Long Term Surveillance of the Vojvodina Agricultural Soil

LONG TERM SURVEILLANCE OF THE VOJVODINA AGRICULTURAL SOIL

Ištvan Bikit, Sofija Forkapić, Dušan Mrđa, Miroslav Vesković, Nataša Todorović, Jovana Nikolov and Kristina Bikit Department of Physics, Faculty of Science, University of Novi Sad, Novi Sad, Serbia [email protected]

INTRODUCTION Vojvodina is the northern province of Serbia. It is an agricultural region located in the Pannonian Plain of Central Europe. The soil of Vojvodina may contain radioactive contaminants from different sources. There are nuclear power plants in the South East Europe region that could contaminant this soil through the release of radionuclides into air and water. The application of phosphate fertilizers with high uranium concentration may also cause a gradual increase of the uranium series activity concentration in soil. The concentration of uranium and thorium in Earth’s crust is in the range 1.1 – 10 ppm [1] for uranium and 10 ppm [2] for thorium. These values correspond to an activity concentration range of 13.5 – 123 Bq/kg for 238U and 39.4 Bq/kg for 232Th. The intensity of the processes of soil sorbtion/desorbtion, migration, retention and translocation is influenced by the nature of the given radionuclide, the type of soil and of crops grown on it, and the climatic conditions. Nuclear Physics Group in Novi Sad has more than 30 years long tradition in low-level gamma spectrometry measurement of agricultural soil in Vojvodina region and the area of Novi Sad. In this paper we compared the results of soil monitoring from 2011 with those results obtained during the year 2001. We used similar measurement technique, and the samples were collected from the same locations.

MEASUREMENT METHOD Activity concentrations of radionuclides gamma emitters were determined by the method of low-level gamma spectrometry on actively and passively shielded germanium detectors with maximal background reduction. Two high resolution HPGe detectors were used. First of them,

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produced by CANBERRA, has nominal efficiency of 36 % and resolution of 1.79 keV. The detector was operated inside the 12 cm thick lead shield with 3 mm Cu inner layer. Second one, germanium detector made by ORTEC, was extended range GMX type detector (10 keV – 3 MeV) with nominal efficiency of 32 % and resolution of 1.9 keV. The detector was shielded with the cylindrical lead shield of 12 cm wall thickness. Surrounding the lead shield, the five 0.5 m × 0.5 m × 0.05 m plastic veto detectors, produced by SCIONIX, were placed and operated in anticoincidence mode with Ge detector. The active shield reduces the integral background by factor 3 in the energy range from 50 to 2800 keV [4]. The gamma spectra were acquired and analyzed using the Canberra Genie 2000 software. A special procedure developed in the Novi Sad laboratory was used for the determination of the 238U activity concentration from gamma-lines of the first progeny of this radionuclide, 234Th [5].

RESULTS The samples of soil were collected from 50 locations in Vojvodina region (Figure 1) and about 300 locations in Novi Sad municipality. From each location of an approximate area of 10 m × 10 m subsamples were collected, mixed and homogenized. The soil was sampled from the surface (0 – 5 cm). Soil samples were dried at 105 oC to constant mass and mechanically fragmented and homogenized as fine powder. Prepared soil samples were packed in cylindrical measurement utensils 62 mm in height and 67 mm in diameter. Typical measurement time was 80 ks.

Figure 1. The sampling locations in Vojvodina

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Obtained results of gamma spectrometry measurements of soil samples for Vojvodina region are presented in Table 1. Activity concentrations of fission and corrosion products (except 137Cs) were below detection limits, therefore in the final results only the activity concentrations of 137Cs, the natural radioactive series of 238U and 232Th, and the natural radionuclide 40K are presented.

Table 1. Arithmetic means (A), standard deviations () and range of measured radionuclide activity concentrations in agricultural soil of Vojvodina region

radionuclide A [Bq/kg] σ(A) [Bq/kg] Range [Bq/kg] 40K 554 92 238 – 1000 226Ra 35.4 8.1 9.7 – 49.1 232 Th 38 10 6 – 64 238 U 42 20 5 – 80 137Cs 8.8 6.2 0.7 – 55

The activity concentration of the natural radioactive series of 238U and 232Th, and the natural radionuclide 40K are also at the normal environmental levels. The activity concentrations of natural radionuclides in soil samples confirm the ln-normal distribution and good correlation (Figure 2). The distribution of 137Cs is skewed which is typical for a man-made contaminant.

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150 30

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50 number of soilnumber samplesof 10

activity concentrationsTh-232activity [Bq/kg] of 0 0 0 10 20 30 40 50 1 2 3 4 5 6 activitiy concentrations of Ra-226 [Bq/kg] Ln of U-238 activity concentrations Figure 2. Correlation of natural radionuclides in agricultural soil

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CONCLUSION The activity concentrations of the natural radioactive series of 238U, 232Th, 226Ra and the natural radionuclide 40K in agricultural soil in Vojvodina region are at the normal environmental levels. The radionuclide 137Cs was identified in all soil samples. It originates from nuclear weapons test fall-out and, after 1986, mostly from the accident of the nuclear power plant "Lenin" in Chernobyl. Due to the 30 year half-life of this radionuclide, it will be relocated, washed out and redistributed. However, it will be present for a long time in the Vojvodina ecosystem. If we compare 137Cs activity concentration results with previous measurements, we note a slight decrease after 2001. Since the 238U activity concentration in all samples is at the natural environmental level, and the 238U/226Ra ratio is not substantially changed, one can conclude that in the measured samples there is no indication of depleted uranium presence. If we compare the recent 238U activity concentrations with previous measurements, we note a slight discrepancy probably due to use of phosphate fertilizers. Taking into account the transfer factors of present radionuclides to plants, the measured activity concentrations of radionuclides in Vojvodina agricultural soil should not endanger the health safety of the produced food.

Acknowledgment The authors acknowledge the financial support of the Ministry of Education, Science and Technological Development of Serbia, within the Projects No171002 and No43002.

REFERENCES [1] Andrejeva OS, Badjin VI, Kornilov AN. Natural and depleted uranium. Atomizdat, Moscow 1987. (in Russian). [2] Kikoina IK. Tables of physical constants. Atomizdat, Moscow 1976. (in Russian). [3] Bikit I, Slivka J, Čonkić Lj, Krmar M, Vesković M, Žikić-Todorović N, Varga E, Ćurčić S, Mrđa D. Radioactivity of the soil in Vojvodina (northern province of Serbia and Montenegro). Journal of Environmental Radioactivity 2005;78:11-19 [4] Bikit I, Forkapić S, Mrđa D, Todorović N. Study of Active Shielding for Gamma Spectrometers in FINUSTAR-2005. In: HarissopulosSV et al. (eds). AIP Conference Proceedings 831, AIP, Melville, NY, 2006: 409 – 411. [5] Bikit I, Slivka J, Mrđa D, Žikić-Todorović N, Ćurčić S, Varga E, Vesković M, Čonkić Lj. Simple method for depleted uranium determination. Japanese Journal of Applied Physics 2003;42:5269–5273.

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LONG TERM SURVEILLANCE OF THE VOJVODINA AGRICULTURAL SOIL

Nuclear Physics Group in Novi Sad has more than 30 years long tradition in low-level gamma spectrometry measurement of agricultural soil in Vojvodina region and the area of Novi Sad. When the issue of depleted uranium emerged, the experimental advantages of the measuring equipment (GMX type of HPGe detector with enhanced efficiency below 100 keV, and iron low-level shielding) were fully exploited. A detection technique selective for depleted uranium was developed. The details of this method together with the results for about 400 samples of soil are presented and discussed. Based on the assessment of radionuclides transfer factors from soil to plants the safety of the food grown on the investigated soil is considered.

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