Kinetic Approach to Model Reactive Transport and Mixed Salt Precipitation in a Coupled Free-Flow–Porous-Media System

Abstract

Evaporative salinization of the agricultural soil is a chronic problem in arid, semiarid and coastal regions worldwide. As a shallow subsurface issue, it is strongly influenced by the free-flow (henceforth, FF) and porous-media (henceforth, PM) flow and transport processes. In specific, it is mainly affected by the mass, momentum and energy exchange between the FF and the PM regions. Furthermore, salt precipitation in such systems is strongly determined by the interaction between different dissolved ionic species. In the scope of this work, we extend the REV-scale coupled FF–PM model concept for salinization proposed by Jambhekar et al. (Transp Porous Media, 2015), to describe reactive transport of ionic species Na +, Cl - and I - and mixed salt precipitation (here, NaCl and NaI). In the first part of our numerical analysis, we illustrate precipitation behavior of NaCl using equilibrium and kinetic approaches. Both approaches are found to be in good agreement with experimental observations. However, in the literature it is often discussed that for mixed salt systems found in the nature, the equilibrium precipitation–dissolution assumption is not easily justifiable. Therefore, in the second part of our numerical analysis, we extend the kinetic approach to describe mixed salt precipitation in an NaCl–NaI system and compare it with the equilibrium precipitation approach. Our numerical analysis indicate that the simulation results for these approaches are very similar and analogous with the phenomenological explanations.