Multidisciplinary Approach to Conceptual Modelling of Hydrothermal Systems in Croatia (HyTheC)

The Pannonian part of Croatia has favorable geothermal characteristics that promote the development of natural thermal springs (temperatures up to 65 °C) in around 20 localities. These waters have been used for millennia and are the basis for tourism and health care centers. Due to an increase in the water demand, higher quantities were abstracted threatening the availability of the thermal resources. These thermal springs are generally part of intermediate scale, topographically driven, hydrothermal systems fed by local meteoric waters with: i) recharge areas in the mountainous hinterlands of the springs, ii) Mesozoic carbonate geothermal aquifers with high secondary porosity and permeability, and iii) discharge areas with favorable structural settings increasing the bedrock fracturing and the permeability field.

The continuous functioning of such systems depends on a delicate balance between: i) infiltration, ii) groundwater flow velocities and precipitation/dissolution processes in the reservoir, iii) active tectonics maintaining the permeability field, iv) heat flow from the deeper parts of the crust, and v) natural outflow of thermal waters and/or their exploitation. In order to maintain this balance and use thermal water resources in a sustainable manner, a system-level understanding is needed. The HyTheC project promotes a multidisciplinary approach (structural geological, hydrogeological, geothermal, hydrogeochemical, geophysical, and remote sensing investigations) to: i) propose a conceptual model of the system, ii) perform a 3D geological model of the study area, iii) conduct hydrogeological and thermal parametrizations of the main hydrostratigraphic units, and iv) perform numerical simulations of the system functioning in undisturbed natural conditions and with different extraction scenarios. The methodology was tested in three pilot areas in Croatia where thermal water is used, but the levels of knowledge on the systems were quite different.

The Daruvar hydrothermal system was the most investigated. The investigations were conducted to detail the regional and local structural settings promoting the thermal water flow and to quantify the impact of geological and physico-chemical processes on the development of the system through 3D numerical simulations. The Topusko hydrothermal system was poorly investigated, and even the recharge area was not determined. The multidisciplinary approach was applied for detailed hydrochemical and hydrogeological characterizations and for the physical validation of the proposed conceptual model through 2D numerical simulations. The Hrvatsko zagorje hydrothermal system is the largest one, with a diffuse outflow of thermal water in several thermal spring areas. Since previous investigations were available, a 3D regional geological reconstruction of the study area was conducted, while numerical simulations of fluid flow and heat transport are under development.

The increase in thermal water utilization is foreseen by many European and Croatian strategic documents regulating energetics, tourism, and environmental protection. The results of the HyTheC project highlight the importance of conducting multidisciplinary investigations for the characterization of hydrothermal systems. The obtained results will serve to protect these economically and culturally important sites preserving them for future generations.

Acknowledgment: This research was funded by the HyTheC project of the Croatian Science Foundation, grant number UIP-2019-04-1218.