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Clay is widely encountered in nature and directly influences seepage behaviors, exerting a crucial impact on engineering applications. Under low hydraulic gradients, seepage behaviors have been observed to deviate from Darcy's law, displaying a non-linear trend. However, the impacts of clay content on non-linear seepage behavior and its pore-scale mechanisms to date remain unclear. In this study, constant-head seepage experiments were conducted in sand–clay porous media under various hydraulic gradients. Low-field nuclear magnetic resonance (LF-NMR) technology was utilized to monitor the bound-water and free-water contents of sand–clay porous media under different seepage states. The results show a threshold hydraulic gradient (i0) below which there is no flow, and a critical hydraulic gradient (icr) below which the relationship between the hydraulic gradient (i) and seepage velocity (v) is non-linear. Both hydraulic gradients increased with clay content. Moreover, the transformation between bound water and free water was observed during the seepage-state evolution (no flow to pre-Darcy or pre-Darcy to Darcy). As the hydraulic gradient reached the i0, the pore water pressure gradually overcame the adsorption force of the bound-water film, reducing the thickness of the bound-water film, and causing non-linear seepage behavior. When i0 < i < icr, the enlarging hydraulic gradient triggers the thinning of bound water and enhances the fluidity of pore water. Moreover, the increasing clay content augments the bound-water content required for the seepage state's change. [ABSTRACT FROM AUTHOR]

Three-phase displacement events often occur in unsaturated porous media, especially when immiscible contaminants (e.g., non-aqueous phase liquid, NAPL) inadvertently enter the subsurface. In this study, the influence of initial water saturation on the three-phase displacement efficiency and residual distribution during NAPL invasion into unsaturated porous media was experimentally investigated. The low-field nuclear magnetic resonance (LF-NMR) technique was used to dynamically monitor the spatial distribution of water in unsaturated porous media during the invasion of NAPL. The residual distributions (e.g., volume fraction) of NAPL/water/gas were quantitatively analyzed. The results showed that both the initial water saturation and the NAPL injection rate had a significant influence on the three-phase displacement. We found that the displacement efficiency of water/gas exhibited a non-monotonic relationship with the initial water saturation in the unsaturated porous media at certain NAPL invasion rates. A method was proposed to characterize the three-phase displacement regime in terms of the classical two-phase Lenormand's diagram. A significant increase in the volume fraction of residual NAPL after NAPL invasion was observed during the transition from the strong drainage region to the transitional displacement region. However, the initial water saturation was insensitive to the volume fraction of residual NAPL, once the characterized three-phase displacement regime transitioned to the stable displacement region. This study not only highlighted the influence mechanism of initial water saturation and NAPL injection rate on the three-phase displacement efficiency, but also endowed the classical two-phase Lenormand's diagram with the three-phase displacement regime. [ABSTRACT FROM AUTHOR]

• Bound water (BW) enhanced non-Fickian characteristics. • Immobile water fraction increased with increasing estimated bound water. • Solute exchange rate decreased with increasing bound water. • Moderately non-Fickian solute transport occurred in porous media with BW < 16.4 %. • Highly non-Fickian solute transport occurred in porous media with BW ≥ 16.4 %. Bound water (BW) has significant effects on the physical and chemical properties of soils and other geologic formations. However, little is known about its effect on the non-Fickian transport of solute in porous media. This paper investigated the effect of BW on the non-Fickian solute transport characteristics in mixed porous media. Different porous media of quartz sand with variable contents of powdered minerals were prepared. The low-field nuclear magnetic resonance (LF-NMR) was used to quantitatively characterise BW and its spatial distribution. Then solute transport experiments were conducted in different prepared porous media and the measured breakthrough curves (BTCs) were analysed using the advection–dispersion equation (ADE), mobile-immobile (MIM) and continuous time random walk (CTRW) models. The results revealed that all measured BTCs exhibited the characteristics of early arrival and late-time tailing. However, stronger non-Fickian characteristics were observed as BW increased in content, indicating that BW influenced the non-Fickian transport of the solute. The optimised immobile water fraction of the MIM model consistently increased with increasing estimated BW fraction. Moreover, the coefficient of mass transfer rate (α) of MIM decreased as BW increased, meaning that the solute exchange between the mobile and immobile fluids became slower and consequently yielded strongly tailed BTCs. From the CTRW model, the solute transport velocity (v CTRW ) increased with increasing BW, leading to a reduction in the breakthrough time of the solute. An increase in BW yielded a decreasing power law exponent (β CTRW ) of the CTRW model, implying an enhancement of the non-Fickian transport characteristics. Furthermore, the porous media with B W < 16.4 % showed moderately non-Fickian solute transport whereas those with B W ≥ 16.4 % exhibited highly non-Fickian solute transport. [ABSTRACT FROM AUTHOR]

• Redistribution of NAPLs driven by freeze–thaw (FT) cycles was confirmed in a wide saturation range (14.34% to 72.05%). • Substantial remobilization of NAPLs occurred between macropores and mesopores during FT cycles. • Distribution of NAPLs in micropores remained almost unchanged during FT cycles. • Contribution of micropores to NAPLs redistribution was much smaller than that of mesopores and macropores. • A model related to freezing-induced pressure was proposed to explain the change in the redistribution rate of NAPLs. Increasing human activities in cold regions have led to serious environmental problems due to non-aqueous phase liquids (NAPLs) contamination. However, the NAPLs redistribution driven by freeze–thaw (FT) cycles is not well understood. In this study, the redistribution of NAPLs in porous media subjected to FT cycles in a temperature range from 20 °C to −15 °C was investigated using the low-field Nuclear Magnetic Resonance (NMR) technique. The redistribution of NAPLs in the prepared samples with NAPLs saturation ranging from 13.56% to 72.05% was confirmed using the transverse spin-spin relaxation time (T 2) distribution and the magnetic resonance imaging (MRI). Experimental results revealed that substantial remobilization and distribution of NAPLs occurred between the macropores (T 2 greater than 300 ms) and the mesopores (60 ms < T 2 < 300 ms) while the NAPLs in the micropores (T 2 < 60 ms) remained almost unchanged during 30 FT cycles. The main reason is attributed to the fact that the redistribution rate in micropores is much lower than in macropores and mesopores. Furthermore, the NAPLs content and the number of FT cycles were positively correlated for macropores while negatively correlated for mesopores. For the samples with low NAPLs saturation (e.g., S N =13.56% and 13.56%), the redistribution rate of NAPLs versus FT cycles exhibited an overall linear relationship for macropores and mesopores while a non-liner relationship for micropores. For all pore categories, the increment in S N can cause a remarkable deviation extent. The balance between capillary pressure and freezing-induced pressure was evaluated by the newly introduced dimensionless pressure ratio Δ P , which could help determine the ultimate mobilization and redistribution of NAPLs during FT cycles. Experimental tests suggested that the Δ P was much larger in macropores than in mesopores and micropores. This can explain the dramatic difference in the redistribution rate among different pore categories. This fundamental study is helpful in understanding the mobilization and redistribution mechanisms of NAPLs in cold regions. [ABSTRACT FROM AUTHOR]

• The change behavior of θ u with soil properties in frozen soils was investigated. • Two cutoffs were proposed to identify three different types of unfrozen water. • The clay content and mineral composition affect the changes of SFCC of bound water. • The ω couldn't reflect the influence of the clay mineral or clay content on θ u. In this study, a low-field nuclear magnetic resonance (NMR) apparatus with the ability to adjust sample temperatures was used to measure the unfrozen water content of frozen soils sampled from the Qinghai-Tibet Plateau. Based on low-field NMR theory, two cutoff values were proposed to quantitatively identify three types of unfrozen water (bulk, capillary, and bound water) in the frozen soils. The influences of soil properties (the clay content, particle size distribution, and clay mineral content) on these three different types of unfrozen water during the soil freezing process were analyzed. The results showed that the soil-freezing characteristic curve of unfrozen water could be divided into three stages, namely, the unfrozen, the rapid-drop, and the residual stages. In the rapid-drop stage, the unfrozen bound water content increases gradually with the increasing clay or clay mineral content, and at a certain temperature, the unfrozen capillary water content increases as the sand and silt content increases. A published power function model is then used to fit the measured data in this study. We found that the single parameter related to soil properties in this model could not sufficiently reflect the influence of clay mineral content or clay content on unfrozen water content. [ABSTRACT FROM AUTHOR]