Tracing Nitrate Sources and Transformations in a Cold-Region River Basin: Dual Isotopes and a Bayesian Mixing Model

Agricultural nitrogen loss accounts for more than half of China’s water pollutant discharge, yet flood-season nitrate sources and transformations remain poorly quantified in cold-region basins under combined agricultural and industrial pollution. We investigated the Woken River, an agricultural–industrial tributary of the Songhua River, during the 2023 flood season (June–August), collecting water, soil, and precipitation samples at 31 sites in three monthly campaigns. Hydrochemical parameters and nitrate dual isotopes (δ15N-NO3−, δ18O-NO3−; denitrifier method) were analyzed, and the contributions of five potential sources were quantified using the MixSIAR model constrained by correlation and principal component analyses. δ15N-NO3− ranged from +6.033‰ to +13.313‰ and δ18O-NO3− from −2.195‰ to +6.060‰. The NO3− dual-isotope study indicates that the primary sources of NO3− in the Woken River basin are soil nitrogen (S), paper mill wastewater (PMW), and domestic sewage and livestock manure (DS&LM). Model estimation results indicate that PMW and S were the dominant sources, each contributing approximately 43% of riverine nitrate. These results support the targeted control of industrial discharge and agricultural soil nitrogen loss in cold-region basins.