ABSTRACT: The article presents conceptual model of surface and groundwater interactions in part of the inland delta of the Danube River, south Slovakia inferred from isotopic and water chemistry data. Quaternary sediments from the Žitný ostrov Island provide one of the largest sources of high quality drinking water in Europe. Žitný ostrov Island is bordered by the Danube (Dunaj) River in the south, its Malý Dunaj left branch in the north and the Váh River to the east. The groundwater in this area mainly comes from Danube bank infiltration. In the north of the Žitný ostrov Island approximately 2,000 L/s flow (Klátovské rameno) gradually rises to the surface over 25 km. Its precise origin has not been determined before. Since the Danube water isotopic composition differs vastly from that in local water, d2H and d18O monitoring is an appropriate method of identifying the exact origin of groundwater discharging into the Klátovské rameno stream. Results from over one year of monitoring show that at least part of this groundwater originates from bank infiltration of local rivers and reaches the surface after flowing beneath the Malý Dunaj River bed. Isotopically enriched groundwater forms an approximately 30 meters deep layer on both Malý Dunaj River banks, while there is isotope depletion in groundwater apparently of Danube origin in deeper parts of the Quaternary. In addition, the groundwater in underlying Neogene sedimentary-confined aquifers is also isotopically depleted and this is most likely recharged in a colder climatic era.