Sudden groundwater inflow into open‑pit mines can create significant technical, safety, and economic challenges, especially in mining areas where comprehensive hydrogeological investigations have not been conducted prior to development. The Mehdiabad Pb–Zn mine, one of the largest metallic deposits in Iran, has recently experienced groundwater inflow through the pit floor, despite the absence of detailed prior groundwater studies in the region. This study aims to determine the origin, flow pathways, and hydraulic connectivity of groundwater entering the open-pit by using an integrated approach that combines groundwater‑potential mapping, hydrochemical analyses, and stable‑isotope investigations. Groundwater potential zoning in karstified carbonate formations was performed by integrating seven controlling factors—lithology, annual precipitation, distance to faults, lineament density, vegetation cover, elevation, and slope—within a GIS environment using the Analytical Hierarchy Process (AHP). The results indicate that carbonate formations intersected by major structural features exhibit the highest groundwater potential and spatially coincide with the location of the open pit. Hydrochemical analyses reveal a dominant Ca–Mg–HCO₃ water type, indicating that carbonate dissolution is the primary process governing groundwater chemistry. Saturation indices calculated using PHREEQC show that the waters are at equilibrium to supersaturated with respect to calcite and dolomite. Stable‑isotope compositions (δ¹â¸O and δ²H) suggest a modern meteoric origin and demonstrate strong hydraulic connectivity between the pit water and the surrounding groundwater system. Integrating spatial, hydrochemical, and isotopic evidence supports the development of a conceptual hydrogeological model in which a fault‑controlled karst system directs groundwater flow toward the open pit. The findings provide a solid scientific basis for groundwater management and inflow‑control strategies in the Mehdiabad mine and other large open‑pit mines situated in karst environments.