Vertical Heat Transfer Through the Unsaturated Zone in an Urban Alluvial Aquifer and Its Influence on Shallow Geothermal Plumes

Urban shallow geothermal systems are increasingly adopted for low-carbon heating and cooling, yet their performance and environmental impact depend on vertical heat transfer processes that are often simplified, particularly across the unsaturated zone that links the urban surface and groundwater. This study quantifies the buffering role of the unsaturated zone and assesses how its explicit representation affects predicted geothermal thermal impacts in an urban alluvial aquifer. We combine multi-depth temperature observations from instrumented piezometers and thermocouple arrays in the Zaragoza alluvial aquifer (NE Spain) with a three-dimensional transient groundwater-flow and heat-transport model implemented in FEFLOW. Model performance was evaluated by comparing simulated temperature profiles against field observations at −2 m, −5 m, and the water table, yielding root mean square errors (RMSE) of 1.24 °C, 0.58 °C, and 0.42 °C, respectively. Scenario simulations show strong damping and phase delay of seasonal signals through the unsaturated zone and indicate that surface heat exchange controls shallow thermal amplitudes (up to approximately 10 °C at approximately 1 m). Simplified configurations that neglect the unsaturated zone and/or surface heat transfer bias impact assessments by increasing simulated aquifer warming (up to 1 °C at the end of summer injection periods) and altering plume intensity and geometry (plume extents on the order of 80 m laterally in the analyzed configuration). These results underline that urban geothermal assessments require field-constrained representations of unsaturated-zone heat transfer and realistic surface boundary conditions to support sustainable subsurface energy planning.