Fingering flow in the subterranean estuary - A multiplier for the iron curtain and reactive transport processes?

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Submarine groundwater discharge (SGD) is an important component of the hydrological cycle significantly contributing to the nutrient, carbon and metal fluxes from coastal aquifers to the sea and has been found to affect coastal waters and ecosystems. It is the common scientific view that in the subterranean estuary, tides and wave set-up induce a saline recirculation cell, often referred to as the upper saline plume (USP) overlying a so-called “tube” of discharging fresh groundwater. In contrast, our own recent research suggests that instabilities and fingering flow appear in the tide-affected near-shore aquifers under certain boundary conditions, disturbing USP and freshwater tube and considerably changing the SGD pattern in the sub-surface. This would consequently have important implications on reactive transport processes in the subterranean estuary. Combining physical experiments and numerical modelling, the project objectives are to (i) assess if and where fingering flow could exist in nature, (ii) delineate the impact of sediment characteristics (e.g., heterogeneity) and boundary conditions (e.g., seasonally variable freshwater flux) on salt-fingering flow, (iii) investigate to what degree 3-dimensional effects will alter the current understanding on the generation of salt-fingering, (iv) improve the prediction capability of stability diagrams and finally (v) study and predict the potential implications of fingering flow during SGD on the biogeochemical processes and the development of highly adsorptive iron(II)hydroxide surfaces (“Iron curtain”) in sandy beach aquifers and the corresponding solute matter fluxes to the sea.