Fingering flow in the subterranean estuary - A multiplier for the iron curtain and reactive transport processes?
Final Report Abstract
Submarine groundwater discharge is an important component of the hydrological cycle contributing to the nutrient, carbon and metal fluxes from coastal aquifers to the sea and has been found to affect coastal waters and ecosystems. Iron precipitates, the “iron curtain” forming in beach aquifers, so-called subterranean estuaries (STEs), can retain some of the nutrients and contaminants before they enter the sea. The iron curtain forms when oxic (mainly seawater) and anoxic, iron rich (mainly meteoric fresh groundwater) mix in the STE. Under certain conditions, the density contrast of sea and freshwater may lead to unstable flow patterns (fingering flow) in tide affected STEs where seawater recirculates as upper saline plume (USP) in the shallow underground, thereby overtopping discharging fresh groundwater. These unstable flow conditions may increase mixing and amplifying iron curtain formation. The project aimed to delineate unstable flow conditions in tide-affected STEs via sand tank experiments for various combinations of aquifer parameters and boundary conditions. Further, it was aiming at demonstrating their effect on mixing and travel times and the formation of the iron curtain under controlled laboratory conditions, which has so far only been done in generic numerical modelling. The results show that fingering flow can appear in a sand tank simulated STE. Stable and unstable conditions could be predicted reasonably well from a previously developed stability diagram that includes all relevant system parameters via non-dimensional analysis and which resulted from generic field-scale simulations. The sand tank results also showed amplified mixing of fresh and seawater as well as elongated travel times in the USP during fingering compared to stable flow conditions. Further reactive sand tank experiments with anoxic ferrous iron containing freshwater and oxic seawater illustrated the formation of the iron curtain in the narrow mixing zone of these two endmembers, which for the first time, was evidenced experimentally. Generic field-scale reactive transport modelling revealed increased iron formation and a larger precipitation zone for fingering compared to stable flow conditions in the STE.
Publications
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Exploring the phenomena of fingering flow upon oceanic oscillations [Oral Presentation]; Groundwater – Climate – Society; 28th Conference of the FH-DGGV, Jena, Germany
Delwar, R.B., Thissen, L., Grünenbaum, N., Greskowiak, J. & Massmann, G.
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Fingering flow in the subterranean estuary under tidal influence (laboratory experiments) [Oral Presentation]; Groundwater Sustainability and Poverty Reduction; 49th IAH Online Congress, Wuhan, China
Delwar, R.B., Grünenbaum, N., Greskowiak, J. & Massmann, G.
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Fingering Flow in the Subterranean Estuary under Tidal Influence. Copernicus GmbH.
Delwar, Rezwana Binte; Grünenbaum, Nele; Greskowiak, Janek & Massmann, Gudrun
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Visualizing reaction zones in tidal subterranean estuaries using physical tank experiments. Copernicus GmbH.
Grünenbaum, Nele; Greskowiak, Janek; Binte Delwar, Rezwana & Massmann, Gudrun
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Groundwater flow dynamics in tide dominated heterogeneous aquifers under laboratory conditions [Poster presentation]; Interacting Groundwater; 51st World Groundwater Congress (IAH); Davos, Switzerland
Delwar, R.B., Grünenbaum, N., Greskowiak, J. & Massmann, G.
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Investigating the occurrence of flow instabilities in coastal unconfined aquifers using sand tank experiments. Journal of Hydrology, 631, 130741.
Delwar, Rezwana Binte; Grünenbaum, Nele; Greskowiak, Janek & Massmann, Gudrun
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Visualizing the ‘iron curtain’ formation in the subterranean estuary using sand tank experiments. Hydrological Processes, 38(4).
Grünenbaum, Nele; Volk, Johanna; Delwar, Rezwana Binte; Bräunig, Laura; Massmann, Gudrun & Greskowiak, Janek
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Visualisation of Flow Dynamics in Tide‐Dominated Unconfined Coastal Aquifers Using Physical Experiments. Hydrological Processes, 39(4).
Delwar, Rezwana Binte; Grünenbaum, Nele; Greskowiak, Janek & Massmann, Gudrun
