Résultat de la recherche
Relations between retreating alpine glaciers and karst aquifer dynamics: Tsanfleuron-Sanetsch experimental test site, Swiss Alps
Karst aquifers are of major importance for groundwater resources world-wide. Especially in high mountain catchment areas, where different types of recharge occur, karst systems represent one of the first water supplies for human activities. However, the current available volume of water is highly dependent on surface water storage during the winter season. Snowfields and glaciers in high mountain areas store considerable amounts of freshwater contributing to groundwater recharge in warmer periods. As a consequence of climate change, these frozen areas are rapidly shrinking, decreasing the amount of water available to recharge the karst aquifers. With a general increase in temperature and a slight diminution of precipitations prognosed for the next 50 years, alpine water resources during dry periods are expected to reduce in the coming decades.
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.