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DIVISION S-1 - SOIL PHYSICS

Tortuosity of an Unsaturated Sandy Soil Estimated using Gas Diffusion and Bulk Soil Electrical Conductivity

Comparing Analogy-based Models and Lattice–Boltzmann Simulations

^a Unilever Research Colworth, Colworth House, Sharnbrook, Bedford MK 44 1LQ, UK
^b Institute for Biodiversity and Ecosystem Dynamics, Physical Geography, Faculty of Science, Universiteit van Amsterdam, Nieuwe Achtergracht 166 1018 WV Amsterdam, The Netherlands
^c Kramers Laboratorium voor Fysische Technologie, Faculty of Applied Sciences, Technical Univ. Delft, Prins Bernhardlaan 2628 BW, Delft, The Netherlands
^d Instituut voor Informatica, Faculty of Science, Universiteit van Amsterdam, Kruislaan 403 1098 SJ Amsterdam, The Netherlands

* Corresponding author (albrecht.weerts{at}unilever.com)

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.

Abbreviations: 2D, two dimensional • 3D, three dimensional • BGK, Bhatnagar Gross Krook Method • D_p,w, effective ion diffusion coefficient of the bulk soil • D_w,diffusion coefficient of an ion in free water • F_g, hydraulic conductivity of bundled straight capillaries (K_soil/K_cap) • GIS, geographical information system • l.u., lattice untis • LBM, lattice-Boltzman method • n_a, gas phase tortuosity parameter • n_w, water phase toturosity/connectivity and correlation factor • s_w, relative saturation • _a, soil surface conductivity • _w, soil water conductivity • _s, saturatedwater content • _w, water content • , pressure head • _d, finite value of pressure head • ₀, air entry value

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