Abiotic Iron Precipitation and Its Influence on Groundwater Flow in Subterranean Estuaries Under Tidal Forcing

Abstract Within subterranean estuaries (STEs), Fe 2+ from freshwater is oxidized by O 2 (aq) from seawater to form Fe precipitates and low‐permeability zones (“Fe curtains”). Tidal fluctuations generate an upper saline plume (USP) above the saltwater wedge (SW), enhancing freshwater–seawater mixing and associated chemical reactions. However, existing studies did not resolve how tidal forcing influences Fe curtain formation or how the resulting precipitates alter groundwater flow. Here, TOUGHREACT was employed to numerically investigate abiotic Fe precipitation and its consequent impacts on groundwater flow in STEs under tidal forcing. The simulation results indicate that Fe oxidation and precipitation predominantly occur within the USP and SW mixing zones. Fe accumulation within the USP is approximately twice that within the SW, especially near the high and low tidal marks, due to the faster tide‐driven seawater circulation. The resulting Fe precipitates reduce sediment porosity and permeability, locally decrease porewater velocity, and alter porewater flow patterns and the freshwater–seawater interface. The horizontal and vertical porewater velocities are reduced by about 20%–40% and 40%–80%, and ∼20% and ∼40% within the USP and SW Fe‐precipitated zone, respectively. Variations in freshwater flux and tidal amplitude regulate Fe precipitation primarily by modifying the configuration of the freshwater–seawater interface, whereas the Fe 2+ and O 2 (aq) concentrations impact Fe precipitation mainly by the supply of reactants. These insights improve understanding of abiotic Fe precipitation and its hydrological implications under tidal forcing in STEs.