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

Abstract

Tidal wetlands are connected with coastal estuarine systems and typically have a low-lying intertidal area with a more elevated supratidal zone (Sheaves & Johnston, 2008;Snedden et al., 2012). The low-lying intertidal wetland surface is regularly inundated by saline seawater from tidal creeks and drainage channels within the wetland (Wolanski & Elliott, 2016). This induces complex porewater-seawater mixing in the wetland aquifer. The abundance of salts in the wetland soil creates a brackish-saline environment, which selectively accommodates halophytic (salt-tolerant) species in the intertidal zone (Barbier et al., 2011).A large number of studies have previously been carried out to understand coastal hydrogeological processes in beach settings, including saltwater intrusion, submarine groundwater discharge (SGD), and upper saline plumes (e.g., Evans & Wilson, 2016;Li et al., 1999;Robinson et al., 2007). Similarly, the hydrology and ecology in tidal wetlands are strongly affected by the tide-influenced porewater flow and salt transport in the wetland subsurface (Hughes, 2016;Moffett et al., 2012;Ursino et al., 2004). However, subsurface hydrology in tidal wetlands, particularly the salt transport patterns, is understudied compared to beach environments (Guimond & Tamborski, 2021;Xin et al., 2022). The sand-dominated beach is characterized by receiving fresh groundwater from inland and discharging it through a submarine zone above the seawater wedge (Figure 1b) (e.g.

Sustained upward groundwater discharge through salt marsh tidal creeks

“…The near-surface layer (< 7 m deep) had an averaged resistivity of 1.17 AE 0.08 and 1.31 AE 0.24 ΩÁm for A-A' and B-B 0 , respectively. Relatively lower and more spatially variable salinity in this layer is a combined result of multiple processes including tidal flooding, rainwater infiltration and evaporation at the atmosphere-soil interface (Liu et al 2022). Resistivity gradually increased for the lower layer (35-58 m deep), indicating a salinity transition in the aquitard between the unconfined aquifer and the confined aquifer.…”

Quantifying Seepage‐Face Evaporation and Its Effects on Groundwater Flow and Solute Transport in Small‐Slope Tidal Flat

“…Numerous studies have revealed that evaporation from beaches underlain by saline groundwater leads to significant salt accumulation near the surface, thereby altering the magnitudes and pathways of groundwater flow (Liu et al., 2022; Yu et al., 2021). However, quantitative studies of these effects have primarily focused on evaporation from the unsaturated beach surface, where pore‐water pressure is below atmospheric levels (e.g., Geng et al., 2023; Shen et al., 2018; Xin et al., 2017).…”

Impacts of Evaporation‐Induced Groundwater Upwelling on Mixing Dynamics in Shallow Wetlands

“…In wetland systems with a shallow groundwater table, the near‐surface region usually has a relatively high water content (due to strong capillary forces), which facilitates pore water evaporation from the sediments (Mahfouf & Noilhan, 1991; Xin et al., 2017). The shallow groundwater table also builds a strong hydraulic connection between the evaporation front and deep surrounding groundwater, which efficiently replenishes the loss of water evaporated near the wetland surface, resulting in a sustainable evaporation rate on the surface (Liu et al., 2022). In wetlands, the large‐scale hydraulic gradients induce lateral groundwater flow to the depression areas, while strong evaporation drives pore water to flow upward.…”