Evaporation and Salt Accumulation Effects on Riparian Freshwater Lenses

While previous studies conclude that riparian freshwater lenses are the result of buoyancy forces that drive river water circulation in the adjacent aquifer, only highly idealized conceptual models have been assessed. A numerical study is conducted to investigate the influence of evaporation from unsaturated bare soils, including accumulation and precipitation of salt, on riparian freshwater lenses, assuming otherwise similar conditions to those adopted in recent studies. Non-evaporating simulations that account for the unsaturated zone are first compared to previous saturation-only numerical and analytical results, and reasonable matches are obtained. The modeling analysis of evaporative simulations includes three different approaches to the treatment of solute in evaporated water: (a) no evaporative salt accumulation, (b) evaporative salt accumulation, and (c) as for (b) except resistance to evaporation caused by salt accumulation is considered. Significant evaporative fluxes caused gaining river conditions to change to losing conditions, leading to the cessation of buoyancy-driven freshwater circulation and to freshwater bodies that are not lenticular. Rather, evaporation reduces regions where the water table and/or unsaturated zone are fresh relative to the freshwater lens volume. Evaporation rates depend on the hydraulic connection between the floodplain surface and water table, which is limited where the water table is deep, the soil hydraulic conductivity is low, and under higher potential evaporation rates. The results show that floodplain evaporation may control the freshwater-saltwater distribution and lead to hypersaline groundwater under certain conditions, although earlier buoyancy-driven conceptual models of riparian lenses appear to remain valid where the thickness of the unsaturated zone is sufficiently large.