Anthropogenic impacts on the nitrate pollution in an urban river: Insights from a combination of natural-abundance and paired isotopes
Wenjing Guo, Dong Zhang, Wenshi Zhang, Shen Li, Ke Pan, Hao Jiang, Quanfa Zhang
Abstract
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.