water content, while the combined parallel-series conceptual model assuming different configurations of film and pore water is the best for the clay soil. In this study, unequal solute diffusivity at the same water content or matric potential when a soil is subjected to drying or wetting process, which is referred as solute diffusion hysteresis, was also observed in all three test soils.