Distribution and transfer of naturally occurring radionuclides and 137Cs in the freshwater system of the Plitvice Lakes, Croatia, and related dose assessment to wildlife by ERICA Tool

The aim of this study was to investigate the natural radioactivity of Plitvice Lakes, under the assumption that due to its status as a National Park, the area can be considered an example of a natural freshwater system. Also, considering the transfer parameter data as the largest source of uncertainty in radiological risk assessments, the impact of site-specific data on dose rate assessment, as opposed to currently available data, was investigated. The study included gamma and alpha spectrometric measurements of ²³⁸U, ²²⁶Ra, ²¹⁰Pb, ²²⁸Ra, and ⁴⁰K in water, sediment, and fish samples, as well as ¹³⁷Cs due to the coinciding of the study with the Fukushima accident. The content of naturally occurring radionuclides significantly varied in sediments of different Lakes, probably as a reflection of the different underlying geology of the area. Also, the ²¹⁰Pb distribution in sediments indicated an up to 312 Bq kg⁻¹ of the allochthonous contribution of this radionuclide at the beginning of the Lake’s watercourse, which probably entered into the lake system by the major inlet river with its steady decrease along downstream lakes. Low ⁴⁰K activity concentrations (27.5 ± 20.1 mBq L⁻¹) in the Lake’s waters might be one of the causes of increased ¹³⁷Cs activity concentrations in fish samples (1.5 ± 0.4 Bq kg⁻¹), which was found to be an order of magnitude higher than average values for different fish species from other Croatian freshwater systems (0.2 ± 0.1 Bq kg⁻¹). A temporary increase of ¹³⁷Cs activity concentrations was measured in water samples collected immediately after the Fukushima accident. Calculated site-specific sediment/water distribution coefficients and fish/water concentration ratios for radium and caesium were on average lower than generic ones found in the literature. Background dose rate assessments performed by the ERICA Tool indicated a profound impact of different input data on assessment results with water activity concentrations resulting in significantly higher dose rates (0.1–67 μGy h⁻¹) in comparison to sediment activity concentrations (0.03–9 μGy h⁻¹). An incremental dose rate due to ¹³⁷Cs was found to be in the range of < 0.001–0.023 μGy h⁻¹ which, in comparison to background dose rates, can be considered negligible.