Topothermohaline convection – from synthetic simulations to reveal processes in a thick geothermal system

Abstract

The water table topography, temperature, and solute content of groundwater all influence regional groundwater flow. Two-dimensional synthetic numerical calculations were performed to investigate the dynamic interaction between topography-driven forced convection and buoyancy-controlled free thermohaline convection. In the coupled topothermohaline model, the recharge and flow-through zones are dominated primarily by topography-driven regional groundwater flow, which drifts warm upwellings towards the discharge zone. Beneath the discharge zone, a dome with high temperature, salinity, and water age is formed in which time-dependent thermohaline convection develops. It was established that (1) increasing the water table gradient suppresses the thermohaline dome, resulting in a near-steady-state solution. (2) Increasing the bottom heat flux strengthens the warm upwellings, which ultimately leads to the break-up of the thermohaline dome, thus paradoxically reducing the average temperature. (3) Increasing the bottom salt concentration weakens the topography-driven groundwater flow, leading to the formation of a multi-layered thermohaline dome with extremely high temperature, salinity, and age. The operation of the topothermohaline model was demonstrated along a hydrogeological section crossing the Buda Thermal Karst (BTK) in Hungary. We found that the unconfined karstic areas are dominated by topography-driven water flow, while in the confined, deep reservoirs, thermohaline convection is the prevailing flow regime. The thermally and compositionally mixed water promotes karstification and reaches the surface near the Danube River, the main discharge area. In the eastern, confined areas of the BTK, significant amounts of heat may be retained on a geological timescale, making it a promising site for geothermal exploration.