engleski

Preliminary results of Uranium and Radium content in Krka River Estuary

Radioactive material in the Šibenik area comes from phosphates that are unloaded at the Šibenik port. Phosphate materials used for the production of phosphates fertilizers contain a minor quantity of radioactive material, mainly the disintegration products of the 235U series (Barišić at al. 1992). Shipments of phosphates to the port of Šibenik port commenced around 1966. Maximum traffic was at the year 1988 (740 000 t) with break (1992-1995) during war. Last year phosphates traffic was around 300 000t. Samplings of marine sediments were performed by scuba diving with plastic corer during 2001 & 2002 at 26 underwater locations (03-38 meters depth). Also were sampled terrestrial lend samples in direction of prevailing winds (Jugo and Bura). After sampling, the sediments were frozen at - 18°C and kept until analyses. Before the measurements the samples were thawed at room temperature and dried at 106°C. The activities of 40K, 232Th, 137Cs, 226Ra and 238U in samples were determined by gamma-ray spectrometry using low background hyper pure germanium (HPGe) semiconductor detector system coupled to a 4096-channel analyzer. Depending on sample mass and activity, spectra were recorded for times ranging between 80, 000 and 150, 000 second. Double counting errors were taken as detection limit. Sediments with highest (226Ra and 238U) activity were found near "phosphate" harbor, while lowest activities were near island Zlarin and this sampling point represent normal values for marine carbonate sedimentation. Normal values for sedimentation with terrigenous influence we found at stations E2, E3, E6 and E9 (Prokljan Lake). High values of 226Ra and 238U are concentrated near (few hundred meters) the location where phosphates are unloaded at Šibenik Port. Sub aquatic topography (shelf parallel with coast line) separates these values from middle ones and show as that main transport was with bottom currents. The Krka River estuary functions as a big pump (fresh water flow on the surface towards the sea and opposite flow of salt water in estuary) and because of that, phosphates were mostly transported toward to waterfalls. Terrestrial samples show that air transports by wind were probably very rare and low. At several locations increased values of 226Ra and 238U in sediment were found in more than 20 cm depth, and sedimentation rate in the estuary was found to be very low, less then 0, 5 mm at the Prokljan Lake (Juračić & Prohić, 1991). This implies that 226Ra and 238U were migrated through sediment, probably by bioturbation. Since uranium does not seem to be concentrated to any significant extend by marine organisms, apart from skeletal material and it is improbable that biological process should play an important role in enrichment of uranium in marine sediment (Kniewald & Branica, 1988). Even strong bioturbation does not fully explain distribution of 137Cs in sediment. The sedimentation rate in Krka River Estuary were probably at least 2 mm per year (even much more in the harbor) and several geochemical processes are responsible for enrichment of 226Ra and 238U in marine sediment. Those processes and influence of bioturbation in marine sediments of the Krka River Estuary will be the focus of the future investigation.

sediments; Uranium; Radium; Krka River Estuary

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