Tortuosity of an unsaturated sandy soil estimated using gas diffusion and bulk soil electrical conductivity: Comparing analogy-based models and lattice-Boltzmann simulations

Prediction of gas and ion diffusion in unsaturated soils has been obstructed by the inability to independently measure tortuosity parameters. Analogies between transport processes are often suggested to assess these nonmeasurable parameters. However, it is unclear whether these analogies are valid in unsaturated soils. Therefore, to obtain a more fundamental insight into the differences between ion and gas diffusion in an unsaturated sandy soil a different approach than current empirical models is required. In this study, fitted tortuosity parameter values of analogy-based models describing the gas diffusion coefficient and bulk soil electrical conductivity (ion diffusion) of a sandy soil are compared. Gas diffusion in unsaturated soil is most affected by connectivity in contrast to electrical conductivity that is among other factors primarily determined by tortuosity. Measured gas diffusion and electrical conductivity are also compared with lattice-Boltzmann simulated gas and ion diffusion as a function of water content in a thin section (two dimensional) of a sandy soil. Results of the lattice-Boltzmann simulations are in qualitative agreement with measured diffusional ratios and support the findings with the analogy-based models. Lattice-Boltzmann simulations may be a valuable tool to overcome empirism of current models and increase our insight in transport properties of unsaturated soils in the coming years.