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

Abstract

Lab: Lucerne is divided by sills into four major and two minor basins. During winter, differential vertical mixing due to non-uniform wind exposure and different total dissolved solid concentrations in the main tributaries of the lake cause considerable interbasin density gradients. These gradients induce density-driven currents across the sills that contribute significantly to the deep water exchange in the basins and gradually reduce the density gradients during summer.Over a period of 2 yr (1990)(1991)(1992), the spatial and temporal evolution of water density and water age (measured by the 3H-3He method) was investigated. Variation in water age between the 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 the density gradients cause. 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 and average oxygen depletion rates were determined for the main basins of the lake. The helium flux was between 1 and 2 x lOlo atoms m-2 s-l, indicating dynamic equilibrium with 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-l), whereas volumetric depletion rates varied from 1.8 g m-3 yr-r in the deepest basin to 10 g m-3 yr-L in the shallowest.

Noble Gases in Lakes and Ground Waters

“…... to analyze deep water exchange and determine mixing rates in lakes (Torgersen et a1. 1977;Aeschbach-Hertig et al 1996a;Hohmann et a1. 1998) and oceans (Peeters et al 2000a;Schlosser and Winckler 2002, this volume ).…”

“…Aeschbach-Hertig et al (1996a) estimated the horizontal and vertical exchange between neighboring basins responsible for deep-water renewal by a simple calculation of mixing ratios based on 3H-3He ages. During the winter in all basins the water deeper than the sills was exchanged more than once by water from above, whereas this exchange was close to zero in most basins during the summer.…”

Distribution of helium and tritium in Lake Baikal

“…Since then, the method has been applied successfully in the analysis of vertical exchange, exchange between lake basins, oxygen consumption and gas exchange, groundwater influx and helium flux from the lake bottom (e.g., in Lake Baldegg (Switzerland) [Imboden et al, 1981 ], Green Lake (New York) [Torgersen et al, 1981], K6nigsee (Germany) [Fischer et al, 1985], Lake Constance (Germany) [Zenger et al, 1990], Lake Van (Turkey) [Kipfer et al, 1994], and Lake Lucerne (Switzerland) [Aeschbach-Hertig et al, 1996a]). …”

A time series of environmental tracer data from deep meromictic Lake Lugano, Switzerland

“…Transient environmental tracers such as the tritiumhelium-3 ( 3 H-3 He) isotope pair or chlorofluorocarbons (CFCs) have proved to be excellent tools for studying the mixing dynamics of lakes (e.g., Torgersen et al 1977;Weiss et al 1991;Aeschbach-Hertig et al 1996). The methods are…”

“…Although the inflow of nutrients has been reduced and the water quality of the surface water has improved during the last decades, the regeneration of the deep water body has been a very slow process, impaired by sluggish vertical mixing due to a chemical stratification of the water column (Wü est et al 1992). A thorough understanding of the mixing dynamics of the lake, including the response to changing biogeochemical and climatic conditions, is therefore important for the management of the lake.Transient environmental tracers such as the tritiumhelium-3 ( 3 H-3 He) isotope pair or chlorofluorocarbons (CFCs) have proved to be excellent tools for studying the mixing dynamics of lakes (e.g., Torgersen et al 1977;Weiss et al 1991;Aeschbach-Hertig et al 1996). The methods are…”