Groundwater and geothermal archetypes in Berlin, Germany

Abstract Urban aquifers are influenced by several natural and anthropogenic factors, such as geological and hydrogeological conditions and built infrastructure, such as heated basements, underground car parks, and train tunnels. Realistic 3D city-scale physics-based models of complex and heterogeneous aquifers must balance accuracy and efficiency to support scenario-based subsurface management. Hence, this study aims to provide an overview of the 3D thermal state of the urban subsurface of Berlin, Germany, with the goal of identifying groundwater and geothermal archetypes. Based on a detailed 3D geological model, covering an area of 118 km 2 and a depth of 250 m, block-divided (500 m × 500 m × 50 m), steady-state groundwater flow and heat transport models are created. These block models serve as a basis for identifying groundwater archetypes representing areas with similar hydrogeological and infrastructure conditions. The simulated, large-scale groundwater temperature patterns are generally in good agreement with interpolated temperatures from depth-oriented measurements. In addition, the block-scale models capture thermal hot spots and low spots that are not detected by interpolated maps. Using regression-based decision trees, 38 groundwater archetypes are identified for the shallow anthropogenically influenced layer of blocks and 21 archetypes at deeper layers (> 50 m bgl). Heated basements and groundwater head difference are the most contributing features in differentiating archetypes for the shallow layer of the blocks, while lower temperature boundary dominates selection of archetypes in deeper layers. Similarity of large-scale groundwater temperature patterns across different numbers of selected archetypes shows the robustness of the approach. Using thermal and geological criteria, 10 of the identified archetypes are classified as geothermal archetypes that indicate suitable conditions for ground source heat pump systems. The archetypes approach could be further developed to support other groundwater and subsurface uses, e.g., by considering groundwater-dependent ecosystems, legal aspects (e.g., groundwater contamination), and the interactions between different uses.