Hydrological status prior to installation of filter bed - Alte Donau Water Park

The Alte Donau was created when the former main channel of the Danube was cut off from the river in the course of the regulation works carried out in 1870 to 1875. As a result, it lost its original character as a flowing waterway.

The separation also meant that the hydrological link to the Danube was essentially limited to the groundwater connection. The Alte Donau was no longer affected by flooding, though its water level was still very strongly influenced by the level of water in the Danube: when the water level in the Danube was low, water from the Alte Donau would seep into the groundwater, and when the Danube was rising the Alte Donau would be refilled with fresh groundwater, thus ensuring a regular exchange of water.

This replenishment process was severely restricted by the construction of the Neue Donau flood relief channel and the Danube Island (Donauinsel), and was eventually eliminated completely with the construction of the Freudenau hydroelectric plant. Because the water level in the backwater zone of the power plant is kept constant, groundwater flows in the Vienna metropolitan area that are dependent on changing water levels in the Danube have been drastically reduced. In the case of the Alte Donau, the inflow of groundwater is additionally restricted by a series of barrier wells installed to remediate a contaminated site in the Bruckhaufen area.

Eutrophication in the 1990s

In the early 1990s there was a huge deterioration of the water quality in the Alte Donau, caused by excess nutrients (eutrophication). These problems were remedied by a comprehensive restoration programme implemented by Municipal Department 45 - Water Management ( MA 45). However, sustainable hydrological management measures are required to safeguard the water quality in the long term.

Restoration of the Alte Donau in the 1990s (German)

Elevated pH values

Calcium-rich water is constantly replenished via the groundwater system. In the long term, therefore, the extremely limited inflow of groundwater  most pronounced in the upper section of the Alte Donau  gives rise to deficiencies, especially in conjunction with the dense growth of water plants in the lake.

The water plants have many beneficial effects on the water quality in the Alte Donau: Water plants act as a biological filter, binding nutrients and thus preventing turbidity and strong growth of algae. At the same time, water plants provide a valuable habitat for numerous aquatic animals. During photosynthesis water plants enrich the water with oxygen, which is vitally important for fish and other water-dwelling creatures.

The essential role of water plants in the Alte Donau (German)

During photosynthesis the plants extract carbon dioxide (CO 2 ) from the water, converting it into the carbohydrates they need in order to grow. When the growth of water plants is extremely dense, however, vast quantities of CO 2 are extracted from the water. This can subsequently lead to a shift in the chemical balance of the water, detectable by elevated pH values. This phenomenon is most likely to occur in the summer months, when the plants are especially active and water temperatures are high. These elevated pH values are detrimental to fish and other aquatic organisms.

Water management measures

In order to safeguard the water quality for the long term, sustainable hydrological management measures are required to refresh the lake water.

Municipal Department MA 45 regularly carries out two important water management measures, namely reducing the water level in the Alte Donau every spring and exchanging water between the Alte Donau and the Neue Donau.

However, these measures are relatively complex and can only be implemented for a limited period of time. The intention is that the biological filter bed in the Alte Donau will provide a permanent solution.