Anthropogenic impacts on the nitrate pollution in an urban river: Insights from a combination of natural-abundance and paired isotopes.

Urban rivers are often characterized by high nitrate (NO ₃ ^- ) loadings. High NO ₃ ^- loadings cause water quality and ecological damages, which undermines the sustainable development of cities. To date, the drivers of these high NO ₃ ^- loadings remain unclear. This study, for the first time, integrated natural-abundance isotopes (δ ¹⁵  N/δ ¹⁸ O-NO ₃ ^- and δD/δ ¹⁸ O-H ₂ O) and ¹⁵ N-pairing techniques to comprehensively reveal the anthropogenic impacts on the NO ₃ ^- pollution in an urban river. Natural-abundance isotopes suggested that in both the wet and dry seasons, the NO ₃ ^- was predominantly from the conservative mixing of different sources, and biological NO ₃ ^- removal was minor. The ¹⁵ N-pairing experiments supported the natural-abundance isotope data, quantitatively showing that in-soil nitrification was prevailing, while NO ₃ ^- removal processes (denitrification, anammox, and dissimilatory NO ₃ ^- reduction to ammonium) were weak. A Bayesian isotope-mixing model showed that soil sources (soil organic nitrogen and chemical fertilizer) dominated the NO ₃ ^- in the upper reaches, while in the lower reaches, the impermeable riparian zone short-circuited the access of soils to the river. Here, the wastewater treatment plants became a significant source of NO ₃ ^- . This study quantitatively revealed the drivers of high NO ₃ ^- loadings in an urban river, and generated important clues for effective NO ₃ ^- pollution control and remediation in urban rivers.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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