The key to previous hydrogeological knowledge when determining the best solution in shallow geothermal systems. The case of a karstic aquifer in the city of Girona (Catalonia, NE Spain).

The European energy market must move quickly to achieve the decarbonization objectives of the economy in 2030-2050, including the domestic and tertiary sectors. In this context, the market for Surface Geothermal Energy (SGE) is growing rapidly due to its applicability in almost any geological and climatic conditions. The area of city of Girona (Catalonia, NE of Spain), is one of the urban areas in Catalonia where the SGE is progressing more rapidly, but is mainly focused on closed-loop (CL) geothermal heat pump systems despite the existing potential in the area for open-loop systems (OL). The urban area of Girona sits on Paleozoic rocks followed by Paleogene series with limestones, sandstones, and marls, configuring a Neogene basin full filled by continental alluvial deposits and Quaternary fluvial sediments, whose aquifers are being mainly used for water supply. Within the Paleogene series, there is the fractured and karstified Girona limestone aquifer (GLA). These materials outcrop in the upper part of the city and dip to the West, positioning themselves below the Neogene basin. The groundwater temperature in the karst aquifer ranges between 14ºC and 15ºC and the average annual temperature in the urban area of Girona is 14.7ºC. This contribution will present a case of a shallow geothermal installation for heating and cooling spaces for a private leisure building with a demand of 33 kWt whose initial solution was proposed with a classical CL system with about 7 BHE 100m depth. The initial project was carried out without conducting any prior hydrogeological assessment or conducting any preliminary drilling investigation to execute a TRT which could have been useful in verifying the issues that were later encountered. The implementation of the drilling campaign during works made it possible to verify the difficulty of drilling the fractured and karstic limestone aquifer and cementing its annulus for the installation of the geothermal single-U probes. The emergency solution taken directly on site - and indeed the most suitable, economical, and efficient solution considering the hydrogeological settings of the site- was to drastically change the chosen solution (CL) for an OL system and drill groundwater wells. Finally, a triplet of wells 20, 30, and 70m deep was executed (two injection and one production well) and a water well test was done and interpreted to acquire the corresponding permits. The case shows that having prior hydrogeological knowledge is essential when choosing the best solution for the client.