Source Apportionment and Seasonal Variation in Nitrate in Baiyangdian Lake After Restoration Projects Based on Dual Stable Isotopes and MixSIAR Model

Nitrate in Baiyangdian Lake is directly linked to the sustainability of watershed ecological functions, acting as a key priority for regional ecological protection. Subsequent to the completion of a series of ecological restoration projects, its sources have undergone inevitable shifts, rendering the original pollution control framework incompatible with the new context. Thus, accurate identification of nitrate sources and their seasonal variation characteristics constitutes a core prerequisite for enhancing the targeting of pollution management. This study integrated dual stable isotopes (δ15N-NO3− and δ18O-NO3−) in water and potential source samples, along with hydrochemical data, and applied the Bayesian stable isotope mixing model (MixSIAR) to elucidate the sources of NO3− in Baiyangdian Lake. The results indicated that denitrification exerted a weak influence on the isotopic composition of NO3− in Baiyangdian Lake. Plots of the NO3−/Cl− versus Cl− ratios for water samples and δ15N-NO3− versus δ18O-NO3− ratios for both water samples and potential sources confirmed anthropogenic sources as the primary nitrate contributors. The δ15N-NO3− vs. 1/NO3− plot revealed that the number of NO3− sources exceeded two. The MixSIAR model demonstrated that wastewater treatment plant (WWTP) discharge was the dominant source throughout the four seasons, accounting for 49–62% with the highest contribution in winter and the lowest in summer. Soil nitrogen release contributed 19–32%, reaching its annual peak in summer. Sediment release accounted for 11–13%, maintaining a relatively low contribution across all seasons. Chemical fertilizer, manure, and sewage (M&S), and atmospheric deposition each contributed less than 6.5%, with negligible contributions. A significant reduction in the contributions of sediment release and M&S reflected the optimization effect of long-term regional ecological restoration efforts. WWTPs point source discharge and seasonal non-point source input from soil nitrogen collectively constituted the core sources of nitrate in Baiyangdian Lake. These findings provide crucial scientific support for the precise source apportionment and differentiated management of nitrate pollution in the basin.