Gas diffusion-derived tortuosity governs saturated hydraulic conductivity in sandy soils

The saturated hydraulic conductivity (K_sat) is an essential effective parameter for the development of improved distributed hydrological models and area-differentiated risk assessment of chemical leaching. Basic soil properties such as the particle size distribution or, more recently, air permeability are commonly used to estimate K_sat. Conversely, links to soil gas diffusivity (D_p/D_o) have not been fully explored even though gas diffusivity is intimately linked to the connectivity and tortuosity of the soil pore network. Based on measurements for a coarse sandy soil, potential relationships between K_sat and D_p/D_o were investigated. A total of 84 undisturbed soil cores were extracted from the topsoil of a field site, and D_p/D_o 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/D_o relationship. All analyses implied strong and fundamental relationships between K_sat and D_p/D_o.