Hydrochemical evolution and nitrate sources, migration, and transformation in surface water and groundwater of a typical tributary of the Yellow River.

Nitrate pollution poses a severe threat to ecological security and the sustainable utilization of water resources. Accurately identifying its sources and transformation processes is crucial for water quality protection. This study investigated the hydrochemical evolution, nitrate sources, and transformation mechanisms in surface water and groundwater within the Dahei River Basin, a typical tributary of the Yellow River, across wet and dry seasons. A comprehensive approach integrating hydrochemical analysis, nitrogen-oxygen stable isotope techniques, and the MixSIAR model framework was employed. The results indicated that the water bodies in the study area were weakly alkaline freshwater, with hydrochemical characteristics primarily classified as the HCO-Ca·Mg type. The major hydrochemical ions were governed by silicate and evaporite weathering, with additional influence from cation exchange processes. Nitrate formation was mainly driven by nitrification, while denitrification played a minor role. Quantitative source apportionment using MixSIAR revealed that the dominant sources of nitrate, in descending order of contribution, were manure and sewage wastes (MS), soil organic nitrogen (SON), chemical fertilizers (CF), and atmospheric precipitation (AP). MS accounted for the largest proportion, contributing 32.4 % to surface water and 24.6 % to groundwater during the wet season, and 28.2 % to surface water and 33.5 % to groundwater during the dry season. Its impact was particularly pronounced in urban and agricultural areas, with a notable accumulation effect in groundwater during the dry season. This study highlights the pronounced spatiotemporal variations in nitrate sources and provides scientific guidance for developing region-specific pollution prevention and control strategies in urban, agricultural, and hilly regions. The findings also offer a valuable reference for water quality management in similar watersheds.