Preventing Salt Precipitation in Soils through Density-Driven Salt Instabilities

Soil salinization causes severe problems in agriculture, especially in arid and semi-arid regions, as it leads to soil degradation and reduces plant growth. During evaporation from a saline-water-saturated soil, salt accumulates near the top of the soil. Depending on the conditions, the increasing salt concentration will either lead to precipitation once the solubility limit is reached or due to the increase in the liquid density a gravitationally unstable situation is given, where instabilities in the form of fingers will develop. Hence, salt can be transported downwards. The development of these instabilities and the potential salt precipitation have been analyzed using numerical simulations on the REV-scale. The simulations were performed by using the numerical simulator DuMuX.   We analyzed the relevant processes to identify the influence of different parameters like soil-hydraulic properties, evaporation rate, or salt properties on precipitation. In Bringedal et. al., 2022, the appearance of instabilities during evaporation from a one-phase system was investigated using a linear stability analysis and numerical simulations on the REV-scale. The linear stability set criteria for the onset of instabilities for a large range of parameters, whereas the numerical simulations provide information about the development of the instabilities after onset. By combining both methods, we can predict the occurrence of instabilities and their effect on the salt concentration near the top boundary. This analysis has been extended to two-phase systems to analyze the impact of phase saturation on the development of salt instabilities.  In future work,  we plan to improve the REV-scale models with the help of the pore-network model. This will be done by identifying relevant parameters for salinization processes on the pore scale and using suitable upscaling methods for the use on the REV-scale.