Numerical study of the land-sourced iron precipitation and its effects on porewater flow at subterranean estuaries

During terrestrial freshwater discharge to the ocean, the freshwater containing high concentrations of dissolved ferrous iron (Fe²⁺) come into contact with the saltwater enriched in dissolved oxygen (O₂(aq)) within the coastal aquifer, resulting in the oxidation and precipitation of iron (Fe) as Fe oxy(hydr)oxides. The accumulated Fe precipitate forms a low-permeability zone, known as the “iron curtain” at subterranean estuaries (STEs), which influences the porewater flow. However, this process is not quantitatively well understood, and the underlying mechanisms remain unclear. Given the processes of variable density flow, Fe oxidation and oxidative precipitation, and the effects of Fe precipitation on porosity and permeability, the process of land-sourced Fe precipitation and its interactions with porewater flow at STEs have been numerically studied using the TOUGHREACT simulation program, which facilitates multiphase flow and reactive transport processes. Results indicate that Fe precipitates correspondingly reduce the porosity and permeability (by approximately 20 %–40 %) of the aquifer. These reductions subsequently decrease the porewater velocity (by approximately 6 % to 18 % seaward and 8 % to 32 % upward) over Fe-precipitated areas and increase the porewater velocity (by approximately 6 % to 12 % seaward and 4 % to 8 % upward) above and below Fe-precipitated areas. In turn, the porewater velocity changes decline the accumulation of Fe precipitates by approximately 0.001 to 0.003 in volume fraction or 3 % to 18 %. These findings highlight the interaction between groundwater-derived Fe and porewater flow at STEs, providing insights that enhance the understanding of the hydrochemical processes in coastal zones.