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ISSSD 2019 October 7 to 11th, 2019. Zacatecas, Zac. Mexico. ______Cs-137 activity concentration in the soil of State,

Rogério Filgueiras1, Ricardo Garcêz1*, Leandro da Silva1, José Lopes2, Dejanira Lauria3, Fernando Ribeiro3, Eduardo P. Viglio4 & Ademir X da Silva1,5

1Department of Nuclear Engineering, Federal University of , Rio de Janeiro, RJ, BRA 2Institute of Physics, Federal University of , Salvador, Ba, BRA

3Institute of Radioprotection and Dosimetry, Rio de Janeiro, RJ, BRA

4Geological Survey of Brazil, Rio de Janeiro, BRA

5Polytechnic School of the Federal University of Rio de Janeiro, Rio de Janeiro, BRA

*Email: [email protected]

Abstract

Alagoas is a state of Brazil Northeast, which has an area of approximately 27779 km², borders the states of and Bahia to the south and with to the north. The Cs-137 is an artificial radionuclide and is present in the whole planet mainly due to the tests and nuclear accidents. A total of 41 samples were collected by the CPRM (Geological Service of Brazil) under the PGAGEM BRASIL Project, in the first 25 cm of soil, spaced approximately 25 to 30 km apart, forming a regular mesh. The samples were identified and georeferenced with a GPS. From 2 to 3 kg of soil were collected per sample, then dried, homogenized, sieved and dried in the oven at 40ºC. Finally, they were packed in polyethylene pots of 300 ml and sealed to achieve the secular equilibrium. Samples were analyzed using the gamma spectrometry technique using hyperpure germanium detectors (HPGe), which have a relative efficiency of 20, 40, 45 and 100%. The activity concentration of the samples and their respective uncertainties were determined with the aid of Genie2000 software. The counting time used was 60,000 seconds. Among the 41 collected samples, 6 presented activity concentration for cesium 137, with activity concentration ranging from 0.3 to 1.1 Bq/kg, with a geometric mean of 0.53 Bq/kg. The samples were found in the high rainfall regions of the State, on the and one near the state capital: Maceió.

Keywords: Cs-137; Activity Concentration; soil; HPGe.

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ISSSD 2019 October 7 to 11th, 2019. Zacatecas, Zac. Mexico. ______1.- INTRODUCTION

Soils may change due to anthropogenic activities, increasing exposure to radiation in individuals beyond natural radiation. Among artificial radionuclides, 137Cs are one of the most important for radiological protection (UNSCEAR, 1982). 137Cs is an artificial radionuclide that is produced through the fission process of 235U and 239Pu. The dispersion of 137Cs on the planet is due to an atmospheric process called fallout, where smaller particles containing 137Cs are suspended for a long period and settle away from the release site. However, the distribution of 137Cs across the planet is not uniform, approximately 25% of all 137Cs released into the atmosphere have been deposited in the southern hemisphere (UNSCEAR, 2008).

Local weather conditions have a significant impact on the deposition and transport of this radionuclide. Rainier sites tend to accumulate more 137Cs, ie the spatial variability of 137Cs fallout seems to be strongly related to rainfall rates and annual rainfall distribution patterns (DAVIES, 1963). The Alagoas State is located between latitudes -8 and -10, has an area of approximately 27,779.343 km² (IBGE, 2002), located in the northeast region of Brazil. Its rainfall regime is directly related to the large-scale atmospheric and ocean circulation configurations on the tropics, but it is also subject to the influence of mesoscale systems such as the convective complexes and the and terrestrial breezes that influence all sub- regions, besides the orographic circulations and small convective cells that constitute the microscale phenomena (EMBRAPA, 2012). Figure 1 shows the rainfall distribution in the state. The Alagoas relief is divided into: coastal plain (east), plateau (north) and depression (midwest). The local relief stands out the Borborema Plateau that separates the east from the midwest, has peaks over 1000 m, and is responsible for establishing the differences between the east and the midwest of the state.

The objective of this paper is to calculate the 137Cs activity concentration in soils in the Alagoas State and to compare the results found with other . And as secondary objectives, the article discusses the factors that may influence the increase of 137Cs concentration in the studied soils: relief, rainfall, latitude, soil type, and the accident at the Fukushima Nuclear Power Plant, which is the most recent nuclear accident. ______234 Proccedings of the ISSSD 2019 Volume 1

ISSSD 2019 October 7 to 11th, 2019. Zacatecas, Zac. Mexico. ______

Figure 1.- Average annual rainfall (EMBRAPA, 2012).

2.- Materials and Methods

2.1.- Sampling For this research 41 samples were collected in an approximate distribution of 25 to 30 km from each other, where the collection points were established by intersecting longitude and latitude lines forming a regular mesh, as shown in Figure 2. The collection points were georeferenced through GPS. The samples were collected in the first 25 cm of soil, from 2 to 3 kg of soil. They were then labeled, numbered and field sieved in the 60 mesh fraction (nylon sieve).

2.2.- Sample preparation The samples were again sieved in the laboratory now with 230 mesh screen, ie a sieve with a maximum particle size of 63 µm was used for the soil samples, and dried in the oven at 40ºC. Each sample was recorded on a field form containing the location and sampling parameters. The characteristics of the sampling site were passed on to the data matrix, such as: rainfall close to collection; vegetation type and geographical coordinates.

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Figure 2.- Distribution of sample collection sites in the Alagoas State.

Approximately 300g of soil sample was packed in 300 ml polyethylene pots, sealed and lined in foil and allowed to stand for at least 30 days to achieve radioactive secular equilibrium.

2.2.- Instrumentation The samples activity concentrations were measured by gamma spectrometry on hyperpure germanium (HPGe) detectors designed by Canberra Inc. (Meriden, CT, USA). HPGe detectors (gc2020, well, gx45a and gx100) and their respective electronic devices (HV sources, amplifiers, multichannel analyzer) from the Laboratory of Environmental Radiometry, the Division of Environmental and Occupational Radioprotection (DIRAD), the Institute of Radioprotection and Dosimetry (IRD / CNEN) were used to measure the specific activity of 137Cs in soil samples. These detectors have individual shielding made of 15 cm thick old lead and 2 mm copper layer, also produced by Canberra Inc. The detectors used for the measurements have a relative efficiency of 20%, 25%, 40% and 100%. ______236 Proccedings of the ISSSD 2019 Volume 1

ISSSD 2019 October 7 to 11th, 2019. Zacatecas, Zac. Mexico. ______The 137Cs specific activity was measured by means of the 661.6 keV gamma phototypes. The counting time used for the samples was 60,000 seconds, and the system background was measured by the time of 230,000 seconds.

Detector efficiency calibration was performed with a standard nitric acid solution (HNO3) containing several radionuclides (57Co, 109Cd, 203Hg, 139Ce, 210Pb and 241Am), provided by the National Laboratory of Ionizing Radiation Metrology (LNMRI) belonging to the IRD / CNEN. LNMRI is the Brazilian radiation metrology laboratory, tracked by the Bureau des Poids et Measures (BIPM) in France. Detectors energy calibration used in the present study was routinely performed using a 152Eu source. In order to ensure measurement quality, a certified soil standard (089 / ERA), provided by the National Institute of Standards and Technology (NIST), was analyzed as a blind sample during sample measurement.

3.- RESULTS Among the 41 samples analyzed, only 6 presented 137Cs above the minimum detectable activity (MDA), which corresponds to 15% of the soil samples. Activity concentration values are shown in Table 1.

Table 1.- Activity concentration values of 137Cs above MDA.

137Cs Uncertainty MDA Sample (Bq/kg) (Bq/kg) (Bq/kg) 3 0.46 0.08 <0.32 11 0.47 0.10 <0.41 13 0.30 0.08 <0.24 14 1.13 0.46 <0.71 22 0.66 0.20 <0.59 24 0.46 0.10 <0.43

MDA ranged from 0.22 to 3.39 Bq/kg, and the latter was considered as an outlier by the Rosner method performed by ProUCL 5.1. ______237 Proccedings of the ISSSD 2019 Volume 1

ISSSD 2019 October 7 to 11th, 2019. Zacatecas, Zac. Mexico. ______For the statistical analysis of the results obtained in this article, half of the activity concentration values that were below the MDA value were considered. The results of the descriptive statistics applied to the data are shown in Table 2.

Table 2.- Descriptive statistics of activity concentration results for 137Cs in soils sample of Alagoas State. 137Cs Arithmetic mean (Bq/kg) 0.49 Standard error 0.03 Median (Bq/kg) 0.46 Geometric mean (Bq/kg) 0.45 Percentile 75 (Bq/kg) 0.60 Percentile 90 (Bq/kg) 0.83 Standard Deviation 0.21 Minimum (Bq/kg) 0.11 Maximum (Bq/kg) 1.13 Score 40

4.- DISCUSSION Table 1 shows that few samples had results with activity concentration above the MDA, this feature is compatible with predictions that low fallout levels are expected in equatorial regions (UNSCEAR, 1969). And research conducted in Brazilian soils has shown that activity concentrations of 137Cs range from 0.3 to 5.2 Bq/kg (medium values), and their deposition in the soil tends to increase according to the distance from the equator line (Ribeiro et al., 2016), which is shown in Table 3.

Table 3 shows some results found in the literature for activity concentration of 137Cs in soil samples. Highlighting the research conducted by Ribeiro et al. (2016), in the Rio de Janeiro State, which is located between latitudes -20 and -23 approximately. For research conducted by Hiromoto et al. (2010), who conducted a survey in the state of ,

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ISSSD 2019 October 7 to 11th, 2019. Zacatecas, Zac. Mexico. ______located between latitudes -19 and -25. For research by Andrello and Apolloni (2004), located in the Paraná State, between latitudes -22 and -26 approximately. And for the research carried out by Schuch et al. (1995), held in the State, located between latitudes -27 and -33 approximately.

Table 3.- Activity concentration of 137Cs found in the .

Activity Concentration Depth (cm) Researchers State (Bq/kg) 0.49 (0.30 – 1.13) 0 – 25 Present study Alagoas 1.25 (<0.20 – 4.23) 0 – 20 Ribeiro et al., 2016 Rio de Janeiro 1.90¹ (0.8 – 4.1)) 0 – 20 Hiromoto et al., 2010 São Paulo 2.26 (1.30 – 3.62) 0 – 30 (soil 1) Andrello and Apolloni, Paraná 1.74 (0.90 – 2.22) 0 – 30 (soil 2) 2004 Rio Grande do 5.2 (2.2 – 7.2) 0-50 Schuch et al., 1995 Sul ¹Geometric mean

4.1.- Nuclear accident in Fukushima The Fukushima nuclear accident occurred on March 11, 2011 in Japan. The samples used in this work were collected between January 11, 2011 and October 31, 2011, ie the Fukushima accident occurred during the collection period. of the samples.

Among the 41 samples, only four samples were collected before the accident on January 11, 2011. Of these four samples, two (samples 13 and 24) showed activity concentration above MDA with 0.30 Bq/kg and 0.46 Bq/kg respectively. Two samples were collected on the day of the accident and no contamination is expected in Alagoas on the same day of a nuclear accident that occurred in Japan. These two samples showed activity concentration below MDA.

The samples collected after the accident that showed activity contraction above MDA were as follows: sample 11, collected on 04/11/2011 (one month after the accident), showed ______239 Proccedings of the ISSSD 2019 Volume 1

ISSSD 2019 October 7 to 11th, 2019. Zacatecas, Zac. Mexico. ______activity of 0.47 Bq/kg; sample 3, collected on 06/11/2011 (three months after the accident), showed activity of 0.46 Bq/kg; and samples 22 and 14, collected on November 29 and 31, 2011 (just over eight months after the accident), showed activities of 0.66 and 1.13 Bq/kg respectively. Most of the samples (24 samples) were collected in November 2011.

Thus, four samples were collected before the accident and two samples were collected on the day of the accident. Of these six samples, two showed activity above MDA, ie 1/3 of the samples. Of the 35 remaining samples collected after the accident, four showed activity above MDA, 11.42%. It is noted that although the proportion of samples with activity concentration above MDA is lower after the Fukushima accident, the highest activity concentration values were found after the accident. Nevertheless, it is not possible to state that the 137Cs found came from the Fukushima accident.

4.2.- Relief No sample collected in the Midwest of the state showed activity concentration of 137Cs above MDA. Most of the samples where 137Cs were detected are in eastern Alagoas, the region with the highest rainfall in the state. The relief for all samples was the ―domain of dissected hills and low hills‖, with the amplitude ranging from 20 to 80 meters and the slope up to 20 degrees. Most of the samples in which the activity concentration was determined were in Argisols soils, also found in mangrove soils, which have high organic matter content.

Figure 3 shows the location of samples that showed activity concentration of 137Cs, four of the six samples are in the same line, parallel to the coast at a distance of approximately 50 km.

A possible explanation for the arrangement of the four samples in a line parallel to the coast is the occurrence of orographic rainfall in the region due to the Borborema Plateau. The two samples (13 and 14) near the coast also coincide with a region of greater precipitation in the Alagoas State as shown in Figure 1.

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Figure 3.- Samples location with activity concentration of 137Cs.

5.- CONCLUSIONS

The low activity concentration of 137Cs in Alagoas State soils is consistent with predictions that low fallout levels are expected in equatorial regions and the southern hemisphere (UNSCEAR, 1969). That is, the values found for the specific activity of 137Cs in the Alagoas State are within the concentration range of activity found in the literature for the southern hemisphere.

The activity concentrations of 137Cs ranged from (0.30 ± 0.08) Bq/kg to (1.13 ± 0.46) Bq/kg. Four samples with activity concentration were located parallel to the coast at a distance of approximately 50 km, indicating a relationship with the local relief due to the Borborema Plateau. And the other two samples are located in an area that has high rainfall.

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ISSSD 2019 October 7 to 11th, 2019. Zacatecas, Zac. Mexico. ______REFERENCES

Andrello, A. C.; Appoloni, C. R. (2004). Spatial variability and Cesium-137 inventories in native forest. Brazilian Journal of Physics 34: 800-803

Davies, J. J. (1965). Cesium and its relationship to potassium in ecology. Radioecology: 539-556.

EMBRAPA. [online]. Climatologia do Estado de Alagoas. Boletim de Pesquisa Número 9 - Embrapa. (2012). Homepage of Embrapa. .

Hiromoto, G.; Peres, A. C.; Tadei, M. H.; Soares, M. R.; Alleoni, L. R. F. (2010). Radioactive Soil Characterization Of The State Of Sao Paulo, Brazil. Annual International Conference on Soils, Sediments, Water and Energy.

IBGE. (2002). Resolução Nº05, de 10 de outubro de 2002.

Ribeiro, F. C. A.; Lauria, D. C.; , J. I. R.; Lima, E. S. A.; Amaral Sobrinho, N. M. B.; Pérez, D. V. (2016). Concentration of fallout Cesium-137 in soils of the Rio de Janeiro State, Brazil. Brazilian Journal of Radiation Sciences 4: 1-20.

Schuch, L. A., Nordemann, O. J. R., Zago, A., Godoy, J. M., Pecequilo, B. (1995). Radionuclídeos naturais e artificiais em seis perfis de solos do estado do Rio Grande do Sul. Ciência e Natura 17: 17-32.

UNSCEAR-United Nations Scientific Committee on the Effects of Atomic Radiation, 1969, Annex A: Radioactive contamination of the environment by nuclear tests. New York.

UNSCEAR-United Nations Scientific Committee on the Effects of Atomic Radiation, 1982, Ionizing Radiation: Sources and Biological Effects. Report to the General Assembly. United Nations, New York.

UNSCEAR-United Nations Scientific Committee on the Effects of Atomic Radiation, 2008. Annex B. Exposures of the Public and Workers from Various Sources of Radiation. United Nationas Publications. 2008.

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