Capillary flows across layers and textural interfaces - Pathways and colloid transport considerations in unsaturated layered porous media

Abstract

Soils and other geologic porous media often display layering and associated textural contrasts that may alter unsaturated flow and transport pathways. Such natural interfaces are usually represented empirically in continuum flow models with limited mechanistic account of the effect of adjusted pathways across layers on colloid transport and on the nature of unsaturated flow. In this study we present the soil foam drainage equation (SFDE) as alternative framework to simulate unsaturated flow in layered porous media.

Hypothesis: In contrast to standard continuum flow models, the SFDE explicitly accounts for capillary flow pathways and their adjustment across textural interfaces that in turn define flow geometry for colloidal transport not predicted by continuum models.

Experiments: Pore scale water distribution in a layered sand sample was measured using X-ray tomography to quantify capillary flow pathways. Literature values of measured saturation and capillary pressure profiles in layered porous columns were used to evaluate solutions based on the SFDE framework and to deduce velocity profiles and their effects for colloid transport.

Findings: The SFDE framework provides new insights into capillary architecture in unsaturated layered media and opens a new way for using pore scale information (from imaging) to provide the necessary SFDE parameters and potentially improve on the standard continuum representation of capillary flows in layered media.

Keywords: Capillary flow pathways; Layered porous media; Particle straining; Richards equation; Soil foam drainage equation.