“The Spree could even flow backwards”: the engineers preparing Berlin for future water problems.

Berlin is located in one of the driest regions of Germany. The prolonged dry spells during recent summers have shown that climate change is affecting both Berlin and Brandenburg. At the same time, the capital has also experienced the impact of heavy rainfall. Improved water management aims to address both of these issues. Researchers are modelling how future conditions and measures will affect different parts of the city.

By 2038, when the last coal mine in the Lausitz region shuts down, Berlin could face a water crisis. Currently, only 30% of the city's drinking water comes directly from groundwater, while 70% comes from bank filtration – a process where water near the surface, fed by rivers or lakes, is collected. One of these sources is the River Spree. However, more than a third of the Spree's water comes from mine drainage in the Lausitz, where groundwater is pumped to enable coal mining and then channelled into the river. In times of drought, this water makes up half of the river's flow. With coal mining set to end by 2035, Berlin may lose a crucial water source in just a few years.

In its early years, Berlin was built on a swamp, and the challenge was getting rid of excess rainfall. Today, however, water scarcity is a growing problem, worsened by climate change. In 2019 and 2020, the flow of the Spree and Havel was halved, and in 2023, parts of the Panke River dried up completely. "Some forecasts suggest this could happen again, even more severely," says Dr Daniel Wicke from the Competence Centre Water Berlin (KWB). "We need to prepare for that."

Circular water use

Berlin has taken steps to address its water management challenges. The city's water utility, Berliner Wasserbetriebe (BWB), already reintroduces treated wastewater into surface water sources, which is then recaptured downstream through bank filtration. However, this creates another issue: pharmaceutical residues and other substances that aren't fully removed in wastewater treatment can find their way back into drinking water supplies. While there are no legal limits for these substances, Berlin has health guidelines that the water must meet. At some waterworks, certain substances have already reached 60% of the recommended limits, and the trend is rising.

Berlin has implemented several measures to manage its water resources. In 2018, the city and BWB founded the Rainwater Agency to reduce the amount of rainwater flowing into the sewers, encouraging its absorption into the ground or reuse instead. For the past six years, new construction projects have been required to manage rainfall on-site, whether for irrigation, infiltration, or non-drinking uses. The city is also installing large catchment basins to prevent sewer overflow during heavy rains and stop untreated wastewater from being discharged into the city centre.

"The total amount of rainfall hasn’t changed much due to climate change, but its distribution has, with longer dry periods and more intense storms," explains Dr Nasrin Haacke from KWB. Berlin’s "Water Master Plan" aims to secure the drinking water supply, improve water protection, and expand wastewater management.

32 measures – but what are the consequences?

The Water Master Plan outlines 32 specific measures, from protecting water conservation areas to modernising sewage treatment plants and reactivating old waterworks to increase groundwater replenishment. But how will these measuresand the end of coal mining impact Berlin’s various areas? "How will the mix of trace substances change when the Spree brings less water into the city? And how will altered wastewater outflows affect the system?" asks Wicke. These and many other questions are being studied by the KWB, BWB, and Berlin’s Senate Department for Mobility, Transport, Climate Protection, and the Environment, as part of the EU research project IMPETUS.

Using hydrodynamic models, the project simulates the impact of climate change and various interventions on Berlin’s water system. For example, what happens to underground water flows if more drinking water wells are added? Or how will water levels respond to reduced flow rates? "In some areas, the Spree could even flow backwards through Berlin," says Wicke. A simplified mixing model developed by the BWB also calculates the proportion of treated wastewater in Berlin’s rivers, helping assess the impact of reduced water flow and planned measures.

What happens if sewage plants discharge their water elsewhere? This occurred at the Schönerlinde plant, which usually discharges into the Tegel Lake via the North Canal. However, the local waterworks found that pharmaceutical residues were accumulating in the lake, despite existing treatment facilities. Now, the plant discharges into the Panke River, bypassing Tegel Lake. This could be reversed in the future, as the sewage plant is being upgraded with ozone treatment to further reduce pharmaceutical residues. In fact, all Berlin’s sewage plants are planned to be retrofitted with this technology.

Strong data foundation, but complex interactions

The researchers have a solid data foundation to work with, incorporating past studies and environmental impact assessments. However, for some factors, such as the proportion of bank filtrate, there are only modelled estimates, leading to some uncertainty in the calculations.

The complexity of the water system means there is no comprehensive groundwater model for the entire city yet. For example, a lowering of the water table changes underground flow paths and how long water takes to travel through the ground. "Given the current knowledge, it makes more sense to use detailed models where there is uncertainty, as simplified approaches may not provide reliable results," explains Haacke. Each scenario the experts simulate takes a long time to compute and generates vast amounts of data, which must be analysed to present the findings clearly.

A tool for quick scenario comparison

The KWB has developed a tool to quickly visualise the key findings from simulations: a map that uses colours to show areas where, for example, the proportion of treated wastewater is high, or where the flow might even reverse. "This allows us to compare scenarios and their consequences quickly," says Wicke. The goal is to help prioritise which actions from the Water Master Plan will be most effective and urgent. The project will continue until September 2025, with further simulations helping Berlin prepare for the challenges of water scarcity and heavy rainfall in a changing climate.