Preprints
https://doi.org/10.5194/egusphere-2025-3132
https://doi.org/10.5194/egusphere-2025-3132
04 Aug 2025
 | 04 Aug 2025

Field-scale modelling reveals dynamic groundwater flow and transport patterns in a high-energy subterranean estuary

Janek Greskowiak, Rena Meyer, Jairo Cueto, Nico Skibbe, Anja Reckhardt, Thomas Günther, Stephan Ludger Seibert, Kai Schwalfenberg, Dietmar Pommerin, Mike Müller-Petke, and Gudrun Massmann

Abstract. Subterranean estuaries (STEs) are biogeochemical reactors modifying the chemistry of salt- and freshwater as they flow through the subsurface sediments. Boundary conditions such as tides, waves, beach morphology, seasonal meteoric groundwater recharge and storm events control end member mixing and residence time distributions within STEs. These in turn affect biogeochemical reactions and thus elemental fluxes discharging to the ocean via submarine groundwater discharge. Especially at high-energy beaches exposed to high tidal ranges and high wave energy, boundary conditions are very dynamic and likely imprint on groundwater flow and reactive transport within the STEs. A quantitative understanding of mixing processes and residence time distributions is necessary in order to adequately describe biogeochemical processes and can be achieved with the help of numerical modelling. Yet, transient field-scale modelling approaches calibrated to comprehensive observational data sets are still lacking, in particular for real-world high-energy STEs. In the present study, for the first time a density-dependent groundwater flow and transport model was developed and calibrated for a high-energy beach. The north beach of the barrier island Spiekeroog, northern Germany, thereby served as an example field site exposed to high-energy characteristic boundary conditions. The model was calibrated to a 1.5-year extensive dataset of groundwater heads, salinities, temperatures and 3H/He groundwater ages at various cross-shore locations along the beach at depths down to 24 m below ground surface. The calibrated model is able to replicate the principal behaviour of the highly transient system and enabled the identification of temporal variability hotspots. The dynamics in salinity are most intense at the in- and exfiltration locations of the tide-induced recirculating seawater. The groundwater age variability was largest seawards of the low tide mark as well as below the deep recirculating seawater cell at around 20–30 m depth near the dunes, where very old freshwater from the islands’ freshwater lens mixes with young brackish water from the upper beach. Temperature variations were seasonal and confined to the upper 5–10 m below the beach. Overall, the model provided important insights into the dynamics of the flow and transport processes.

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Janek Greskowiak, Rena Meyer, Jairo Cueto, Nico Skibbe, Anja Reckhardt, Thomas Günther, Stephan Ludger Seibert, Kai Schwalfenberg, Dietmar Pommerin, Mike Müller-Petke, and Gudrun Massmann

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2025-3132', Alicia Wilson, 05 Sep 2025
    • AC1: 'Reply on RC1', Janek Greskowiak, 02 Oct 2025
  • RC2: 'Comment on egusphere-2025-3132', Anonymous Referee #2, 12 Sep 2025
    • AC2: 'Reply on RC2', Janek Greskowiak, 02 Oct 2025
Janek Greskowiak, Rena Meyer, Jairo Cueto, Nico Skibbe, Anja Reckhardt, Thomas Günther, Stephan Ludger Seibert, Kai Schwalfenberg, Dietmar Pommerin, Mike Müller-Petke, and Gudrun Massmann
Janek Greskowiak, Rena Meyer, Jairo Cueto, Nico Skibbe, Anja Reckhardt, Thomas Günther, Stephan Ludger Seibert, Kai Schwalfenberg, Dietmar Pommerin, Mike Müller-Petke, and Gudrun Massmann

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Short summary
Mixing of fresh groundwater and circulating seawater below beaches triggers water-rock chemical reactions and may affect coastal water quality. The subsurface of so-called high-energy beaches that are exposed to high tides, waves and storm floods are understudied as monitoring under these conditions is difficult. For the first time, this study quantifies the subsurface flow and mixing processes of a high-energy beach with the help of computer simulations based on an extensive set of field data.
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