Treated wastewater and weak removal mechanisms enhance nitrate pollution in metropolitan rivers.

The focus of urban water environment renovation has shifted to high nitrate (NO 3 −) load. Nitrate input and nitrogen conversion are responsible for the continuous increase in nitrate levels in urban rivers. This study utilized nitrate stable isotopes (δ15N–NO 3 − and δ18O–NO 3 −) to investigate NO 3 − sources and transformation processes in Suzhou Creek, located in Shanghai. The results demonstrated that NO 3 − was the most common form of dissolved inorganic nitrogen (DIN), accounting for 66 ± 14% of total DIN with a mean value of 1.86 ± 0.85 mg L−1. The δ15N–NO 3 − and δ18O–NO 3 − values ranged from 5.72 to 12.42‰ (mean value: 8.38 ± 1.54‰) and −5.01 to 10.39‰ (mean value: 0.58 ± 1.76‰), respectively. Based on isotopic evidence, the river received a significant amount of nitrate through direct exogenous input and sewage ammonium nitrification, while nitrate removal (denitrification) was insignificant, resulting in nitrate accumulation. Analysis using the MixSIAR model revealed that treated wastewater (68.3 ± 9.7%), soil nitrogen (15.7 ± 4.8%) and nitrogen fertilizer (15.5 ± 4.9%) were the main sources of NO 3 − in rivers. Despite the fact that Shanghai's urban domestic sewage recovery rate has reached 92%, reducing nitrate concentrations in treated wastewater is crucial for addressing nitrogen pollution in urban rivers. Additional efforts are needed to upgrade urban sewage treatment during low flow periods and/or in the main stream, and to control non-point sources of nitrate, such as soil nitrogen and nitrogen fertilizer, during high flow periods and/or tributaries. This research provides insights into NO 3 − sources and transformations, and serves as a scientific basis for controlling NO 3 − in urban rivers. • Urban rivers have received significant amounts of nitrate through direct exogenous input and ammonium nitrification from sewage. • A weak permanent nitrate removal mechanism results in the accumulation of nitrate. • The main source of nitrate pollution in urban rivers is mainly due to the substantial input of treated wastewater. [ABSTRACT FROM AUTHOR]