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Urbanization has altered the fate and transport of anthropogenic nitrogen\ud(N) in rivers and estuaries globally. This study evaluates the capacity of\udan urbanizing river–estuarine continuum to transform N inputs from the\udworld's largest advanced (e.g., phosphorus and biological N removal)\udwastewater treatment facility. Effluent samples and surface water were\udcollected monthly along the Potomac River estuary from Washington D.C. to\udthe Chesapeake Bay over a distance of 150 km. In conjunction with box model mass balances,\udnitrate stable isotopes and mixing models were used to trace the fate of\udurban wastewater nitrate. Nitrate concentrations and δ15N-NO3− values were higher down-estuary from the Blue\udPlains wastewater outfall in Washington D.C. (2.25 ± 0.62 mg L−1 and\ud25.7 ± 2.9 ‰, respectively) compared to\udupper-estuary concentrations (1.0 ± 0.2 mg L−1 and 9.3 ± 1.4 ‰, respectively). Nitrate concentration then\uddecreased rapidly within 30 km down-estuary (to\ud0.8 ± 0.2 mg L−1),\udcorresponding to an increase in organic nitrogen and dissolved organic\udcarbon, suggesting biotic uptake and organic transformation. TN loads\uddeclined down-estuary (from an annual average of 48 000 ± 5000 kg day−1\udat the sewage treatment plant outfall to 23 000 ± 13 000 kg day−1 at the\udestuary mouth), with the greatest percentage decrease during summer and\udfall. Annually, there was a 70 ± 31 % loss in wastewater\udNO3− along the estuary, and 28 ± 6 % of urban wastewater TN\udinputs were exported to the Chesapeake Bay, with the greatest contribution\udof wastewater TN loads during the spring. Our results suggest that\udbiological transformations along the urban river–estuary continuum can\udsignificantly transform wastewater N inputs from major cities globally, and\udmore work is necessary to evaluate the potential of organic nitrogen and\udcarbon to contribute to eutrophication and hypoxia

Sources and transformations of anthropogenic nitrogen along an urban river–estuarine continuum