Natural and artificial radionuclides distribution in soil in Murfatlar area, Dobrogea region, Romania

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1 Natural and artificial radionuclides distribution in soil in Murfatlar area, Dobrogea region, Romania ROMUL MIRCEA MARGINEANU 1*, AURORA RANCA 2, PETRU BOLOS 2, MARIANA CLAUDIA GOMOIU 1, CORINA SIMION 1, VIOREL TOBOSARU 3, ANA-MARIA BLEBEA- APOSTU 1 1 Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering, IFIN-HH Bucharest-Magurele, Romania 2 Research and Development Station for Vineyard and Vinification, SCDVV Murfatlar, Romania 3 University of Bucharest, Faculty of Physics, Romania Abstract. In this paper we intend to study the distribution of natural and artificial radionuclides in a small area, namely Murfatlar vineyard Romania. The Research and Development Station for Vineyard and Vinification covers an area of about 160 ha. Geographically, the investigated area is located in the South East of Romania in Dobrogea, about 18 km from the Black Sea. From this place we took 82 soil samples, from surface to a depth of 10cm. Soil samples, after conditioning, were measured by high resolution gamma spectrometry in underground laboratory of IFIN-HH situated in Unirea salt mine from in Slanic-Prahova. In these samples we determined the specific activity of Cs137, K40, Ac228 and Ra226. For Cs137 the average value is of 6.3 ± 1.8 Bq/kg and for K40 we have average value of 582 ± 64 Bq/kg. The average values for Ra226 is 45.9 ± 5.4 Bq/kg and for Ac228 is 44.1 ± 3.9 Bq/kg. Also, the data were statistically analyzed using three different tests for normal distribution. Key words: Natural and artificial radioactivity, radiation background, underground laboratory, radioactive isotopes, nuclear accident, statistical tests for normal distribution * Corresponding Author: romulus@ifin.nipne.ro

2 2 1.1 Environmental conditions 1. INTRODUCTION The investigated area is situated in the center of Constanta County, on the slight slopes that surround the Danube River - Black Sea Canal, fig. 1, [1]. Geographically, the area is situated at ' North latitude and ' East longitude, in the Carasu valley, clogged during time, due to movements of the earth crust and erosion of adjacent coasts. Fig. 1 Dobrogea map with Murfatlar position, image from Google Earth The investigated area is located generally on the plateau with beveled sides and gently sloping hillsides and having an expositions predominantly Southern and South-Western on Tortoman plateau. Carasu valley that separates the Tortoman and Cobadin plateaus can be considered as a synclinal depression, this valley is currently Danube River-Black Sea Canal. The slope of fields in the Murfatlar area shows different inclinations varying from 2.0% to 6%. Groundwater has little influence on the soil of this area because it is located at great depths, at the base of tens of meters thicknesses leoss layer, without affecting the soil profile.

3 Concerning the soil, the predominance of loess deposits is evidentiated, interrupted by appearances of shaped island of Sarmatian limestone. The loess has a loamy texture in the area of Murfatlar region. Calcareous cernoziom soil is dominant in areas under plants and, following agricultural works, anthrosols were formed. Murfatlar area, located in the South-East of Romania, is close to the 30 km radius intervention zone of Cernavoda Nuclear Power Plant under the conditions in which the Black Sea vicinity influences the climate of this region. The Danube and the Danube - Black Sea Canal is an important source of water for the population and for irrigation in the investigated area. For this reason, there is a great need for monitoring the Danubian and Dobrogean areas around nuclear facilities Environmental radioactivity About a radionuclide we say that it is natural, when its existence is the result of processes that occur or have occurred in nature without human intervention. In contrast a radionuclide is called artificial, when its existence is the result of human activities. Radioactive substances in the environment can be divided into two large groups after their origin. Depending on the origin, we are talking about natural or artificial radioactivity. By environmental radioactivity, we understand all radioactive phenomena occurring in environment. Basically, it refers to all radioactive substances which are present in the environment. Natural radioactivity of the environment is due primarily of radionuclides from the series U238, U235, Th232 and 40. Due to nuclear power plants, laboratory facilities and nuclear explosions in the atmosphere, a new source of environmental radioactivity, artificial radioactivity has occurred. Cs-137 has been recognized as one of the radiologically most important long-lived radionuclides of anthropogenic origin, which has accumulated in the environment. This was considered to be the most important for long-term radiological impact due to large emissions, relatively long half-life and relatively high bioavailability. After its release into the atmosphere, Cs137 was quickly associated with aerosols, which are a major reservoir of pollutants in the atmosphere, [2]. The deposition of Cs-137 in the atmosphere is coming from nuclear tests and the Chernobyl accident. Cs137 reached Earth's surface, including the oceans, by wet and dry deposition. On 26th April 1986 at Chernobyl in Ukraine, the largest nuclear accident at non-enveloped reactor 4 happened. The explosion of Unit 4 reactor followed by fire spread in the environment more than 6.3*10 18 Bq (170 million Ci) of radioactive noble gases. Besides the noble gases, large amounts of I133, Cs137,

4 Cs134, Sr90 and other radionuclides were released into the atmosphere, with particular impact on public health and the environment, [3]. In Romania, atmospheric deposition due to the Chernobyl accident, were characterized by complex weather conditions, with rain affected areas, but also areas where it has not rained. This resulted in a heterogeneous distribution of the Cs137 deposition on the ground. 2. MATERIALS AND METHODS 4 The salt mines are characterized by ultra low nuclear radiation background, because neither sodium, nor chlorine has long live natural radioactive isotopes. For this reason, the underground salt mines offer unique conditions for research of nuclear physics, dosimetry, astrophysics, biophysics etc. In such spaces, the dose rate of radiation, at a physical depths of about 300 m (~ 600 m.w.e) is <2 µsv/h, [4]. The underground laboratory, the only one of its kind in Romania, is located in Unirea salt mine in Slanic-Prahova. The gamma spectrometry underground laboratory is equipped with high resolution HPGe detectors, first one with relative efficiency of 120% and the second one with relative efficacy of 22.3%. Both detectors are housed in radiation shields so that the radiation background is times smaller relative to a background spectrum measured at the surface without shield. The spectrometric systems are produce by CANBERRA, [5, 6]. Spectrometric systems are operated with GENIE 2000 software. Soil samples were taken in 82 places in Murfatlar vineyard, covering virtually the entire surface of Research Station, see fig. 2. After conditioning, the soil samples were placed in cylindrical plastic boxes, the mass of a sample is 100g. Fig. 2. Sampling places in SCDVV Murfatlar vineyard, image from Google Earth

5 The efficiency versus energy of the detection systems was determined in two ways: (i) experimentally, by participating in intercomparison runs organized by IAEA-Vienna and (ii) using GeometryComposer routine of GENIE2000 software, supplied by CANBERRA. The geometric parameters used in GeometryComposer routine to get energy efficiency function, were selected to reproduce the experimental values obtained in proficiency tests with IAEA RESULTS AND DISCUSSIONS The specific activities of Cs137, K40, and Ac228 Ra226 were determined by high resolution gamma spectrometry. For Cs137 the values range from 2.6 Bq/kg to 13.6 Bq/kg, with an average of 6.3 ± 1.8 Bq/kg and for K40 we have values from 359 Bq/kg to 714 Bq/kg with an average of 582 ± 64 Bq/kg. The average value for Ra226 is 45.9 ± 5.4 Bq/kg ranging from 30.4 Bq/kg to 60.2 Bq/kg and for Ac228 we measured values between 28.9 Bq/kg and 50.9 Bq/kg with an average of 44.1 ± 3.9 Bq/kg. Similar values for specific activities of Cs137 and K40 were reported in [7, 8, 9]. The data for the above mentioned isotopes were statistically analyzed. Pearson correlation coefficients are shown in the tab. 1. The values for the Pearson coefficients demonstrates no correlation between Ra226-K40, K40-Ac228 and Ra226-Ac228 due to the fact that for these three pairs, the correlation values are <0.25. Pearson coefficient for Cs137-K40 is 0.53, showing that these radionuclides are moderately correlated. Tab. 1, Pearson correlations coefficients Radioisotopes Pearson coefficients Cs137-K Ra226-Ac K40-Ac K40-Ra The four datasets for Cs 137, K40, Ac228 and Ra226 were analyzed using Shapiro-Wilk test to check if they are normally distributed, [10]. Data obtained from measurements by high resolution gamma spectrometry were statistically analyzed to verify that they are normally distributed. For this we

6 used the Shapiro-Wilk test where if the null hypothesis is rejected,it means that the data set is not normally distributed, otherwise, we can say for α = 0.05 with a confidence level of 95% that it is normally distributed. The analysis method of Shapiro-Wilk expanded test for n>50, is well described in [10]. Calculation of parameters was performed in Excel by the above method and the results are presented in Tab. 2. Table 2. Shapiro-Wilk expanded test results Radionuclid p-value Test result Null hypothesis (H 0 ) Cs rejected K rejected Ra accepted Ac rejected From tab. 2 it is observed that the null hypothesis was accepted only for Ra226. For the other three radionuclides the test results show that the null hypothesis were rejected because p-value<0.05. Also, for all four sets of values we performed P-P and Q-Q plots in order to test the same hypothesis. The P-P plot reprezents the absolute frequency of values occurrence in predefined intervals. Q-Q graph is a quantiles-quantiles representation, where we have experimental data on the x axis and on the y-axis are the calculated values of the inverse of the standard normal distribution integrated for a given probability. This type of chart proves the existence of a normal distribution of data if these lie on a straight line. In case of a visual analysis on the distribution of Cs137 we can see from Fig. 3 and 4 apparently a bimodal distribution. We can also notice the existence of an outlier value from distribution, which was not considerated in the contour maps. Both the P-P, Q-Q graphs confirm that the data are not normally distributed. 6

7 7 Fig. 3. P-P graph for Cs137 Fig. 4. Q-Q graph for Cs137 Fig. 5. P-P graph for K40 Fig. 6. Q-Q graph for K40 Fig. 7. P-P graph for Ra226 Fig. 8. Q-Q graph for Ra226

8 8 Fig. 9. P-P graph for Ac228 Fig. 10 Q-Q graph for Ac228 The P-P chart of K40 shows that it is not a Gaussian distribution, this being confirmed by the Q-Q graph and the p-value of Shapiro-Wilk expanded test. This distribution is caused by not uniform fertilizations during the years of the investigated area. Ra226 distribution proves to be symmetric, which can be seen from the P- P graph of absolute frequencies, fig. 7, the Q-Q graph, fig. 8 and the statistical p- value of Shapiro-Wilk test. All three methods of statistical analysis performed on the data set of Ac228 show that its distribution is not Gaussian, as can be verified from p-value given by Shapiro-Wilk test and seen in Figure 9 and 10. Also, in fig. 15 of the spatial distribution it is observed that the values of specific activities show large variations over short distances. Fig. 11 Contour map of SCDVV vineyard elevation

9 9 Fig. 12. Contour map on Cs137 Specific activity Fig. 13. Contour map on K40 Specific activity Fig. 14. Contour map on Ra226 Specific activity Fig. 15. Contour map on Ac228 Specific activity For generating these contour maps we realized an algorithm based on a function that creates a rectangular network of points from our coordinates. Then we eliminate areas containing buildings or other constructions. The function made a coordinates square matrix of size 82x82, over which maps of the surface from above figures were built. Interpolation method used was "nearest neighbor". Then, the new matrix of three dimensions is plotted as a surface plot. Fig. 11 contains a contour map of the altitude above sea level from the locations where sampling was conducted. This was carried out with the altitude data provided by GoogleEarth. It can be seen that the elevation drops from 65 m in N to 20 m in S, which may explain the higher values for specific activity of K40 in the lower elevation zones, fig 13 because one of its sources is the fertilizer that is soluble, and it migrates with the solution to lower altitudes. Fig. 12 presents the spatial distribution of Cs137. From the figure it is noted that the Cs137 has higher specific activities in the upper part of the plot, and at the bottom of the slope. Small amounts are found in the eastern part of the plot

10 where the altitude has intermediate values. This can be explained by the existence of small processes of erosion and sedimentation. The erosion is poor at the top of the slope, increases towards the middle, and at its base sedimentation takes place CONCLUSIONS The values for the Pearson coefficients demonstrate no correlation between Ra226-K40, K40-Ac228 and Ra226-Ac228. Pearson coefficient for Cs137-K40 is 0.53, showing that these radionuclides are moderately correlated. K40 is not a Gaussian distribution, this being confirmed by the P-P and Q- Q graphs and the p-value of Shapiro-Wilk expanded test. This is due to not uniform fertilizations during the years of the investigated area. The symmetric distribution of Ra226 is confirmed by P-P graph, the Q-Q graph and the statistical p-value of Shapiro-Wilk test. Ac228 spatial distribution it is observed that the values of specific activities show large variations over short distances. Analysis on the distribution of Cs137 shows apparently a bimodal distribution. All three tests for normal distributed data are in a perfect agreement with each other for all four datasets, proving that the results are correct. ACKNOWLEDGMENTS This work was supported by the Romanian National Authority for Scientific Research Projects No. PN and No. PN REFERENCES 1. Aurora Ranca, Maria Iliescu, Victoria Artem, Petru Bolos, AcademicPress, 2010, ISBN Povinec, P., Biogeosciences, 10, , OCDE Committee on the safety of nuclear installations OECD Nuclear Energy Agency, R. Margineanu, C. Simion, S. Bercea, O.G. Duliu, D. Gheorghiu, A. Stochioiu, M. Matei, Applied Radiation and Isotopes 66 (2008)

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