Tortuosity, Diffusivity, and Permeability in the Soil Liquid and Gaseous Phases

Abstract

Tortuosity phenomena of pore space influence the transport of water, solutes, and gases in soil. This study presents three analyses linking tortuosity and transport in unsaturated soil. The first is a diffusion‐based analysis of tortuosity in the soil water and soil air phases, related to soil surface area (SA) and pore‐size distribution (PSD) (characterized by Campbell b and content of pores >30 μm). The analysis is based on recent models to predict the diffusion coefficients, Dp, of (i) a solute in soil, (ii) a gas in repacked soil, and (iii) a gas in undisturbed soil, each as a function of fluid‐phase (soil water or soil air) content, α. For use in the analysis, the relation between SA and the threshold water content where solute diffusion ceases due to disconnected water films was measured for eight soils (5–46% clay). The tortuosity analysis supported by measured Dp(α) data shows that SA governs and has a larger impact on liquid‐phase tortuosity than PSD has on gaseous‐phase tortuosity. At the same value of α, the tortuosity is typically larger in the soil water than in the soil air phase, and the difference becomes more pronounced with increasing SA and at low α. In the second analysis air permeability, ka, and gas diffusivity, DP,g, are linked in the Millington and Quirk fluid flow model to describe soil structure‐forming potential and to establish a model platform to describe ka as a function of DP,g and α. Measurements on repacked, nonaggregated soil support the ka(DP,g;α) model platform, while measurements on repacked, aggregated soils and on undisturbed soils show that ka is greatly affected by soil aggregation and structure and DP,g is not. In the third analysis, a constitutive parameter model is applied to gas and solute diffusivities and air and water permeabilities in six soils along a soil texture gradient. This illustrates the different behavior of the four transport parameters with PSD and α. The liquid‐phase transport parameters show a steeper decrease with α compared with the gaseous‐phase parameters, in part due to the higher tortuosity in the liquid phase. Also, ka in undisturbed soil exhibited a less steep decrease with α compared with DP,g, probably due to preferential air flow in larger pores during convective transport. Any attempt to develop a unifying and PSD‐dependent model for transport parameters in the soil liquid and gaseous phases will require careful distinction between repacked and undisturbed soils.

The role of terrestrially derived organic carbon in the coastal ocean: A changing paradigm and the priming effect

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Pore-scale investigation on the response of heterotrophic respiration to moisture conditions in heterogeneous soils

“…The diffusivity of dissolved and gaseous species in soils is dependent on the saturation degree as well as pore water and pore air connectivity (Moldrup et al 2001). In this study, this dependency is described using the following equations (Archie 1942;Hunt 2004), ), below which water is disconnected and the effective solute diffusion coefficient becomes zero; e th is the gas percolation threshold (m 3 /m 3 ), below which air is disconnected and gas phase diffusion ceases; m1 and m2 are cementation exponents (-); n1 and n2 are saturation exponent (-).…”

Transport study in unsaturated porous media by tracer experiment in a dichromatic X-ray experimental device

“…In this context, dispersion also depends on water content, and the links between tortuosity, pore velocity and dispersivity are not entirely understood [3][4][5]. Solute dispersion studies in unsaturated porous media suggest that the dispersion coefficient increases with decreasing water content [3,4,6,7].…”

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