Impact of soil texture and salt type on salt precipitation and evaporation under different hydraulic conditions

Abstract

Salt precipitation and evaporation in porous media is an important research topic. However, several aspects remain controversial; notably, whether efflorescence inhibits evaporation in wet soil, the influence of soil texture on salt crust formation and the effect of sulphate precipitation on evaporation under changing hydraulic conditions. Therefore, this study investigates the influence of salt type (NaCl and Na2SO4) and soil texture (sandy soil, sandy loam and silt loam) on salt precipitation and evaporation under different hydraulic conditions (with and without fixed groundwater). Our results demonstrated that a NaCl salt crust strongly inhibited evaporation, even from wet soil (with groundwater), owing to its denser salt structure, which inhibited fluid flow. Moreover, the salt crust pattern differed based on soil texture. Relatively thin, flat and uniform salt crusts were developed in finer soils (silt loam), with greater evaporation resistance, whereas thick and rough salt crusts were developed in coarser soil (sandy soil), with less evaporation resistance. Na2SO4 was precipitated as subflorescence in drying soil, whereas wet soils exhibited a mixed salt precipitation pattern including both efflorescence and subflorescence. Notably, evaporation was primarily inhibited by efflorescence, rather than subflorescence. Our research provides novel insights into the dynamics of salt precipitation and evaporation in natural soils.

Effect of salt types on salt precipitation and water transport in saline sandy soil

“…Salt crystallization continued to grow slowly until the end of the experiment (15 days), when efflorescence completely covered the soil surface in treatment S1, while efflorescence in treatment S2 covered only 70% of the soil surface area. This result is similar to that reported by Li et al (2022), where the efflorescence of NaCl treatments exhibited a higher coverage rate and covered a larger area compared to Na 2 SO 4 treatments.…”

“…Soil evaporation processes are usually affected by aerodynamic resistance and soil dry layer resistance. Moreover, previous studies reported that evaporation in saline soils is also affected by the resistance arising from salt crystallization (Fujimaki et al, 2006; Li et al, 2022; Li & Shi, 2019; Zhang et al, 2014). In this study, the effect of wind speed on evaporation was not considered; therefore, for the evaporation process, dry layer resistance and salt crystallization resistance were primarily considered, and they were calculated as follows: $$$$ E=\frac{C_{vs}-{C}_{va}}{R_s+{R}_{sc}} $$$$ where C v is the water vapour concentration (kg m −3 ) and subscripts s and a denote soil and air, respectively, R s is the soil dry layer resistance (m s −1 ), and R sc is the salt crystallization resistance (m s −1 ). $$$$ {C}_v=\frac{e_s\times {M}_w\times {H}_r}{R\left(T+273.15\right)} $$$$ $$$$ {e}_s=611\times \exp \left(\frac{17.502\times T}{T+240.97}\right) $$$$ where e s is the saturated water vapour pressure (Pa), M w is the molar mass of water (kg mol −1 ), H r is the relative humidity (%), R is the universal gas constant (J mol −1 K −1 ), and T is the soil surface temperature or air temperature (°C).…”

“…In porous media, salt crystallization grows continuously with time. Therefore, in this study, an exponential relationship was assumed between the area covered by salt crystallization and time, and the variation of the area covered by salt crystallization was quantified according to the fitting equation of Li et al (2022), as follows: $$$$ {A}_{sc}=\frac{A_s}{1+{e}^{-\alpha \left(t-\beta \right)}} $$$$ where A sc is the area covered by salt crystallization (cm 2 ), A s is the surface area of the soil column (cm 2 ), t is the experimental period (d), α and β are the fitting parameters. In the process of salt crystallization covering the soil surface, the area covered by salt crystallization will not continue to grow when it reaches a certain value, which is equal or smaller than the surface area of the soil column.…”

“…Although NaCl can also precipitate as subflorescence, its effect on water transport is primarily dependent on efflorescence (Desarnaud et al, 2015; Sghaier & Prat, 2009). In contrast, Na 2 SO 4 and MgSO 4 mostly precipitate as subflorescence and rarely as efflorescence (Li et al, 2022; Nachshon & Weisbrod, 2015; Ruiz‐Agudo et al, 2007). Nachshon and Weisbrod (2015) reported that MgSO 4 subflorescence under the condition of continuous water supply without efflorescence.…”

“…According to Nachshon and Weisbrod (2015), NaCl efflorescence leads to lower hydraulic conductivity and permeability coefficients than MgSO 4 subflorescence, thus water flux is smaller, and the efflorescence layer with a denser structure has stronger resistance to water vapour diffusion. Li et al (2022) reported that NaCl efflorescence is more effective at inhibiting water transport than the subflorescence and mixed crystallization of Na 2 SO 4 . In general, it is suggested that NaCl efflorescence affords stronger resistance to water vapour diffusion compared to the subflorescence of salts such as Na 2 SO 4 and MgSO 4 .…”