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

Abstract. Urbanization has altered the fate and transport of anthropogenic nitrogen (N) in rivers and estuaries globally. This study evaluates the capacity of an urbanizing river–estuarine continuum to transform N inputs from the world's largest advanced (e.g., phosphorus and biological N removal) wastewater treatment facility. Effluent samples and surface water were collected monthly along the Potomac River estuary from Washington D.C. to the Chesapeake Bay over a distance of 150 km. In conjunction with box model mass balances, nitrate stable isotopes and mixing models were used to trace the fate of urban wastewater nitrate. Nitrate concentrations and δ¹⁵N-NO₃⁻ values were higher down-estuary from the Blue Plains wastewater outfall in Washington D.C. (2.25 ± 0.62 mg L⁻¹ and 25.7 ± 2.9 ‰, respectively) compared to upper-estuary concentrations (1.0 ± 0.2 mg L⁻¹ and 9.3 ± 1.4 ‰, respectively). Nitrate concentration then decreased rapidly within 30 km down-estuary (to 0.8 ± 0.2 mg L⁻¹), corresponding to an increase in organic nitrogen and dissolved organic carbon, suggesting biotic uptake and organic transformation. TN loads declined down-estuary (from an annual average of 48 000 ± 5000 kg day⁻¹ at the sewage treatment plant outfall to 23 000 ± 13 000 kg day⁻¹ at the estuary mouth), with the greatest percentage decrease during summer and fall. Annually, there was a 70 ± 31 % loss in wastewater NO₃⁻ along the estuary, and 28 ± 6 % of urban wastewater TN inputs were exported to the Chesapeake Bay, with the greatest contribution of wastewater TN loads during the spring. Our results suggest that biological transformations along the urban river–estuary continuum can significantly transform wastewater N inputs from major cities globally, and more work is necessary to evaluate the potential of organic nitrogen and carbon to contribute to eutrophication and hypoxia.