Environmental monitoring of radionuclide pollutants in Black Sea sediments

Sediment samples from the Bulgarian Black Sea shelf were analyzed for radionuclide content during eleven consecutive years (1991 - 2002) from 35 locations. These data together with heavy metal content were compared with samples taken from definitely clean areas. The coast of Bulgaria, extending some 170 km is mainly soft sedimentary limestones or sandstones, overlaid in many areas with beach or wind-blown sand. In the north, suspended material from the Danube delta has settled, carrying a lot of intertidal mud. The distribution of anthropogenic 137Cs, 134Cs and natural 238U, 232Th, 226Ra radionuclide content in Black Sea bottom sediments - slime, silt and sandy is obtained by low level gamma spectroscopy. The obtained data show that radionuclides concentrations strongly depend on the sediment nature - results for sandy sediments are within close range, while higher concentrations are measured in slime and silt shelf samples. The near-shore sediment matrixes - sandy, slime or silt are compared with deep-sea ones to determine the effect of depth on radionuclide sorption. The dependencies of radionuclide concentration on sampling year and location are shown. Cs is found to be strongly bound by slime sediments and also disequilibrium between natural nuclides is observed. Statistical treatment of the obtained data is performed, showing correlations between sediment nature depending on sampling year and location. A comprehensive database for the nuclide concentration values for near-shore and deep-sea sediments is created to enable the modeling of radionuclide transfer, accumulation and influence on marine ecosystems. (authors)