The Radioactivity of River Sediments in Slovenia as a Consequence of Global and Local Contamination Denis Glavič-Cindro, Matjaž Korun "Jožef Stefan" Institute Jamova cesta 39, 1000 Ljubljana, Slovenia [email protected], [email protected] Milko Križman Slovenian Nuclear Safety Administration, Železna cesta 16 1000 Ljubljana, Slovenia [email protected] ABSTRACT In this article we discuss the activity concentrations of natural and man-made radionuclides in moving and bottom river sediments. The moving sediments were collected during a six-week period, while the bottom sediments were obtained by grab sampling. The concentrations of the radioisotopes were systematically larger in the moving sediments than in the bottom sediments. The contamination of the sediments with man-made radionuclides Cs-137 and I-131 was observed, as well as enhanced concentrations of natural radionuclides. Cs-137 was identified in all the samples; I-131 was identified at the locations where the influence of discharges from hospitals was expected, as well as at locations where there is no direct influence of these discharges. From a comparison between the concentrations of radioisotopes of the uranium and thorium decay series in different samples, the influence of industrial activities was identified. Elevated concentrations of U-238, Ra-226 and Ra-228 identified the probable influences of coal, lead and uranium-ore mining and the processing of monazite in the production of TiO2. 1 INTRODUCTION The Euratom Treaty [1] requires all the Member States of the European Union to monitor continuously the level of radioactivity in the air, water and soil. Whereas monitoring the air and soil is relatively easy, by pumping air through aerosol filters and by measuring the dose rate above the soil, the continuous monitoring of water is more demanding. The continuous pumping of water through filters in order to collect suspended particles in filters and filtrated water samples requires sophisticated sampling equipment, which needs frequent maintenance and is prone to malfunctions. In Slovenia there are only three continuous water sampling stations for radioactivity measurements in operation: one upstream and two downstream of the Krško NPP. However, the continuous monitoring of surface waters can also be achieved by sampling and making measurements of sediments, if the sampling is 911.1 911.2 preformed continuously, e.g., with sediment traps. The sediments adsorb the radioactive isotopes dissolved in the water and offer an insight into the pollution of the water. The floating sediment, which these traps collect, is more efficient in absorbing dissolved radionuclides than bottom sediment. Namely, the particles forming the floating sediment are smaller and the organic fraction is larger. For example, information on pollution due to anthropogenic activities (hospitals, nuclear and coal-fired power plants, uranium mining) can be obtained. Man-made fission products from atomic bomb testing and from the Chernobyl accident that originate from the upper soil layer and which are washed out by precipitation can also be measured. The radioactivity of river sediments is not controlled on a regular basis, except in the programs of the off-site monitoring of facilities, which are restricted to the immediate surrounding of the facility and to the area where the influence of discharges is expected. In this study the bottom-sediment samples were collected from all the main rivers in Slovenia at locations downstream of discharge points, where pollution is expected. In addition to these locations some locations where no influence of pollution is expected were included for a comparison. In addition to bottom sediments, moving sediments, collected by traps, were sampled at the same locations as the bottom sediments. Since some traps were lost, the number of moving-sediment samples is smaller. In this paper we will present the results of these measurements and draw some conclusions about the influence of anthropogenic activities on the radioactive contamination of river sediments. 2 METHODS The grab sampling was performed at the same locations as the moving sediments were collected with the traps. Since the material is taken from the ground, the bottom sediments resemble the properties of the riverbed to a greater extent than the moving sediments. During the sampling procedure 13 samples of moving sediments and 20 samples of bottom sediments were collected. The moving sediments were collected in traps during a 6-week period. The water flows more slowly through the traps than outside the traps and consequently the sediments tend to settle. However, the velocity of the water, which depends on the water level, determines the size of the particles that settle. Therefore, it should be noted that the conditions of the sampling are neither predictable nor reproducible. To achieve a high sensitivity of the measurements an amount of material greater than 200 g should be collected. The sample preparation includes drying at 60 °C and homogenisation of the sampled material. After their preparation the sediments were measured by gamma-ray spectrometry. The gamma-ray spectrometers that were used for these measurements are sensitive across a broad energy range, and therefore the concentrations of low-energy gamma-ray emitters, U-238 and Pb-210, were also measured. The concentrations of the nuclei in the uranium decay series, U-238, Ra-226, and Pb-210, Ra-228 and Th-228 from the thorium decay series, the primordial isotope K-40, the cosmogenic isotope Be-7, and the artificial isotopes I-131 and Cs-137 were measured. The sample preparation and measurements need to be done as quickly as possible to minimize the decay of short-lived artificial radionuclides, e.g., I-131. Proceedings of the International Conference Nuclear Energy for New Europe, Portorož, Slovenia, Sept. 8-11, 2008 911.3 3 RESULTS An example of the measurement results is shown in Table 1. Here, the concentrations of the radionuclides in moving and bottom sediments, collected in the rivers Koritnica and Voglajna, are presented. Table 1: Concentrations of radioactive isotopes in samples collected in the rivers Koritnica and Voglajna. Sampling location Koritnica - Log pod Mangartom Voglajna - Teharje Sampling date 14. 6. 2007- 26. 7. 2007 26. 7. 2007 11. 6. 2007- 23. 7. 2007 23. 7. 2007 Sample quantity (kg) 0,06 0,57 0,13 0,28 Sample code RP07SD52371 RP07SD52372 RP07SD32271 RP07SD32272 RADIONUCLIDE SPECIFIC ACTIVITY (Bq/kg) SPECIFIC ACTIVITY (Bq/kg) U-238 15,1±6,115,0±2,664,5±5,423,5±4,1 Ra-226 28,0±3,024,6±2,591,7±8,540,0±3,9 Pb-210 113,6±90,2< 78,8 136,0± 7,143,4±5,7 Ra-228 2,4±1,12,0±0,462,2±1,832,8±1,2 Th-228 1,9±0,61,7±0,247,3±1,331,4±1,0 K-40 32,5±6,330,8±3,6 515,0±50,1 492,7±48,1 Be-7 23,9±5,215,7±1,5 361,4±16,437,1±2,6 I-131 5,9±1,3 Cs-137 6,2±0,65,8±0,422,0±0,93,8±0,3 The radioisotope Cs-137 was detected in all the sediment samples. The largest concentration was measured in the river Drava in Ruše (47 ± 2) Bq/kg and in Maribor (21 ± 1) Bq/kg in the bottom sediment and (36 ± 3) Bq/kg in the moving sediment in Maribor. The sample of moving sediment from Ruše was not measured because the trap was lost. These high concentrations are due to the washout of the upper soil layer in the high mountain range in Austria, where this river flows. The mountain ranges in Austria received a comparatively high deposit of Cs-137 from the Chernobyl accident, up to 100 kBq/m2 [2, 3]. In general, the concentration of Cs-137 was higher in the moving sediments than in the bottom sediments. The lowest concentrations of bottom sediments, below 4 Bq/kg, were measured in the rivers Sava, Ljubljanica, Voglajna, Selška Sora and Poljanska Sora. The other artificial radioisotope detected in the sediments was I-131, which is used in nuclear medicine. Its concentration exceeded the detection limit in seven out of the thirteen moving sediments, and in four out of the twenty bottom sediments. The average concentration in the moving sediments was (7.8 ± 2.5) Bq/kg and in bottom sediments (1.1 ± 0.7) Bq/kg. At all locations where I-131 was detected in the bottom sediments the concentration in the moving sediment exceeded the concentration in the bottom sediment. The concentration of I-131 exceeded the detection level at the locations where the influence of the emissions from hospitals using I-131 is expected (Ljubljanica in Zalog, Sava in Hrastnik and Brežice, Drava in Ruše and Savinja in Tremerje) as well as at locations where there is no direct influence of these emissions (Voglajna in Teharje, Paka in Šoštanj, Selška Sora in Škofja Loka). At these locations I-131 was only detected in the moving sediment. It is assumed that the source of I-131 at these locations is patients being released from the hospitals, where they received therapeutic doses of I-131 and which through the sewage contaminate the rivers. Since the activities of I-131 are calculated assuming a constant contamination of the moving sediments throughout the period of its collection, the peak concentration in a contamination of short duration may be substantially higher. Proceedings of the International Conference Nuclear Energy for New Europe, Portorož, Slovenia, Sept. 8-11, 2008 911.4 In general, the concentrations of natural radioisotopes belonging to the uranium and thorium decay series and K-40 exhibited similar concentrations in the bottom sediments as in the upper soil layer. An exception was the sediment from Koritnica in Log pod Mangartom, where the concentrations of the nuclei from the thorium decay series were only around 2 Bq/kg.