Processes Controlling Formation of Salt Efflorescence in Coastal Salt Flats

Salt flats are bare soil surfaces with elevated salinity levels which inhibit vegetation. They are commonly found in areas of coastal wetlands where salt in surface soil accumulates due to evaporation from the shallow saline water table. The expansion of these vegetation-free zones may severely affect the bio-ecological function of these coastal wetland systems. In this study, sand flume experiments and numerical modeling were carried out to investigate the mechanisms underpinning the development of salt flats in the elevated areas of coastal wetlands. We found that salt precipitation occurred on the soil surface where water content is low, but the hydraulic connection to the saline groundwater table is maintained. This connection is required for evaporation to promote upward porewater flow and bring salt to the surface. However, the surface salinity level decreases with increasing surface saturation due to the density-driven removal of salt. As a result, salt efflorescence is absent on the surface with higher water saturations, despite strong evaporation. Density-driven flows can be triggered by the evaporation-induced upward salinity gradient and transfer salt from the surface back to the water table. Numerical simulations confirmed this salt removal process and suggested the intensity of the density-driven flow is mainly dictated by the soil permeability in the unsaturated zone. The findings from this study increase understanding of the processes involved in the formation and evolution of salt flats. Salt precipitated on the soil surface with continuous evaporation and low water saturationEvaporation-induced upward salinity gradient can trigger salt fingersSalt fingers tended to form near the soil surface with higher water saturation and inhibited surface salt accumulation

Automated summary

This study investigates the mechanisms underlying the development of salt flats in elevated areas of coastal wetlands through sand flume experiments and numerical modeling. It is found that salt precipitation occurs on the soil surface with low water content, where hydraulic connection to the saline groundwater table is maintained. This connection is necessary for evaporation to promote upward porewater flow and bring salt to the surface. However, the surface salinity level decreases with increasing surface saturation due to the density-driven removal of salt. Numerical simulations confirm this salt removal process, with the intensity of density-driven flow mainly dictated by soil permeability in the unsaturated zone.