Hydrogeochemical characteristics and potential risks within a headwater catchment, Taihu Lake Basin, China

The Yuying River catchment is a hilly watershed that serves as a high-quality water source for Taihu Lake. Its hydrochemical characteristics is crucial to the safety of water supplies around Taihu Lake. In order to evaluate the potantial risk to Taihu Lake, samples of rainwater, surface water, and groundwater from the Yuying River catchment were gathered to conduct a comprehensive hydrochemical analysis, the isotope analysis of δ²H, δ¹⁸O, and ³H. The results showed that the hydrochemical types of the groundwater were mainly HCO₃·SO₄-Na·Ca and HCO₃-Na·Ca. The renewal rate of groundwater in the Yuying River catchment is slightly greater (+10% to +20%) than 34% a^-1. The formation process of hydrochemical characteristics was controlled by the process of cation exchange and leaching (mainly based on silicate dissolution). Combined with historical data, the concentration of NO₃^- in groundwater of the Yuying River catchment was increased from 2000(4.7 mg/L) to 2019(15.8 mg/L). Both the constructed land area (increase 377.31%) and NO₃^- concentration in groundwater (increase 336.17%) elevated about 3 times compared with thoses in 2000. Combined with PCA statistical analysis of nitrate in groundwater of different land use, it was found that the discharge of domestic sewage was the main reason of the abnormal increase of NO₃^- in the study area. It suggested that the enhanced of human activities (unscientific domestic sewage disposal) has exceeded the self-purification ability of the Yuying river catchment water environment. It is urgent to adopt scientific and effective domestic sewage treatment methods to ensure the water environment and water ecological security of the Yuying river catchment and the downstream Taihu Lake.