Based on field observations and previous numerical simulations, combined effect of external forces (e.g. water table elevation) and buoyancy forces can cause a complex groundwater flow pattern characterized by mixed thermal convection, not only in siliciclastic sedimentary basins, but also in adjoining karstified deep carbonates.
The interaction of different driving forces was examined along a two-dimensional west-east geological section across Buda Hills (Rózsadomb) to Gödöllő Hills. On the course of this study three simulation scenarios were systematically examined in the last evolutionary stage of Buda Thermal Karst: (a) a purely topography-driven steady-state groundwater flow, (b) a topography-driven steady-state flow system with forced thermal convection, and (c) a time-dependent flow system with mixed thermal convection. The numerical model was verified using the results of basin-scale hydraulic evaluation (observed pressure elevation profiles, tomographic potentiometric maps) of Mádl-Szőnyi ($2019$) complemented by estimated recharge rates, and calculated heat fluxes. The effect of different flow and thermal boundary conditions was systematically tested during the simulations.
Effect of thermal convection was studied in order to examine its influence on the temperature distribution and on the groundwater flow pattern. It was established that existence of thermal convection increases the heat flux compared to the conductive model in agreement with values of the Nusselt number from $1.5$ to $5$ in model (b) and (c), respectively. The pure advective heat transfer due to forced convection causes a large hot upwelling with a surface temperature of $60–80$ °C beneath the regional discharge area in agreement with the temperature depth profiles and appearances of thermal springs. However, the effect of time-dependent free thermal convection also facilitates small hot upwellings in the unconfined karstified carbonate system which might elucidate the unexplained heat anomalies in temperature maps and profiles. In addition, the effect of free thermal convection increases the value of the monitoring parameters in the models (e.g. Darcy flux, temperature and hydraulic head).
These simulations draw attention to the importance of different driving forces of groundwater flow, especially at the margin of unconfined and confined carbonate sequences, such as in Buda Thermal Karst. Thorough comparison of the numerical results and the observation data could improve understanding the interaction caused by mixed thermal convection.
This research is a part of a project that has received funding from the European Union’s Horizon $2020$ research and innovation program under grant agreement No $810980$. The project was supported by the Hungarian Scientific Research Fund (K $129279$) and by the János Bolyai Research Scholarship of the Hungarian Academy of Sciences.
Mádl-Szőnyi J. ($2019$) Pattern of groundwater flow at the boundary of unconfined and confined carbonate systems on the example of Buda Thermal Karst and its surroundings. DSc thesis. p. $150$. (in Hungarian)