<p>Groundwater salinization severely constrains water quality in arid and semi-arid regions. This study investigates the origin and mechanisms of groundwater salinization in the Fahalian River Basin, southwestern Iran, focusing on interactions between karst and evaporite formations. The Hamzeh Beigi sub-basin (HBSB), a key contributor to salinization of the Fahalian River, was examined using hydrochemical, isotopic, and hydrogeological analyses. Over 13&#xa0;months, 162 water samples were collected from 15 freshwater and saline springs, streams, and lakes. Field parameters, including electrical conductivity (EC), pH, temperature, and discharge, were measured in situ. Major ions were analyzed using ion chromatography and titration, and stable isotopes (δ¹⁸O and δ²H) were measured using isotope ratio mass spectrometry (IRMS). X-ray diffraction (XRD) analysis of Gachsaran Formation samples was conducted to determine mineralogical contributions to salinity. Total dissolved solids (TDS) ranged from 180 to 108,000&#xa0;mg L⁻¹, classifying waters as fresh (Ca²⁺–HCO₃⁻), brackish (Ca²⁺–SO₄²⁻), and saline (Na⁺–Cl⁻). Isotopic signatures indicate a meteoric origin, excluding evaporation or deep brine intrusion as dominant factors. Hydrochemical tracers indicate that salinity is primarily derived from halite dissolution, with additional sulfate input from gypsum layers. Water balance results indicate that saline springs discharge water originating from the Asmari-Jahrum karst aquifer within the Anar Anticline. Intense fracturing and faulting enhance hydraulic connectivity between the karst aquifer and the overlying evaporites, enabling upward flow and localized halite dissolution. A conceptual groundwater flow model illustrates this interaction. The findings clarify salinity evolution in mixed karst–evaporite systems and support groundwater protection strategies in the Zagros region.</p>