Natural Isotope Constraints on River Nitrate Sources in Polluted Regions

Abstract River nitrogen (N) pollution drives eutrophication and dead zones in more than two‐thirds of global estuaries and bays. Nitrate (NO 3 − ) is a highly mobile form of reactive N. Elevated NO 3 − concentrations in rivers deteriorate water quality and threaten aquatic biota and human health through toxic and carcinogenic effects. Mitigating river NO 3 − in polluted regions is pivotal to achieving Sustainable Development Goal 6 (SDG 6. Clean Water and Sanitation). However, the contributions of major sources to river NO 3 − loads remain poorly resolved, hindering ongoing mitigation efforts. Prevailing source apportionment models attribute river NO 3 − primarily to atmospheric deposition and agricultural non‐point sources. Here we establish a multi‐isotope mass‐balance framework that integrates natural N and oxygen isotopes ( δ 15 N, δ 18 O, Δ 17 O) with concentration, flux, and isotope effect pertaining to river NO 3 − sources, pools, and fates. Application of this framework to polluted rivers in Europe, Asia, and North America reveals that atmospheric deposition contributes only 5 ± 3%, non‐point sources account for 30 ± 9% (lower than previously reported >60%), wastewater remains a dominant contributor at 37 ± 8%, and previously neglected in‐stream nitrification contributes 28 ± 7%, effectively offsetting the fraction formerly overattributed to non‐point sources. This finding demonstrates the importance of strengthening wastewater treatment and controlling in‐river secondary NO 3 − production to reduce river NO 3 − pollution, and significantly advances our knowledge of river NO 3 − sources, which is essential for optimizing mitigation strategies to achieve SDG 6.