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

The focus of urban water environment renovation has shifted to high nitrate (NO₃⁻) 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–NO₃⁻ and δ¹⁸O–NO₃⁻) to investigate NO₃⁻ sources and transformation processes in Suzhou Creek, located in Shanghai. The results demonstrated that NO₃⁻ 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–NO₃⁻ and δ¹⁸O–NO₃⁻ 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 NO₃⁻ 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 NO₃⁻ sources and transformations, and serves as a scientific basis for controlling NO₃⁻ in urban rivers.