City-wide groundwater temperature profiles reveal underground urban heat islands in Vienna

Summer periods of warm weather become longer and hotter causing “Urban Heat Islands” in large cities. Rising temperatures do not stop at the surface, but migrate into the underground, where infrastructure – such as sewage and district heating systems, buildings and shallow geothermal energy systems for cooling – amplifies the increase of groundwater temperatures. In order to be able to quantify this ongoing process and predict future temperature developments, a sound data basis is necessary. As of now, only groundwater temperature measurements have been available for different depth (one-point or multiple depth measurements) and time intervals (varying from two weeks to months) at a limited number of wells within the shallow urban aquifers in Vienna. To increase the spatial information content, the goal of this study was to measure groundwater temperature and level within the urban shallow aquifers of Vienna in two extensive field campaigns for more than 800 wells and to analyze those data statistically

To document the warmest and coldest conditions, measurements took place in one week each in October (2021) and April (2022). In total, groundwater temperatures in 1 m depth intervals and groundwater level were measured at 812 locations. Out of these ones, at 150 wells, water temperature was measured in pumped water. The average value of the profile equals best the pumped groundwater and thus represents the average aquifer temperature. According to our data analysis, the groundwater temperatures in Vienna vary between 6.9 °C and 30.6 °C. The highest temperatures were detected in close proximity to possible heat sources and a rapid drop in temperature with increasing distance could be demonstrated.

Based on the collected data, temperature maps for both measurement dates and for different depth-intervals were derived, and display the underground urban heat islands in Vienna. The temperature maps enable the estimation of the potential for sustainable heating and cooling with groundwater in the capital of Austria.

Together with historic long-term temperature data, trend analyses will be performed to allow a prognosis of thermal changes in the groundwater. The results, together with an extensive analysis of the groundwater chemistry and ecology, will feed into the development of a catalogue of measures for authorities and policymakers. Intention of the included recommendations is to counteract further groundwater warming and to ensure an efficient and sustainable use of groundwater for heating and cooling. The guidelines will therefore not only contribute to cooling the groundwater, but also to decarbonize the heating and cooling supply of Vienna.