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4.2. LAKE RESEARCH (LIMNOPHYSICS) 161
4.2 Lake Research (Limnophysics)
Names of group members
Dr. Johann Ilmberger, head of group
Dr. Christoph von Rohden, postdoc
Dipl. Phys. Karl Wunderle, diploma student (completed April 2005)
Abstract Water quality of lake waters is, among other factors, affected by mixing and transport
processes in the interior, as well as by the exchange with the ambient ground water. These internal
processes and the ground water exchange are the main subjects of our investigations.
Figure 4.7: Schematic diagramm of transport processes in lakes.
Background Transport processes in lakes are important for the lake water quality. Especially
mining lakes tend to have a poor quality because of the disturbed landscape due to the mining
activities.
Methods The investigations are based on tracers and direct measurements. Mainly we are using
the tracer SF6, but also stable isotopes and now upcoming Radon (see section 4.1.6).
The background level of SF6 can be used to study the interaction of ground water with lake water.
This is based on the equilibration of rain and surface waters with atmospheric gases and the increase
of the atmospheric level of the man made SF6 during the last decades. Therefore ground water -as it
normally infiltrated a few decades ago- has a low SF6 content, while the lake water SF6 concentration
is, at least during overturn, close to the (higher) atmospheric concentration. This difference of the
signals can be used to study ground water - lake water exchange. We will try to apply this method
to the mining lake RL117.
We use SF6 spike experiments to investigate vertical mixing and ground water exchange. There we
inject a small amount SF6 (few hundred mg) into the (hypolimnic) water body and trace the vertical
spread and the balance by profile measurements (see section 4.2.1). We applied this method to Lake
Constance, Lake Hufeisensee, Lake Merseburg Ost 1a and 1b. We will apply it at Lake Waldsee.
Direct measurements include the profile measurements of temperature and electrical conductivity with
a CTD-probe at a vertical resolution of ∼2cm (see section 4.2.2), continuous temperature measurements
at different water depths using logging temperature probes and measurements of water currents
with an Acoustic Doppler Current Profiler (see section 4.2.3).
Lakes in investigation: Mining lake Merseburg Ost 1a, Mining lake Merseburg Ost 1b, Lake Willersinnweiher,
Lake Waldsee, Lake RL117.
The tracer methods, especially the spike experiments, are very well suited for the transport investigations.
Projects and funding
Our work at Willersinnweiher, where we did current measurements, continuous temperature and CTD-