Groundwater gravitational circulation of Karst Veliko Vrelo and Malo Vrelo springs by isotope and the noble gas method: case study of the Beljanica Massif

Defining groundwater circulation velocity and residence time in the karst is of particular importance, especially for purposes of water supply use. Investigations were performed at Veliko Vrelo and Malo Vrelo springs, which are typical gravitational circulation karst springs of the Beljanica massif, for determining groundwater residence time, both during the high-flow period, which is characterized by rapid water circulation (spring period), as well as in the recession period (end of summer) when water circulates significantly more slowly through smaller and relatively deeper karst channels. For purposes of defining the different groundwater residence times and circulation velocity, besides tracer test (Na-fluorescein), stable isotope methods ¹⁸O, ²H, ¹³C as well as radioactive isotopes ³H and ³H + ³He were used. Additionally, to determine the temperature conditions that prevail, as well as recharge duration in the vadose zone during the groundwater recharge period, the content of noble gases in groundwater was measured. The results reveal the dual development of karst conduits, where the first circulation zone is characterized by larger channel dimensions where groundwater rapidly circulates over a period of only several days, while the second zone consists of smaller channels and cracks, in which the waters reside 3 or 4 months during the recession period.