Salty groundwater flow in the shallow and deep aquifer systems of the Schleswig–Holstein area (North German Basin)

Abstract In the Schleswig–Holstein area, salty groundwaters are not always spatially related to the presence of shallow sat structures. Furthermore, hydrochemical data point to instable salinity profiles and to the occurrences of deep brines close to the surface. Therefore, complex interaction between shallow and deep solute migration must occur. Numerical simulation of fluid flow, mass and heat transport have been carried out in order to understand the role of shallow salt dissolution and young geological features on groundwater flow as well as to investigate the interrelationships between shallow and deep aquifer systems. For these purposes, a shallow (− 500 m) and a deep profile model (− 5 km) have been constructed. The results indicate that different flow regimes coexist within the study area. Shallow brine migration is strongly controlled by the geological features of the basin such as salt flank, glacial channels and sand layers. Furthermore, shallow salt dissolution is the major cause for gravitational convection in the deeper aquifers. This source of salinization from above leads to the formation of instable salinity profiles at 2 km depth. Further interactions between shallow and deep fluid flow exist in the eastern part of the basin where brine upconing is due to both topography-driven flow and thermohaline convection. The simulations also showed that the hydraulic conductivity of the stratigraphic units influences the brine regime on a regional-scale. Gravitational convection is likely to occur in permeable units located near shallow salt structures, while thermohaline convection persists within thick permeable areas in which salt dissolution is not the dominant process. The presented study provides new insights into fluid basin processes. The described flows could develop in any geothermal basin hosting salt diapirs which pierce shallow aquifer systems.