Effects of unstable flow on solute transport in the marsh soil and exchange with coastal water

Recent studies of marsh hydraulics have focused on tide‐induced pore water circulation as the main drive for solute transport in the marsh soil and exchange with coastal water. Our study revealed another important mechanism provided by unstable fingering flow, which largely modified solute transport paths. In the marsh interior, downward penetration of salt fingers forced ambient pore water and solute plumes to move upward and exit the marsh soil through marsh platform at relatively high concentrations, up to 2 orders of magnitude higher than exit solute concentrations at the tidal creek bed. The mixing of solute with ambient pore water in the marsh interior was intensified greatly by fingering flow. A critical distance to the creek was determined based on a field‐scale model simulation to distinguish tidal circulation‐dominated and fingering flow‐dominated solute transport zones. The new transport mechanism has implications for understanding the fate of solutes in particularly salt marshes of low creek densities. Key Points Unstable fingering flow leads to great modifications of solute transport paths Salt fingers cause solute plumes to exit from marsh platform with relatively high concentrations New solute transport mechanism is more important in salt marshes of low creek densities