Hydrochemical evolution driven by hyporheic exchange of the Weihe River Basin, Northwest China

Hyporheic exchange (HE) is a critical process driving groundwater-surface water interactions in riverine ecosystems and plays a key role in regulating hydrogeochemical cycling. However, previous studies have primarily focused on reach-scale processes, leaving the watershed-scale spatial heterogeneity of HE and its hydrochemical consequences insufficiently understood. This study systematically quantified the spatial patterns of HE and associated hydrochemical responses across an entire semi-arid river basin, using in-situ temperature profiles, sediment properties, and hydrochemical analyses in the Weihe River Basin. HE exhibited a pronounced longitudinal gradient, with fluxes ranging from 17.71 to 316.04 mm d^-1, transitioning from strong and highly variable upwelling in the upper and middle reaches to weak and stable downwelling in the downstream plains. These contrasting HE regimes generated distinct hydrochemical signatures. Upwelling transported Ca-HCO₃-rich groundwater into the hyporheic zone, leading to elevated major-ion concentrations (porewater TDS 531.29 mg L^-1) conditions, whereas downwelling introduced oxygenated, low-salinity river water that enhanced dilution and redox buffering within the hyporheic zone. Pronounced variations in nitrogen were observed between upwelling and downwelling HE regimes across the basin. Surface-water TN concentrations were higher in upwelling reaches (5.99 mg L^-1) than in downwelling reaches (2.94 mg L^-1), while porewater TN remained low throughout. Surface water quality in downwelling zones was significantly better than in upwelling zones (p < 0.05). PCA identified four components explaining 86.8% of the total variance. Natural environmental processes, including evaporite weathering, evapotranspiration, and cation exchange, dominated the evolution of major-ion hydrochemistry, whereas nitrogen and organic carbon patterns were primarily influenced by anthropogenic inputs. By modifying vertical flow paths, residence times, and redox conditions, HE mediates the transport and reactive mixing of solutes across the surface water-hyporheic interface, shaping spatial contrasts in water chemical composition and quality trajectories of rivers. This watershed-scale integration of HE spatial heterogeneity and hydrochemical sensitivity provides new mechanistic insight into river-corridor functioning and offers a framework for managing groundwater-surface water interactions in semi-arid basins.