Basin-scale organization of groundwater and surface water during the monsoon season: Hydrochemical and isotopic insights from the Brahmaputra River Basin, Northeast India

Study region The Indian sector of the Brahmaputra River Basin (BRB). Study focus We co-evaluated groundwater (GW) and river water (RW) across the Indian BRB using a synoptic monsoon-season survey (July 2022; n = 83; 36 GW, 47 RW). Hydrochemistry, stable isotopes (δ²H, δ¹⁸O), water-quality indices, and principal component analysis (PCA) were interpreted within a geomorphic–hydrogeological framework to resolve solute-acquisition processes, water-quality controls, and setting-dependent GW–RW relationships. New hydrological insights for the region GW–RW organization varied systematically with geomorphic setting and subsurface storage. Monsoon-season RW was comparatively dilute and carbonate-weighted, consistent with rapid routing and preferential expression of carbonate-derived solutes during high flow, whereas GW recorded greater solute accumulation, stronger silicate-weathering signatures, and widespread cation exchange. Stable isotopes showed pronounced GW–RW separation in low-gradient alluvial settings, where GW integrated recharge across broader spatial and temporal scales, but greater correspondence in mountainous settings characterized by shorter flow paths and limited storage. PCA independently reproduced this setting-dependent hydrochemical structure. Glacier- and snow-fed rivers preserved depleted high-elevation signatures that were progressively modified by downstream mixing. Drinking-water suitability was constrained principally by Fe, Mn, and Al, with As and NO₃⁻ contributing locally, whereas irrigation suitability was broadly favorable. Exceptional antecedent rainfall coincided with multi-constituent enrichment in two Himalayan tributaries, demonstrating that event-scale perturbations can locally disrupt basin-scale river-water patterns.