Topusko is the second warmest natural thermal water spring area in Croatia, located at the southwest edge of the Pannonian Basin System. Due to favourable geothermal properties, these waters have been used for heating and health and recreational tourism since the 1980s. Thermal springs with temperatures up to 50 °C are the final part of an intermediate-scale hydrothermal system. However, systematic research on the Topusko spring area has not been conducted to lay the foundation for sustainable resource utilisation. Multidisciplinary research including the hydrogeochemical characterisation of naturally emerging thermal water, an electrical resistivity tomography (ERT) investigation conducted to reconstruct the subsurface geology, and hydrogeological parametrisation of the geothermal aquifer was carried out to refine the existing local conceptual model. The results show Ca-HCO₃ facies of Topusko thermal waters, which get heated in a Mesozoic carbonate aquifer. The water equilibrium temperature in the geothermal aquifer is estimated to be 78 °C based on the SiO₂-quartz geothermometer. The fault damage zone, which enables the upwelling of thermal water, was identified by ERT investigations. The transmissivity values of the aquifer derived from the results of step-drawdown tests range from 1.8 × 10⁻² to 2.3 × 10⁻² m²/s. Further multidisciplinary research is necessary to improve the existing conceptual model of the Topusko hydrothermal system.

The sustainable utilization of geothermal energy mostly depends on the characteristics of the geothermal resource from which it is extracted. Among others, detailed geological modeling is a key factor for estimating the potential of a geothermal resource. This research focuses on the modeling and reconstruction of the geological setting of the Daruvar thermal spring area using geophysical techniques. An integrated geophysical approach based on electrical resistivity tomography (ERT) and both active and passive seismic (MASW and HVSR) methods was used. Based on ERT results and the stratigraphic logs of the wells in Daruvar, three resistivity layers/geological units were identified. The deepest layer with resistivity < 150 Ωm is the Triassic carbonate that constitutes the thermal aquifer. Sharp lateral variations in the resistivity distributions within the bedrock were interpreted as fault damage zones saturated with thermal waters. Integrating the results of the seismic methods, the thickness of the first seismic layer that corresponds to the Quaternary cover was estimated from 5 to 20 m. Here, results of the geophysical investigations were combined into a 3D geological model highlighting the occurrence of subvertical N-S and E-W trending faults in the Daruvar spring area. The N-S-trending fault was interpreted as a fault plane parallel to the regionally mapped Daruvar fault. This fault juxtaposes the Triassic carbonate complex of the thermal aquifer with a Neogene sedimentary sequence of significantly lower permeability. Neogene–Quaternary tectonic activity further increased the fracturing and the permeability field in the Daruvar spring area, as proven by the smaller scale E-W faults and the well logs. This fracture network permits a quick upwelling of thermal fluids resulting in thermal springs with temperatures up to 50 °C. This work proves that the construction of a detailed geological model is crucial for assessing the reservoir and fault geometries in thermal systems hosted in fractured carbonate rocks.

A hydrochemical and geothermometric study of thermal waters in Topusko (Croatia) was conducted in order to improve the existing conceptual model of the Topusko hydrothermal system. The town of Topusko is located 80 km south of Zagreb at the SW edge of the Pannonian Basin System, which has favourable geothermal characteristics. The natural thermal springs of Topusko with temperatures up to 53°C and thermal waters from shallow wells with temperatures up to 65°C represent the second warmest natural thermal water source in Croatia. Water samples have been collected monthly since March 2021 from two natural thermal springs, i.e. Livadski izvor and Blatne kupelji springs, and the TEB-4 thermal well in the discharge area of the hydrothermal system. Furthermore, rainwater was sampled in the supposed recharge area. Chemical composition of groundwater is determined by the original composition of the infiltrated water as well as chemical reactions with various rocks along its circulation path. In-situ parameters, major anions and cations, stable water isotopes, radioactive isotope analysis of tritium (3H), and silica geothermometers were used to assess the origin of thermal waters in Topusko and their interaction with the thermal aquifer. Furthermore, a local meteoric water line was constructed and compared with the isotope ratio of the thermal waters. The results pointed to: i) the meteoric origin of thermal waters according to stable water isotopes; ii) Ca-HCO3 hydrochemical facies suggesting that carbonate dissolution occurs in the aquifer, being consistent with the presence of carbonates in the stratigraphic logs of wells; iii) an equilibrium temperature in the reservoir of 88°C according to silica geothermometers; and iv) low tritium activity being consistent with sub-modern waters and a recharge before 1955. The monitoring of these thermal springs will continue for another year to collect a larger dataset and reinforce our conclusions. Additional analyses including radiocarbon dating and stable isotope composition from SO42- will be conducted to provide a comprehensive hydrochemical characterisation of these thermal waters, which have been utilised since ancient Roman times, and intensively since 1980s, but have not been thoroughly investigated.Acknowledgments: Presented research has been conducted in the scope of the project “Multidisciplinary approach to hydrothermal system modelling” (HyTheC) funded by the Croatian Science Foundation under grant number UIP-2019-04-1218.

Thermal imaging was utilized across three hydrothermal systems in Croatia for preliminary screening of low-temperature surface hydrothermal manifestations, particularly subthermal springs (13–20 °C). This study assessed the effectiveness of this remote sensing method for initial geothermal surveys and monitoring of surface geothermal anomalies. The methodology integrated GIS tools (Esri, Redlands, CA, USA) to identify potential zones of subthermal water outflow using thermal orthomosaics derived from the photogrammetric reconstruction of thermal infrared images. Thermal images were acquired using a fixed-wing eBee Plus RTK unmanned aerial vehicle (UAV) (senseFly Ltd., Cheseaux-sur-Lausanne, Switzerland) equipped with a senseFly thermoMap camera (senseFly Ltd., Cheseaux-sur-Lausanne, Switzerland) and processed using Pix4Dmapper software. Due to COVID-19 restrictions, the intended methodology was simplified by discarding temperature calibration. Temperature calibration of thermal images was performed only for a few smaller areas to address discrepancies between measured ground temperatures and temperature values depicted in the maps. The methodology was validated near the Daruvar hydrothermal system, where a thermal anomaly corresponding to the subthermal spring location was visually detected, demonstrating the applicability of the method for surface investigation of low-temperature geothermal systems. No new subthermal or thermal springs were discovered. In total, 45.35 km² was surveyed over 9 days, making this a viable and low-cost methodology for preliminary exploration. However, limitations, including the narrow time window for thermal data acquisition, regulatory requirements for drone flights, and subjectivity of the methods used, were identified.

The increase in thermal water utilisation is foreseen by many European and Croatian strategic documents regulating energetics, tourism, environmental protection and sustainable development. Croatian Geological Survey wishes to establish a multidisciplinary group for hydrothermal systems research which will contribute to responsible geothermal development in our country through a 5-year research project HyTheC which started in 2020.Pannonian part of Croatia has favourable geothermal characteristics and natural thermal water springs emerge at two dozen localities, with temperatures up to 65 °C. These waters have been used for millennia, and in the past fifty years they are a basis for the development of tourism and health care centres which use the thermal water resource for heating, therapy and recreation (Borović & Marković, 2015). As their water demand increased, higher quantities were abstracted and additional intake structures and wells were constructed.Thermal springs are part of hydrothermal systems which include: recharge areas in the mountainous hinterlands of the springs; geothermal aquifers - in Croatia mostly fractured and karstified Mesozoic carbonate rocks (Borović et al., 2016) - in which water resides and gets heated due to heat flow from the Earth; and discharge areas in places with favourable structural characteristics of higher permeability. The continuous functioning of such systems depends on a delicate balance between groundwater flow velocities, precipitation/dissolution processes and structural framework.In order to maintain that balance and use thermal water resources in a sustainable manner, a system-level understanding is required. Multidisciplinary methodology (structural geology, hydrogeology, geothermal, hydrogeochemical and geophysical research and remote sensing) will be used to construct conceptual models of systems, perform 3D geological modelling, hydrogeological and thermal parametrisation of the geological units involved in the thermal fluid flow, and conduct numerical simulations of system functioning in undisturbed conditions and with different extraction scenarios.This methodology will be tested in three pilot areas in Croatia where thermal water is being utilized (Daruvar, Hrvatsko zagorje and Topusko). These three areas have significantly different levels of initial data availability and it shall therefore be determined which methodology and order of application of different methods should be applied while researching the systems with considerable existing data, medium amount of data and very scarce data, respectively.Keywords : hydrothermal system, natural thermal spring, multidisciplinary research, CroatiaReferencesBorović, S. & Marković, T. 2015 : Utilization and tourism valorisation of geothermal waters in Croatia. Renewable and Sustainable Energy Reviews, 44, pp. 52-63.Borović, S., Marković, T., Larva, O., Brkić, Ž. & Mraz, V. 2016 : Mineral and Thermal Waters in the Croatian Part of the Pannonian Basin. U: Papić, P., ur., Mineral and Thermal Waters of Southeastern Europe. Cham: Springer, pp. 31-45. ACKNOWLEDGMENTThe Installation Research project HyTheC (UIP-2019-04-1218) is funded by the Croatian Science Foundation.