Effect of Diffusive Tortuosity on Longitudinal Dispersivity in Undisturbed Soil.

The fate of agricultural chemicals and different contaminants in soil depends on various transport, reaction, and transformation processes. The tortuous pore geometry in soil influences primarily the transport processes which include diffusion-dispersion and advective transport. A large body of literature exists on the relationship between dispersivity(λ) of soil and advective velocity(ν) of solutes during transport. The effect of tortuosity (τ) is generally embedded in the relation between ν and λ. Direct measurements of τ and resulting λ are rarely reported in the literature. In this study the longitudinal (λ_L) dispersivity was estimated from laboratory scale transport experiments using undisturbed soil spheres having τ ranging from 1.07 to 3.05. The soil spheres were collected from a large region covering the states of West Bengal and Odisha in eastern India. A one-dimensional solute transport model and the Hydrus 2D/3D modeling environment was used to estimate the λ_L values. Results showed that λ_L follows a logarithmic relationship with &tau (R² =0.92). Because λ is a function of both the geometry of the porous media and the microscopic variation of ν inside the flow domain, our result for the first time shows the nature of the relationship between pore geometry (as captured by τ) and λ values. Such relationships may be built into flow and transport models to directly estimate dispersivity in the soil.