Establishment and application of a new method identifying nitrogen pollution sources using CHEMTAX model combined...

Source identification of total nitrogen (TN) in rivers is important for water environment protection. This study aimed to establish a TN source identification method using CHEMTAX combined with fingerprint indicators and to apply this method in the basin of the upper Hanjiang River. Forty-eight end-member samples, including cropland, forest, grassland, wastewater, litter and manure samples, were collected and mixed to obtain 95 receptor samples with preset proportions. Four cations (K⁺, Ca²⁺, Na⁺, and Mg²⁺), two anions (Cl^- and SO₄^2-), five trace elements (B, Sr, Si, La, and Se) and four dissolved organic matter (DOM) spectral indices (FI, S_R, SUVA₂₅₄, and HIX) of both the end-member and receptor samples were measured as fingerprint indicators. Finally, a TN source identification method based on CHEMTAX using concentrations of four cations (K⁺, Ca²⁺, Na⁺, and Mg²⁺) and two elements Sr and Si was established. In addition, methods of hydrochemical fingerprint indicator combined with the CHEMTAX model and Bayesian mixing model based on nitrogen and oxygen stable isotopes were employed for the nitrate source identification in the upper Hanjiang River Basin. A weak-to-moderate statistical correlation was found between the contribution rate of each source to riverine TN in the Hanjiang River calculated by CHEMTAX and MixSIAR. The results of principal coordinates analysis, variance partitioning analysis and random forest models analysis suggested that the contribution rate of each pollution source to TN was consistent with the current situation of land uses, basin characteristics and the intensity of human activities. The established method not only reduced test cost, but also showed potential for online nitrogen source identification monitoring. This study opened a new window for TN source identification.