Isotopes Trace Nitrogen Pollution in Water

Nitrogen pollution is an increasing threat to rivers, lakes and groundwater worldwide. Excess nitrate from wastewater discharges, agricultural activities and other human sources can degrade water quality, fuel harmful algal blooms and damage freshwater and coastal ecosystems. Yet identifying the origin of nitrate contamination is often difficult because nitrogen undergoes complex biological and chemical transformations as it moves through the environment.

To help address this challenge, the IAEA led a four-year coordinated research project bringing together scientists from 14 countries to jointly develop and test innovative methods for tracing nitrate pollution in connected river and groundwater systems.

The project combined stable isotope analysis of nitrogen and oxygen in nitrate with hydrochemical measurements and contaminants of emerging concern, including pharmaceuticals, artificial sweeteners and other compounds linked to human activity.

Using these complementary tracers, researchers were able to distinguish between different sources of nitrate and better understand how nitrogen moves through aquatic environments. The integrated approach provided greater confidence in source identification than any single monitoring method alone.

"This multi-tracer approach transforms how we understand water quality by providing greater clarity on pollution sources, transport pathways and environmental processes in river systems that influence both terrestrial and downstream marine ecosystems," said Astrid Harjung, an isotope hydrologist at the IAEA. "It enables scientists, policymakers and water managers to make informed decisions that support pollution reduction, ecosystem protection and the sustainable management of water resources."

Across the participating countries, the project identified nitrate contributions from urban wastewater, agricultural activities and legacy nitrogen stored in soils and groundwater.

Wastewater-related indicators such as caffeine, sucralose (an artificial sweetener) and pharmaceutical compounds were frequently detected alongside elevated nitrate concentrations, providing clear evidence of human-derived contamination. These tracers proved especially valuable in separating wastewater inputs from agricultural and natural sources.

The approach was particularly effective in highly urbanized environments, such as the Kifissos River Basin in Athens, Greece.

"Through this IAEA project, we combined nitrate isotope techniques with contaminants of emerging concern to better understand pollution sources and pathways in a highly urbanized catchment," said Ioannis Matiatos, a senior scientist at the Hellenic Centre for Marine Research. "This integrated approach revealed the dominant role of human wastewater in nitrate contamination and demonstrated the value of combining conventional and emerging tracers for water-quality assessments."

Beyond source identification, the project generated new insights into the environmental processes that control nitrogen behaviour in aquatic systems. Isotope data helped researchers evaluate natural microbial processes such as nitrification and denitrification, which can increase or decrease nitrate concentrations depending on local conditions.

Advanced analytical tools, including machine learning and Bayesian statistical methods, also revealed seasonal and regional patterns in contamination and transport, providing a stronger scientific basis for water management decisions.

The project also strengthened scientific and technical capacity in participating countries. Researchers developed georeferenced databases, assessment tools and new expertise for evaluating vulnerability to nitrate pollution.

"By integrating nitrate isotopes with contaminants of emerging concern, we improved our ability to identify contamination sources and transport pathways, generating scientific evidence that can support more effective water-resource management," said Asunción Romanelli, a researcher at Argentina's National Scientific and Technical Research Council and the National University of Mar del Plata.

The project's findings, published in IAEA TECDOC No. 2126 (2026), demonstrate that combining isotope techniques with chemical tracers provides a robust framework for distinguishing agricultural, wastewater-derived and legacy nitrogen sources. The methodology offers countries practical tools to improve water quality assessments, support evidence-based decision-making and protect vital freshwater resources for future generations.