Eddy-driven effects on solute transport in porous media

Abstract

Groundwater pollution poses a significant threat to water resource sustainability. However, while the hydrodynamic impact of pore-scale eddies has been investigated, their quantitative linkage to upscaled macroscopic solute transport parameters remains insufficiently established. This study investigates the effects of hydrodynamic conditions (flow velocity) and porous media structural parameters (particle size, arrangement) on eddy development and solute transport through laboratory experiments and numerical simulations. A novel three-dimensional (3D) quantitative method for characterizing eddy zones was proposed, revealing the mechanisms of eddy formation and their impact on solute breakthrough curves (BTCs). Results indicate that higher flow velocities and larger particle sizes amplify eddy proportions, leading to pronounced BTC tailing due to delayed solute exchange between main flow stream and eddy zones. The mobile-immobile model (MIM) parameters, particularly the immobile zone ratio (1 - β), showed strong alignment with eddy proportions, reducing inversion ambiguity. Smaller particle sizes diminished early solute breakthrough, while random-packed (RP) media exhibited the slowest solute penetration compared to structured arrangements (SC, FCC, BCC). The study establishes exponential relationships between dilution index and eddy-dominated solute heterogeneity, highlighting structural controls on diffusion coefficients. These findings enhance theoretical frameworks for groundwater solute transport and provide practical insights for optimizing pollution remediation strategies in porous media systems.