Reactive transport modeling to reveal the impacts of beach morphodynamics, storm floods and seasonal groundwater recharge on the biogeochemistry of sandy subterranean estuaries

• Effects of morphodynamics, floods, and recharge on STE biogeochemistry were studied. • 2-D reactive transport models with complex chemical reaction networks were employed. • Beach morphodynamics enhance the transience of oxic-to-suboxic redox zones. • Floods amplify species dynamics at depth, while recharge has less impact. • Mineral reactions and surface complexation are key besides primary redox processes. Subterranean Estuaries (STEs) are important biogeochemical reactors at the land-ocean interface. They transform dissolved species prior to discharge, thereby influencing chemical fluxes from land to sea. The coupling between physical flow and biogeochemical reactions in the STE is complex, and a deeper process understanding demands the application of reactive transport modeling (RTM). Most previous RTM studies focused on idealized STEs, investigating the impacts of relevant oceanic forcings, such as tides and waves. The aim of this study is to investigate the presently unknown interplay between STE biogeochemistry and beach morphodynamics, storm floods as well as seasonal groundwater recharge. 2-D cross-sectional RTMs for a sandy beach aquifer were developed for this purpose, assessing the effects of the three individual as well as all combined dynamic coastal forcings, respectively. We find that beach morphodynamics enhance the transience of aerobic-to-suboxic zones in near-surface groundwater, whereas storm floods cause temporal concentration changes at greater depth. The impact of seasonal groundwater recharge is less pronounced. The concentrations of dissolved species are further impacted by precipitation/dissolution of the minerals calcite, goethite, siderite, iron sulfide and hydroxyapatite as well as complexation at goethite surfaces. Our study contributes to an advanced understanding of the interplay between STE biogeochemistry and the dynamics of relevant coastal forcings encountered at high-energy beaches. However, further field-based investigations are needed to verify conclusions of our generic RTM study.