engleski

Study of the bank filtered Zagreb aquifer system using isotope analyses near well field Kosnica

The Sava River is the longest river in Croatia and the main source of water for the Zagreb aquifer system. The fluctuations of the Sava River water levels cause fluctuations in the groundwater levels of the Zagreb aquifer, especially in the vicinity of the Sava River. Due to constant decrease of groundwater levels and possible groundwater quantity issues, especially in dry periods, construction of a new regional well field Kosnica is in progress. Kosnica well field is considered as a main well field supplier for Zagreb and determination of recharge areas is of prime concern, as such areas have to be protected in order to preserve good groundwater quality and quantity. Groundwater often consists of a mixture of recharge from surface water (lakes or rivers) and local precipitation. It is important to know the proportions of the recharge components in order to increase the sustainable supply of drinking water through bank infiltration, and to prevent drinking water pollution by infiltration of water from a contaminated surface water source. Different recharge components can be identified through the stable isotope composition of groundwater (2H and 18O) because of different isotope composition of end components and evaporation in surface water bodies. Tritium has also been used as a good tracer. In the case of the Sava River there is additional source of anthropogenic tritium from Krško Nuclear Power Plant (30 km upstream from Zagreb) which can be used as a tracer. Samples for determination of isotopic composition, 3H activity and 2H and 18O, were collected during 2016 in the Sava River, precipitation (monthly samples from Zagreb–Grič, meteorological station included in GNIP–Global Network of Isotopes in Precipitation) and groundwater at 10 monitoring wells close to the Sava River. 3H activity was measured by liquid scintillation counter (Quantulus 1220) and samples were electrolytically enriched before measurement. 2H and 18O were determined by Liquid water isotope analyser, Los Gatos. Significant increase of 3H activity in the Sava River was observed in October 2016, (178 ± 21) TU, and in groundwater of the observation well nearest to Sava River with damped response (maximum 35 TU) and with delay of 2.5 months related to the Sava River. This increase was explained by release of tritiated water from the Krško Nuclear Power Plant. Stable isotope analyses showed similar range of δ2H and δ18O values for the Sava River and groundwater samples with higher variations in surface water. Differences in monthly variations of δ18O values between particular monitoring wells, together with 3H values, indicated different infiltration times of surface water of the Sava River to different observation wells in the vicinity of Kosnica well field. Research and purchase of stable isotope analyser was financed by IAEA TC Project, CRO 7001.

Sava River ; Kosnica well field ; 18O ; 2H ; tritium

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