The Hasouna Wellfields of the Great Man-Made River Project (GMRP) in Libya constitute an important source of groundwater from the Cambro-Ordovician Hasouna aquifer. This study investigates the groundwater chemistry of the Hasouna Wellfields based on field measurements and laboratory analyses from 484 wells. The main objectives are to characterize the physicochemical properties and major dissolved constituents of the groundwater, evaluate its suitability for drinking and irrigation purposes, and assess the potential implications of groundwater chemistry for wells, pumping equipment, and water-conveyance infrastructure. The results indicate that the groundwater is generally neutral to slightly alkaline, with a mean pH of 7.17. The mean electrical conductivity (EC) is 1609.08 µS/cm, while the mean total dissolved solids (TDS) concentration is 1039.42 mg/L. The groundwater is hard to very hard, with a mean total hardness of 400.79 mg/L as CaCO₃ and a mean total alkalinity of 139.81 mg/L as CaCO₃. The major cations include Ca²⁺, Mg²⁺, Na⁺, and K⁺, with mean concentrations of 114.57, 28.33, 188.27, and 8.44 mg/L, respectively. The principal anions include HCO₃⁻, CO₃²⁻, Cl⁻, SO₄²⁻, and NO₃⁻, with mean concentrations of 169.17, 0.30, 297.35, 210.19, and 53.59 mg/L, respectively. The results indicate relatively high mineralization, particularly reflected by TDS, EC, chloride, and sulphate concentrations. The mean nitrate concentration is also an important water-quality parameter that requires continued monitoring. Irrigation assessment indicates a significant salinity hazard, whereas the sodium-related indices indicate a comparatively lower sodium hazard. The calculated saturation and corrosion indices indicate that the groundwater is slightly undersaturated with respect to calcium carbonate and has potential corrosive characteristics. Overall, the findings highlight the importance of continued groundwater-quality monitoring and appropriate management of salinity, nitrate, calcium-carbonate saturation, and corrosion potential in the Hasouna Wellfields and the associated Great Man-Made River water-conveyance system......................Keywords: ........Groundwater; Hasouna Wellfields; Hasouna Aquifer; Great Man-Made River; Hydrochemistry; Groundwater Quality; Salinity; Irrigation Water; Corrosion.