<p>The Bolje Sestre karst spring has been supplying potable water to the entire Montenegrin Coast since 2010, representing one of the largest projects completed in the entire Mediterranean Basin over the last 50&#xa0;years. The considerable depletion of spring discharge that happened in the meantime required multidisciplinary research aimed at conceptualising and properly characterising this aquifer system. Research conducted in 2021–2022 has helped to understand the Bolje Sestre karst recharge–discharge mechanisms, estimate water budgets and quantify available groundwater resources, and created a platform to stabilise the capacity of the source. Bolje Sestre has a dual recharge mechanism: (1) “winter” recharge consisting of autogenic recharge occurring within the karst aquifer and partly allogenic recharge from the extended karst catchment during the wet season; and (2) “summer” recharge in the dry season when supplementary allogenic recharge from the adjacent alluvial aquifer of Morača River becomes active. The water budget analysis estimates that “winter” recharge results in karst spring discharge of 1.39–1.57 m³/s, while binary “summer” recharge (from autogenic karst and allogenic alluvium contributions) during summer months results in discharges ranging from 1.03 to 1.12 m³/s. It has been found that the main cause of the discharge depletion is the unsustainable alluvial sediment excavation from the Morača riverbed and banks, which disturbs the hydraulic connection and reduces the recharge to the composite aquifer system. Along with the stopping of excavation of alluvial sediments, some technical measures will be required to stabilise groundwater depletion and increase aquifer storage.</p>