Iron Curtain Formation in Coastal Aquifers: Insights from Darcy-Scale Experiments and Reactive Transport Modelling

Although many studies have examined reaction zones in groundwater–seawater mixing areas, little attention has been given to how subsurface processes drive changes in iron (Fe) precipitation over time and space. This gap has limited our understanding of the “iron curtain” phenomenon in coastal aquifers. To address this, this study developed a reactive transport model to investigate how porosity evolves during the oxidative precipitation of Fe(II) in porous media. The model incorporates the dynamic effects of tortuosity, diffusivity, and surface area as minerals accumulate. Validation experiments, conducted with syringe tests that simulated Fe precipitation during freshwater–saltwater mixing, showed that precipitates formed mainly near the inlets, reflecting the development of a geochemical barrier at the groundwater–seawater interface. Scanning electron microscopy confirmed that Fe precipitates coated the surfaces of spherical particles. Numerical simulations further revealed that high Fe(II) concentrations drove pore clogging near the inlet, creating a dense precipitation zone akin to the iron curtain in coastal aquifers. At 10 mmol/L Fe(II), local clogging was observed, while at 100 mmol/L Fe(II), outflow rates (i.e., discharge) were substantially reduced. Together, the experiments and simulations highlight how hydrogeochemical processes influence hydraulic properties during the oxidative precipitation of Fe(II) in mixing zones.