Abstract
Terrigenous (Sc, Fe, K, Mg, Al, Ti) and anthropogenic (Pb and Cu) element fluxes were measured in a new sediment core from Lake Biel (Switzerland) and in previously well-documented cores from two upstream lakes (Lake Brienz and Lake Thun). These three large peri- alpine lakes are connected by the Aare River, which is the main tributary to the High Rhine River. Major and trace element analysis of the sediment cores by inductively coupled plasma mass spectrometry (ICP-MS) shows that the site of Lake Brienz receives three times more terrige- nous elements than the two other studied sites, given by the role of Lake Brienz as the first major sediment sink located in the foothills of the Alps. Overall, the terrigenous fluxes reconstructed at the three studied sites suggest that the construction of sediment-trapping reservoirs during the twentieth century noticeably decreased the riverine suspended sediment load at a regional scale. In fact, the extensive river damming that occurred in the upstream watershed catchment (between ca. 1930 and 1950 and up to 2,300 m a.s.l.) and that significantly modified seasonal suspended sediment loads and riverine water discharge patterns to downstream lakes noticeably diminished the long-range transport of (fine) terrigenous particles by the Aare River. Concerning the transport of anthropogenic pollutants, the lowest lead enrichment factors (EFs Pb) were measured in the upstream course of the Aare River at the site of Lake Brienz, whereas the metal pollution was highest in downstream Lake Biel, with the maximum val- ues measured between 1940 and 1970 (EF Pb ( 3). The following recorded regional reduction in aquatic Pb pol- lution started about 15 years before the actual introduction of unleaded gasoline in 1985. Furthermore, the radiometric dating of the sediment core from Lake Biel identifies three events of hydrological transport of artificial radionuclides released by the nuclear reactor of Muhleberg located at more than 15 km upstream of Lake Biel for the time period 1970-2000.
Highlights
Terrigenous (Sc, Fe, K, Mg, Al, Ti) and anthropogenic (Pb and Cu) element fluxes were measured in a new sediment core from Lake Biel (Switzerland) and in previously well-documented cores from two upstream lakes (Lake Brienz and Lake Thun)
Concerning the transport of anthropogenic pollutants, the lowest lead enrichment factors (EFs Pb) were measured in the upstream course of the Aare River at the site of Lake Brienz, whereas the metal pollution was highest in downstream Lake Biel, with the maximum values measured between 1940 and 1970 (EF Pb [ 3)
The terrigenous fluxes reconstructed at our studied sites indicate that sediment-transfer processes at the catchment scale are significantly altered by artificial hydropower reservoir lakes located in the upper reaches of the catchment
Summary
Terrigenous (Sc, Fe, K, Mg, Al, Ti) and anthropogenic (Pb and Cu) element fluxes were measured in a new sediment core from Lake Biel (Switzerland) and in previously well-documented cores from two upstream lakes (Lake Brienz and Lake Thun). The riverine sediment transport in the European Alps, which is primarily controlled by environmental factors (climate, vegetation cover and topography) influencing the rate and process of weathering, has been significantly altered by the extensive construction of reservoirs and hydroelectric dams that drastically changed hydrological patterns and reduced suspended sediment loads (Vorosmarty et al 2003; Finger et al 2006; Thevenon et al 2012). The numerous hydropower dams constructed upstream of Lake Brienz in the highalpine Grimsel area (up to 2,300 m a.s.l; Fig. 1) have strikingly altered the seasonality of the river flow (shift of the particle input from summer to winter) and have considerably reduced the overall particle input from upstream glaciers to Lake Brienz (Wuest et al 2007) In addition to these hydropower operations, the Aare River, which is the main water supplier of the High Rhine River, was redirected into Lake Biel in 1878 and has been regulated by the Hagneck hydroelectric dam since 1900, in an effort to prevent flooding of the nearby area called ‘‘Seeland’’ (Fig. 1). In the absence of long-term environmental records that could provide pre-anthropogenic levels of pollutants (i.e. natural background levels), the enrichment of trace metals can be expressed relative to the lowest values in the record, which are generally found in the oldest sections of the analysed sediment records (Shotyk et al 1998; Arnaud et al 2004; Thevenon et al 2011b)
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