Tracing nitrate contamination sources and dynamics in an unconfined alluvial aquifer system (Velika Gorica well field, Croatia)†

Nitrate ions (NO₃⁻) are one of the most common contaminants in the groundwater of the Zagreb alluvial aquifer, which hosts strategic groundwater reserves of the Republic of Croatia and supplies drinking water to one million inhabitants of the capital city. To better understand the origin and the dynamics of NO₃⁻ in the unsaturated and saturated zones, the stable isotopes of nitrogen (δ¹⁵N) and oxygen (δ¹⁸O) in dissolved nitrate, combined with physico-chemical, hydrogeochemical and water stable isotope data, were used in the current work, together with statistical tools and mixing models. The study involved monthly sampling of groundwater, surface water, precipitation and soil water samples. Additionally, the isotopic composition of total nitrogen (δ¹⁵N_bulk) was determined in solid samples representing the local nitrate sources. The combination of a nitrous oxide isotopic analyzer and the titanium(III) reduction method provides reliable measurements of δ¹⁵N_NO3 and δ¹⁸O_NO3, with optimal stability achieved under specific conditions. Nitrate in the study area predominantly originates from organic sources, with nitrification as the main biogeochemical process, while denitrification was identified at sampling sites under specific anaerobic conditions. Although statistical analysis can be a valuable tool, it should be applied with caution if NO₃⁻ originates from multiple sources. The isotopic composition of water showed that groundwater is predominantly recharged by the Sava River but its contribution varied spatially. The results also show the existence of a different recharge source in the southern part of the aquifer. Our findings highlighted the importance of employing a diverse range of analytical methods to obtain reliable and comprehensive understanding of nitrate contamination. By integrating multi-method approaches, stakeholders can better understand the complexities of groundwater contamination and implement more targeted measures to safeguard the water supplies for future generations.