Upwelling mechanisms of deep saline waters via Quaternary erosion windows considering varying hydrogeological boundary conditions

Abstract

Abstract. Intrusion of deep saline waters into freshwater aquifers does not only endanger the regional drinking water supply, but also rivers and stagnant waters and their fauna are threatened by salinisation. The upwelling of highly mineralised saline waters in large parts of the North German Basin is favoured by the presence of Elsterian glacial erosion windows in the Lower Oligocene Rupelian Clay, the most important hydraulic confining unit in this region. Lower precipitation rates and decreasing groundwater levels as a consequence of global climate change, but also anthropogenic interventions, such as increasing extraction rates or the use of the deep geologic subsurface as a reservoir, decrease the pressure potential in the freshwater column and may possibly accelerate this primarily geogenic salinisation process in the coming years. Density-driven flow and transport modelling was performed in the scope of the present study to investigate the upwelling mechanisms of deep saline waters across Quaternary window sediments in the Rupelian. Simulation results show that the interactions between the groundwater recharge rate and anthropogenic interventions such as extraction rates of drinking water wells or the utilisation of the deep subsurface, have a significant influence on the groundwater pressure potential in the freshwater aquifer and associated saltwater upwelling. In all scenarios, salinisation is most severe in the sediments of the erosion windows. Hydraulically conductive faults also intensify salinisation if located nearside erosion windows or induce a more distributed or localised salinisation in aquifers with drinking water relevance in areas that do not intersect with erosion windows. A decline in groundwater recharge thereby significantly favours upward saltwater migration. The simulation scenarios further show that a decrease in groundwater recharge also results in freshwater salinisation occurring up to 10 years earlier, which underlines the need for waterworks to initiate effective countermeasures quickly and in time.

Modeling the influence of climate on groundwater flow and heat regime in Brandenburg (Germany)

“…Areas where the hydraulic gradient forces upward groundwater fluxes can provide flow paths for outflow of deep Cl − -rich groundwater via hydrogeological windows thereby posing a potential risk of salinization to the drinking-water aquifers. Initial evidence from brackish springs and lakes above the shallow-buried salt domes has been verified by groundwater sampling (Rößling et al, 2010) and modeling (Kaiser et al, 2013;Chabab et al, 2022). At the typical depths of this aquifer complex (>500 m), we did not observe any influence of recharge variability on the groundwater dynamics during the simulation period.…”

“…In our study, we have approximated this complex architecture of fluvio-glacial deposits characterized by mixed lithologies and abundant discontinuities via unitaveraged effective rock properties, typical for poorly consolidated sand. Recent efforts have been focused on providing a more accurate parameterization of these aquifers, either through higherresolution structural models (Frick et al, 2019;Chabab et al, 2022) or via stochastic parameter distribution (Truong et al, 2023).The incremental level of heterogeneity in the shallow geology is expected to locally decelerate or enhance surface-togroundwater interactions.…”