Variation in nitrate isotopic signatures in sewage for source apportionment with urbanization: a case study in Beijing, China

Nitrate (NO 3 − ) pollution is a severe problem in urban aquatic systems especially within megacity undergoing rapid urbanization, and mostly, sewage is supposed as the prevailing NO 3 − source. A dual isotope approach ( δ 15 N-NO 3 − and δ 18 O-NO 3 − ) was applied to explore the variation in NO 3 − 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 NO 3 − isotopic signatures, including δ 15 N from 1.1 to 24.7 ‰ and δ 18 O from 1.6 to 22.8 ‰ in raw sewage, as well as δ 15 N from 6.1 to 22.8 ‰ and δ 18 O from 1.6 to 13.2 ‰ in treated effluents. The WWTP processing would result in the enrichment of NO 3 − isotopic compositions in discharged effluents with NO 3 − concentrations increasing. Besides, advanced sewage treatment technology with more pollutant N reduction may raise the heavier NO 3 − isotopic compositions further. The NO 3 − isotope value ranges of urban sewage and manure should be separated, and the seasonal and tighter NO 3 − isotope value ranges are supposed to improve the accuracy of source apportionment. The NO 3 − isotope value ranges conducted in this study might provide useful information for tracing NO 3 − sources towards the implementation of efficient water pollution control in Beijing.