Natural and Artificial Radionuclides in the Sediment, Water, and Fish Samples from the Sava River in Serbia, Slovenia and Croatia: Comparing Past and Recent Findings

This study investigates the activity concentrations of natural (40K, 238U, 226Ra, 232Th) and artificial (137Cs) radionuclides in the sediment, water, and fish samples collected along the Sava River in Serbia, Croatia, and Slovenia in 2022. Gamma spectrometric measurements showed that in the sediment samples from Serbia, the mean activity concentrations were 423 Bq kg−1, 31 Bq kg−1, 31 Bq kg−1, and 32 Bq kg−1, for 40K, 238U, 226Ra, and 232Th, respectively, in agreement with the natural background levels. Sediment samples from Slovenia and Croatia showed similar results, indicating uniformity along the riverbed. The water samples were within the acceptable range of the radionuclides’ concentrations for drinking water, while in fish samples, only 40K was detected. The study also presents long-term monitoring (1999–2022) of the 137Cs activity concentration in the Sava River sediment, combining data from three national programs. Cesium-137 was primarily present in sediment, with significant spatial and temporal variations. Due to the small 137Cs discharges from the Krško Nuclear Power Plant and relatively significant uncertainties in sampling and measurements, we can conclude that the radiation effects of the power plant are undetectable, and that the 137Cs activity concentrations in the Sava River sediment result from global contamination.