Numerical modeling and analysis of evaporative salinization in a coupled free-flow porous-media system

Abstract

Interaction between fluid in the porous medium and the conjugate free flow is observed in many natural and technical applications. In these applications, the flow and transport processes in the porous medium and in the free flow display strong interdependency. This is also influenced by the mechanisms of interaction at the common interface between the porous medium and the free flow. Studying these mechanisms plays a significant role in understanding the interaction behavior of these systems. Moreover, it is also important for the numerical modeling of such systems. An environmental example of such a system is evaporative drying of the shallow sub-surface subjected to the atmospheric free flow. As it significantly influences the terrestrial water balance and soil salinization, developing a detailed understanding about such a coupled system is very important. Furthermore, salt precipitation associated with evaporation from the shallow sub-surface directly affects the fertility of the soil. This becomes significantly important in the arid, semi-arid or coastal regions, where water from the deep hydrological systems is largely used for irrigation. Literature review shows that the soil salinization associated with the promoted irrigation is a major problem for farmers worldwide, as it directly influences the crop yield. In such systems, the evaporative salinization is influenced by the ambient air velocity, temperature and humidity on the free flow side and, by the soil properties and properties of water used for irrigation on the porous-media side. Therefore, for studying the evaporation and salinization dynamics in such systems, developing a detailed understanding of the underlying processes is very important and, elaborations using the modeling tools are very crucial. The main focus of this PhD thesis is to study evaporative salinization in the shallow sub-surface exposed to the adjacent atmospheric free flow. For this purpose, in the first part of this work, we develop a coupled model concept which describes a two-phase compositional porous-media system coupled with a single-phase compositional free flow. The developed REV scale model is robust and capable of comprehensive analysis of the exchange processes for mass, momentum and energy between the free flow and the porous medium. The developed model concept is implemented in the numerical framework of the open source porous-media simulator DuMux. In a natural hydrogeochemical system mixed salt precipitates is frequently observed. Therefore, in the second part, the model is extended to describe reactive transport of dissolved ionic species. Here, chemistry driven approaches are used to describe the salt precipitation processes. In addition to this, the model also accounts for solid salt accumulation driven changes in the porous media properties. Moreover, it provides the flexibility to analyze the influence of different free flow and porous media processes and parameters on evaporation and salinization dynamics. Evaporative salinization experiments was performed by the research group of Professor Nima Shokri at the University of Manchester. During these experiments saline-water saturated sand-columns were exposed to drying in an environmental chamber. The data from these experiments is used for model validation in this work. The first validation study is carried out using the simplified model for stage SS1 of evaporative salinization. Here, dissolved salt (NaCl) is assumed to be a single component and equilibrium based approach is used for salt precipitation. Thus, during evaporation, spontaneous salt precipitation is expected as dissolved salt in the solution reaches its solubility limit. For this approach, the validation study has shown excellent agreement between the model results and experimental observations. This simplified model is then applied for all stages of saline water evaporation. The comparison between numerical and experimental results was promising. However, the numerical results has shown divergence from the experimental observations in late stage SS1 and during transition to stage SS2. To understand these differences, a details parameter analysis is carried out. For this, influence of the free flow, porous medium and their interface on evaporation and salinization dynamics is analyzed. The parameter analysis highlighted that the variation in individual parameters or processes in the free flow, porous medium or at their interface can significantly influence evaporation and salt precipitation behavior. In the second validation study, the extended model is applied to analyze precipitation dynamics in a mixed salt (NaCl-NaI) system. Here, in order to account for salt precipitation process both equilibrium and kinetic reaction based approaches are developed. These approaches are first validated using the available experimental data for single salt (NaCl) precipitation. This validation indicated that both the equilibrium and kinetic precipitation approaches are well in accordance with the experimental observations. Furthermore, for mixed salt precipitation (NaCl-NaI), the simulation results are found to be in good accordance with the phenomenological explanations discussed in the literature. Due to lack of experimental data, the mixed salt precipitation model will be validated in the near future. We conclude that the developed model concept offers a comprehensive and robust framework for modeling dissolved salt transport and precipitation processes in the drying shallow subsurface interacting with the free flow. Moreover, the validation studies indicate that the model is reliable to study and analyze the reactive transport and mixed salt precipitation processes in a naturally occurring hydrogeochemical system. Furthermore, generic nature of the developed model allows direct extension to other salinization applications where salt precipitation is observed at the interface between free flow and porous medium.