TRITIUM/HELIUM-3 DATING OF BASEFLOW Southern Vienna Basin

摘要

Long-term time series of tritium in baseflow provide an indicator of the meansubsurface residence time (MRT) and hence a useful index of vulnerability/sustainabilityof surface water. Unfortunately, long-term times series are uncommon. We areinvestigating the use of tritium/helium-3 dating as a substitute for long-term time seriesin the Southern Vienna Basin, Austria. Preliminary results indicate that the apparentage of the water depends on sampling location. Apparent tritium/helium-3 ages rangefrom about 2 to 12 years with the oldest water occurring at head-water springs. We haveevaluated the exchange of helium-3 with the atmosphere by performing a gas tracer testand applying a gas exchange model. The model was then used to predict the observedchange in the helium-3 (and hence apparent tritium/helium-3 age) with excellentagreement to observed values. This result suggests that while gas exchange is an activemechanism for noble gases in streams, it is possible to predict this exchange and hence“correct” tritium/helium-3 ages for this process.