Surface water bodies in urban areas are often polluted by substances from mixed sewer systems. Especially lakes and water bodies with reduced flow velocity are exposed to high loads of contaminants. The Grunewald chain of lakes in Berlin, Germany, is a relevant example for an anthropogenic affected water system. The flow direction of the main channel (Schlachtensee → Krumme Lanke → Grunewaldsee → Hundekehlesee → Dianasee → Königssee → Halensee) is changed to the opposite of the natural flow direction from south west to north west. The tributary channel (Herthasee → Hubertussee → Fennsee) is flowing against the natural flow direction as well, and in addition it has an underground pipe to the river Spree. The environmental and nature protection authorities of the district Charlottenburg-Wilmersdorf are responsible for the maintenance for most of the lakes and partner in this project. The majority of the lakes is used as receiving water bodies to collect the precipitation water from the urban areas such as housing and traffic areas in the catchment. As a consequence, the water quality of the lakes, especially the oxygen amount, is very much impaired, which caused a high fish mortality due to reduced oxygen availability. After heavy rain events in summer 2021, around 400 kg fishes in the Hubertussee were dead. Moreover, some of the lakes are used as bathing lakes and for leisure activities. The high loads of contaminants affect the water quality strongly, which is found by measured data that shows high loads of phosphate and algae blooms. The Grunewald chain of lakes is a complex system with biogeochemical processes that are influenced by human impacts. Some mitigation measures have been taken to improve the water quality. One of the measures is the implementation of the surface water treatment plant Beelitzhof in 1981 to clean the water and reduce the phosphate load to a range between 0.01 and 0.04 mg/l, as well as the implementation of the corresponding pumps to transport the water against the natural gradient. Other measures are the implementation of retention ponds and filter systems to reduce the contaminant loads of precipitation water and sewage water. Since the implementation of the surface water treatment plant Beelitzhof, the water quality of the lakes Schlachtensee, Krumme Lanke and Grundewaldsee has improved to a good status. But the water quality of the following lakes is still in a poor status. Especially the eutrophication and the odour of the rotten water in the Fennsee have an impact on the population in that area. In 2007 the precipitation filtration plant has been built at the lake shore of the Halensee with the aim to reduce the contaminant loads of the sewage water from the urban areas. The phosphate loads have been reduced by 80% in the Halensee. Furthermore, the high loads of contaminants are a risk for the other lakes, especially during high precipitation events that cause the transport of high loads of contaminants. Therefore, additional efforts are needed to improve the ecological status and the water quality of the lakes.
The aim is to analyse potential measures and their effectiveness that can be taken to reduce the loads of contaminants in the Grundewald chain of lakes, especially phosphate loads in sewage waters, by using model simulations. For instance, an optimized operation of the pump stations, the reduction of the contaminant loads and lake internal measures will be analysed. Using a hydrodynamic water quality model can help to get an impression about the effectiveness of measures before they are implemented in the real world system. One of the main goals is to find effective measures that can be implemented in the Grunewald chain of lakes to improve the water quality as well as the ecological and socioecological value of the chain of lakes. Considering other literature, no other digital water management systems for urban chains of lakes have been developed and used for solution finding in terms of improving the water quality.
For the maintenance of the Grunewald chain of lakes and the analysis of the effectiveness of measures, an integrated simulation model system (urban water management system for lakes) is built in the uWMS project. Based on the hydrodynamic and water quality model CE-QUAL-W2 (Wells, 2023) the chain of lakes are represented and analysed with regard to water quality and special focus on phosphate loads due to sewage water immissions. With the uWMS project the effect and efficiency of measures are investigated. During workshops with the stakeholders, possible solutions to improve and stabilize the ecological function of the lakes for long term are developed. The model development is supported by a monitoring program to get deeper insights into causes and the chain of effects for diverse pollution sources. The uWMS is aimed to be transferred to other urban lakes and lake systems and by this it is a new tool to investigate urban water management.
Deutsche Bundesstifung Umwelt (DBU)
February 2024 – January 2027
Bezirksamt Charlottenburg-Wilmersdorf
Ingenieurgesellschaft für Stadthydrologie mbH