Definition of Groundwater Genesis of the Vidlič Mt. Complex Karst System as a Basis for Groundwater Utilization

The investigation of complex karst systems has always been a difficult task for hydrogeologists, especially related to the spatial position of karst channels. The city of Pirot, which is located in southeastern Serbia, taps karst water for water supply from three karstic springs (Kavak, Krupac and Gradište), which are characterized by extremely good and stable groundwater regime and quality. According to the general water regime, it can be concluded that in addition to the development of shallow and large karst conduits (as proven by tracer tests), there are also deeper karst channels, in which water circulates very slowly and remains for much longer. In order to understand the genesis and characteristics of karst springs used for water supply, multi-parameter research was conducted, which, in addition to monitoring the yield on a daily basis, also included detailed hydrochemical research together with an analysis of microelements and stable and radioactive isotopes. Water from springs has a stable hydrochemical composition highlighting prolonged contact with the host rock. Isotopic analysis showed that the water is a mixture of young waters (residing in the system for a few days, as determined by tracer tests); semi-young waters that, based on the radioactive isotopes 3T and 3He, have resided in the system for 53 years; and very old waters that have spent more than 3000 years in the system. Finally, such new data on significant dynamic as well as static reserves of quality drinking water are of particular importance for long-term sustainable water utilization.