Evaporation-driven convective mixing and mineral precipitation in groundwater

The management of groundwater under arid climate conditions represents one of the biggest challenges, and the ongoing trend of climate change suggests that it will result to be even more urgent. This is especially true if we consider the interaction between water, the porous medium and other phases, such as minerals. For example, exploitation/conservation of salt lakes/lagoons, limit salinization of soil and land desertification, study the alteration of soil’s mechanical properties and reliability of wastewater disposal are human activities that are strictly connected to groundwater dynamics and climate regimes. For this reason, more investigation is needed to understand how the complex system composed of surface conditions and the groundwater body, including chemical reactions, works. In this study, the focus is on the interplay between the mixing that occur in the aquifer due to variable density flow and mineral precipitation from the saline water. Variations in groundwater density are relevant in such systems, where the evaporation drives upward the water flow and reconcentrates the solutes at the exposed aquifer surface. The increase in water density with salinity confined to a layer which lays on less dense water may lead to a gravitationally unstable condition. From this stage, saline fingers originate, grow, and sink in the aquifer, establishing a free convective flow throughout the whole thickness. This mechanism forces the solutes to sink to the deeper zones and leave the saline water, from which minerals can precipitate. Here we present a variable-density flow model coupled to reactive transport to replicate a typical salt-lake environment connected to the aquifer beneath, i.e., under saturated conditions. These conditions are obtained by imposing a constant evaporation rate at a segment of the top boundary and a constant pressure at the inlet top boundary, to allow the water to enter while it evaporates. The chemistry of the initial and recharge water is the same to observe the modifications to different parts of the system due to convective flow and reactions. A series of simulations was run, changing the evaporation rate and permeability, corresponding to different Rayleigh numbers, to understand their effects on fluid dynamics and mineral formation. The results show that a saline layer forms at the surface, diffuses, and eventually becomes unstable under steep density gradients. The formation of fingers and dynamic of convection are different among those simulations but for long times the system faces a chemical (and density) stratification where the freshwater flow is constrained progressively toward the surface by the convective cell, which affect in turn the transport toward the unstable saline layer. Evaporation and permeability have different influence and weight in the system, determining how a system would evolve in time. The precipitation of minerals limits the convective flow, on the other hand, spatial distribution of minerals depends on the features of convection, and so, evaporation and permeability. In addition, these two parameters are modified by minerals in porosity.