Reactive transport modeling to study the impact of mineral reactions and surface complexation on the transport of dissolved species in a subterranean estuary

Dissolved species of terrestrial and marine origins are transformed in Subterranean Estuaries (STEs) before they flow into the coastal oceans. The occurring biogeochemical reactions are highly complex, demanding the application of numerical reactive transport modeling (RTM) approaches to achieve a deeper process understanding. The objective of this study was to quantify the impact of organic matter degradation and secondary mineral reactions on the fate of dissolved species in a generic sandy STE. A comprehensive RTM approach was developed for this purpose, investigating the effects of ion activities, pH, pe, redox reactions, mineral equilibria (goethite, siderite, iron sulfide, hydroxyapatite and vivianite) as well as surface complexation. We found that the STE biogeochemistry was very sensitive to the assumed reaction network. For example, dissolved inorganic carbon and pH were mainly controlled by calcite and siderite dynamics. Dissolved Fe²⁺ and HS^- were precipitated as goethite, siderite and/or iron sulfides, respectively. PO₄^3- concentrations were strongly controlled by the formation of P-bearing minerals, e.g., vivianite and hydroxyapatite, as well as surface complexation. Our work helps to establish the relative importance of some of the major biogeochemical processes in the STE. In a next step, field data from a high-energy STE site on Spiekeroog (‘DynaDeep observatory’) will be used to explore which processes take place in real-world STEs.