Canton planning an aeration system for Lake Zug

As a result of this project, the phosphorus levels in Lake Zug are expected to fall below the legally required level by 2070. The cost of the new system is around CHF 12 million.

The phosphorus problem is now being tackled: Lake Zug has been heavily polluted by the nutrient for a long time. And the efforts made so far to reduce pollution – specifically with better urban drainage and agricultural measures – are not enough. That’s why additional so-called ‘internal lake measures’ are needed.

The canton wants to use a compression system to pump compressed air into the southern lake basin in the winter to improve circulation. A kind of ‘ventilation system’ for Lake Zug. This measure should now be set out in the Structural Plan for the canton (Richtplan). The changes will be publicly available for participation until 6th November.

Compression plant near Walchwil
The Building Department (Baudirektion) has to act because the federal government is demanding action to reduce the phosphorus content to the legally required level. The compression system that the canton is now planning is to be set up near Walchwil. The natural circulation of the lake is to thereby be restored using compressed air, which is transported underground via pipes. “This process will slowly removes the phosphorus deposit in the deep water from the lake via the Lorze river in a controlled process and, at the same time, will enrich the deep water with oxygen,” says the documents for the adaptation of the Structural Plan.

The low oxygen levels have a negative effect on fish and many other creatures in the lake. For example, it restricts the spawning areas of, for example, the Rötel (red rock fish). In addition, Lake Zug produces a lot of algae due to its high phosphorus content, which can be unpleasant for bathers.

In combination with the previous lake-external measures, the canton expects to achieve a “sustainable recovery” of Lake Zug by around 2070. As construction director Florian Weber explained at the media conference on the structural plan adaptation, the project is expected to cost around CHF 12 million. After public participation, the adaptation will go to parliament, and the government council (Regierungsrat) will later propose a loan. In addition to the canton of Zug, Schwyz and Lucerne also border Lake Zug. “We are having discussions about their financial participation in the project,” said Florian Weber.

Nice to look at, but sick: Lake Zug has too high a nutrient content.              Photo: Mathias Blattmann
The intended location of the system        
The system, explained in a simple model                 Pictures: PD

Variant has proven successful in other lakes
The system will be installed in the southern lake basin, because the circulation there is even worse than in the northern area of the lake due to the lack of major inflows and outflows. A study by Eawag (ETH's water research institute) shows that the supplemented circulation is the most effective measure, explained Florian Weber. He and his team came to the conclusion that this variant had the best cost-benefit ratio and has proven itself in other Swiss lakes, for example in Lake Baldegger (Baldeggersee).

The so-called diffusers are anchored at the deepest point of the southern basin. “They create a vertical flow in which the water above mixes in a controlled manner in the cold months,” the documents say. ‘Controlled’ means that strict monitoring is planned, as cantonal planner René Hutter explained at the media event. The mixing depth of the circulation can be adjusted at any time in case there are undesirable side effects. If, for example, there were too high a phosphorus concentration in the surface layer, those responsible could react immediately.

The only visible sign of the facility are buoys (Auftriebsbojen) indicating the location of the diffusers. Florian Weber and René Hutter emphasized that shipping and fishing would not be affected. The operation will be limited to the winter months because it is easier to affect the circulation at colder water temperatures. The operating costs amount to around CHF 450,00 per year.

The phosphorus content in Lake Zug is currently just under 80 milligrams per cubic metre. The legally required target level is 30 milligrams. “What we are now planning is a kind of contaminated site remediation of Lake Zug,” said René Hutter. The value was 200 milligrams in 1975, the highest value. Since then, the nutrient content has been continuously reduced, but has recently stagnated.