Combining the multivariate statistics and dual stable isotopes methods for nitrogen source identification in coastal rivers of Hangzhou Bay, China

Coastal rivers contributed the majority of anthropogenic nitrogen (N) loads to coastal waters, often resulting in eutrophication and hypoxia zones. Accurate N source identification is critical for optimizing coastal river N pollution control strategies. Based on a 2-year seasonal record of dual stable isotopes (delta N-15 - NO3- and delta O-1(8) - NO3) and water quality parameters, this study combined the dual stable isotope-based MixSIAR model and the absolute principal component score-multiple linear regression (APCS-MLR) model to elucidate N dynamics and sources in two coastal rivers of Hangzhou Bay. Water quality/trophic level indices indicated light-to-moderate eutrophication status for the studied rivers. Spatio-temporal variability of water quality was associated with seasonal agricultural, aquaculture, and domestic activities, as well as the seasonal precipitation pattern. The APCS-MLR model identified soil + domestic wastewater (69.5%) and aquaculture tailwater (22.2%) as the major nitrogen pollution sources. The dual stable isotope-based MixSIAR model identified soil N, aquaculture tailwater, domestic wastewater, and atmospheric deposition N contributions of 35.3 +/- 21.1%, 29.7 +/- 17.2%, 27.9 +/- 14.5%, and 7.2 +/- 11.4% to riverine NO3- in the Cao'e River (CER) and 34.4 +/- 21.3%, 29.5 +/- 17.2%, 27.4 +/- 14.7%, and 8.7 +/- 12.8% in the Jiantang River (JTR), respectively. The APCS-MLR model and the dual stable isotope-based MixSIAR model showed consistent results for riverine N source identification. Combining these two methods for riverine N source identifications effectively distinguished the mix-source components from the APCS-MLR method and alleviated the high cost of stable isotope analysis, thereby providing reliable N source apportionment results with low requirements for water quality sampling and isotope analysis costs. This study highlights the importance of soil N management and aquaculture tailwater treatment in coastal river N pollution control.

Automated summary

Coastal rivers are a major contributor of anthropogenic nitrogen loads to coastal waters. Accurate identification of nitrogen sources is crucial for effective pollution control strategies. This study used stable isotopes and water quality parameters to identify nitrogen dynamics and sources in two coastal rivers. The results showed that soil and domestic wastewater, as well as aquaculture tailwater, were the major sources of nitrogen pollution. Combining stable isotope analysis with the APCS-MLR model provided reliable and cost-effective results for nitrogen source identification. The study emphasizes the importance of soil nitrogen management and aquaculture tailwater treatment in coastal river pollution control.