Реферат: This study is focused on the groundwater system of the Rashche Spring which serves as the main source of drinking water
for the City of Skopje, located at the foot of the widely karstifed Zeden massif. Investigations were made to defne the
mechanisms of groundwater recharge and its residence time. This was done using isotope tracers including 1H, 2H, 3H, and 18O in water samples from the local hydrological cycle, as well as noble gases for the application of 3H/3He dating method. A linear regression model of stable isotope data of precipitation samples resulted in the best-ft equation of δ2 H = 7.46·δ18O + 9.56, and shows a δ2H dependence from the altitude of precipitation formation with a gradient of −3.06‰/100 m. Local precipitation, surface waters and Rashche Spring water showed a relatively narrow range of tritium values of 5 ± 1 TU, whereas the deep artesian water from Polog Valley yielded very low tritium contents (0.5 TU). In addition to this, we applied a one-cell mixing model with three inlet components (precipitation, Vardar River and deep artesian water). Numerical simulation based on tritium data of local precipitation and based on a defned ratio of the inlet components allowed calculation of the MRT (mean residence time) of 30 years for the water of the aquifer located below the Zeden Massif which is recharging the Rashche Spring. Using the 3H/3He dating method, the ages of observed ground water including the water from the Rashche Spring were determined showing a good agreement with the calculated MRT. This result also provides substantial information about the vulnerability of this water resource and the necessity for better conservation of the investigated local surface and groundwater.