A comparison of geophysical and hydrogeological
models: Examination of the threats to wetland habitats by groundwater
exploitation in Ostfriesland (Germany)
Geophysical methods (electric and electromagnetic) allow mapping the resistivity
distribution in the underground in multiple dimensions. Different soils show
different resistivity; e.g. clay usually has a lower resistivity than sand. This
allows us to differentiate between different sediment types using geophysical
techniques. Given appropriate data coverage the geophysical resistivity models
can be directly compared with geological models showing the distribution of
soils in the underground. Here, a case history is described, where both
geophysical and hydrogeological methods were applied to estimate the interaction
between wetlands and groundwater exploitation.
Wetland habitats are rare ecosystems, whose specific ecologic conditions (soils,
climate,...), but especially their high humidity, allow the survival of specific
flora and fauna. Due to their nature wetland habitats depend on a high
groundwater level and are very sensitive to periods of drought. These wetland
habitats are often linked to zones of lower permeability in the underground
(such as clay structures), which cause a reduced drainage of surface water from
the uppermost aquifer. In the examined area south of Esens in Ostfriesland
groundwater is pumped from a deeper aquifer in the vicinity of the wetlands. It
is obvious that a missing low permeability layer leads to stronger interaction
between different aquifers. This drainage by the exploitation of groundwater can
lead to the destruction of wetlands, especially if it coincides with strong
seasonal variations of the water table during summer.
The area under investigation is located in Ostfriesland (NW-Germany), south of
the city of Esens. The hydrogeological model, which was constructed by S.
Schade, is compared with the resistivity model from Radiomagnetotellurics and
direct current Geoelectrics. In general the hydrogeological sections match the
resistivity models (see figure). A conductor (less than 50 Ohm-m) is linked to a
structure with high clay content, whereas a resistor (more than 100 Ohm-m) is
usually linked to sand or gravel. Furthermore interfaces between different
sediment types, which can be seen in the boreholes, often correspond to a change
of the resistivity model. As the resistivity models give a higher spatial
resolution than the geological models, the information derived from the
geophysical measurements allows the correction and improvement of geological
models.
The continuous description of underground structures leads to more reliable
interpolation between boreholes. Furthermore the resistivity measurements allow
an independent estimate of the porosity, giving valuable information for the
derivation of hydrogeological models. The integration of different geoscientific
approaches allows a better estimation of the interdependency of wetlands and
groundwater exploitation.
Caption Figure (Comparison.jpg): Comparison of hydrogeological model (a) with
the resisitivity model (b) derived from direct current geoelectric measurements.
The boreholes shown for comparison are located in the vicinity of the profil (less
than 200 m). The geological section was derived from a three-dimensional model
provided by S. Schade using the software GSI3D.
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References:
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S. SCHADE, Ein genetisch-lithostratigraphisches
Untergrundmodell quartaer Schichten auf Grundlage vernetzter Profilschnitte in Harlingerland, Ostfriesland, Diplom-Arbeit, University of Cologne
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T. SEHER, Untersuchung von Feuchtbiotopen in
Ostfriesland: Gefaerdungsabschaezung mit Multielektroden-Geoelektrik und
Radiomagneto-tellurik, Diplom-Arbeit, University of Cologne
............................................................
Tim Seher,
Dipl.
Geoph.
E-mail:
tim.seher@arcor.de
Web:
www.geosum3d.de
