Groundwater Urban Heat Island (GUHI) and Climate Warming: Insights from Two Decades of Monitoring in Wrocław, Poland

Climate change and intensive urbanisation contribute to an increase in groundwater temperatures, leading to the development of the subsurface urban heat island (SubUHI) and Groundwater Urban Heat Island (GUHI) phenomenon. This article presents the results of comparative studies conducted in Wrocław (Poland) in two measurement campaigns in 2004–2005 and 2022–2024. The study aimed to assess long-term thermal changes in groundwater in relation to air temperature using statistical and spatial analyses. The data were processed using descriptive methods, interpolation (ordinary kriging), map algebra, and correlation analysis (Pearson). The results indicate a clear warming of groundwater, especially in the winter season, where the average temperature increase ranged from 2 to 4 °C compared to the first measurement period. The most substantial changes were observed in shallow Quaternary aquifers, showing a strong correlation with air temperature (r ≈ 0.8). Spatial analyses revealed the major groundwater temperature (GWT) increases in the city centre and densely built-up districts, confirming the intensification of the GUHI effect. In addition, a decrease in the seasonal amplitude of GWT and a local lowering of the water table were observed.