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 U-238, Ra-226, Pb-210, Ra-228, and K-40 in water, sediment, and fish samples, as well as Cs-137 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 Pb-210 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 K-40 activity concentrations (27.5 +/- 20.1 mBq L-1) in the Lake's waters might be one of the causes of increased Cs-137 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 Cs-137 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 mu Gy h(-1)) in comparison to sediment activity concentrations (0.03-9 mu Gy h(-1)). An incremental dose rate due to Cs-137 was found to be in the range of < 0.001-0.023 mu Gy h(-1) which, in comparison to background dose rates, can be considered negligible.

Automated summary

The study investigated the natural radioactivity of Plitvice Lakes and found significant variations in naturally occurring radionuclides in sediments of different lakes. The low K-40 activity concentrations in the Lake's waters may have contributed to the increased Cs-137 activity concentrations in fish samples. The influence of different input data on dose rate assessments was profound, with Cs-137 causing a negligible incremental dose rate compared to background levels.