Abstract
Many environmental and agricultural applications involve the transport of water and dissolved constituents through aggregated soil profiles, or porous media that are structured, fractured or macroporous in other ways. During the past several decades, various process-based macroscopic models have been used to simulate contaminant transport in such media. Many of these models consider advective-dispersive transport through relatively large inter-aggregate pore domains, while exchange with the smaller intra-aggregate pores is assumed to be controlled by diffusion. Exchange of solute between the two domains is often represented using a first-order mass transfer coefficient, which is commonly obtained by fitting to observed data. This study aims to understand and quantify the solute exchange term by applying a dual-porosity pore-scale network model to relatively large domains, and analysing the pore-scale results in terms of the classical dual-porosity (mobile-immobile) transport formulation.We examined the effects of key parameters (notably aggregate porosity and aggregate permeability) on the main dual-porosity model parameters, i.e., the mobile water fraction (ϕm) and the mass transfer coefficient (α). Results were obtained for a wide range of aggregate porosities (between 0.082 and 0.700). The effect of aggregate permeability was explored by varying pore throat sizes within the aggregates. Solute breakthrough curves (BTCs) obtained with the pore-scale network model at several locations along the domain were analysed using analytical solutions of the dual-porosity model to obtain estimates of ϕm and α. An increase in aggregate porosity was found to decrease ϕm and increase α, leading to considerable tailing in the BTCs. Changes in the aggregate pore throat size affected the relative flow velocity between the intra- and inter-aggregate domains. Higher flow velocities within the aggregates caused a change in the transport regime from diffusion dominated to more advection dominated. This change increased the exchange rate of solutes between the mobile and immobile domains, with a related increase in the value of the mass transfer coefficient and less tailing in the BTCs.
Highlights
Soil and groundwater pollution by a broad range of industrial and agricultural contaminants is an ever-increasing problem worldwide
We investigated the effects of multi-scale pore sizes in a dualporosity porous medium on fluid flow and solute transport processes
The resulting breakthrough curves (BTCs) were fitted analysed in terms of the conventional macroscopic dual-porosity (MIM) transport model to estimate several key macroscopic transport parameters
Summary
Soil and groundwater pollution by a broad range of industrial and agricultural contaminants is an ever-increasing problem worldwide. Much evidence exists that preferential flow through especially the vadose zone is contributing to surface and subsurface pollution problems (e.g., Flury et al, 1994; Abbaspour et al, 2001; Hendrickx and Flury, 2001; Allaire et al, 2009, Vogel et al, 2010; Zhang et al, 2013; Mahmoodlu et al., 2013, 2014). Comprehensive reviews of alternative modelling approaches are provided by NRC (2001), Šimunek et al (2003), Gerke (2006), Jarvis (2007), Šimunek and van Genuchten (2008) and Köhne et al (2009). E.T. de Vries et al / Advances in Water Resources 105 (2017) 82–95 term is used, to lead to mobile-immobile (MIM) type dual-porosity models of the form (Coats and Smith, 1964; van Genuchten and Wierenga, 1976): φm θ
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