Soil erosion is a major concern for both agricultural and natural resources. Soil water repellency (SWR) is known to hinder wetting of soils, decreasing infiltration of water and thus increasing overland flowthe driving force for erosion. The latter, namely the hydrological impacts of SWR on erosion, are quite well established. In contrast, the mechanical impacts of SWR, namely on the resistance to erosion, are poorly understood. Here, we provide a critical review of the current understanding of the combined impacts of SWR on erosion. Analysis of recent experimental data provides contradictory evidence: an increase in erosion with increasing SWR in some cases, versus a decrease in others. We offer a plausible explanation for this contradiction-the interrelated soil mechanical and hydrological impacts of SWR on erosion, which we exemplify by a simple 1D model. We identify research gaps and suggest ways to resolve them.

Flow through a granular, natural porous media can erode it chemically by dissolving and thus shrinking the solid particles, or mechanically, by removing them. The two-way interplay between the transport of fluids and dissolved solutes and alteration of the porous structure and hence the medium's transport properties is of interest to processes ranging from subsurface energy storage to contamination in groundwater. Here, we conduct a quantitative pore-scale analysis through a combination of numerical simulations and microfluidic experiments. We find that erosion enhances solute dispersion at low Pe (diffusion-dominated) and diminishes it at high Pe. Residence time distribution reveals that mechanical erosion tends to induce non-Fickian transport more than chemical erosion, which we attribute to the differences in their effects on pore size distribution.

We show that the return-point memory of cyclic macroscopic trajectories enables the derivation of a thermodynamic framework for quasistatically driven dissipative systems with multiple metastable states. We use this framework to sort out and quantify the energy dissipated in quasistatic fluid-fluid displacements in disordered media. Numerical computations of imbibition--drainage cycles in a quasi-2D medium with gap thickness modulations (imperfect Hele-Shaw cell) show that energy dissipation in quasistatic displacements is due to abrupt changes in the fluid-fluid configuration between consecutive metastable states (Haines jumps), and its dependence on microstructure and gravity. The relative importance of viscous dissipation is deduced from comparison with quasistatic experiments.

Understanding the fate of microplastics (MPs) in soil is one of the most urgent environmental tasks we face. It is also a very challenging one, as there are numerous properties of both MPs and the soil, as well as hydrological and geochemical conditions, that interplay to affect MPs transport. In this study, we conduct laboratory experiments in which two of the most commonly used MPs types in agriculture (polyethylene terephthalate (PET) and polypropylene (PP)) are leached into an idealized soil analog (glass beads). We find that MPs inhibit water flow, delaying its passage through the sample and making it more tortuous, forcing the flow to occur through preferential pathways (fingers). These effects are more pronounced for PP, which is more hydrophobic, than for PET. The transport of PP is inhibited relative to that of PET, which is attributed to its both its impeding effect on water flow (the driving force), as well as its surface charge which increases its tendency to adsorb onto soil particles, and lower density which curbs downward transport.

A document by Ran Holtzman. Click on the document to view its contents.

Pockmarks are morphological depressions commonly observed in ocean and lake floors. Pockmarks form by fluid (typically gas) seepage thorough a sealing sedimentary layer, deforming and breaching the layer. The seepage-induced sediment deformation mechanisms, and their links to the resulting pockmarks morphology, are not well understood. To bridge this gap, we conduct laboratory experiments in which gas seeps through a granular (sand) reservoir, overlaid by a (clay) seal, both submerged under water. We find that gas rises through the reservoir and accumulates at the seal base. Once sufficient gas over-pressure is achieved, gas deforms the seal, and finally escapes via either: (i) doming of the seal followed by dome breaching via fracturing; (ii) brittle faulting, delineating a plug. The gas lifts the plug and seeps through the bounding faults; or (iii) plastic deformation by bubbles ascending through the seal. The preferred mechanism is found to depend on the seal thickness and stiffness: in stiff seals, a transition from doming and fracturing to brittle faulting occurs as the thickness increases, whereas bubbles rise is preferred in the most compliant, thickest seals. Seepage can also occur by mixed modes, such as bubbles rising in faults. Repeated seepage events suspend the sediment at the surface and create pockmarks. We present a quantitative analysis that explains the tendency for the various modes of deformation observed experimentally. Finally, we connect simple theoretical arguments with field observations, highlighting similarities and differences that bound the applicability of laboratory experiments to natural pockmarks.

Solute transport in unsaturated porous media is of interest in many engineering and environmental applications. The interplay between small-scale, local forces and the porous microstructure exerts a strong control on the transport of fluids and solutes at the larger, macroscopic scales. Heterogeneity in pore geometry is intrinsic to natural materials across a large range of scales. This multiscale nature, and the intricate links between two-phase flow and solute transport, remain far from well understood, by and large. Here, we use high-resolution direct simulation to quantify solute mixing and dispersion behavior within correlated porous media during drainage under an unfavorable viscosity ratio. Through analysis of flow and transport at multiple realizations, we find that increasing spatial correlations in pore sizes increase the size of the required Representative Elementary Volume (REV). We show that increasing the correlation length enhances solute dispersivity through its impact on the spatial distribution of low-velocity (diffusion-dominated) and high-velocity (advection-dominated) regions. Fluid saturation is shown to directly affect diffusive mass flux among high-and low-velocity zones. Another indirect effect of correlated heterogeneity on solute transport is through its control of the drainage patterns via repeated alteration in the connectivity of flowing pathways. Our findings improve quantitative understanding of solute mixing and dispersion under two-phase conditions, highly relevant to some of our most urgent environmental problems.

Solute transport in unsaturated porous media is of interest in many engineering and environmen- tal applications. The interplay between small-scale, local forces and the porous microstructure exerts a strong control on the transport of fluids and solutes at the larger, macroscopic scales. Heterogeneity in pore geometry is intrinsic to natural materials across a large range of scales. This multiscale nature, and the intricate links between two-phase flow and solute transport, remain far from well understood, by and large. Here, we use high-resolution direct simulation to quantify solute mixing and dispersion behavior within correlated porous media during drainage under an unfavorable viscosity ratio. Through analysis of flow and transport at multiple realizations, we find that increasing spatial correlations in pore sizes increase the size of the required Representative Elementary Volume (REV). We show that increasing the correlation length enhances solute dis- persivity through its impact on the spatial distribution of low-velocity (diffusion-dominated) and high-velocity (advection-dominated) regions. Fluid saturation is shown to directly affect diffusive mass flux among high- and low-velocity zones. Another indirect effect of correlated heterogene- ity on solute transport is through its control of the drainage patterns via repeated alteration in the connectivity of flowing pathways. Our findings improve quantitative understanding of solute mixing and dispersion under two-phase conditions, highly relevant to some of our most urgent environmental problems.