Groundwater–surface water interactions in major rivers of Henan Province, China: Insights from stable hydrogen and oxygen isotopes and hydrochemistry

Study region Ten major rivers spanning four drainage basins (Hai, Yellow, Huai, and Yangtze) across the alluvial plains of Henan Province, China. Study focus Groundwater–surface water (GW–SW) exchange dynamics across these anthropogenically disturbed river systems were systematically investigated using an integrated approach combining hydrochemical analysis, dual stable isotopes (δD and δ 18 O), and end-member mixing analysis. The study aimed to quantitatively determine the direction and mixing fraction of GW–SW exchange across different river reaches. New hydrological insights for the region Three distinct exchange regimes were identified: (i) gaining streams (Luo, Yi, Guo, and mainstem Huai Rivers), where groundwater contributes 21.2%–94.1% of river discharge, predominantly in deeply incised mountain valleys and piedmont zones; (ii) losing streams (Yellow and Ying Rivers), with river-to-aquifer recharge proportions of 16.9%–87.8%; and (iii) alternating gaining–losing systems (Wei, Bai, Ru, and Sha Rivers), where exchange direction reverses spatially between reaches or cross-sectionally between banks. The Wei River exemplifies the latter asymmetry, with west-bank groundwater discharge (8.7%–40.8%) contrasting sharply with east-bank river leakage (27.0%–79.5%). Considered alongside previous studies, our results suggest that groundwater abstraction, perched river conditions associated with channel-bed aggradation, and hydraulic structures may contribute to the increasing spatial complexity across the plains. These findings establish a quantitative framework for GW–SW exchange across Henan's major rivers, providing a scientific basis for precise water resource assessment and sustainable allocation strategies.