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Zwahlen, François
Nom
Zwahlen, François
Affiliation principale
Fonction
Professeur.e émérite
Email
francois.zwahlen@unine.ch
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Résultat de la recherche
Voici les éléments 1 - 2 sur 2
- PublicationAccès libreRelations between retreating alpine glaciers and karst aquifer dynamics: Tsanfleuron-Sanetsch experimental test site, Swiss Alps(2011)
The Tsanfleuron area, located in the Southwestern Swiss Alps, was found to be an ideal experimental test site for research on alpine aquifers, glaciers, snowpack, freshwater resources and climate variations. The rapidly retreating Tsanfleuron glacier overlies a large regional karrenfield and its meltwater directly recharges the karst aquifer. Relations between stratigraphic and tectonic settings, recharge processes and underground drainage were validated by the means of 22 tracer experiments. Groundwater flow towards the main spring occurs in the superficial aquifer, parallel to stratification, while flow towards a western spring crosses the entire stratigraphic sequence, consisting of about 800m of marl and limestone, along deep faults. The diurnal variability of glacial meltwater production during the warm season influences the shape of tracer breakthrough curves and, consequently, flow and transport in the aquifer.
The Tsanfleuron glacier currently loses an estimated thickness of about 1.5 m per year according to field observations and water balance calculations. Flow measurements and glacial tracer tests allowed characterization of meltwater drainage and aquifer recharge. Three pathways have been clearly identified between glacier and the karst aquifer.
Recharge and spring discharge display strong diurnal and seasonal variability, with a general high- flow period during snow and glacier melt from spring to autumn. Annual meteorological variations in the Tsanfleuron area were well defined by a snow cover, a snow melt and an ice melt season. Snow, ice or rain volumes were characterized through water stable isotopes in order to estimate the different recharge contributions to the available volume at the main output. As expected, the glacier has a great influence on the groundwater quantity and moreover drives the diurnal variations of many physical parameters. A time shift of about 10 hours between the lowest electrical conductivity (ice meltwater signature) and the equivalent isotopic ratio was observed.
Calculated water balance, without applying uncertainties, presents a system close to equilibrium without considering the glacier melt. By modifying measured values by uncertainties, balances vary from a deficit of 18% to an excess of 61%. Therefore worst prediction of the future availability of spring water after disappearance of the glacier suggests that the discharge may decrease by 22%. However, nearly all of the loss will occur in summer and autumn, presumably resulting in temporary water shortage. Best cases are on contrary sufficient excess balance, where glacier disappearance would not have any incidence on water availability. For equilibrium scenarios glacier currently completes any deficit, but this completion will vanish in future and decreases the output volume at the main spring. - PublicationAccès libreOrigin and behaviour of microorganisms and particles in selected karst aquifer systems(2008)
; ; Goldscheider, NicoKarst aquifers represent an important groundwater resource world-wide. They are, however, highly vulnerable to contamination due to fast transport and limited contaminant attenuation processes encountered in such systems. Pathogenic microorganisms are among the most frequent and problematic contaminants in groundwater from karst aquifers. It is generally accepted that the presence of faecal indicator bacteria, such as Escherichia coli and enterococci, in groundwater indicates the possible presence of pathogenic bacteria, protozoa and viruses. Monitoring microbial water quality relies, however, on sterile water sampling and subsequent laboratory analyses. Moreover, the sampling intervals and the relatively long time lag between sampling and results are often too long to prevent microbially polluted water entering the distribution network. Therefore, establishing a correlation between the microbial water quality and parameters that can easily be measured online – using it as an “early-warning” parameter – constitutes the first main objective of this work. Most microorganisms in groundwater are harmless. Enumeration of pathogenic and water quality indicator bacteria thus provides an incomplete picture of the microbial assemblages. These microbial communities are of growing interest, as aquifers, which include habitats for a wide range of organisms, are increasingly considered as ecosystems harbouring their proper biocenoses. These ecosystems have to be better understood in order to ensure the sustainable management of groundwater resources. Furthermore, as bacteria are abundant and ubiquitous, thriving even in extreme habitats, the potential use of microbial communities for groundwater biomonitoring has been raised. However, the understanding of microbial diversity, their distribution patterns and their dynamics in karst aquifer ecosystems are actually still in an early stage and rather sparse and constitute therefore the second main objective of this research project. The occurrence, dynamics and transport processes of suspended matter and faecal indicator bacteria were investigated at two karst aquifer systems (the Yverdon-les-Bains and Noiraigue test sites, Switzerland), which are characterised by allochthonous point recharge, and several shallow cave sites located in the unsaturated zone of karst aquifers (the Vers-chez-le-Brandt, Grand-Bochat and Gänsbrunnen test sites, Switzerland). Classical hydrogeological parameters, such as discharge, temperature, electrical conductivity, organic carbon and turbidity, were monitored continuously online, along with event-based, high-frequency analyses of Escherichia coli and enterococci. In order to gain more insight into the origin and behaviour of suspended particles in karst groundwater, particle-size distribution (PSD) was also monitored continuously. Results demonstrated that suspended particles in karst spring water either originated from the remobilisation and scouring of intrakarstic particulate material due to increasing flow velocities (i.e. autochthonous turbidity) and / or the transfer of particles from land surface and sinking streams (i.e. allochthonous turbidity), which entered the system following rainfall events. These allochthonous turbidity events coincided with increased faecal indicator bacteria and organic carbon levels. PSD allowed distinguishing both types of turbidity: autochthonous turbidity consisted of particle concentration increases over a wide range of particle sizes (from colloidal sizes up to 0.1 mm), whereas allochthonous turbidity periods were characterised by a predominance of finer particles (0.9 to 10 µm). PSD is therefore proposed as surrogate indicator for possible microbial contamination of groundwater from karst aquifers. The structure, diversity and temporal variability of microbial communities from a swallow hole draining agricultural land and two connected karst springs (Yverdon-les-Bains karst aquifer system) were investigated using molecular microbiological methods (PCR-DGGE and cloning / sequencing) and related to hydrological and physico-chemical parameters. Storm responses and an annual hydrological cycle were monitored to determine the short- and long-term variability, respectively, of bacterial communities. Statistical analysis of bacterial genetic fingerprints (16S rDNA PCR-DGGE profiles) of spring water samples revealed several clusters that corresponded well with different levels of the allochthonous swallow hole contribution. Microbial communities in spring water samples highly affected by the swallow hole showed low similarities among them, reflecting the high temporal variability of the bacterial communities in the water entering the swallow hole. Conversely, high similarities among spring water samples with low allochthonous contribution provided evidence for a stable autochthonous endokarst microbial community. This autochthonous endokarst microbial community was characterised by a high diversity with only a few dominant species. δ-Proteobacteria, Acidobacteria and Nitrospira species were important members of this community. The high percentage of unknown sequences (i.e. sequences revealing similarities lower than 97 % to already known sequences) suggests that many karst aquifer bacteria are still undiscovered. Moreover, it highlights that karst aquifers are rarely studied, unique habitats and that sequences from shallow subsurface ecosystems are currently underrepresented in databases. In conclusion, the high sensitivity of PSD analysis allows consequently proposing it as an “early-warning” surrogate for real-time monitoring of possible microbial contamination of groundwater from karst aquifers. The method permits optimising water treatment and identifying periods when spring water must be rejected. Furthermore, this study represents a first step towards a better understanding of the microbial ecology in karst aquifer systems and may serve as a starting point for developing biomonitoring tools.