<p>The present study investigated the hydrogeochemical characteristics and spatial variability of subsurface water quality in the Pairi River Basin (≈1305.82&#xa0;km²), Chhattisgarh, India. A total of 79 groundwater samples were collected during the premonsoon phase (May–June 2024) and analyzed for 13 physicochemical and 5 heavy metal parameters following standard protocols. The suitability of groundwater for drinking purposes was evaluated using the water quality index (WQI) and heavy metal pollution index (HMPI). The results revealed that carbonate‒silicate rock weathering, ionic action, and groundwater residence time regulate mineralization, resulting in predominant Ca²⁺–Mg²⁺–Cl^––SO₄^2– and mixed ionic facies. The strong association between Ca²⁺–NO₃^––Cd–Cr ions resulted from carbonate dissolution and metal mobilization coupled with unsustainable agricultural practices, whereas elevated Mn and Fe levels indicated redox–controlled geogenic release. The WQI classified 49.4% of the samples in the poor to unsuitable category, whereas the HMPI identified 35.4% of the sampling stations as unsuitable for human consumption. This study highlights significant spatial heterogeneity in groundwater quality and associated health risks in vulnerable tribal–dominated terrains of the Pairi River Basin.</p>