Hydrochemical and isotopic fingerprints of groundwater origin and evolution in the Urangulan River basin, China's Loess Plateau

The origin and evolution of groundwater in the Urangulan River basin area under growing concern as its situated in an economically and ecologically crucial area of China. In the present study, a combination of different methods (i.e. self-organizing maps (SOM), piper diagrams, ionic ratios, multiple isotopic analyses and Bayesian isotope mixing model) provided an efficient way for analysing groundwater origin and evolution. The hydrochemical type was found to be Ca-HCO₃ in low TDS and Na + K-Cl or Na + K-SO₄ in high TDS groundwater. According to the δ²H and δ¹⁸O_water values, groundwater in the study area mainly originated from atmospheric precipitation and was influenced by evaporation. In addition, the rock weathering in conjunction with the cation exchange completely dominated the geochemical evolution process. The dual SO₄²⁻ isotope and Bayesian isotope mixing model showed that gypsum dissolution, fertilizer input and sewage input were the main sources of SO₄²⁻ in the study area, accounting for an average of 30.2 %, 28.5 %, and 17.3 % of SO₄²⁻ in the groundwater, respectively. Other than water-rock interactions, human activity (mining and irrigation) distributed throughout the study area in combination with the spatial characteristics was the dominant factor controlling the hydrochemical evolution. The results of this study provided a basis for understanding groundwater origin and evolution while facilitating the effective management and utilization of groundwater.