Thermal impact of underground car parks on groundwater

In addition to the continuous increase of groundwater temperatures due to global warming, heat losses from infrastructure, (underground) buildings and geothermal use lead to thermal anomalies on regional to local scales. Often, these local heat accumulations (hot spots) of groundwater temperatures are associated with underground car parks (UCP). They represent sizeable infrastructures that are typical for densely built-up areas and are numerous in many cities. Unlike regular basements, they often reach beneath the groundwater table and heat up due to frequent traffic. They therefore act as heat sources for groundwater. By analysing long-time data from 31 sites in Germany, Austria, and Switzerland, we discovered seasonally varying heat flux intensities and even directions. While all UCPs heat the groundwater during the warm period, most UCPs cool the groundwater in the cold period. Only few act as continuous heat source all year round. We also discuss characteristics and their influence on the temperature such as the type of use (public/private) and the depth of the UCP. Furthermore, we present the results of a spatial analysis of heat fluxes and flows from over 5000 UCPs in Berlin, Germany. By discussing the range of heat fluxes and the hydrogeological conditions that lead to regional differences, we demonstrate the role of UCPs for subsurface urban warming. The results show that about 40 % of Berlin’s total heat flow from UCPs occurs in the “Mitte” district, where the density of UCPs is highest and the distance to the groundwater table is typically below 4 m. Finally, the knowledge gained about subsurface heat sources can help improve urban thermal groundwater management and highlights the potential for recovering waste heat from UCPs through geothermal applications.