On February 12, 2026, C²TN Academy, as part of its Thematic Line “Earth Systems, Radioactivity & Cultural Heritage,” held a seminar dedicated to the Zagreb aquifer, the main source of drinking water for around a quarter of Croatia’s population.
The seminar, entitled “Zagreb Aquifer – water and nitrate origin” took place during the visit of experts from the International Atomic Energy Agency (IAEA), Zoran Kovač and Gabrijela Ljubek, to the Centro de Ciências e Tecnologias Nucleares (C²TN), Loures Campus of Instituto Superior Técnico. The initiative was part of the “POR-7004” project, dedicated to mitigating nitrate contamination in the Vouga River Basin in Estarreja.
Paula Carreira, a researcher at C²TN, opened the session by welcoming the participants and introducing the guest speaker, Zoran Kovač, Associate Professor at Faculty of Mining, Geology and Petroleum Engineering from University of Zagreb. The collaboration between Técnico and University of Zagreb is part of projects of IAEA Isotope Hydrology Section (IAEA-IHS), bringing together teams facing common challenges in the management and protection of water resources, using isotopic techniques to develop new scientific solutions.
The Zagreb aquifer plays a strategic role in supplying the capital and the surrounding region and has been the subject of intensive research over the past two decades, with particular focus on groundwater quality. This unconfined hydrological system is in direct contact with Sava River. ´Almost all research shows that Sava River is the main source of recharge for the Zagreb aquifer, although its influence varies according to hydrodynamic conditions`, explained Zoran Kovač.
During the meeting, several results obtained from this aquifer system were presented through the combination of nuclear techniques and classical hydrogeological analyses, such as piezometric level monitoring and hydrogeochemical characterisation. Stable isotopes of hydrogen and oxygen from water make it possible to identify recharge sources, while tritium, a radioactive isotope of hydrogen, was used to calculate the mean groundwater velocity. In the Zagreb region, higher tritium levels were associated with the discharge of cooling water from the Krško Nuclear Power Plant in Slovenia. Analysis of tritium activity peaks in the river and in piezometers allowed researchers to estimate the groundwater.
One of the most significant topics addressed during the seminar was nitrate contamination. In the urban area of Zagreb, wastewater was identified as the main source of contamination of the system, whereas in predominantly agricultural areas contamination results from a mixture of sources, including manure, wastewater, synthetic fertilisers and, to a lesser extent, natural soil nitrogen. Surprisingly, synthetic fertilisers are not the primary source of nitrates in agricultural zones.
Accurate identification of the origin of these contaminants requires the use of stable nitrogen and oxygen isotopes present in nitrates, which is essential for implementing effective protection measures. Water intended for human consumption must contain nitrate concentrations below 50 mg/L, while higher levels may directly affect health, causing conditions such as Methemoglobinemia, and may also contribute to the eutrophication of aquatic ecosystems.
Professor Zoran Kovač also stated that, in the study area, “all pumping wells used for public water supply are protected by sanitary protection zones and continuous monitoring programs”, stressing that this oversight enables the early detection of potential problems.
According to the researchers, any strategy to reduce contaminants must begin with a rigorous identification of their origin and an analysis of the processes governing their transformation within the aquifer system. “It is a demanding field, but essential to safeguard human health and the environment. Patience and persistence are required”, Zoran Kovač emphasized, addressing students and early-career scientists.
A large share of the research carried out by the University of Zagreb is published in open access and publicly available, while more information about the articles can be found on academic platforms, from Google Scholar to Web of Science (WoS) and Scopus.