Numerical modelling as a tool to improve conceptual models: the case study of a regional hydrothermal system in NE Italy

The construction of a reliable conceptual model is a crucial aspect in modern hydrogeology. A detailed conceptual model can be used as a starting point for a subsequent numerical model aimed to plan a sustainable utilization of the hydrogeological resource. Usually, conceptual modelling of a regional, deep hydrothermal system is puzzling due to its geological complexity or the lack of exhaustive data over a wide area. In this peculiar case, numerical modelling can be used to validate the conceptual model evaluating some aspects of the flow. The Euganean Geothermal System in Veneto Region (NE Italy) is the subject of still ongoing studies because the naturally emerging thermal water (temperature from 65°C to 86°C) represents a profitable resource for the touristic industry, impacting the regional economy. The thermal water is of meteoric origin, and infiltrates in Veneto Prealps to the north of the main geothermal area (i.e., Euganean Geothermal Field, EGF). The water flows for 100 km in the subsurface of the central Veneto heating up to 100°C by the crustal heat flux. The conduits hosting the fluid flow are identified in the damage zones of the Schio-Vicenza fault system (SVFS), while the outflow in the EGF is associated to an interaction zone between the SVFS faults. The stress concentration produces a localized high secondary permeability, maintaining the aperture of the fractures. This work aims to validate this conceptual model through 3D coupled flow and heat transport numerical simulations unravelling the impact of fracture networks on the regional-to-local groundwater circulation. A reconstruction of the geological setting, including the high permeable zones, is performed. The distribution of the hydraulic and thermal properties is reproduced using the equivalent porous medium approach and discrete elements. The modelled temperature in the EGF reservoir results higher than in the surrounding areas, according to the measured temperature distribution. This local increase is produced by the rising of deep-seated groundwater driven by the local fractures. The results validate the proposed conceptual model. In addition, they suggest that the temperature in the deeper part of the reservoir could be higher than the expected value, opening a new scenario for the utilization of the thermal resource.