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Research topic 1: Groundwater flow pattern

A central component of this research topic is the reliable termination of submarine groundwater discharge (SGD) fluxes and their spatial and temporal variability. We apply a novel integrated approach by combining groundwater level information, tritium/helium groundwater ages, stable isotopes, GIS-based (geographic information system) groundwater recharge estimates and density-dependent groundwater flow modeling. Through this integrated approach we aim at (i) coherent estimates of the fresh and saline SGD fluxes and their seasonal variation and (ii) quantification of the uncertainty of these estimates. We also investigate short term variability driven by periodic and aperiodic events using a physical-optical rapid sensor assembly. Complementary to the hydrological modelling approach, radon tracing is used to acquire mass balances and estimate water residence times in the subterranean estuary. Finally, direct measurements using Lee-type seepage meters and a new automated seepage meter, developed in the scope of BIME, are employed to complement SGD volumetric flux estimates from numerical models and radon-based mass balances. Our results on SGD flow patterns and volumetric fluxes are fed into research topic 2 (Groundwater composition) and research topic 3 (Groundwater microbiology).

Project Hydrology

Lead
Prof. Dr. Gudrun Massmann (Hydrology and Landscape Hydrology Group)

PhD student
Nele Grünenbaum (Hydrology and Landscape Hydrology Group)

Research scientist
Dr. Janek Greskowiak (Hydrology and Landscape Hydrology Group)

  

Objective
Budgeting nutrients and trace elements that enter the oceans via submarine groundwater discharge (SGD) requires detailed knowledge on the groundwater flow system and its dynamics in the subterranean estuary, especially for meso- to macrotidal high energy-beaches which are currently understudied. Spiekeroog Island was chosen as study site because of its exposed high-energy beach system with a meso-tidal amplitude. A combination of field investigations and numerical modelling in this study will help to extent the previous information about subterranean estuaries, quantifying subsurface residence times and delineating flow and salinity patterns.  

 

Groundwater-Surface Water Dynamics
In coastal aquifers, SGD includes a terrestrial freshwater component as well as saline proportion originating from tidally driven recirculation of seawater. However, there is hardly any field data about the actual volumes of water circulating in the intertidal zone in such high energy systems. Accordingly, it is necessary to collect long time data about the dynamics and volumes of submarine groundwater discharge depending on tides, storm floods and seasonal groundwater recharge variations.
Self-made lysimeters and seepage meters are used to estimate the volumes of infiltrating seawater and exfiltration groundwater. The rates are assumed to be dependent on the beach morphology and tidal amplitudes. Therefore, a 3D-laser scanner is used to determine the change in the beach topography on short and medium time scales. Additionally, observation wells are installed and equipped with data loggers to monitor water levels and electrical conductivities with a high frequency over several tidal cycles.

 

Numerical Modelling
Numerical models are set up with the density-dependent groundwater flow modeling code SEAWAT in order to process the collected field data and existing hydrological and hydrogeological information of the barrier island. The models will continuously be updated, refined and recalibrated as soon as new data becomes available within the later stages of proposed projects. Tritium-helium measurements in the intertidal zone and the islands freshwater lens will help to improve the knowledge of groundwater pathways and residence times in the subterranean estuary and serve as calibration constraint in the numerical models.

 

Internship
Alina Harms: "Untersuchung der (ungesättigten) Rezirkulation am Strand Spiekeroogs von In- bis Exfiltration"

 

Presentations at national and international symposia

Greskowiak J, Ahrens J, Ahmerkamp S, Grünenbaum N, Kossack M, Schnetger B, Ehlert C, Holtappels M, Beck M, Pahnke K, Brumsack H-J, Massmann G (2018). Modelling reactive transport of Si and 222Rn to constrain tide-induced seawater rates at a meso-tidal beach. SWIM, Gdansk, Poland (Oral presentation).

Grünenbaum N, Greskowiak J, Sültenfuß J,  Massmann G (2018) Messung und Modellierung der Grundwasseraltersverteilung im subterranen Ästuar unter einem hochenergetischen und mesotidalen Strand einer Barriereinsel (Spiekeroog, Nordwestdeutschland). 26. FH-DGGV-Tagung, Bochum, Germany (Oral presentation).

Grünenbaum N, Greskowiak J, Massmann G, Harms A, Waska H (2017) Recirculation rates within beach sediments under meso-tidal conditions (Spiekeroog, Northern Germany). IAH44th, Dubrovnik, Croatia (Oral presentation).

Grünenbaum N, Greskowiak J, Massmann G (2017) Investigation of the recirculation on the beach of Spiekeroog from in- to exfiltration. EGU, Wien, Austria (Poster presentation).

Grünenbaum N, Greskowiak J, Massmann G (2017) Investigation of the recirculation on the beach of Spiekeroog from in- to exfiltration. PhD Student Meeting, Erlangen, Germany (Oral presentation).

Project Marine Sensors

Lead
Prof. Dr. Oliver Zielinski (ICBM Marine Sensor Systems Group)

PhD student
M. Sc. Kai Schwalfenberg (ICBM Marine Sensor Systems Group)

  

Objective
Our objective is planning, preparing and performing the sensor system development and experiments for the determination of submarine groundwater discharge (SGD) over temporal and spatial scales as well as the composition of nutrients in the porewater. We develop a new type of seepage meter, equipped with sensors for conductivity, temperature, pressure, oxygen, pH, FDOM (fluorescent dissolved organic matter), turbidity and flowrate. With this seepage meter field observations of seep fluxes at Spiekeroog and other locations, where SGD is directly observed at the surface, will be performed. Additionally, a lander system is used for measuring conductivity, temperature, pressure, oxygen, FDOM, turbidity and nitrogen in the corresponding water column. These data will support a better understanding of the characteristics of SGD at coastal systems.

 

Seepage meter and lander system
The seepage meter is designed as a rectangular dome equipped with sensors for different physical parameters. We use a CTD equipped with sensors for oxygen, pH, FDOM and turbidity. An additional essential parameter of a seepage meter is the flowrate of the water discharging. To measure this parameter we use two different flowmeters. For weaker seeps, like Spiekeroog, we apply an electromagnetic flowmeter. For stronger seeps we use a mechanical flowmeter.
The lander system is a common measurement device in physical oceanography. Since the focus of the measurements with the seepage meter is groundwater outflow, the lander system complements through sampling the surrounding water column to reach a better understanding of SGD.

 

CDOM/FDOM analyses
Pore water is sampled at different depths (down to 100 cm) for lab analyses. The abundance and composition of CDOM (colored dissolved organic matter) and FDOM in pore water samples are analyzed through UV-VIS spectrophotometry and excitation-emission-matrix-fluorescence spectrometry (EEMs). Spectra are analyzed via specific MatLab scripts and PARAFAC analysis. Optical fingerprints can help to distinguish proteins in the pore water as well as potential terrestrial or marine origin of dissolved organic matter.

 

 

Seepage meter

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