This study evaluated the hydrogeochemical characteristics and drinking-water quality of the Sarakhs plain aquifer. Data from 292 wells were analyzed for physicochemical parameters (pH, EC, TDS, TH) and major ion concentrations (Ca²⁺, Mg²⁺, Na⁺, K⁺, CO₃²⁻, HCO₃⁻, SO₄²⁻, Cl⁻). Saturation indices were computed using PHREEQC, and a weighted-arithmetic water quality index (WQI) was applied to assess suitability. Results showed slightly alkaline conditions (pH: 6.4–8.6, mean 7.97). EC values ranged from 1020 to 37,500 μS cm⁻¹ (mean 3870 μS cm⁻¹), indicating high salinity. Sodium and chloride dominated the ionic composition, with the Piper diagram identifying mixed Na⁺–Cl⁻ and Na⁺–Mg²⁺–Ca²⁺–Cl⁻ facies. WQI analysis revealed that fewer than 12% of samples were good-to-excellent, while over 76% were poor-to-very poor. Spatial mapping showed better quality in the eastern plain near the Harirud River, highlighting its role in recharge and salinity dilution. Unlike previous studies that focused solely on quantitative aspects or individual water quality parameters, this study provides the comprehensive hydrogeochemical assessment of the Sarakhs plain aquifer integrating spatial analysis, saturation indices, and drinking water quality indexing across. Overall, groundwater quality in the Sarakhs plain is critically poor for drinking purposes, with serious constraints on its use. However, the Harirud River's recharge capacity offers a strategic opportunity for sustainable water management in the region. These findings underscore the urgent need for protective measures and informed resource planning.