5th Swiss Geoscience Meeting, Geneva 2007
Sedimentary and geochemical balance and impact of
floods on the Wettingen reservoir (Switzerland)
Jüstrich Stéphanie* & Wildi Walter*
*Institut F.-A. Forel, 10, rte de Suisse, Case Postale 416, CH-1290 Versoix
(stephanie.justrich@terre.unige.ch)
The Wettingen dam is located on the Limmat river downstream of Zürich city.
The reservoir is fed by clear outflowing waters of Lake Zürich and by
suspension-rich waters of the Sihl river (Wildi et al. 2004). Two small effluents
(Furtbach and Reppisch rivers) enter the reservoir as well as sewage waters
from the main sewage treatment plant in Zürich city (Werdhölzli) and from two
local treatment plants (Limmattal and Killwangen-Spreitenbach).
Periodic sampling of suspensions (continuous flow centrifuge system) and
sediments (sediment traps) in the inflowing and outflowing waters during a one
year period, from May 2005 to April 2006, allowed to assess the sedimentary
and geochemical balance as well as the trap efficiency of the reservoir.
Suspensions were also sampled during the flood that occured in August 2005
with bottle samples used for filtration (0.7µm glass microfibre filters). The same
method was used to check suspension concentrations along two river profiles at
the entrance and the exit of the reservoir to follow the evolution of concentration
between the campaigns and after April 2006. Concentration for 14 trace metals
and grain size were analysed for suspensions and sediments from the periodic
sampling.
The actual sediment fill reaches almost half of the reservoir volume.
Sedimentation from suspensions has a deposition rate of 50% during periods of
meanflow but reaches 90% by high flow events. Reservoir sedimentation is
closely related to high flow events indeed. Concerning contaminants
nevertheless, a dilution phenomenon of their concentration can be observed
during floods.
The contribution of the minor effluents (rivers and sewage waters from two local
facilities) reaches only one percent of the total water and contaminant flux by
meanflow. Concerning contaminants concentration and flux, a flood occuring in
an effluent can have significant effect (up to 30% of the total input to the
reservoir for the Reppisch river, assuming that the Limmat river is under mean
flow regime).
The Wettingen reservoir traps the main of the forwarding contamination coming
mainly from Zürich city and settling down with fine particles near the dam (see
also: Horowitz & Elrick, 1987). Contamination values for suspensions in the
reservoir are higher than mean values for suspensions in some important Swiss
rivers (Pardos et al. 2003). The environmental risks involved are essentially the
potential uptake of contaminants from sediments by in-stream erosion.
5th Swiss Geoscience Meeting, Geneva 2007
Figure 1. Situation of the Wettingen reservoir and principal effluents concerning
suspensions and contaminants.
REFERENCES
Horowitz, A.J. & Elrick, K.A. 1987: The relation of stream sediment surface
area, grain size and composition to trace element chemistry. Applied
Geochemistry, Vol. 2, pp 437-451.
Pardos, M., Houriet, J.P. & Dominik, J. 2003: Micropolluants dans les
sédiments. Documents environnement n° 353, OFEFP, 58p.
Phys. Chem. Earth (B), vol 26, n°1, 77-82.
Wildi, W., Dominik, J., Loizeau, J.L., Thomas, R., Favarger, P.Y., Haller, L.,
Perroud, A. & Peytremann, C. 2004: River, reservoir and lake sediment
contamination by heavy metals downstream from urban areas of Switzerland.
Lakes and Reservoirs: Research and Management 9, 75-87.