Variation in nitrate isotopic signatures in sewage for source apportionment with urbanization: a case study in...

Nitrate (NO₃^-) pollution is a severe problem in urban aquatic systems especially within megacity undergoing rapid urbanization, and mostly, sewage is supposed as the prevailing NO₃^- source. A dual isotope approach (δ ¹⁵N-NO₃^- and δ ¹⁸O-NO₃^-) was applied to explore the variation in NO₃^- isotopic signatures in sewage processed by wastewater treatment plants (WWTPs) in Beijing from 2014 to 2015. We found that the raw and treated sewage owned the different NO₃^- isotopic signatures, including δ ¹⁵N from 1.1 to 24.7 ‰ and δ ¹⁸O from 1.6 to 22.8 ‰ in raw sewage, as well as δ ¹⁵N from 6.1 to 22.8 ‰ and δ ¹⁸O from 1.6 to 13.2 ‰ in treated effluents. The WWTP processing would result in the enrichment of NO₃^- isotopic compositions in discharged effluents with NO₃^- concentrations increasing. Besides, advanced sewage treatment technology with more pollutant N reduction may raise the heavier NO₃^- isotopic compositions further. The NO₃^- isotope value ranges of urban sewage and manure should be separated, and the seasonal and tighter NO₃^- isotope value ranges are supposed to improve the accuracy of source apportionment. The NO₃^- isotope value ranges conducted in this study might provide useful information for tracing NO₃^- sources towards the implementation of efficient water pollution control in Beijing.