No abstract

In the Schleswig-Holstein area, salty groundwaters are not always spatially related to the presence of shallow salt structures. Furthermore, hydrogeochemical 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 faults, Pleistocene 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 topographydriven 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 units in which salt dissolution is not the dominant process.The presented study provides new insights into fluid flow processes at basin scale. The described flows could develop in any geothermal basin hosting salt diapirs which pierce shallow aquifer systems.

No abstract

In the Schleswig-Holstein region (S-H) of Germany, most observed near-surface saline ground waters originate from dissolution of shallow salt domes. Previous numerical simulations of thermohaline flow clarified the major mechanisms controlling large-scale density-driven flow. It has been found that, in addition to topographically driven flow, gravitational and thermohaline convection are the primary mechanisms for extensive solute exchange between shallow and deep aquifers. Geological features such as glacial channels control recharge/discharge processes at the surface. Here we address several previously unresolved issues: (1) the impact of a permeable unit (transition zone) between the salt and adjacent units; (2) the role of variable brine viscosity in affecting regional-(i.e., km-) scale heat and mass patterns; and (3) the influence of anthropogenic activities such as pumping stations on density-driven flow. We found that geophysical factors play a major role in 2 determining the dynamics of fluid processes. The transition zone significantly influences the flow field and the distribution of heat, slowing the formation of highly concentrated salty plumes. The impact of variable fluid viscosity on the coupled heat and brine flow is twofold.