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When surface water infiltrates soil, the fine soil particles carried in water gradually clog soil pores and form a low-permeability soil layer. Clogging impacts the variations in pore water pressure heads in soil and effective hydraulic conductivity. However, few studies have connected field measurements of pore water pressure heads to clogging in soil. This study proposed a diagram to demonstrate the relationship between the normalized pore water pressure head (λ) and effective hydraulic conductivity (Keff) based on a conceptual 1-D vertical infiltration model. The coevolution of λ and Keff indicated the occurrence of clogging and its location relative to the pore-pressure measurement point. We validated the λ-Keff diagram based on a series of numerical simulations of infiltration experiments in a lysimeter. The simulation results showed that the proposed diagram not only indicated the occurrence of clogging but also the development of the unsaturated zone beneath the upper clogging layer. Furthermore, we used a diagram to analyze the spatiotemporal changes in permeability in a lysimeter during three cycles of physical infiltration experiments. The experimental data presented with λ-Keff diagram indicated cracking on the soil surface, and clogging gradually developed at the bottom of the lysimeter.

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Understanding the displacement of the resident wetting fluid in porous media is crucial to the remediation strategy. When pollutants or nutrients are dissolved in the surface wetting fluid and enter the unsaturated zone, the resident wetting fluid in the porous system may remain or be easily flushed out and finally arrive in the groundwater. The fate and transport of the resident wetting fluid determine the policy priorities on soil or groundwater. In this study, the displacement of the resident wetting fluid by the invading wetting fluid in porous media was simulated using direct numerical simulation (DNS). Based on the simulations of the displacements in porous media, the effect of the non-wetting fluid on the displacement was evaluated by observation and quantification, which were difficult to achieve in laboratory experiments. The result can also explain the unknown phenomenon in previous column experiments, namely that the old water is continuously released from the unsaturated porous media even after a long period of flushing with the new water. The effects of the interfacial tension, contact angle, and injection rate, which affected the immiscible fluid–fluid flow pattern, were also evaluated. Since pollutants dissolved in the wetting fluid could change the physical properties of the wetting fluid, the interfacial tensions of the resident wetting fluid and the invading wetting fluid were set separately in the simulation. Moreover, our simulation demonstrated that the consecutive drainage–imbibition cycles could improve the displacement of the resident wetting fluid in porous media. The successful simulation in this study implied that this method can be applied to predict other immiscible fluid–fluid flow in natural or industrial processes.