Linking groundwater hydrochemistry to vegetation dynamics in arid inland river basins, Northwest China

The middle reaches of the Heihe River Basin, an arid inland river system in northwest China, where groundwater plays a key role in sustaining oasis ecosystems under strong evaporative conditions. This study investigated groundwater hydrochemical characteristics and their linkages with vegetation dynamics using major ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻, HCO₃⁻, NO₃⁻) and physicochemical parameters (pH, EC, TDS). Multivariate statistical methods, including K-means clustering and principal component analysis (PCA), were applied to identify hydrochemical types and controlling processes. Relationships among groundwater chemistry, normalized difference vegetation index (NDVI), and potential evapotranspiration (ET₀) were analyzed to explore ecohydrological interactions. Groundwater is weakly alkaline and dominated by Na⁺ and HCO₃⁻, mainly controlled by mineral dissolution, evaporation–concentration, and anthropogenic inputs. A clear salinity gradient was observed, with higher TDS in northern irrigated areas. Strong ecohydrological coupling is evident, as evaporation-driven salinity accumulation (positive ET₀–TDS relationship) constrains vegetation growth. NDVI shows negative correlations with TDS and major ions, indicating salinity stress. Notably, elevated HCO₃⁻ further suppresses vegetation, suggesting alkalization as an additional ecological constraint. These findings provide a scientific basis for salinity management and ecosystem protection in arid regions. • The dominant facies was Na–Ca–HCO₃, mainly controlled by rock weathering. • PCA extracted three components explaining 86.7% of variance, reflecting mineralization and anthropogenic influences. • Major ions were higher in northern and central oasis areas. • NDVI declined with increasing TDS and ions, indicating salinity-induced vegetation stress.