D10: Impact of Contaminants - Hydromod

Integrated Water Resource Management for Important Deep European Lakes and their Catchment Areas
EUROLAKES
D10: Impact of Contaminants
FP5_Contract No.: EVK1-CT1999-00004
Version: 4.0
Date: 25/07/01
File: D10-vers.4.0.doc
Page 62 of 136
With exception of Loch Lomond, that turns out to be a “reference area”, the 3 lakes are,
in terms of pollution, characterised by domestic sewage and its breakdown products.
Although there is also industrial’s effect on the environment, the areas are not dominated
by typical industrial pollution sources. The section hereafter gives a concise description
of relevant pollutants in the catchment areas of the lakes. For some pollutants
little information is currently available and (in contrast to e.g. heavy metals), and as yet
no test methods and effective screening methods known. Therefore, the picture is still
incomplete.
10.1 CATCHMENT AREA OF LAKE CONSTANCE (BODENSEE)
After Lake Geneva Lake Constance is the second largest lake on the northern edge of
the Alps [BROMBACH and MICHELBACH 1998]. The lake basin was formed by the
erosive forces of the Rhine glacier which excavated a 500 m deep valley during its last
extension, about 30.000 years before the present. At the end of this glaciation, 15.000
years ago, the lake area was more than twice its present cover, including large parts of
the Rhine Valley and both Lake Walen and Lake Zurich. Lake Constance and its 11
487 km² large catchment area belong to the flow system of River Rhine [BROMBACH
and MICHELBACH 1998]. The lake is divided into two parts, the upper lake (including
the Überlingersee) and the lower lake (including the Gnadensee and Zellersee). The
Rhine bridge Constance can be taken as the parting line. The contact between both
parts of Lake Constance is named Seerhein. The catchment area of the upper lake is
10919 km² large. The catchment area of the lower lake is only 568 km² large [GURTZ
et al 1997]. The surface of the lake is ca. 540 km² large. The catchment area includes
parts of the German "Bundesländer" Baden-Wuerttemberg and Bavaria, Austria,
Liechtenstein, Switzerland and Italy. The volume of Lake Constance amounts to 48.5
billion m³ and the maximum depth is ca. 254 m. The mean discharge referring to the
tide gauge Constance is ca. 372 m³/s approximating 1.000 mm discharge rate per annum
or a mean dwell of 4.1 years in the lake [BROMBACH and MICHELBACH 1998].
In its natural state, Lake Constance was a typical oligotrophic pre-alpine lake with low
concentrations of nutrients, low densities of phytoplankton, high water transparency and
high hypolimnic oxygen concentrations. Until the early 1970's, the major part of this
sewage entered the lake without any treatment, resulting in a strong increase of the Pload,
additionally enhanced by increasing inputs from agricultural sources and from
precipitation. At present, 75 % of the sewage phosphorus is chemically precipitated in
treatment plants. Further reduction of the P-load is expected after completion of the
sewage purification programme in large treatment plants. Depending on the remaining
P-load, new lower equilibrium concentrations will be reached in the lake. Thus, this provides
an example of a successful restoration programme beyond national borders.The
Lake Constance and its environs is a densely populated area with a Europe-wide relevance.
The ecosystem Lake Constance has an unique flora and fauna and therefore
having a high ecological value. The drinking water for more than 4 million people living
in the neighbouring countries is gained from the lake. The whole drinking water draw-off
from Lake Constance amounts 0.14 km³/a being 1.2 % of the yearly intake of 11.6
km³/a [BROMBACH and MICHELBACH 1998]. Table 10-1 shows the portion of area of
the countries in the catchment area of Lake Constance.
Country Area Part of the Forest Agricul- Non- Resi-