Climate-driven groundwater recharge decline as a potential accelerant of geogenic salinisation in Brandenburg: A regional-scale 3D hydrogeological modelling study

Geogenic salinisation threatens groundwater in the German State of Brandenburg, where 25% of drinking water aquifers are already affected. The source are deep-seated brines originating from Upper Permian (Zechstein) salt dissolution, migrating upward via structural flow paths. The Oligocene Rupelian Clay isolates freshwater aquifers, but Quaternary glacial erosion created localised "windows" filled with permeable sands, enabling saline water ascent. Climate change drives declining groundwater recharge (GWR) in Brandenburg, projected to worsen and, coupled with extraction, increasingly compromise the freshwater-saline interface. Understanding the interplay between erosion windows, reduced recharge, and extraction is paramount for sustainable water management in this climate-vulnerable region.

A high-resolution 3D density-dependent flow and transport model is developed and employed using the Geomodelator-GUI [1] and TRANSPORTSE software [2] to investigate salinisation mechanisms in Brandenburg’s Lower Spree catchment area. The 3D framework integrates detailed hydrostratigraphy, capturing complex window geometry and anisotropic flow, assessing preferential pathways and clay barrier efficacy. Simulations assess four 100-year scenarios: (i) a Zero-Extraction Baseline, ZE, (ii) a Constant Recharge Baseline, CR, (iii) a Uniform Recharge Decline, UR, (linear 42% GWR reduction by 2050), and (iv) a Differential Recharge Decline, DR (spatially variable reductions: -20% to -60% in recharge/depletion zones). The model incorporates seven waterworks and utilises strata-specific porosity and hydraulic conductivity parameters derived from regional studies. Maximum salt concentrations are 10 g/L below the Rupelian.

Results demonstrate increased salinisation from upwelling under reduced recharge and extraction, particularly in deeper Tertiary aquifers and Quaternary window sediments itself. UR causes the highest intrusion: salt concentrations increase by 17% (9.6 mg/L) in the erosion windows. DR reduces intrusion by maximum 38% vs. UR at 100 years, but deep aquifers remain critically vulnerable. Shallow aquifers show minor changes (from an initial 0.1 mg/L to 0.17 mg/L), indicating salinisation predominantly affects deeper aquifers. Critically, even constant recharge with extraction drives salinisation, proving that groundwater extraction over long time periods is a decisive factor that is exacerbated by GWR decline.

3D model outcomes have elucidated critical processes inaccessible to 2D approaches [3] and provide an essential scientific foundation for proactive water resource management in Brandenburg and analogous basins. Results will directly support strategies to mitigate salinisation risks, such as adjusting sustainable extraction rates under future climate scenarios, and prioritising areas for enhanced monitoring or managed aquifer recharge. As subsurface utilisation for energy storage increases, this work also offers insights for safeguarding freshwater resources from potential deep brine mobilisation. Ultimately, the study underscores the urgency of integrating climate adaptation and detailed subsurface characterisation into groundwater governance to secure freshwater supplies in the face of escalating geogenic and anthropogenic pressures.

References:

[1] Kempka, T. et al. (2026, in review): GEOMODELATOR-GUI: A Web-Based Graphical User Interface for 3D Geological Modeling. SoftwareX.

[2] Kempka, T. (2020): Verification of a Python-based TRANsport Simulation Environment for density-driven fluid flow and coupled transport of heat and chemical species, doi: 10.5194/adgeo-54-67-2020.

[3] Chabab, E. et al. (2022): Upwelling mechanisms of deep saline waters via Quaternary erosion windows considering varying hydrogeological boundary conditions, doi: 10.5194/adgeo-58-47-2022.