A Groundwater Recharge Model for the Zagreb Aquifer System (Croatia): Current Status and Proposed Enhancements

Groundwater of the Zagreb alluvial aquifer in northwestern Croatia represents an extremely important natural resource that supplies drinking and industrial water to the City of Zagreb and its surrounding area. The aquifer system consists of Quaternary sediments, within which an alluvial aquifer of intergranular porosity has developed. The thickness of the aquifer varies from approximately 10 m in the western part of the area to up to 100 m within the eastern structural depression. Hydraulic conductivity is very high and, based on pumping test results, reaches values of up to 3600 m/day in the vicinity of the Sava River. The aquifer is unconfined and is recharged through both infiltration of precipitation and leakage from the Sava River riverbed. Previous studies have estimated groundwater recharge using groundwater balance calculations, which were subsequently verified by a numerical groundwater flow model. For the purposes of numerical modelling, the alluvial aquifer area was divided into three polygons with different groundwater recharge values: (i) areas with a thin or absent semipermeable covering layer at the top of the aquifer, (ii) marginal aquifer areas to the north and south, composed predominantly of fine-grained deposits, and (iii) urban areas. Although the calibration results of the model are decent and the obtained recharge values appear realistic, there is still a need for more accurate and detailed determination of groundwater recharge in both spatial and temporal domains, with respect to both leakage from the Sava River and precipitation percolation. To address these limitations, extensive investigations are planned within the framework of the bilateral Croatian–Slovenian project GWQualityPath2070 (HRZZ: IPS-2024-02-5367). The research program comprises the analysis of the isotopic composition of groundwater, river water, and precipitation, the investigation of surface water and groundwater dynamics, and the integration of the SWAT semi-distributed hydrological model with a numerical groundwater flow model. Expected results of the research activities, in general—and in particular those related to the groundwater recharge model—will provide a solid basis for further research, primarily focused on analyses of climate change impacts on the quantitative status of groundwater, as well as the effects of different agricultural practices and land use on groundwater quality.