Sources, Transformation, and Fate of Groundwater Nitrate Under Intensive Anthropogenic Pressures in the Lower Yellow River Basin: Insights From Hydrochemistry and Multi-Isotope Analysis.

Enhanced human activities have increased nitrate contamination in groundwater, threatening water-supply safety and public health. This study examined nitrate sources and transformation processes in 29 shallow groundwater samples from the lower Yellow River Basin, a region subject to intensive agricultural, industrial, and domestic pressures. The observed NO3 --N concentrations ranged from 0.37 to 177.95 mg/L. Self-organizing maps, the MixSIAR model, and hydrochemical and multi-isotope analyses identified five hydrochemical clusters. Across all samples, the mean proportional contributions of soil nitrogen and chemical fertilizers were 52.6% and 31.0%, respectively, indicating that these two sources dominated groundwater nitrate. Sewage/manure and industrial or mining wastewater made smaller overall contributions, but their relative importance increased in areas affected by stronger human activities. The isotope and hydrochemical evidence indicated that nitrification was a key process increasing groundwater nitrate concentrations; this effect was particularly pronounced in the northwestern groundwater convergence zone, where soil nitrogen accumulation was relatively high. These findings suggest that anthropogenic inputs not only introduce nitrate directly but also enhance the leaching and transformation of soil- and fertilizer-derived nitrogen. The results support targeted groundwater management through precision fertilization, soil nitrogen monitoring, wastewater control, and priority protection of vulnerable recharge and convergence areas.