This study demonstrates an approach to estimate the background value of nitrate as a basis for better groundwater management and protection in areas under long-term human impact. The aim was to determine the ambient background value (ABV) of nitrate in the catchment area of the Velika Gorica well field, a hydrogeologically homogeneous area within the Zagreb aquifer. ABVs are determined using four well-known model-based objective methods (the iterative 2-σ technique, IT; the calculated distribution function, CDF; the cumulative frequency curve, CFC; and the probability plot, PP), while simultaneously testing the reliability of the results of each method. If the results are not statistically significant, data selection is performed. The results show that using data without selection can lead to statistically non-significant ABVs, but with the additional selection of data, a statistically non-significant result became a statistically significant one. In summary, all final ABVs must be statistically significant and determined using as large a data set as possible. Reducing the size of the data set is acceptable only in the case of a statistically non-significant result.

The diversity of processes taking place in hillslope agro-ecosystems makes the estimation of vadose zone dynamics rather challenging. This paper presents the first insight into the research of volumetric water content, granulometric composition, mete...

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.

The unsaturated zone plays a crucial role in the hydrological cycle and is a key factor in modeling and understanding hydrological processes. It regulates water transfer from the land surface to the groundwater while providing protection, transfer, and attenuation of potential groundwater contaminants. This research presents the first results of continuous in situ monitoring of all soil horizons of the Eutric Cambisols and the remaining unsaturated zone of the Zagreb aquifer (Velika Gorica well field). The Zagreb aquifer is the main source of potable water for the inhabitants of the City of Zagreb and Zagreb County, which is protected by the Republic of Croatia. Cross-correlation analysis and water stable isotopes (${\delta }^2\mathrm{H}$ and ${\delta }^{18}\mathrm{O}$ ) were used to investigate whether there is a hydraulic connection between precipitation and groundwater, the approximate duration of percolation, and whether there are indicators that suggest the presence of preferential flow or processes such as water mixing and/or evaporation. The cross-correlation analysis showed and confirmed the assumption of the presence of preferential flow with average lag times ranging from 1.59 up to 3.63 h. The results also indicate that low organic matter content may contribute to higher lag times associated with precipitation infiltration. It was found that only high-intensity precipitation events allow effective infiltration, and that the amount of infiltration depends on the water content of the soil at the beginning of each precipitation event. The isotopic results showed that the isotopic composition of the soil water in the first three soil horizons follows the one of precipitation, while deuterium excess confirmed that the sampled water is mobile water and that an evaporation process is possible in the A horizon. The results also show that suction cups can be used to determine the isotopic signature of both bulk and mobile water, depending mainly on the granulometric composition of the material in which they are installed. Intensive and growing groundwater extraction in past decades now challenges humans to maintain the resource, especially in regions where climate is drying. Knowing which factor and to what extent drives the process of resource replenishment is the key to maintaining and securing the resource and thus sustainable water supply. Knowing what we can measure, and which methodologies we can use for processing the data we have measured, is of value to practitioners and water supply managers. An original combination of well-known methodologies (water stable isotopes analysis and cross-correlation analysis) as presented in this article suggests how can we process measured precipitation data in order to obtain the knowledge of the processes that drive resource replenishment. In addition, it suggests what kind of data we should measure and what kind of equipment we need for such a task.

Natural potential sources of nitrate contamination involve decaying of organic matter, bacterial production, atmospheric deposition, and soil N. The study presents the first results of nitrate distribution and migration through soil horizons of the Eutric Cambisols, one of the most common soils developed in the area of the Zagreb aquifer and situated in an area without potential anthropogenic sources of nitrate (first sanitary protection zone of the Velika Gorica well field). A total of 16 parameters of soil water and 16 parameters of soil were used to conduct statistical techniques and analyse associated factors within the soil zone. The results indicate that in the deepest soil horizon, nitrogen is present mostly as nitrate due to nitrification under aerobic conditions which promote stability and the potential for nitrate transport. It was found that nitrate concentrations are the result of soil N nitrification, caused by a NO₃⁻/Cl⁻ molar ratio higher than 1 and the absence of precipitation isotopic signature. The results also show that within the coarse-grained Eutric Cambisols N primarily migrates to deeper parts of unsaturated zone in the form of nitrate and nitrite.