Visualizing reaction zones in tidal subterranean estuaries using physical tank experiments

In coastal aquifers, terrestrial freshwater and seawater mix and form typical compartments in the subsurface (subterranean estuary, STE), a zone of intense biogeochemical reactions. Flow and transport processes are mainly driven by density differences and- if present- tides. Typically, an upper saline plume is encountered overtopping a freshwater discharge tube and a saltwater wedge. Contrary to the general view of the hydraulic conditions in the STE, latest studies proclaim that the subsurface salinity distribution is less stable and more dynamic than previously thought, especially under the influence of strong morphodynamics. Also, the occurrence of the phenomena of fingering flow has been observed in modelling studies. In this study, physical tank experiments were conducted to compare unstable and stable flow conditions in the STE with respect to the formation of iron oxides that may form in the transition zone between the oxygen-free, iron(ii)-containing, terrestrial freshwater and the oxygen-rich seawater. The results illustrate how biogeochemical processes in the STE are linked to the hydrodynamics as salt fingering flow strongly influenced the location and extent of ferric iron oxidation and the precipitation of Fe(III)hydroxides.