Numerical analysis of the abiotic formation and distribution of the "iron curtain" in subterranean estuaries

Precipitation of Fe (hydr)oxides (such as Fe(OH)3) in subterranean estuaries can form a low-permeable zone, referred to as the "iron curtain", of which the genesis, distribution, and influencing factors remain unclear. This study investigated the abiotic formation of iron curtains under variable seawater and freshwater conditions using the numerical model code TOUGHREACT. Results indicate that Fe(OH)3 accumulates on the freshwater side of the freshwater-seawater mixing zone, whereas the precipitation on the seaward side is inhibited by locally high H+ concentrations. Fe(OH)3 initially precipitates in the shallow layer of the aquifer and subsequently extends to deeper layers, owing to the increased residence time of both freshwater and seawater with depth. The accumulation amount and the spatial distribution of Fe(OH)3 precipitation are influenced by the water flux and the O2(aq) to Fe2+ concentration ratio. Consistent with field observations, these findings enhance a better understanding of iron transformation and distribution in subterranean estuaries.