Implication of self-organizing map, stable isotopes combined with MixSIAR model for accurate nitrogen control in a...

Nitrogen pollution and eutrophication in reservoirs is a global environmental geochemical concern. Occasional algal blooms still exist in reservoirs that have undergone pollution treatment. The lack of quantitative evidence of nitrogen sources and fate limits long-term stable ecological safety management. This work applied an approach integrated zonal mapping, stable isotopes (δ¹⁸O_H2O, δ¹⁵N_nitrate, δ¹⁸O_nitrate, and δ¹³C-DIC) and a Bayesian isotope model to analyze regional and seasonal differences in the contribution and sources of nitrogen to a well-protected reservoir. The values of δ¹⁸O_nitrate and the positive relationship between NO₃^- and δ¹³C-DIC suggested that nitrification was the primary NO₃^- production in the rivers. While Denitrification was present at only a few sites. Results of the MixSIAR model coupled the NO₃^-/Cl^- indicator revealed that the domestic sewage contributed high riverine NO₃^- loading (68.6 ± 10.6 %) in the dry season. In the wet season, the main nitrate sources of upper watershed were ammonia and carbamide fertilizers (47.5 % and 40.3 %). While the domestic sewage was still the major contributor of downstream region (a dense residential area), indicating possible problems with rainwater and sewage drainage networks. The results implied that the colleting and treatment of sewages were the priority in downstream region, and non-point source pollution control and wastewater treatment plant upgrading were essential to control nitrate pollution in the two upstream regions. These findings provide new insights into precise nitrogen pollution traceability and identification of treatment priorities in the sub-region, and promote the management other well-protected watershed in similar need of further nitrogen contamination control.