Evaporation of saline solutions from porous media is governed by the complex interactions between the transport properties of the porous media, the evaporating solution, and the external boundary conditions. In the present study, we have investigated the effects of salt concentration on the evaporation process from porous media in the presence of a sharp textural discontinuity, a common heterogeneity in natural porous media formed due to the weathering or formation of soil horizons, wind deposition, and erosion. We have conducted a comprehensive series of macroscale and microscale experiments to delineate how the precipitation pattern is modified as salt concentration varies from relatively low values to a concentration close to the solubility limit. For concentrations much less than the solubility limit, the precipitation begins at the coarse‐textured part of the heterogeneous porous media (which is a counter‐intuitive result considering the preferential evaporation of water from the fine‐textured part of the heterogeneous surface). However, when the concentration is close to the solubility limit, precipitation initiates preferentially at the fine‐textured part of the heterogeneous porous surface. This behavior results from the interaction between the transport properties of the porous media and the properties of the evaporating solution which must be considered. Additionally, using pore‐scale images obtained by X‐ray microcomputed tomography (CT), we have visualized the dynamics of precipitation in the presence of heterogeneity at high spatial and temporal resolution. The pore‐scale results corroborate the mechanisms controlling the precipitation patterns in the presence of textural discontinuities inferred from the macroscale experiments.

Evaporation of saline water from porous media is ubiquitous in many processes including soil salinization, crop production, and CO2 sequestration in deep saline acquirer. It is controlled by the transport properties of porous media, atmospheric conditions, and properties of the evaporating saline solution. In the present study, the effects of mixed wettability conditions on the general dynamics of water evaporation from porous media saturated with NaCl solution were investigated. To do so, we conducted a comprehensive series of evaporation experiments using sand mixtures containing different fractions of hydrophobic grains saturated with NaCl solutions. Our results showed that increasing fraction of hydrophobic grains in the mixed wettability sand pack had minor impact on the evaporative mass losses due to the presence of salt whose precipitation patterns were significantly influenced by the mixed wettability condition. Through macroscale and microscale investigations, we found formation of patchy efflorescence in the case of mixed wettability sand pack as opposed to crusty efflorescence in the case of completely hydrophilic porous media. Furthermore, the presence of salty water and hydrophobic grains in the sand pack significantly influenced the general dynamics and morphology of the receding drying front. Our results extend the understanding of the saline water evaporation from porous media with direct applications to various hydrological and engineering processes.

Increasing salinity in groundwater and soil poses a threat to water and land resources. With the expectation of major changes to the hydrological cycle through climate change, the need for understanding the fundamental processes governing solute transport through soil has grown significantly. We provide experimentally verified insights into the influence of particle size distribution on solute transport in porous media during evaporation at the pore- and macro-scales. To do so, we utilized four-dimensional (space plus time) synchrotron X-ray tomography for iodine k-edge dual energy imaging to obtain solute concentration profiles in every single pore during saline water evaporation from coarse- and fine-grained sands. Close to the surface of the coarse-grained sand significantly higher salt concentrations were observed when compared to fine-grained sand with the same porosity under similar cumulative evaporative mass losses. The physics behind this behaviour was delineated using the recorded data with high spatial and temporal resolutions. Moreover, the measured data enabled us to quantify the variations of the effective dispersion coefficient during evaporation and how it is influenced by the particle size distribution. We show that, contrary to common assumption in modelling of solute transport during evaporation, the effective dispersion coefficient varies as a function of liquid saturation and the length of the invaded zone during evaporation from porous media, and that it increases as liquid saturation decreases.

Evaporation of saline water from porous media and associated salt deposition are important for many applications ranging from soil salinization to the protection of archeological and civil structures. We investigate the effects of textural heterogeneity of porous media on evaporative salt precipitation patterns. Textural heterogeneity has been introduced into sand columns to form capillary‐interacting porous domains comprised different particle sizes. Several levels of textural contrasts were experimentally studied in terms of their effects on drying rates, salt precipitation patterns, and drying front displacement rates. Results show that in contrast with evaporation of pure water, evaporation of saline solutions from media with textural contrasts exerted only a minor effect on evaporation rates. This difference is attributed to enhanced liquid transport within the highly porous precipitated salt on the surface. The presence of textural contrasts significantly enhanced preferential spread of salt precipitation on the surface. The enhancement is attributed to low surface water content of the coarse domain and larger spacing between remaining evaporating wet pore clusters in which greater local evaporation rates are accelerating salt precipitation. The study illustrates the interactions between the water characteristic curve, preferential drying front displacement and subsequent salt deposition patterns, as also seen in high resolution X‐ray Computed tomography (CT) used to verify the suggested mechanisms. The study provides new insights into mechanisms of saline water evaporation and the complex effects of porous media heterogeneity on salt precipitation patterns and dynamics; all critical ingredients for enhanced understanding of the salt distribution in and transport in soils.