Topothermohaline convection – from synthetic simulations to reveal processes in the thick geothermal system of the Buda Thermal Karst, Hungary

In real carbonate geothermal systems, the groundwater flow can be influenced by both forced convection driven by the water table topography, as well as free thermal and haline convection induced by buoyancy forces (topothermohaline convection). The interaction of different driving forces has an impact on reservoir parameters such as water temperature and salt content, the most accurate knowledge of which is extremely important for the planning, implementation, and operation of successful geothermal projects. In this study, (1) we performed a series of 2D synthetic simulations to quantify the role of each driving force in the topothermohaline convection system, and (2) we applied the model to the Buda Thermal Karst (BTK) system as a demonstration area (Galsa et al. 2025).

During the synthetic model study, the effect of the water table gradient, the bottom heat flux, and the bottom salt concentration were examined on the dynamics of the coupled system using control parameters. We found in the base model that the recharge area is controlled by cold, fresh, and young infiltrated waters driven by water table topography, while a thermohaline dome characterized by warm, saline, and old waters forms beneath the discharge area. By increasing the water table gradient, topography-driven forced convection becomes dominant, so that intense regional groundwater flow sweeps warm, saline and old water out of the model, leading to a purely advective, stationary solution. By increasing the bottom heat flux, thermal convection becomes prevailing, and thus, paradoxically, intense thermal convection effectively cools the model. By enhancing the salt concentration, the density of the water increases, and a multi-layered thermohaline dome forms beneath the discharge zone with extremely high temperature, salt content, and water age.

This methodology was applied along a 2D section crossing the Buda Thermal Karst system to examine the evolution of the geothermal reservoir. After 10 kyr, precipitation saturated the western, unconfined karst by cold, young and fresh water, while in the eastern, confined part of the deep reservoir, thermohaline convection dominated the groundwater flow. The boundary separating the two regions shifted slowly but steadily to the east, so after 1 Myr, the zone with high-temperature (>200 °C), old (>100 kyr) brackish water retreated to the eastern, deeper (>2–3 km) part of the BTK. Overall, the thermally and chemically mixed water moves westward directly beneath the clayey Oligocene aquitard, can promote karstification and reach the surface in the vicinity of the Danube River, the main discharge zone of the area, producing both cold and lukewarm springs.

By taking into account the interaction of various forces driving groundwater flow, we can obtain a more accurate picture of the temperature, salinity, and water age distribution of the reservoir, which facilitates the precise design and efficient and sustainable operation of deep geothermal systems, minimizing the vulnerability of water resources.

 

References

Galsa, A., M. Szijártó, Á. Tóth, J. Mádl-Szőnyi, Topothermohaline convection – from synthetic simulations to reveal processes in a thick geothermal system, Hydrology and Earth System Sciences, 29/17, 4281–4305, 2025.