Elena Anovska-Jovcheva’s research while affiliated with Saints Cyril and Methodius University of Skopje and other places

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 (< 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 defined 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.

In addition to the relevant geological, hydro-geological and hydro-meteorological data, the distribution of the Environmental isotopes, O-18/O-16, H-2/H-1 and Tritium concentrations in the local hydrological cycle (precipitation, surface, ground and Spring waters), have been observed and analyzed, all in order to determine the mechanism of recharge of the Rashce Spring, main supplier with potable water of the Skopje City, Republic of Macedonia. Although the Tritium content into the precipitation is approaching the natural level 4-10 TU, it still can be used as an efficient tool in hydrological investigations and in particularly, for the observed hydro systems where historical Tritium data are available. The obtained so far results, by creating computer model, offered determination of the volume of the groundwater aquifer that is feeding the Rashce Spring, receiving a figure of 4,683x109m3with a MRT = 27 years. These results were confirmed by T ( 3He )dating and noble gas data,as well, giving an additional possibility to estimate the vulnerability and better conservation of the investigated Spring waters. Acknowledgement: Authors are expressing their thankfulness to the Municipality of Skopje and to the IAEA (International Atomic Energy Agency), for their appreciated support of the performed research.

On the borders between FYR of Macedonia, Albania and Greece there are three lakes: Ohrid Lake, and Big and Small Prespa Lakes. Galichica and Dry mountains separate the lakes. Several Projects (Supported by IAEA, NATO, EU, UNESCO and others) have been dealing with the determination of the water balance and the protection of Prespa and Ohrid Lakes with special attention to the explanation of the decreasing of the Prespa Lake water level, in excess of 8m in the last decades. Investigations using Environmental isotopes and artificial tracers showed that Prespa Lake’s waters drain through the Galichica and Dry mountains into Ohrid Lake. By using an appropriate model, relevant hydrogeological, hydro meteorological data, satellite images and a computer program which implements an adequate recursive and exponential equations, the volume of the underground aquifer below Galichica Mountain and the volume of the Prespa Lake have been calculated. Keywords: environmental isotopes, lake’s watershed, tracer experiments, mean residence time.

The Prespa-Ohrid lake system is shared between Greece, Republic of Macedonia (Former Yugoslav Republic) and Albania. In the recent years a water decrease in the Prespa Lake has triggered a number of studies on this very complex and important water system. As part of an international research project in which institutions from the UK, Greece, R. Macedonia and Albania took part, which was supported by the NATO Science for Peace (SfP) programme, the problem of water loss in the Prespa Lake has been investigated. Previous studies have established that the water from the Prespa Lake flows into the Ohrid Lake through the Galichica Mountain. As part of the study the flow through the mountain has been analysed using a free surface flow model. The model has been solved using a multi-domain boundary element method approach. Keywords: Prespa-Ohrid lake system, free surface groundwater flow, Galichica aquifer. 1 Introduction Three lakes: Ohrid, Big Prespa and Small Prespa are on the borders between Albania, Republic of Macedonia and Greece, see Figure 1. A hypothesis published at the beginning of the 20th century [1] predicted that the water from the Prespa Lake drains into Ohrid Lake through the Galichica and Dry mountains, which separate the two lakes. Investigations involving the use of natural isotopes