2015 International Nuclear Atlantic Conference - INAC 2015 São Paulo, SP, Brazil, October 4-9, 2015 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 978-85-99141-06-9

NATURAL RADIONUCLIDES IN SOILS FROM SÃO PAULO STATE CERRADO FOREST

Márcia V. F. E.S. Miranda, Emerson E. G. de Farias, Rebeca S. Cantinha, Elvis J. de França

1Centro Regional de Ciências Nucleares do Nordeste (CRCN-NE) Avenida Professor Luiz Freire 200 50740-540, Recife, PE [email protected]; [email protected]; [email protected]; [email protected]

ABSTRACT

Considering the long life history, forests should be preferentially evaluated for the monitoring of radionuclides, mainly artificial radioisotopes. However, little is known about nuclides from Uranium and Thorium series, as well as, K-40, in soils from the São Paulo State forests. Soils are the main reservoir of natural radionuclides for vegetation, thereby deserving attention. Taking into account the advantages of High-Resolution Gamma-ray Spectrometry – HRGS, diverse radionuclides can be quantified simultaneously. In this work natural radionuclides in soils from the Estação Ecológica de Assis were evaluated by HRGS. Samples of 0-10 cm depth were collected under crown projection of most abundant tree species of long-term plots installed within the Estação Ecológica de Assis, São Paulo State, Brazil. After drying and milling until 0.5 mm particle size, test portions of 30 g were transferred to polypropylene vials, sealed with silicone and kept under controlled conditions until 30 days to achieve secular equilibrium. A group of gamma-ray spectrometers was used to analyze about 27 samples by 80,000 seconds. Activity concentrations of Pb-214, Ac-228 and K-40 and their respective expanded analytical uncertainties at the 95% confidence level were calculated by Genie software from Canberra. Abnormal values were not detected for radionuclides in soils samples, however K-40 activity concentrations changed considerably due to the mineral cycling, in which K and, consequently K-40, is mainly stocked in vegetation in spite of soils.

Keywords: Conservation units, spectrometry, radioactive series

1. INTRODUCTION

Natural radionuclides are present in the environment, contributing to radiation doses received by human population and accumulating in tissues of all live beings. Plants can absorb radionuclides from soil. It is known that radionuclide accumulation in soils is dependent on the physical and chemical conditions concerning to the soil composition, percolation, leaching and sorption rates [1].

Cerrado is considered a Brazilian hotspot, although, it has suffered the most rapid deforestation in South America. More than 70% of its original coverture was lost for agricultural purposes [2, 3]. In South-Eastern Brazil, Cerrado and the Atlantic forest form a boundary in which it is observed the transition between Cerradão (dense woodland savannahs with a semi-closed canopy) and the seasonally dry forest, a closed-canopy vegetation type. Although they are remarkably distinct in species composition, some vegetation species occur in both ecosystems [4]. In this research natural radionuclides present in soils of the Estação Ecológica de Assis, which is a Cerrado type biome, were evaluated by gamma spectrometry.

2. EXPERIMENTAL

2.1. Area of study

Soils were collected in the Estação Ecológica de Assis located at the Assis Municipality, sited between the parallels 22º33’20’’ and 22º37’41’’ and the meridians 50º24’48’’ and 50º21’27’’, inside height levels of 500 and 588 meters of a smooth waved relief. The climate of this area is considered subtropical temperate, inside a transition zone between Cwa and Cfa, according Köppen classification, that means dry winter and rainy summer. Soils were classified as Oxisols and Entisols types, based on International Soil Classification System.

2.2. Sample preparation

Soils were collected at 0-10 cm depth under crown projection of the most abundant trees species of long-term plots installed within the Estação Ecológica de Assis, São Paulo, Brazil. Soil samples were dried at 60 °C and milled in a porcelain mortar until 0.5 mm particle size. Test portions of 30 g were placed in polypropylene vials, with 6 cm of diameter and specific geometry for analysis. Vials were sealed with silicone and kept under controlled conditions until 30 days to achieve secular equilibrium.

2.3. Gamma-ray sepectrometry

Radionuclides present in soil samples were determined by high resolution gamma-ray spectrometry performed at the high-purity germanium detector (HPGe), model 4018 from Canberra®, with resolution of 2.2 keV at the 1.33 MeV 60Co photopeak. Measurements were done during 80,000 seconds and analytical blank was analyzed during 200,000 seconds. An efficiency curve was constructed using internal standards obtained by spiking soil with known activity of the radionuclides 152Eu (27.99 Bq), 133Ba (25.70 Bq), 241Am (30.18 Bq), 137Cs (27.76 Bq).

2.4. Analysis of the results

The activity of radionuclides present in soil samples were calculated based on gamma-rays detection at the HPGe detector, using the software Genie 2000 provided by Canberra®. The analytical uncertainties (u) were considered and expanded at the 95% confidence level, to generate the expanded analytical uncertainty (UA) of the radionuclide activity, as done by [5].

The quality of the analytical procedure was evaluated according to the National Intercomparison Program of the Institute of Radiation Protection and Dosimetry (IRD). In this program an environmental reference sample is distributed three times a year to the participating laboratories and the performance of the laboratory is evaluated considering the obtained result.

INAC 2015, São Paulo, SP, Brazil. 3. RESULTS AND DISCUSSION

The quality of the analytical procedure performed in this research was confirmed as the value of the normalized deviation (D), used to evaluate the quality of gamma spectrometry analysis, was between -2 and 2. As preconized in the protocol of the National Intercomparison Program of the Institute of Radiation Protection and Dosimetry (IRD) the closer to zero the value of D is, the better is the performance of the testing laboratory. For values of │D│≥3, the analysis system is out of control.

The results of gamma spectrometry for soils collected in an area located at the Estação Ecológica de Assis, are presented in Table 1. Values are showed according to soil types the samples were collected under crown projection of the most abundant trees species. Results of K-40 varied from 17.97 to 48.83 Bq.kg-1, Ra-226 varied from 6.30 to 19.48 Bq.kg-1 and Ra- -1 228 varied from 6.36 to 20.06 Bq.kg .

Table 1. Radionuclide activities measured in soils from the Estação Ecológica de Assis.

ESTAÇÃO ECOLÓGICA DE ASSIS Radionuclide K-40 Pb-214 Ac-228 Energy (keV) 1460.82 351.93 911.2 Soil type Entisols Mean (Bq.kg-1) 30.12 13.27 15.67 Standard deviation (Bq.kg-1) 13.44 5.88 3.97 N 8 8 8 Soil type Oxisols Mean (Bq.kg-1) 33.73 11.24 15.22 Standard deviation (Bq.kg-1) 20.03 5.91 5.97 N 21 21 21 Mean (Bq.kg-1) 32.36 11.43 14.91 Standard deviation (Bq.kg-1) 11.88 4.33 4.38 Total of samples 27 27 27

According to UNSCEAR [6], the specific activities for 228Ra and 226Ra in soils ranging from 11 to 64 Bq.kg-1 and 17 to 60 Bq.kg-1, respectively. But, this value depends on the geological characteristics of the site. In the results listed in Table 1 it can be observed that the mean values obtained for these radionuclides are within this range.

Jacomino et al. [7] chemically analyzing two Cerrado soils of Tapira, Minas Gerais State, Brazil, found differences in radionuclides concentrations according to the soil type. This group found values of Pb-214, Ac-228 and K-40 elevated in Cerrado soils from Minas Gerais state than in Cerrado soils from São Paulo State studied here.

INAC 2015, São Paulo, SP, Brazil. 4. CONCLUSIONS

In the present study the activity concentrations of 228Ra and 226Ra were in agreement with the world wide average, but the activity of 40K was lower than the world wide average. The knowledge of natural radioactivity of tropical forest was therefore obtained in this study.

REFERENCES

1. R. Fujiyoshi, S. Sawamura, “Mesoscale variability of vertical profiles of environmental radionuclides (40K, 226Ra, 210Pb and 137Cs) in temperate forest soils in Germany”, Science of the Total Environment, 320, pp. 177-188 (2004). 2. P. Hunke, R. Roller, P. Zeilhofer, B. Schröder, E. N. Mueller, “Soil changes under different land-uses in the Cerrado of Mato-Grosso, Brazil”, Geoderma Regional, 4, pp. 31-43 (2015). 3. O.M. R. Cabral, H. R. da Rocha, J. H. Gash, H. c. Freitas, M. A. V. Ligo, “Water and energy fluxes from a woodland savanna (cerrado) in southeast Brazil”, Journal of Hydrology: Regional Studies, 4, pp. 22-40 (2015). 4. R. A. G. Viani, R. R. Rodrigues, T. E. Dawson, H. Lambers, R. S. Oliveira, “Soil pH accounts for differences in species distribution and leaf nutrient concentrations of Brazilian woodland savannah and seasonally dry forest species”, Perspectives in Plant Ecology, Evolution and Systematics, 16, pp. 64-74 (2014). 5. J. D. S. Paiva, E. E. Sousa, E. G. Farias, A. M. Carmo, C. A. S. Filho, E. J. França. “Applied tools for determining low-activity radionuclides in larges environmental samples”, Journal of Radioanalytical and Nuclear Chemistry, in press. 6. United Nations Scientific Committee on the Effects of Atomic Radiation, Sources and effects of ionizing radiation, UNSCEAR, New York, United States of America (2000). 7. V. M. F. Jacomino, K. A. P. Oliveira, M. H. T. Taaddei, M. C. Siqueira, M. E. D. P. Carneiro, M. R. L. Nascimento, D. F. Silva, J. W. V. Mello, “Radionuclides and heavy metal contents phospogypsum samples in comparison to Cerrado soils”, Revista Brasileira de Ciência do Solo, 33, pp. 1481-1488 (2009).

INAC 2015, São Paulo, SP, Brazil.