<p>Karst springs are essential freshwater resources in semi arid mountain regions, yet their recharge areas, connectivity, and vulnerability remain difficult to constrain in structurally complex settings. This uncertainty limits the ability to separate climate driven variability from structure-controlled flow pathways, which weakens both vulnerability assessment and management decisions. This study investigates the hydrogeological functioning of the transition zone between the High and Middle Atlas Mountains located in the upstream Oum Er Rbia Basin. Utilizing an integrated approach, we combined hydrodynamic time-series analysis with hydrochemical characterization and environmental isotopes (δ¹⁸O, δ²H, ³H) to assess aquifer recharge, connectivity, and vulnerability. Hydrodynamic analysis of spring discharge records from 2000 to 2012 reveals a stark contrast between inertial, storage-dominated systems and flashy, conduit-controlled overflows, a divergence governed largely by local structural compartmentalization. Geochemical data uniformly identify a Ca–Mg–HCO₃ facies derived from carbonate dissolution, with negligible saline influence from Triassic evaporites. Stable isotopes confirm exclusively meteoric recharge following a distinct altitude gradient, identifying a regional mixing zone that integrates convergent flows from both mountain domains. Crucially, a significant positive correlation between nitrate concentrations and tritium activity demonstrates that aquifer vulnerability is intrinsically linked to the rapid transmission of modern recharge. These findings underscore the necessity for differentiated management strategies that incorporate the specific hydrodynamic memory and transit times of these critical karst systems to ensure regional water security.</p>