Radioactive isotope tritium (3H) and stable isotopes of hydrogen (2H/1H) and oxygen (18O/16O) were measured during 2010 in the Sava River, precipitation and groundwater at 3 monitoring wells and 1 production well of the Petruševec aquifer, close to the Sava River in the Zagreb area. Significant increase of 3H activity in the Sava River was observed in June, (200 ± 20) TU, and in groundwater of all wells with damped response (maximum 60 TU) and with delay of 3–5 months related to the Sava River. This increase was explained by release of tritiated water from the Krško Nuclear Power Plant, 30 km upstream from Zagreb in the beginning of June 2010. 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 wells of the Petruševec aquifer.
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