A novel method for assessing Geothermal Aquifer Potential (GAP) in open-loop heat pump systems

Low-enthalpy geothermal energy and groundwater heat pump (GWHP) systems have proven highly efficient for urban heating and cooling applications. However, their deployment still requires reliable resource mapping tools capable of rapidly assessing hydrogeological conditions over large urban areas. To address this gap, the present paper introduces the Geothermal Aquifer Potential (GAP) method, a novel, open and fully GIS-based workflow specifically designed for rapid, scalable and reproducible screening of shallow unconfined aquifer for open-loop groundwater systems. Unlike existing approaches, GAP operationalises a streamlined and fully open-data procedure that allows practitioners and planners to derive geothermal suitability maps with minimal input requirements and high spatial detail. The method integrates open-access datasets (including piezometric levels, aquifer base elevation and hydraulic conductivity) within QGIS software and applies Kriging interpolation to generate continuous spatial surfaces. From these, it computes aquifer thickness, transmissivity, and extractable flow rates using the Thiem–Dupuit equation combined with well-spacing constraints. A distinguishing feature of GAP is its explicit incorporation of key hydrogeological factors such as aquifer heterogeneity, spatial variability in sedimentary deposits and depth-to-bedrock conditions, feature of GAP is its explicit incorporation of recharge capacity, thermal breakthrough risks, and long-term system sustainability. Geological factors, including lithological composition and stratigraphic layering, are equally essential to assess borehole stability and thermal conductivity. Thermal power potential is computed on a cell-by-cell basis and classified into six intuitive power categories aligned with typical urban building demands, enabling immediate interpretability for energy planners. The originality and robustness of the GAP model are demonstrated through its application to the alluvial plain of Turin (Italy), where it successfully identifies high-potential zones (exceeding 500 kW) and reveals additional, previously unrecognised, high-yield areas. A comparison with existing open-loop installations in the city centre confirms its predictive accuracy and practical relevance. The key strengths of GAP include rapid map generation, open-source reproducibility, flexible parametrization, and the creation of a structured geodatabase suitable for subsequent numerical modelling. However, its limitations, such as uncertainty propagation through multiple interpolations, cell-wise independence, and reliance on simplified analytical solutions, underscore its intended role as a preliminary yet unprecedentedly efficient screening tool, not a replacement for detailed design analyses. Future developments will include integrating building-level thermal demand profiles and more detailed subsurface characterization, further enhancing the method's ability to support targeted geothermal system sizing and sustainable urban energy planning.