Advancing Sustainable Water Resource Management Through a Hydrogeological Conceptual Model of a Complex Multi-Aquifer System: A Case Study from the Spree River Basin

Groundwater is the primary water source for urban and industrial use in the lower Spree catchment, southeast of Berlin, Germany, and supports critical groundwater-dependent ecosystems, including lakes, wetlands, streams, and springs. Recent droughts and groundwater overexploitation have caused declines in groundwater levels, lake water levels, and river flows. The region's complex hydrogeology, coupled with extensive human interventions in the hydrological cycle, highlights the pressing need for a comprehensive understanding of groundwater dynamics and aquifer-surface water interactions to ensure effective and sustainable water resource management. This study develops a high-resolution hydrogeological conceptual model for a complex multi-aquifer system, providing insights into groundwater recharge, flow mechanisms, and lake-aquifer interactions. A 3D geological model was constructed to represent aquifer lithology and structural heterogeneity, forming the foundation of the conceptual framework. Hydrogeological, hydrochemical, and isotopic analyses—employing tracers such as Tritium, Oxygen-18, and deuterium—revealed groundwater flow paths, recharge sources, and the aquifers connectivity. The study highlights dynamic lake-aquifer interactions, driven by meteorology, hydrogeological conditions, and human activities such as groundwater abstraction. A comparative water balance study between the two decades revealed significant variations driven by both natural and anthropogenic factors, with minimal groundwater level drawdown during 2000–2009, compared to noticeable drawdown during 2010–2019. During the second decade, groundwater extraction increased by an additional 20.7 million cubic meters (MCM), rising from 196 MCM to 216.7 MCM. At the same time, aquifer recharge decreased by 67.8 MCM, dropping from 643.4 MCM to 575.6 MCM. This imbalance underscores the urgent need for sustainable groundwater management. The conceptual model revealed confined conditions in large parts of the aquifer system due to fine glacial sediments, making subsurface Managed Aquifer Recharge (MAR) methods such as Aquifer Storage and Recovery (ASR) and Aquifer Storage Transfer and Recovery (ASTR) essential. Based on evaluation criteria, including the presence of a high-yield deep aquifer, continuous aquifer thickness (i.e., absence of clay lenses), proximity to the source water, and distance from existing extraction wells, potential MAR sites were identified. Moreover, using historical streamflow data and thresholds such as hydro-ecological limits, median daily flow, channel maintenance flows, the available surface water for injection into MAR projects was estimated for six locations across the study area. The yearly available surface water for MAR was found to range between 0.5 and 3.2 MCM per location, with a total of 7.4 MCM across all sites. These findings provide critical insights for sustainable groundwater management, ensuring water supply security, and protecting ecosystems in the Berlin-Brandenburg region.