Numerical investigation of the combined effect of forced and free thermal convection in synthetic groundwater basins

The theoretical examination of the combined effect of water table configuration and heat transfer is relevant to improve understanding of deep groundwater systems, not only in siliciclastic sedimentary basins, but also in fractured rocks or karstified carbonates. Numerical model calculations have been carried out to investigate the interaction of topography-driven forced and buoyancy-driven free thermal convection in a synthetic, two-dimensional model. Effects of numerous model parameters were systematically studied in order to examine their influence on the Darcy flux, the temperature and the hydraulic head field. It was established that higher geothermal gradients and greater model depths facilitate the evolution of time-dependent free thermal convection in agreement with changes of the thermal Rayleigh number and the modified Péclet number. However, increasing water table slope and anisotropy coefficient favor the formation of stationary forced thermal convection. Free thermal convection mainly affects the deeper part of the midline and the discharge zone of the synthetic model. In the examined model basins, the position of the maximum hydraulic head is located within the bottom thermal boundary layer near the recharge zone. This divergent stagnation point underlies a local downwelling zone characterized by underpressure. These simulations draw attention to the importance of understanding the combined effect of forced and free thermal convection in sedimentary basins regarding regional groundwater flow patterns, and temperature distributions.