Application of environmental isotope techniques for determining the origin and mechanisms of recharge of the Rashche Spring

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 karstified Zeden massif. Investigations were made to define 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-fit equation of δ2H = 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 (