Gas Diffusion-Derived Tortuosity Governs Saturated Hydraulic Conductivity of Sandy Soils.

Accurate prediction of saturated hydraulic conductivity (K_sat) is essential towards development of better distributed hydrological models and area-differentiated risk assessment of chemical leaching. The K_sat is often estimated from basic soil properties such as particle size distribution or, more recently, soil-air permeability. However, similar links to soil-gas diffusivity (D_p/D_o, the ratio of gas diffusion coefficients in soil and pure air) have not been explored although gas diffusivity is a direct measure of the connectivity and tortuosity of the pore networks. Based on measurements for a coarse sandy soil, potential relationships between K_sat and D_p/D_o were investigated. A total of 88 undisturbed soil cores were extracted from the topsoil of a field site, and D_p/D_o (at four different matric potentials) and K_sat were measured in the laboratory. Water-induced and solids-induced tortuosity factors were obtained by applying a two-parameter D_p/D_o model to measured data, and subsequently linked to the cementation exponent of the well-established Revil and Cathles predictive model for saturated hydraulic conductivity. Furthermore, a two-parameter model analogue to the Kozeny-Carman equation was developed for the K_sat - D_p/D₀ relationships. All analyses implied strong and fundamental relations between K_sat and D_p/D_o at all four different matric potentials. The promising links between K_sat and basic pore network characteristics should be further investigated for a wider range of soil types.