The aquifer captured on the wellfield Sikirevci is a part of the strategic water reserves of the Republic of Croatia. For effective management of these important water reserves it is necessary to know the dynamics of groundwater flow in the aquifer system under existing conditions which enables us to analyze the behavior of the system in a variety of future scenarios. The aim of the planned research is focused on a detailed study of hydrodynamic characteristics of the captured aquifer system, on defining recharge and also the origin and age of groundwater. Three-dimensional numerical model was used as a tool of defining recharge of the investigated aquifer system. This paper describes the process of developing a conceptual model and the development and calibration of numerical model of groundwater system. The model was calibrated for a period of five and a half years on the basis of measured groundwater levels and very good agreement between measured and calculated values was achieved. During field works the samples of groundwater, rainfall and river water were collected and the analysis of stable isotopes of oxygen and hydrogen, tritium and physico-chemical parameters were conducted. Isotopic and hydrochemical research helped in the development of a conceptual model. The concentrations of tritium in groundwater showed that the residence time of water is more than 65 years, which is consistent with the results of the simulation of particle tracking. Hydrochemical studies have also shown that flow rates in the aquifer system are relatively low, causing long residence time of water. Mass balance analysis showed that the infiltration of rainfall is the main source of groundwater recharge in the area of the model. The same was confirmed by the results of the analysis of stable isotopes of oxygen and hydrogen. The groundwater discharges in the Sava river, except during periods of high water levels when Sava river recharges the aquifer. Although the Save river forms hydraulic boundary in natural conditions, modeling results have shown that even relatively low pumping rates of 200 L/s cause the flow beneath the river from the neighbouringh Bosnia and Herzegovina to occur. This suggests transboundary character of the investigated groundwater system. The amount of inflow should be confirmed with the calibration of the right bank of the Sava river, which was not possible due to lack of groundwater levels data in that area.