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Urban areas are major contributors to the alteration of the local atmospheric and groundwater environment. The impact of such changes on the groundwater thermal regime is documented worldwide by elevated groundwater temperature in city centers with respect to the surrounding rural areas. This study investigates the subsurface urban heat island (SUHI) in the aquifers beneath the Milan city area in northern Italy, and assesses the natural and anthropogenic controls on groundwater temperatures within the urban area by analyzing groundwater head and temperature records acquired in the 2016–2020 period. This analysis demonstrates the occurrence of a SUHI with up to 3 °C intensity and reveals a correlation between the density of building/subsurface infrastructures and the mean annual groundwater temperature. Vertical heat fluxes to the aquifer are strongly related to the depth of the groundwater and the density of surface structures and infrastructures. The heat accumulation in the subsurface is reflected by a constant groundwater warming trend between +0.1 and + 0.4 °C/year that leads to a gain of 25 MJ/m2 of thermal energy per year in the shallow aquifer inside the SUHI area. Future monitoring of groundwater temperatures, combined with numerical modeling of coupled groundwater flow and heat transport, will be essential to reveal what this trend is controlled by and to make predictions on the lateral and vertical extent of the groundwater SUHI in the study area.

<p>Many cities worldwide extend upon alluvial aquifers which have a great potential for low temperature geothermal installations. Typically, the geothermal potential describes the ability to exchange heat with the subsurface and the relative sustainability.</p> <p>To estimate the geothermal potential of shallow aquifers many techniques have been adopted such as analytical solutions and numerical methods considering aquifer thermal parameters (e.g. porosity, thermal diffusivity) and the system configuration (e.g. diameter of pipes, borehole thermal resistance). Analytical methods are typically fast and easy to implement in a GIS environment but commonly neglect the effects of groundwater advection on heat transfer mechanisms. On the other hand, physically based numerical methods can handle conductive and advective transport and complex 3D geometries but have the limitation of domain size/resolution that makes modeling unfeasible at scales greater than city districts or cities.</p> <p>Hence, a new solution based on a surrogate model is presented to estimate the geothermal potential of aquifers at large scale covering a great variability of Darcy flow velocity. The model is based on the response of a synthetic transient-state 3D FEM model reproducing the infinite line source (ILS) configuration. The simulated thermal perturbation over a representative volume at different time stages was then used to calculate the thermal resistance of the aquifer and the corresponding (energy replenishment) potential combining the most relevant variables that affect the heat transport in porous media: thermal conductivity, specific heat capacity, saturation, porosity and flow velocity.</p> <p>Then, a machine learning regression-based surrogate model was generated by fitting the calculated response (thermal potential) for all possible combinations of input variables. The proposed model well replicates the ASHRAE analytical solution which is based on the ILS method for no groundwater flow, and goes beyond including the effects of thermal transport by groundwater.</p> <p>Finally, the model response was implemented in a GIS to obtain large scale geothermal potential maps in areas with highly variable groundwater flow velocity (between 10<sup>-5</sup> to 10 m/d) highlighting an expected increase of the geothermal potential due to the advective transport. Field experiments are necessary to verify the numerical findings aiming to reconsider the commonly adopted temperature delta thresholds in areas where the energy replenishment potential is high due to groundwater advection.</p>