[en] Full text: For almost 15% of Slovenia's inhabitants living in Ljubljana, the capital of Slovenia, two groundwater sources of drinking water are of great importance. An abundance of groundwater is hidden inside the sandy and gravely Sava river aquifer underneath the urban city area, called Ljubljansko polje, which is one of the largest underground reservoirs of drinking water in Slovenia. Ljubljansko polje is a tectonic basin by its origin and is, together with the second important groundwater resource - Ljubljansko Barje - a part of the Ljubljana basin. Ljubljansko Barje is highly complicated from the hydrogeological point of view - the variety of unconfined and unconfined aquifers stretching along the city suburbs in the South give us just a misty figure of the processes taking place in the sandy layers, in places covered by impermeable clayey layers and surrounded by karst mountains. The Ljubljansko polje aquifer is one of the most investigated Slovenian areas, because its groundwater has been used for public drinking water supply since 1890. Together with the increasing number of Ljubljana's inhabitants and consequently rising withdrawal quantities, groundwater quality began to show unacceptable deviations from the quality standards. The question of acceptable exploitation quantities that would not cause further decrease in groundwater quality was opened. The aim of isotope investigations was to determine the origin of the abstracted groundwater in more detail. The results served as helpful tools in determining priority tasks in planning future water exploitation and protection. Isotope investigations had not been applied in groundwater researches of Ljubljansko polje until recently. As an additional tool for understanding the groundwater recharge and flow of Ljubljansko polje groundwater, oxygen isotope composition was being determined during the period from autumn 1997 to autumn 1999. On the basis of results of previous hydrogeological investigations it was concluded that only two important sources of the Ljubljansko polje groundwater exist, local precipitation and the Sava river water. The two sources showed a noticeable difference in oxygen composition, which served as the basis of groundwater origin determination. The pumping wells included into the public water supply system were used as sampling points. Isotope investigations showed that the share of the river water and of local precipitation strongly depend on the sampling point location, namely on its distance from the Sava river recharging area. As a consequence, the values of physical and chemical parameters show annual changes according to the variations in the portion of the two sources. The sampling points with a high proportion of the recharging river water show better groundwater quality. In other words, human impact in the urban city area is the main reason for deterioration of groundwater partly flowing below the urbanised area of the Ljubljana City. Encouraging results of isotope investigations of the Ljubljansko polje led to the decision to start investigations on the second part of the Ljubljana basin - Ljubljansko barje. The exploitation of the aquifers located south of the city started in the mid-eighties of the previous century. It has been expected that this water resource with its mostly not urbanised recharge area could be protected from the anthropogenic influences more easily than the water resource in the close vicinity of the city. The main aim of the research, based on stable isotope (oxygen, deuterium) techniques, was to confirm the existence of different aquifers determined by former geological investigations and hydrogeological observations and to determine the differences in their recharge dynamics. The investigations started immediately after the conclusion of the Ljubljansko polje research in the autumn of 1999 and took place for two years. The results of isotope composition and chemical parameters, observed simultaneously, contribute to the understanding of groundwater origin, mean altitude of the recharge area, aquifer recharge dynamics and the relation between surface waters and groundwater. Two recent investigations, based on the determination of the stable isotope composition, give important results and form the basis for the decision of the future protection and exploitation of the Ljubljansko polje and Ljubljansko barje aquifers. (author)