The effect of dynamic hydro(geo)logical boundary conditions on redox-zoning in high-energy subterranean estuaries

Subterranean estuaries (STE) below beaches are biogeochemical reactors that modify the composition of fresh meteoric groundwater and recirculating seawater before they enter the ocean via submarine groundwater discharge (SGD), which can affect coastal ecosystems. Thereby, prevailing redox conditions have a major impact on the concentrations and mass fluxes of water constituents, e.g., nutrients, metals and organic molecules within the STE. Due to the transient nature of the flow and transport within STEs as well as the variable hydrogeochemical boundary conditions, redox zoning in the STE is likely highly dynamic. Elucidating the factors that affect redox zoning and its dynamics is essential for the interpretation and understanding of hydrogeochemical data and the prediction of coastal solute fluxes. In the present study we investigated the individual and combined effects of storm floods, seasonal changes of temperatures and groundwater recharge rates, as well as beach morphodynamics on the redox behavior, i.e., redox zoning in the STE in a generic modelling approach. A 2D cross-shore density-dependent flow and reactive transport model was set up for this purpose, mimicking a beach aquifer exposed to high-energy conditions due to high tides, waves and storm floods. The results of this study show that redox dynamics can occur well down to a depth of 20 m. Morphodynamics were shown to be the most important factor for redox zoning in the STE. For cases where morphodynamics are less pronounced, e.g., at low-energy sites, storm floods and the seasonal temperature changes may be dominating. Seasonal changes in meteoric groundwater recharge rates seem to be least relevant for the redox dynamics in STEs. The results of the present study increase the understanding of STEs as biogeochemical reactors.