ELUCIDATING COMPLEX groundwater CIRCULATION in KARST SYSTEMs using ENVIRONMENTAL ISOTOPIC TRACERS: Case of the middle atlas, morocco

The Middle Atlas in Morocco serves as a critical recharge zone for several major river basins, yet its groundwater systems remain poorly constrained at the regional scale. This study presents an integrated hydrogeological assessment based on major-ion chemistry, stable isotopes (δ²H, δ¹⁸O), tritium (³H), and radiocarbon (¹⁴C) from 60 springs and 8 surface waters across the Sebou, Oum Er-Rbia, and Moulouya basins. Results show that groundwater is dominantly of meteoric origin, with minimal evaporative influence, and largely composed of Ca–HCO₃ and Ca–Mg–HCO₃ types. A new local meteoric water line (δ²H = 7.99 δ¹⁸O + 14.56) and δ¹⁸O–altitude gradient (–0.23‰ per100 m) allow estimation of recharge elevations ranging from 700 m to 2,490 m. Tritium and ¹⁴C data distinguish fast-flowing conduit systems with modern water from confined compartments with older, mixed groundwater. Elevated salinity in some springs is attributed to subsurface dissolution of Triassic evaporites rather than surface evaporation. Structural features, particularly fault zones like Tizi N’Tretten, control both vertical circulation and cross-basin flow. These findings provide the first massif-scale isotopic and geochemical baseline for the region can be extended to other karst systems in arid and semi-arid environments to resolve recharge dynamics, flow architecture, and inter-basin connectivity.