Impact of climate change

Vienna’s groundwater is getting warmer and warmer

(Bild: Kybele/stock.adobe.com)

Climate change not only brings with it temperature and weather extremes: it also has an impact on groundwater. In Vienna, this is becoming increasingly warmer: from 2001 to 2010, the temperature rose by an average of 0.9 degrees, from 2011 to 2020 it was 1.4 degrees. 

The main reasons for this are the significant rise in air temperature due to climate change and increasing sealing. This was the result of an investigation by Geosphere Austria as part of the "Heat below the city" project, which also determined the chemical and biological status of the groundwater.

Measurement data from the past 20 years evaluated
The Geosphere project focused on analyzing the temperature of the groundwater and its long-term development, according to a press release on Wednesday. To this end, measurement data from 87 measuring stations over the past 20 years was evaluated. "We also measured temperature profiles in the groundwater at 800 measuring points in April and October, from a few meters to a depth of around 40 to 50 meters," explained Cornelia Steiner, Geosphere expert for geothermal energy.

Temperature differences by season
The water was coolest in April and warmest in October, as winter and summer temperatures have a delayed effect.

The groundwater in many regions of Vienna is several degrees warmer than in the surrounding area. This is known as the "urban heat island effect".

Urban heat island effect

There are two reasons for this:

• the higher air temperature because streets and buildings heat up more
• the extensive underground infrastructure heats up the groundwater, for example subway and road tunnels, parking garages, sewer systems, district heating pipes and cooling-only geothermal applications

The average value of all 800 measuring points was 13 degrees Celsius in April and 15 degrees Celsius in October.

The annual average difference in groundwater temperature between sealed and unsealed areas is around four to six degrees.

Warming has an impact on water quality and geothermal energy
On the one hand, warming changes the biological and chemical quality of the groundwater and, on the other, has an impact on applications such as geothermal energy, which is currently only permitted in the range of five to 18 degrees Celsius in Vienna. Further warming could therefore lead to restrictions here. In order to avoid excessive warming of the groundwater, no systems are now approved in Vienna that are only used for cooling.

Catalog of measures 
Based on the scientific results, a catalog of measures is now being developed to provide authorities and politicians with recommendations for integrative management for the sustainable and efficient thermal use of groundwater. The project, funded by the Vienna Science and Technology Fund (WWTF), was carried out as a collaboration between the University of Vienna (project management), the University of Natural Resources and Life Sciences Vienna, Geosphere Austria and the City of Vienna (M45, Vienna Waters).

The Viennese data can be accessed in the Geothermal Atlas. Among other things, it enables an initial estimate of the energy potential of geothermal probes and groundwater heat for each location and will be gradually extended to other federal states, Geosphere announced.