Density-driven exchange between the basins of Lake Lucerne (Switzerland) traced with the 3H-3He method

W. Aeschbach-Hertig, R. Kipfer, M. Hofer, and D. M .Imboden
Environmental Physics, Swiss Federal Institute of Technology (ETH), Swiss Federal Institute for Environmental Science and Technology (EAWAG), CH-8600 Dübendorf

H. Baur
Isotope Geology, Swiss Federal Institute of Technology, NO C61, CH-8902 Zurich



Lake Lucerne, located in central Switzerland, is divided by sills into four major and two minor basins. During the winter, differential vertical mixing due to non-uniform wind exposure as well as differences in the total dissolved solid concentration between the main tributaries cause considerable inter-basin density gradients. These gradients induce density-driven currents across the sills which contribute significantly to the deep water exchange in the basins and gradually reduce the driving density gradients during the summer.

Over a period of 2 yr (1990-1992), the spatial and temporal evolution of water density and water age (measured by the 3H-3He method) was investigated. Variation of water age between different basins and the occasional occurrence of age inversions (older water overlying younger one) can be explained in terms of the observed density distribution and the exchange flows which the density gradients cause. The water age provides an integral measure of the magnitude of deep water renewal, which in some basins can be roughly separated into contributions from density-driven currents and wind-induced vertical mixing.

By correlating water age with dissolved helium and oxygen concentrations, radiogenic helium fluxes as well as average oxygen depletion rates were determined for the main basins of the lake. The helium flux was between 1 and 2x1010 atoms m-2 s-1 indicating dynamic equilibrium with the helium production in the underlying crust. Oxygen depletion rates per unit sediment area were roughly equal in all basins (between 220 and 290 g m-2 yr-1), whereas volumetric depletion rates varied from 1.8 g m-3 yr-1 in the deepest basin to 10 g m-3 yr-1 in the shallowest one.


Map of Lake Lucerne.


Cross-section along the major axis of Lake Lucerne, showing the distribution of 3H-3He ages measured in March and April of 1991.


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