The main purpose of this study was to understand the interactions between precipitation, surface water, and groundwater in the Zagreb aquifer system using water stable isotopes. The Zagreb aquifer is of the unconfined type and strongly hydraulically connected to the Sava River. As the groundwater is the main source of drinking water for one million inhabitants, it is essential to investigate each detail of the recharge processes of the aquifer to ensure adequate protection of the groundwater. Measuring the content of water stable isotopes in surface waters and groundwater enabled the creation of two- and three-component mixing models based on the isotopic mass balance for the purpose of the quantification of each recharge component. The mixing models gave ambiguous results. Observation wells equally distant from the Sava River did not have the same recharge component ratio. This indicated that there were more factors (in addition to the distance from the river) that were affecting groundwater recharge, and the properties of the unsaturated zone and surface cover data were therefore also taken into consideration. The thickness of the unsaturated zone and the characteristics of different soil types were identified as important factors in the recharge of the Zagreb aquifer. The areas with high thickness of the unsaturated zone and well-permeable soil had a very similar recharge component ratio to the areas with small thickness of the unsaturated zone but low-permeable soil.

Nitrates represent one of the main groups of contaminants in the Zagreb aquifer system. Some natural groundwater quality indicators can have a significant influence on their stability and mobility in the saturated zone. Correlation and multivariate statistical analyses were used to test the correlation of average values of NO 3with O 2 , ORP, pH, EC and temperature of groundwater, and to allocate observation wells that belong to the same clusters. ORP values didn`t relate to any observed variables, which is probably due to their variability which suggests changes in the oxidation-reduction conditions in the aquifer system. Principal component analysis was used for the determination of variables that are related to the nitrate concentrations and which were then used in cluster analysis. Other variables were excluded from cluster analysis. Three methods were used to perform cluster analysis, where the results calculated with Ward`s method were chosen as the most appropriate. In the end, two clusters were identified, one with smaller, and one with higher NO 3 -, O 2 and EC values. Observation wells from cluster 1 are generally located near the Sava River and have similar nitrate concentrations. Lack of other nitrogen species and moderately aerobic conditions suggest very fast nitrification in the shallow Holocene aquifer.

Nitrates present one of the most common groundwater contaminants in the world and one of the five major groups of contaminants in the study area. Gaussian simulation (GS) algorithm was used for determining the spatial distribution of average nitrate concentrations from 2010 to 2015 on 95 sampling points. Results indicate two main focus areas of nitrate contamination, located on the left and right bank of the Sava River. Those areas generally extend according to groundwater flow, while areas near Sava River have much smaller concentrations. GS showed that they can be useful for this kind of mapping because they favor abrupt changes in data values which are in this case a result of heterogeneous lithological composition of the aquifer. ARTICLE HISTORY

Nitrates are among the most common groundwater contaminants worldwide, and the same situation is present within the Zagreb aquifer. The Zagreb aquifer presents the only source of potable water for inhabitants of the City of Zagreb and part of Zagreb County. Isotopic composition of water (δ2H and δ18O) and nitrates (δ15N and δ18O), groundwater chemistry, and molar ratios, in combination with correlation and multivariate statistical methods, have been used for the estimation of nitrate origin. Nitrate stable isotopes excluded synthetic fertilizer as the main source of nitrate contamination. They showed insignificant influence of denitrification on nitrate concentrations but could not define the main source of nitrate contamination. The usage of molar ratios, especially NO3−/K+, helped to clarify this issue. Waste water has been defined as the main source of nitrate contamination. All results indicate that nitrogen in a large extent enters the aquifer in the form of ammonium ion, which is transformed to nitrates by the process of nitrification.

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