Influence of Grain Size Transition on Flow and Solute Transport through 3D Layered Porous Media

Abstract

Soils and other geologic porous media often have contrasting grain size layers associated with a grain size transition zone between layers. However, this transition zone is generally simplified to a plane of zero thickness for modeling assumption, and its influence has always been neglected in previous studies. In this study, an approach combining a deposition process and a random packing process was developed to generate 3D porous media without and with consideration of the transition zone. The direct numerical models for solving the flow and concentration fields were implemented to investigate the influence of the grain size transition on flow and solute transport. Our results showed that although the transition zone occupied 13.6% of the entire layered porous medium, it had little influence on the distribution of flow velocity at the scale of the entire layered porous medium. However, the transition zone had a significant influence on the local flow field, which was associated with the increased spatial variability of velocity and the varied distribution of flow velocity. This varied local flow field could increase the solute residence time and delay the breakthrough time for solute transport. Although using both the advection-dispersion equation (ADE) and the mobile and immobile (MIM) models to fit the breakthrough curves (BTCs) for solute transport through layered porous media resulted in trivial errors, the ADE model failed to capture the influence induced by the local flow field, especially the influence of the transition zone. In contrast, the MIM model was shown to be able to capture the influence of the transition on solute transport. It was found that the mass transfer rate α, a parameter of the MIM model, was significantly improved by the presence of the transition zone, while it decreased as the transition zone fraction increased. Our study emphasized that the transition zone can vary the local flow field at the pore scale, while it has little influence on the hydraulic properties (e.g., hydraulic conductivity) of the macroscale flow field. However, the local flow field varied by the transition zone has a significant influence on solute transport.

Using a Pore-Scale Modelling Approach to Study Solute Dilution Process through Cemented Porous Media

“…The obtained experimental results of three groups of porous media indicated that the cementation degree of the solid grains had significant influencing effects on the temporal evolution of the dilution breakthrough curves. As shown in Figure 3, for the porous media in which the solid grains were slightly cemented (PM35, PM30), the values of the (23,43,63, and 84 mm) increased rapidly to levels higher than the asymptotic values of the transient flux-related dilution index in the noncemented porous medium (PM39). In addition, it was found that the higher the cementation degree, the greater the asymptotic value of the flux-related dilution index.…”

“…For example, incomplete mixing at smaller scales has been found to generally lead to the reaction extent of the solute being lower than the predictions obtained using Darcy-Scale reactive transport models [6]. In addition, previous studies have revealed that the heterogeneity of a porous medium and the characteristics of pore channels, including the topological features, connectivity, and size distributions of pore spaces, have significant influences on solute dilution processes in a porous medium [22][23][24]. In particular, the results of previous studies have indicated a strong link between the enhancement of solute dilution and the flow focusing facilitated by the heterogeneous permeability field in a spatially uncorrelated aquifer [25,26].…”