<p>Groundwater temperature and age are crucial proxy data that play a fundamental role in understanding regional-scale groundwater flow systems and managing drinking and geothermal water resources. To investigate groundwater flow as well as heat and age mass transport processes in a complex hydrogeological system with deep carbonate sequences and adjoining sedimentary basins (DCSBs), numerical simulations were carried out in two-dimensional synthetic and two- and three-dimensional field-based conceptual environments. The simulations carried out for the Buda Thermal Karst (BTK), Hungary, revealed that the increasing asymmetry in the water table and the appearance of DCSB-type heterogeneity could affect the transition from advection-dominated to conduction- and diffusion-controlled transport processes in the models. However, simultaneously, both effects significantly influence the intensity of groundwater flow. Thermal buoyancy was superimposed on the water table-controlled forced convection (mixed convection), causing significant age mass accumulations in the closed convection cells. To quantify and track the changes in physical processes in the DCSB-type system, the simultaneous use of monitoring parameters calculated in the different parts of the model domain (e.g., unconfined vs confined), contours of groundwater age and temperature, and histograms of normalized groundwater age are presented. The numerical results from the preliminary three-dimensional model were compared to the ¹⁴C observation data in the BTK. The groundwater age calculated in the model was of the same order of magnitude as the results of ¹⁴C dating from samples taken at different depths in the unconfined and confined parts, and from the deeper mixing zone of the BTK.</p>