Numerical study of evaporation‐induced salt accumulation and precipitation in bare saline soils: Mechanism and feedback

Abstract

Evaporation from bare saline soils in coastal wetlands causes salt precipitation in the form of efflorescence and subflorescence. However, it is not clear how much the precipitated salt in turn affects the water transport in the soil and hence the evaporation rate. We hypothesized that efflorescence exerts a mulching resistance to evaporation, while subflorescence reduces the pore space for water vapor to move through the soil. A numerical model is developed to simulate the transport of water, solute, and heat in the soil, and resulting evaporation and salt precipitation with the hypothesized feedback mechanism incorporated. The model was applied to simulate four evaporation experiments in soil columns with and without a fixed shallow water table, and was found to replicate well the experimental observations. The simulated results indicated that as long as the hydraulic connection between the near surface soil layer and the water source in the interior soil layer exists, vaporization occurs near the surface, and salt precipitates exclusively as efflorescence. When such hydraulic connection is absent, the vaporization plane develops downward and salt precipitates as subflorescence. Being more substantial in quantity, efflorescent affects more significantly evaporation than subflorescence during the soil-drying process. Different evaporation stages based on the location of the vaporization plane and the state of salt accumulation can be identified for characterizing the process of evaporation from bare saline soils with or without a fixed shallow water table.

Groundwater dynamics in subterranean estuaries of coastal unconfined aquifers: Controls on submarine groundwater discharge and chemical inputs to the ocean

“…Once the saturated salinity level is reached, salt precipitation can occur in the shallow sand layer, which would in turn reduce evaporation from the sand [Nachshon et al, 2011;Zhang et al, 2014]. Despite these studies exploring the effects on evaporation on the salinity distribution in shallow STE sediments, the importance of these effects on the flows and saltwater-freshwater mixing in a STE remains unclear.…”

“…We first synthesize current understanding with respect to the effect of the various driving forces on nearshore groundwater flow and salt transport (Section 2). 9 Second, we review the behavior of nutrients, carbon, greenhouse gases, metals and organic contaminants in CUAs (especially the STE), and the ultimate flux of these chemicals to the coastal ocean (Section 3). Finally, key knowledge gaps and challenges that need to be addressed to advance understanding of this poorly quantified connection between the land and ocean are provided.…”

Beyond the Salt Crust: On Combined Evaporation and Subflorescent Salt Precipitation in Porous Media

“…Currently the impact of salt precipitation on the overall hydraulic parameters of the soil is not very well understood (Zhang et al 2014). In this work, the impact of salt precipitation on hydraulic conductivity was examined.…”

“…The nature of salt precipitation as subflorescent or efflorescent crust is highly dependent on the hydraulic conditions that determine the location of the evaporation front to be at the matrix-atmosphere interface or within the matrix (Gupta et al 2014;Zhang et al 2014). Nevertheless, even when hydraulic conditions permit evaporation to take place at the matrix surface, many salts tend to precipitate in a subflorescent pattern, just below the surface in a thin layer at the order of few millimeters (Nachshon et al 2011b;Ruiz-Agudo et al 2006).…”

“…Efflorescent and subflorescent salt precipitation has markedly different impacts on the evaporation processes from porous media (Nachshon et al 2011a, b;Zhang et al 2014). Nevertheless, the liquid and vapor transport processes through these salt layers are far from being fully understood.…”