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Impact of hydrological extremes on alpine karst groundwater resources

Final Report Summary - IMKA (Impact of hydrological extremes on alpine karst groundwater resources)

The main goal of the IMKA project was to investigate the impact of hydrological extremes on alpine karst groundwater resources. During the investigation period from 2012 to 2016, the major karst springs of the Lechquellen Mountains were characterized hydrochemically and groundwater quality was monitored under different hydrological conditions. Several multi-tracer tests with fluorescent tracers and particles were carried out to identify and characterize the underground flowpaths and develop a conceptual hydrogeological model of the test site.
Most springs in the project area showed fast and remarkable discharge and electrical conductivity reactions to rainfall. The only exception is the Schwarzbach Spring which showed a conductivity response to rainfall, but the discharge remained almost constant. In comparison to other springs that were monitored in more detail, the Gute Laune epikarst spring showed the fastest response to rainfall with a delay of only 1 to 1,5 hours. At the three major karst springs, which are active the whole year, we observed a delay after rainfall of only 8 to 12 h. Many of the smaller springs, which are only active in summer and/or after rain events, showed a major decrease of the discharge rate or even ran dry during very hot and dry summer months.
During the first investigation period, selected springs were monitored during rainfall events. Physico-chemical parameters, turbidity and the particle size distribution indicated a potential contamination risk at these springs following hydrological extremes. The springs also showed distinct changes of the major ions. Long flow distances (several kilometers), high flow velocities (more than 300 m/h) and short transit times were obtained from multiple tracer tests as well as information regarding the catchment areas of the most important springs.
During the second project phase, the springs were monitored in more detail to identify correlations between turbidity, particle size distribution and coliform bacteria and E.coli which are used as indicators for fecal contamination. In addition, water samples were analyzed for total organic carbon (TOC) and background fluorescence for comparison with the fecal contaminants. The detailed observation of the natural parameters under different hydrological conditions made it possible to characterize the dynamics and interaction of organic carbon, turbidity and bacteria in the research area.
Turbidity peaks at the karst springs can be classified as primary (allochthonous) and secondary (autochthonous) turbidity. We found correlations between coliform bacteria and small particles in particular in the 0,8 to 2,0 µm fraction as well as a correlation between coliform bacteria and TOC. The excitation-emission matrices (EEMs) obtained by fluorescence analysis showed well-defined peaks in the area of humic-like, fulvic-like and tryptophan-like substances. We observed higher fluorescence intensities after rainfall events that correlate with the increasing numbers of E.coli and TOC. These measurements give evidence that fluorescence measurements can be used to determine organic substances in karst spring waters and act as early warning indicator for fecal contamination.

Figure captions:
Figure 1: Formarinsee Lake in the Lechquellengebirge mountains. The lake has no surficial outflow and drains underground through conduits into the karst aquifer.
Figure 2: The Gute Laune epikarst spring with students of the Karlsruhe Institute of Technology during a field course.
Figure 3: Sampling at the Gute Laune spring with automatic sampler and particle counter.