The Radioactivity of River Sediments in 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 and , 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 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 , , Voglajna, Selška 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 in Tremerje) as well as at locations where there is no direct influence of these emissions (Voglajna in Teharje, 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. This can be attributed to the type of sediment, which is almost completely limestone sand. As for the other radioisotopes, the concentrations of these radionuclides were higher in the moving sediments than in the bottom sediment. The concentration of Ra-226 exceeded the concentration of U-238. At all the locations the concentration of Pb-210 exceeded the concentrations of U-238 and Ra-226 in the moving and in the bottom sediments. This excess is assigned to the washout from the atmosphere, where Pb-210 is produced in the decay of radon. An important source of Pb-210 in the sediment could also be the decay of radon in the water. The concentrations of the radionuclides of the thorium decay series Ra-228 and Th-228 were almost in equilibrium in the samples of moving sediment as well as the bottom sediments from all the locations. The concentration of K-40 was in the range from 130 Bq/kg to 700 Bq/kg in the moving sediment and from 130 Bq/kg to 650 Bq/kg in the bottom sediment. Such concentrations are usually in the upper layer of the soil. Here, again, the river Koritnica represents an exception. The concentrations of K-40 in the moving and bottom sediments were around 30 Bq/kg. This could be attributed to the type of sediment. Be-7 is produced in the upper layers of the atmosphere by reactions between nitrogen and cosmic rays. It is washed out from the atmosphere by precipitation into the upper soil layer. The contamination of river sediments is due partially to the rainwater directly and partially to the washout of the upper layer of the soil. Since Cs-137 is washed out from the upper soil layer as well, a positive correlation between both concentrations is expected. Indeed, in the moving sediments and in the bottom sediments such a correlation was established. The correlation coefficient was around 0.5.

4 DISCUSSION

It is difficult to estimate the influence of anthropogenic activities on the contamination of sediments with natural radioisotopes belonging to the uranium and thorium decay series, since the initial concentrations are not known and the measured concentrations do not differ to a large extent from the concentrations expected to be present in the absence of pollution. Nevertheless the largest concentration of U-238 in the bottom sediments was measured in the river Rinža near Kočevje, (54 ± 7) Bq/kg. This contamination presents the influence of earlier coal mining near Kočevje. Because of the small influence of the anthropogenic activities on the concentrations, the concentrations themselves may not be a reliable indicator for contamination by anthropogenic activities. As an alternative, the comparison between the different properties of the samples taken at the same location may provide an indication of the influences that may be associated with the pollution from TENORM.

Proceedings of the International Conference Nuclear Energy for New Europe, Portorož, Slovenia, Sept. 8-11, 2008 911.5

4

3 Voglajna

Poljanska Sora 2

1 a(U-238,moving)/a(U-238,bottom)

0 0,50,60,70,80,91,01,11,2 ρ(moving)/ρ(bottom)

Figure 1: The correlation between the ratio of the density of the moving sediment sample versus the density of the bottom sediment sample and the ratio of the uranium concentration in the moving sediment versus the uranium concentration in the bottom sediment

Fig. 1 presents the correlation between the ratio of the densities of the moving and bottom sediments and the ratio between the uranium concentration in the moving and bottom sediments. The ratio of the densities measures the difference in the materials of the moving and bottom sediments. It can be seen that the correlation forms a nearly smooth dependence, with two outstanding points indicating additional contamination sources of moving sediments with U-238 to a larger degree than the bottom sediments. These sources are assigned to the production of TiO2 in the case of Voglajna and uranium mining in the case of Poljanska Sora [4]. The concentrations of U-238 and Ra-226 in the samples from the river Koritnica were around 15 Bq/kg and 26 Bq/kg, respectively, in the moving and bottom sediments. On the other hand, the concentrations of the radionuclides Ra-228 and Th-228, which belong to the thorium decay series, were around 2.2 Bq/kg and 1.8 Bq/kg. This large difference in the concentrations between the radionuclides belonging to the uranium and thorium decay series is attributed to the contamination from the former Italian mining of lead ore in Rabelj with U-238 and Ra-226.

5 CONCLUSION

In this study the sources of contamination of river sediments with artificial and natural radioisotopes were observed and analysed. The artificial isotope Cs-137 originates from the tests of atomic bombs and the Chernobyl disaster and the isotope I-131 because of its

Proceedings of the International Conference Nuclear Energy for New Europe, Portorož, Slovenia, Sept. 8-11, 2008 911.6 application in nuclear medicine. These radionuclides are transferred to the sediments by the erosion of the upper soil layer and via the sewage systems, respectively. I-131 was detected at locations where the influence of emissions from hospitals is expected as well as at locations, where there is no direct influence of these emissions. Both isotopes represent an indication of anthropogenic activities, since they are not produced in naturally occurring processes. The enhanced concentrations of natural radioisotopes belonging to the uranium and thorium decay series were found in moving sediments in Poljanska Sora and Voglajna. In Poljanska Sora enhanced concentrations of U-238 and Ra-226 indicate the influence of former uranium-ore mining and processing. In Voglajna elevated concentrations of U-238, Ra-226 and Ra-228 indicate the influence of the processing of monazite, which is used in the production of TiO2. Large concentrations of radionuclides belonging to the uranium decay series in comparison to the concentrations of the radionuclides from the thorium decay series in the sediments from Koritnica indicate the influence of the earlier mining of lead ore in the mine Rabelj. The largest concentrations of U-238 and Ra-226 in the bottom sediments were found in Rinža, and are recognized as an influence of the earlier coal mining. It should be mentioned that the elevated concentrations of natural isotopes rarely reach values which exceed twice the concentration expected in the absence of anthropogenic activities. It can be concluded, therefore, that the contamination of river sediments is an indication of anthropogenic sources of pollution, but these concentrations themselves do not contribute significantly to the exposure of the public.

REFERENCES

[1] http://europa.eu/abc/treaties/archives/en/entoc38.htm, article 35.

[2] M. Korun, D. Glavič-Cindro, Ovrednotenje rezulatov meritev radioaktivnosti rečnih sedimentov v Sloveniji posledica glabalnega in lokalnega onesnaževanja, IJS-DP-9702, Ljubljana, 2007.

[3] F. J. Maringer, Endbericht Studie “Tschernobylfolgen in Oberösterreich”, Universität für Bodenkultur Wien, Vienna, 2006, pp. 12-16.

[4] L. Benedik, G. Omahen, Nadzor radioaktivnosti v okolju Rudnika urana Žirovski vrh med izvajanjem programa trajnega prenehanja izkoriščanja uranove rude in ocena vplivov na okolje, IJS-DP-8753, Ljubljana, 2003.

Proceedings of the International Conference Nuclear Energy for New Europe, Portorož, Slovenia, Sept. 8-11, 2008