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

Tidal wetland ecosystems are sensitive to porewater salinity dynamics. However, it is unclear how salts move and distribute in these wetlands, particularly how the salts accumulated by evaporation get removed from the wetland soil, so that the salinity levels may stabilize to accommodate vegetation. We conducted a combined field and modeling study to identify the porewater flow and salinity patterns in a subtropical wetland subjected to tidal inundation and evaporation. Measured and simulated salinity contours indicated the formation of hypersaline porewater plumes in the upper intertidal zone and the fresher porewater zones with salinity close to that of seawater near the creek and in the supratidal zone. Simulations indicated the discharge of the hypersaline upper intertidal porewater to the creek with a discharge pathway developed under the fresher near‐creek porewater zone, and this was further confirmed by the field‐observed significant salinity gradient under the creek. Our model suggested that both water and salt discharge from the wetland soil occurred predominantly at the creek bank and creek bed. The porewater discharge is more intensive through the creek bank than the creek bed, while the salt discharge across both the creek bank and creek bed was comparable due to the much higher salinity level under the creek bed. Salt discharge driven by density gradients and tidal‐induced porewater circulation provides a mechanism for removing salts accumulated in the upper intertidal zone due to evaporation and could prevent salt flat formation and marsh plants dieback. Key Points Hypersaline porewater plumes formed in the upper intertidal wetland soil by evaporation with the highest salinity more than 150 ppt Downward migration of hypersaline porewater contributed to deep aquifer salinization Salt discharged to the creek through its bank and bed; the groundwater adjacent to the creek bed showed salinity stratification