Evaporation and Salt Accumulation Effects on Riparian Freshwater Lenses

Abstract

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.

Effects of NaCl and CaCl2 Salinization on Morpho-Anatomical and Physiological Traits of Potted Callistemon citrinus Plants

“…The interest in understanding the mechanisms of plants' response to drought is increasing for both food crops, due to a climate-change driven risk to food security, and ornamental species due to commercial and economic issues [6][7][8]. Especially in arid and semi-arid regions, like the Mediterranean area, climate change is causing the exacerbation of drought, also triggering the rising of the salinity levels in soil, bringing severe limitations to plant growth, development, and production [9,10]. For over 20 years now, salinity in both water and soil has been one of the major constraints to agriculture worldwide, costing about USD 27 billion per year, and also affecting ornamental plants' value [6,7].…”

Salt Transport Under Tide and Evaporation in a Subtropical Wetland: Field Monitoring and Numerical Simulation

“…Numerical simulations were conducted using SUTRASET (Shen et al., 2018), a modified version of the finite element model SUTRA (Voss & Provost, 2010), to simulate the 2D variably saturated porewater flow and solute transport in soil with variable fluid density. SUTRASET is capable of implementing the occurrence of seepage, tide fluctuation, evaporation, and salt precipitation (America et al., 2020; Shen et al., 2018). A summary of the governing equations of the SUTRASET model is presented in Text S1 and Table S1 in the Supporting Information .…”

“…The model domain consists of 47470 nodes and 46900 elements with the mesh size refined near the creek and the intertidal wetland surface. The vertical mesh size Δz ranges from 0.04 to 0.063 m in the unsaturated zone to better describe the water and salt gradients and the horizontal mesh size Δx = 0.5 m. It should be noted that this selection of mesh resolution is a trade-off between the computation times and the simulation accuracy of such a 2D coastal transect with a length of more than 300 m. Our mesh resolution, particularly the vertical resolution in the unsaturated intertidal zone, is similar to that used by Geng and Boufadel (2015) and America et al (2020). Finer mesh (i.e., with 196131 nodes and 194600 elements) and a reduced time step were tested, and the simulation results were found to be independent of these numerical parameters.…”

Transience of Riparian Freshwater Lenses

“…Previous studies of riparian lenses assume steady‐state conditions, except for America et al. (2020), who examined the temporal development of mixed‐convective processes, including the chaotic descent of unstable fingers of hypersaline groundwater created by the evapo‐concentration of saltwater. However, riparian lenses are expected to shrink and grow following changes to river water levels and temporal variations in the heads of adjacent aquifers.…”