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This article in SSSAJ

  1. Vol. 76 No. 5, p. 1509-1517
     
    Received: Jan 24, 2012
    Published: September 12, 2012


    * Corresponding author(s): hamasyo@mail.saitama-u.ac.jp
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doi:10.2136/sssaj2012.0033

Maxwell’s Law Based Models for Liquid and Gas Phase Diffusivities in Variably-Saturated Soil

  1. Shoichiro Hamamoto *a,
  2. Per Moldrupb,
  3. Ken Kawamotoc and
  4. Toshiko Komatsuc
  1. a Graduate School of Science and Eng. Saitama Univ. 255 Shimo-okubo Sakura-ku, Saitama, 338-8570, Japan and Inst.for Environ. Sci. and Technology Saitama Univ. 255 Shimo-okubo Sakura-ku, Saitama, 338-8570, Japan
    b Environmental Engineering Section Dep. of Biotechnology, Chemistry and Environmental Engineering Aalborg Univ. Sohngaardsholmsvej 57 DK-9000 Aalborg, Denmark
    c Graduate School of Science and Eng. Saitama Univ. 255 Shimo-okubo Sakura-ku, Saitama, 338-8570, Japan and Inst.for Environ. Sci. and Technology Saitama Univ. 255 Shimo-okubo Sakura-ku, Saitama, 338-8570, Japan

Abstract

The gas diffusion coefficient (Ds,g) and solute diffusion coefficient (Ds,l) and their dependencies on fluid content (κ) (equal to soil–air content θ for Ds,g and soil–water content ɛ for Ds,l) are controlling factors for gas and solute transport in variably saturated soils. In this study, we propose unified, predictive models for Ds,g(ɛ) and Ds,l(θ) based on modifying and extending the classical Maxwell model at fluid saturation with a fluid-induced reduction term including a percolation threshold (ɛth for Ds,g and θth for Ds,l). Different percolation threshold terms adopted from recent studies for gas (Ds,g) and solute (Ds,l) diffusion were applied. For gas diffusion, ɛth was a function of bulk density (total porosity), while for solute diffusion θth was best described by volumetric content of finer soil particles (clay and organic matter), FINESvol. The resulting LIquid and GAs diffusivity and tortuosity (LIGA) models were tested against Ds,g and Ds,l data for differently-textured soils and performed well against the measured data across soil types. A sensitivity analysis using the new Maxwell’s Law based LIGA models implied that the liquid phase but not the gaseous-phase tortuosity was controlled by soil type. The analyses also suggested very different pathways and fluid-phase connectivity for gas and solute diffusion in unsaturated soil. In conclusion, the commonly applied strategy of using the same, soil-type-independent model for gas and solute diffusivity in analytical and numerical models for chemical transport and fate in variably-saturated soils appears invalid, except for highly sandy soils. The unified LIGA model with differing percolation thresholds for diffusion in the liquid and gaseous phases solves this problem.

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Copyright © 2012. Copyright © by the Soil Science Society of America, Inc.