Tracking Water’s Hidden Polluters: How Scientists Are Tracing Nitrate to Its Source

Excess nitrate threatens rivers, lakes and groundwater worldwide, feeding harmful algal blooms and damaging freshwater and coastal ecosystems.

A river sample can reveal that nitrate levels are too high, but finding out where that pollution came from takes more detective work. Wastewater, farming and nitrogen left in soils and groundwater can all contribute to contamination, leaving water managers with a problem that a simple concentration measurement cannot explain.

Excess nitrate threatens rivers, lakes and groundwater worldwide, feeding harmful algal blooms and damaging freshwater and coastal ecosystems. Tracking its origins becomes difficult because nitrogen changes as it travels through soil and water, with biological and chemical processes altering the clues scientists use to identify its source.

A four-year research project led by the International Atomic Energy Agency brought together scientists from 14 countries to tackle that challenge. Their work combined several kinds of chemical evidence to build a clearer picture of where nitrate comes from, how it moves and what happens along the way.

Everyday Chemicals Help Reveal Pollution Sources

The researchers paired stable nitrogen and oxygen isotope measurements in nitrate with water chemistry analysis and tests for contaminants of emerging concern, including pharmaceuticals, artificial sweeteners and other substances associated with human activity.

Isotopes are forms of the same element with different masses, and their proportions can help scientists investigate nitrate sources and transformations. Reading those measurements alongside other chemical clues gave researchers greater confidence than relying on any single monitoring method.

Caffeine, sucralose and pharmaceutical residues were especially useful because their presence helped identify wastewater inputs that could otherwise be difficult to separate from agricultural or natural sources. The approach can also help distinguish human waste from animal-derived contamination, such as manure.

IAEA isotope hydrologist Astrid Harjung said the combined evidence gives scientists and water managers a clearer understanding of pollution sources, transport routes and environmental processes, helping them make better decisions about protecting water resources.

Wastewater Leaves Its Mark in Urban Rivers

Across participating countries, the project identified nitrate contributions from urban wastewater, agriculture and legacy nitrogen stored in soils and groundwater. This stored nitrogen matters because contamination found today may reflect inputs from earlier years.

Researchers frequently detected wastewater-related compounds alongside elevated nitrate concentrations, providing evidence of human-derived pollution and helping explain contamination patterns that nitrate measurements alone could not resolve.

In Greece's heavily urbanised Kifissos River Basin in Athens, the combined approach revealed the dominant role of human wastewater in nitrate contamination.

Ioannis Matiatos, a senior scientist at the Hellenic Centre for Marine Research, described how pairing nitrate isotopes with emerging contaminants helped his team understand pollution sources and pathways across the catchment, demonstrating the practical value of examining several tracers together.

Microbes Change the Story as Nitrogen Travels

Nitrate does not simply move unchanged from a pollution source into a river or aquifer. Microbial activity can produce nitrate through nitrification or remove it through denitrification, changing both its concentration and the evidence researchers need to interpret.

The isotope data helped teams investigate these processes and understand why nitrate behaves differently under different local conditions. Knowing whether nitrate is being produced or removed helps researchers make sense of measurements taken along connected river and groundwater systems.

Machine learning and Bayesian statistical methods helped uncover seasonal and regional patterns in contamination and transport, giving water managers a stronger scientific basis for interpreting changes across places and sampling periods.

Better Evidence for Protecting Water

Participating teams developed georeferenced databases, assessment tools and expertise for identifying areas vulnerable to nitrate pollution. These resources give countries a foundation for more informed monitoring and water management.

Asunción Romanelli, a researcher at Argentina's National Scientific and Technical Research Council and the National University of Mar del Plata, said combining nitrate isotopes with emerging contaminants improved her team's ability to identify contamination sources and transport pathways.

Published in IAEA TECDOC No. 2126 (2026), the findings offer practical guidance for investigating polluted freshwater systems. Understanding whether nitrate comes from wastewater, agriculture, or stored nitrogen gives authorities better evidence for deciding where pollution controls are needed and how to protect rivers, groundwater and downstream ecosystems.