Groundwater temperatures (GWT) reflect the interactions between climate, geological conditions, and human influence. Changes in the thermal conditions of shallow aquifers can have far-reaching consequences, including altered groundwater quality, increased geothermal potential, and threats to groundwater-dependent ecosystems. Both anthropogenic infrastructure and climate change affect the thermal conditions of groundwater. However, there is still a lack of studies that examine the impacts of built environments on groundwater temperatures at large spatial scales, particularly regarding their seasonality.

Therefore, this study updates an existing global dataset with the newest groundwater temperature observations collected at governmental levels, focusing for the first time not only on annual mean temperatures but also on their seasonal variation.

After calculating temperature anomalies (i.e., identifying deviations from expected thermal patterns), we first link these data to local climate zones (LCZ). LCZs are the state-of-the-art urban land cover classification that describes the impact of the (built) environment on local temperatures. Developed initially with a focus on air temperatures, we find a strong relationship between LCZs and GWT as well. Particularly, sites in compact high-rise locations are on average +2.8 °C warmer than their surroundings, and sites in heavy industry even +4.0 °C (see attached figure).

Furthermore, time series from various wells are analysed to determine the extent to which the seasonal development of GWT follows the atmospheric signal or is superimposed by other—particularly anthropogenic—influences. This analysis provides insights into regional and global patterns of subsurface heat dynamics and their drivers. First results reveal a significant impact of non-atmospheric factors on groundwater thermal regimes at many urban and non-urban locations.