Rapid groundwater warming in an eastern mediterranean coastal aquifer linked to urbanization

• Multi-decal well data from the regional coastal aquifer in a semi-arid area was utilized to follow the trend in groundwater temperature. • Groundwater temperatures under a densely urban area indicated a 0.13 °C per year increase in groundwater temperature. • Rural environments showed a similar groundwater temperature increase as the land surface temperature. The rise in groundwater temperature is a growing concern, primarily driven by urbanization and increasing global temperatures. While long-term trends of groundwater warming have been documented in temperate regions, these trends are rarely studied in semi-arid areas. This study aims to quantify three decades of groundwater temperature changes in an Eastern Mediterranean Coastal Aquifer, a critical freshwater resource affected by hydroclimatic variations and rapid urban expansion. Utilizing temperature records from over 1,000 wells and satellite-derived land surface temperatures (LST), the study reconstructs the spatial and temporal patterns of groundwater warming in both urban and rural areas. The results indicate notable changes in both absolute temperatures and spatial structure. In urban areas of the central part of the coastal aquifer, groundwater temperatures have increased significantly by 0.13 °C per year, which is much higher than regional air temperature trends and global groundwater warming rates. In contrast, the rural northern and southern regions exhibit groundwater warming rates similar to those of LST trends, ranging from approximately 0.02 to 0.07 °C per year. Correlation analyses reveal a shift from natural factors, such as vadose-zone thickness, latitude, and proximity to the sea, to human-induced factors, including declining rural land cover and changes in soil properties associated with urban development. These findings highlight that subsurface warming in semi-arid coastal aquifers is greatly intensified by urbanization, which may have significant implications for groundwater quality.