Radioactivity of bottom sediments sampled in 1996 from the Romanian sector of the Danube river

Man-made and natural radioactivity of 22 bottom sediments sampled in spring 1996 from the principal cross sections along the Danube river, from the entrance in Romania to its mouths (delta included), and from the Romanian coast of the Black Sea were investigated by gamma-ray spectrometry. The gamma ray measurements were carried out by means of a high resolution, low background PC multichannel spectrometer, using a HPGe EG/G ORTEC detector of 30 percent relative efficiency and 2.1 KeV resolution for the 60 Co 1332.5 KeV line. As standards, the reference materials IAEA-135 (Radionuclides in Irish Sea sediment) and IAEA-306 (Radionuclides in Baltic Sea sediment) with certified radioactive concentrations were used. Counting times ranged between 14-24 hours. Specific activities of 134 Cs, 137 Cs, 226 Ra, 228 Ra and 40 K were determined. 137 Cs was present in all the samples with activity levels between 0.5±0.3 and 86.1±3.0 Bq/kg dry, while 134 Cs was measured in the range of 0.6±0.2 to 1.7±0.2 Bq/kg dry only at 5 sampling stations where higher activities of 137 Cs were recorded. 226 Ra, 228 Ra and 40 K concentrations were found to be at natural levels in all the samples. The highest natural and artificial radioactivity was found at Orsova and Svinita upstream to the Portile de Fier (Iron Gates) dam, at Cernavoda, km 300, and in the Black Sea off Razelm Lagoon at Gura Portita. For 226 Ra and 228 Ra, the highest specific activities were also determined at Stambulul Vechi, km 4, in the Danube Delta. Taking into account the mineralogical structure of the bottom sediments, it was observed that, in general, all the radionuclides analyzed were more efficiently retained in the silty clay-muds. It was noted that anthropogenic radioactive pollution of bottom sediments from the Romanian Danube river and the Black Sea is due mainly to the Chernobyl accident, an observation in good agreement with the results obtained during 1993-1995. (authors)