Analysis of the Gamma Spectrometry 210 Pb Radioisotope in River Bottom Sediments of the Hydrographic sub-basins Around the UTM-Caldas

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1 Analysis of the Gamma Spectrometry 210 Pb Radioisotope in River Bottom Sediments of the Hydrographic sub-basins Around the UTM-Caldas Pedro H. Dutra¹; Carlos A. Carvalho Filho¹; Rubens M. Moreira¹; Maria Ângela B.C. Menezes¹; Nivaldo C. Silva²; Aline F.G. de Oliveira¹ and Valquíria F. L. Viana³ 1. Centro de Desenvolvimento da Tecnologia Nuclear Av. Presidente Antônio Carlos, 6.627, Campus da UFMG Pampulha CEP Caixa Postal CEP Belo Horizonte - Minas Gerais, Brazil pedrohenrique.dutra@gmail.com 2. Laboratório de Poços de Caldas Rodovia Poços de Caldas - Andradas, Km 13, Zona Rural CEP: Poços de Caldas - Minas Gerais Brazil ncsilva@cnen.gov.br 3. Instituto de Ciências Biológicas Universidade Federal de Minas Gerais Campus da UFMG Pampulha CEP Belo Horizonte - Minas Gerais, Brazil valquiria.flviana@gmail.com ABSTRACT The uranium mine of Caldas, currently named Ore Treatment Unit (UTM-Caldas), is sited at the Poços de Caldas Plateau (Minas Gerais State) and was the first uranium mineral-industrial complex in Brazil. It has been installed since 1982 and now it is under decommissioning process. Taking into account the potential sources of contamination and the assessment of the impact of the mine, based on the presence of radionuclides from the radioactive decay series of natural 238 U, the aim of the article is to present the distribution of 210 Pb in the stream bottom sediments of the study area that consists of the Taquari watershed, sub-divided by its three major sub-basins: Consulta stream, Soberbo stream and Taquari river. The radionuclide activity concentrations were measured in sediment samples that were collected in twelve collecting points, during four sampling campaigns, carried out in the dry and rainy seasons of 2010 and The results of the 210 Pb concentration activity were obtained by gamma spectrometry performed in both high and low energy CANBERRA detectors. The results point out that the UTM-Caldas is influencing on the bottom sediment distribution of 210 Pb activity in its neighborhood. However, a more detailed study should be done in order to identify if there is another source of 210 Pb in the study area, such as a geogenic anomaly, that may contributing to the local increment of 210 Pb activity. 1. INTRODUCTION A total of four lead (Pb) isotopes are naturally present in the environment. These isotopes are: 204 Pb with 1.4% abundance; 206 Pb, with 24.1%; 207 Pb, with 22.1% and 208 Pb, with 52.4%. The 210 Pb radionuclide is a member of the radioactive starting at 238 U. It is a no-essential and toxic metal whose biogeochemical cycling can be affected by man, causing a high degree of contamination. Lead enters into the environment during production (mining and smelting included), consumption (battery, pigments, ceramics and plastic), recycle, elimination of Pb chemicals, burning of fossil fuels, use of mineral fertilizers, application of sewage sludge, etc [1], [2], [3], [4], [5]. 210 Pb has long half-life of 22.3 years; it is used as a tracer, in sediments dating and in indicating mobilization of some elements, such as radon, radium and uranium.

2 Nittrouer et.al. [6]. and M. Hasan et.al. [7], in their studies on the accumulation rate of Pb isotopes in river bottom sediments noticed that, due to its properties, the element tends to suffer adsorption on siltclay material and organic matter. River sediments are an important compartment when evaluating the intensity and pattern of impact to which aquatic ecosystems are submitted, inasmuch as they are considered as sinks and possible contamination sources in these ecosystems [8]. The site of the former uranium mining and milling facilities, presently named Caldas Ore Treatment Unit (UTM-Caldas), is located at the southern part of the Poços de Caldas. During the period from 1982 to 1995 this location hosted the first uranium mine to be explored in Brazil. Presently the UTM-Caldas is undergoing the closing process, under the supervision0 of the National Nuclear Energy Commission (CNEN) and the Brazilian Institute for the Environment and Renewable Resources (IBAMA). Several joint projects are being carried at the site with a view to an optimum solution for its recovery. The paper presents a methodology for analysis of 210 Pb concentration, using the gamma spectrometry technique adopted by the Nuclear Technology Development Center (CDTN/CNEN), taking as a case study the hydrographic sub-basins around the UTM-Caldas, presently managed by the Nuclear Industries of Brazil (INB). 2. CHARACTERIZATION THE STUDY AREA Located in Poços de Caldas Plateau, which is at the border between the Minas Gerais and São Paulo States, the study area lies at the centre-southeast part of the plateau (Fig. 1). Figure 1: Location and Limits of the Taquari River sub-basin and the UTM-Caldas. The Poços de Caldas Plateau, characterized by the presence of a nearly completed main volcanic caldera, originating from a great volcanic activity, and after extensive weathering and erosion, originated its current line-up. The several processes affecting the local geology during this time span originated a large abundance of minerals of economic interest, such as the uranium-molybdenum ore [9]. The UTM-Caldas

3 is location inside a zone affected by volcanic reactivation processes, which occurred at the time of the emplacement of the massif. Historicaslly the UTM-Caldas was, the first facility producing uranium in Brazil; its mining activity started in 1982 and was definitely discontinued in 1995, due to economic reasons. It was the only one in the world at which uranium was found and mined in deposits related to alkaline rocks [10]. The watercourses draining this region are maintained by rainfall [10]. The springs of the Antas River lie near the south-eastern border of the plateau and the watercourse crosses the all of its extent. The Verde River and its tributaries (e.g.: Taquari River) form a less extensive system that drains the eastern part of plateau. 3. MATERIALS AND METHODS The sampling of river bottom sediments were carried at 10 sampling stations and 2 sources stations. The sampling campaigns covered two years, during dry (September) and rain seasons (March). The sampling equipment used was of a rock islander type, with an average withdrawal capacity of 4 kg of river bottom sediments per station. The material was sieved in situ, using a 10 mesh sieve to separate gravel from fine material. Each sample of the fine material was stored in resistant plastic bottles and appropriately identified. The samples collected at each season was sent to the Soil and Sediment Sample Preparation Laboratory at CDTN for due preparation. There the samples were dried at room temperature, which took about 4 weeks in average. Subsequently, the samples were homogenized and quartered to obtain a uniform aliquot of the total sample.. An agglomerated grain breakdown for grain size analysis was carried off (not shown in this paper). Each sample, containing between 70 to 145 grams, was placed in container and analyzed by gamma spectrometry. For a comparative purposes, an analysis was carried in the same container of a certified sample (code number IAEA-447) obtained from the International Atomic Energy Agency (IAEA). Each group of collected samples collected was analysed at the Nuclear Spectrometry Laboratory (LEN) of CDTN, both with a CANBERRA equipment, model GX5019, type Extended Range Coaxial Ge Detectors (XtRa), called Energy High Detector (EHD) and a model GUL100P, type Ultra-LEGe, called Low Energy Detector (ELD). The exposure time of each sample to gamma detector was 24 hours. Following this elapsed time, an analysis was carried and the gamma peaks in each samples were identified (Fig. 2).

4 Figure 2: Gamma spectrum showing peaks of detected radionuclides. 4.1 IAEA 447 Reference Sample 4. RESULTS AND DISCUSSIONS Table 1 shows the analytic results obtained for reference sample. Table 1. Experimental results and certificate value for reference material IAEA-447 Sample Experimental Value (Bq kg -1 ) Certificate Value (Bq kg -1 ) Experimental Value (Count s -1 kg -1 ) EHD ELD D1 406 ± ± ± D2 422 ± ± ± ± D3 418 ± 52 (June/2013) ± ± D4 418 ± ± ± In order to assure that the obtained results were reliable, a u-score calculation was performed. Comparing of the results with the certified value, the criterion u < 1.96 (P = 95% or ± 2σ) was applied, which is equivalent to the 95% of confidence interval of certified value [12], [13]. All results obtained were below the standard value (Table 2).

5 Table 2. Results of u-score test applied to values Sample u-score D D D D Determination of 210 Pb The results for the specific activity of the radionuclide in calculated samples by the gamma spectrometry detector program Genie 2K are presented in Table 3. Table 3. Specific activity of 210 Pb, Bq.kg -1, in the samples EHD Sample 2ª Rainy Season 3ª Dry Season 4ª Rainy Season 5ª Dry Season C1 214± ± ± ± 25 C ± ± ± ± 135 C3 316 ± ± ± ± 43 C4 590 ± ± ± ± 87 S1 113 ± ± ± ± 14 S ± ± ± ± 275 S3 272 ± ± ± ± 36 S4 331 ± ± ± ± 36 T1 131 ± ± ± ± 39 T2 242 ± ± ± ± 34 T3 310 ± ± ± ± 39 T4 311 ± ± ± ± 71 The results for activity are plotted in charts (Fig. 3) for better visualizing the state of the sub-basin. It can be noticed from the graphs that the activity concentrations at the stations do not maintain a constant value. That variation can be explained by the occurrence of periodical flood and drought in water bodies, by contributions from other sources sources placed near the banks of these sampling stations.

6 Bk/Kg Bq/kg Bk/kg Consulta Stream Soberbo Stream ª 3ª 4ª 5ª C1 C2 C3 C ª 3ª 4ª 5ª S1 S2 S3 S Taquari River 2ª 3ª 4ª 5ª T1 T2 T3 T4 Figure 3: Activity concentrations at the Consulta (left), Soberbo (right) and Taquari (below) subbasins. The results empirically obtained from the spectra are displayed in the Table 4. Sample Table 4. Results of 210 Pb, count s -1 kg -1 ELD 2ª Rainy Season 3ª Dry Season 4ª Rainy Season 5ª Dry Season C ± ± ± ± C ± ± ± ± 0.01 C ± ± ± ± C ± ± ± ± S ± ± ± ± S ± ± ± ± 0.01 S ± ± ± ± S ± ± ± ± T ± ± ± ± T ± ± ± ± T ± ± ± ± T ± ± ± ± 0.005

7 Sample Table 5. Results of 210 Pb, count s -1 kg -1 - EHD 2ª 3ª 4ª 5ª Rainy Season Dry Season Rainy Season Dry Season C ± ± ± ± 0.01 C ± ± ± ± 0.02 C ± ± ± ± 0.02 C ± ± ± ± 0.02 S ± ± ± ± 0.01 S ± ± ± ± 0.07 S ± ± ± ± 0.02 S ± ± ± ± 0.02 T ± ± ± ± 0.02 T ± ± ± ± 0.01 T ± ± ± ± 0.01 T ± ± ± ± 0.02 These observations already allow a finding: the values obtained with the ELD values are well below those acquired using the EHD. Thus, mathematical adjustments were performed in order to allow a comparison between the results of the two detectors. Making use of the data in Tables 4 and 5, verification of the data obtained by the low energy detector was carried out for validation, as shown in Table 6. Sample Table 6. Ratios between the results and their averages Reason (DAE/DBE) c.s -1.kg -1 Average ± 2ª Rainy Season 3ª Dry Season 4ª Rainy Season 5ª Dry Season DP C ± 1.9 C ± 0.4 C ± 2.2 C ± 0.3 S ± 0.7 S ± 0.5 S ± 0.2 S ± 1.5 T ± 2.6 T ± 0.7 T ± 0.9 T ± 0.4 Average ± DP 13.6 ± ± ± ± ± 1.5 With the data obtained by the calculated ratios of the analyzed values, the graph shown in Fig 4 was drawn chart to check if the average of the means at each sampling station was within the margins established with the global average.

8 Average Ratio Ratio between DAE and DBE Média Média Superior Média inferior C1 C2 C3 C4 S1 S2 S3 S4 T1 T2 T3 T4 Sampling Stations Figure 4: Variation of the average of the ratios relatively to the global mean. Observing Fig. 4, it can be noticed that all values of Table 6 are within the margins of the global average, with the exception of the C1 sampling station, which whose value falls above the established upper average. Taking into consideration that there is a total of 12 collection points with 4 results for each campaign at each station, a total of 48 results has been obtained. Table 7 shows that the percentage of results were the average track spacing in a smaller percentage than or equal to 5%, or less than 10% and greater than 10%. Table 7. Average distance normalized campaign as a percentage Percentage class % Number of Results Corresponding Percentage % > As for the causes that led results to be above 10% departure from the average, both positively or negatively, there can be several causes: contribution from other radionuclides in the of high-energy detector, higher errors do to self-absorption effects in the low energy detector, short counting time relatively to that desirable in the ELD, among others.

9 5. CONCLUSIONS Analyzing the outcome of 210 Pb measurements in river bottom sediment samples it has been shown that detection by gamma spectrometry technique using CANBERRA detectors, carbon window HPGe and beryllium window HPGe produced more reliable results. The results do not depend on using the Genie 2000 software with the "Geometry Composer" calculation tool generating activity concentration results with efficiently and self-absorption corrections, or simply in the form of count.s -1.kg -1 directly obtained from the net area under the gamma peak of interest. In case the computer program is not available, analytical calculations can produce complete and confirmatory results. Analyzing the percent departures from the average determined in this work, it can be said that the performances of the detectors were similar. As for the causes that led to the results to exceed 10% departure, either positive or negative, they may be various: effects of contribution from the presence of other radionuclides in the sample during measurement in high-energy detector, increased effect of selfabsorption in the low-energy detector, counting time for gamma radiation shorter than recommended, among others. Finally, this work shows that the determination of only one radionuclide allows the detection of the impact of effluent discharges into a river, proximity to sources, seasonality, anthropogenic activities s well as mapping the distribution of activity levels in the environment. It also suggests that the Caldas- UTM influences the distribution of 210 Pb on its surroundings. More detailed studies should be performed in order to identify if the geological anomalies in the region that can contribute to increased 210 Pb activity in the study area. ACKNOWLEDGEMENTS This manuscript was adapted of part of thesis Distribuição do 210 Pb em Sedimentos de Bacias Hidrográficas Circunvizinhas a Mina de Urânio de Caldas-MG, 2014, submitted to Nuclear Technology Development Center. Acknowledgements by the support of colleagues and the following Brazilian institutions: Nuclear Technology Development Center (CDTN), Brazilian Nuclear Energy Commission (CNEN), Research Support Foundation of the State of Minas Gerais (FAPEMIG), and Brazilian Council for Scientific and Technological Development (CNPq). REFERENCES 1. M. Mihaljevič. Lead. In: C.P. Marshall, R.W. Fairbridge. Encyclopedia of Geochemistry. Springer Netherlands, pp (1999). 2. J.S. Rieuwerts, M. Farago, M. Cikrt, V. Bencko. Heavy Metal Concentrations in and Around Households Near a Secondary Lead Smelter. Environ Monit Assess, 58, pp (1999). 3. D.C. Adriano. Trace Elements in the Terrestrial Environments: biogeochemistry, bioavailability, and risks of metals. Springer, New York (2001). 4. G. Ahlberg, O. Gustafsson, P. Wedel. Leaching of Metals from Sewage Sludge during one Year and their Relationship to Particle Size. Environ Pollut, 144, pp (2006). 5. M. Komárek, V. Ettler, V. Chrastn, M. Mihaljevič. Lead isotopes in environmental sciences: A review. Environment International, 34, pp (2008).

10 6. C.A. Nittrouer, R.W. Stemberg, R. Carpenter, J.T. Bennett. The Use of 210 Pb Geochronology as a Sedimentological Tool: Application to the Washington Continental Shelf. Marine Geology, 31, n. 3-4, pp (1979). 7. M. Hasan; D. Bódiz; Sz. Czifrus. A Simplified Technique to Determine the Self-Absorption Correction for Sediments Samples. Applied Radiation and Isotopes, 57, n. 6, pp (2002). 8. S.N. Santos, J.M. Lafon, J.A.M. Corrêa, M. Babinski, F.F. Dias, M.H.T. Taddei. Distribuição e Assinatura Isotópica de Pb em Sedimentos de Fundo da Foz do Rio Guamá e da Baía do Guajará (Belém - Pará). Quim. Nova, 35, n. 2, pp (2012). 9. C.A. Carvalho Filho. Avaliação da Qualidade das Águas Superficiais no Entorno das Instalações Minero-Industriais de Urânio de Caldas, Minas Gerais f. Tese (Doutorado em Ciência e Tecnologia das Radiações, Minerais e Materiais) Centro de Desenvolvimento da Tecnologia Nuclear, Belo Horizonte, M.O. Fraenkel, R.C. Santos, F.E.V.L. Loureiro, W.S. Muniz. Jazida de Urânio no Planalto de Poços de Caldas Minas Gerais. In: DEPARTAMENTO NACIONAL DA PRODUÇÃO MINERAL. Principais Depósitos Minerais do Brasil: Recursos Minerais Energéticos, Brasília: v. 1, cap. 5, p P.H. Dutra. Distribuição do 210 Pb em Sedimentos de Bacias Hidrográficas Circunvizinhas a Mina de Urânio de Caldas-MG f. Dissertação (Mestrado em Ciência e Tecnologia das Radiações Minerais e Materiais), Centro de Desenvolvimento da Tecnologia Nuclear, Belo Horizonte, C.J. Brookes, I.G. Betteley, S.M. Loxton. Fundamentals of mathematics and statistics. John Wiley & Sons, p. 369 (1979). 13 A. Shakhashiro, A.M.G.F. Azeredo, U. Sansone, C.K. Kim, G. Kis-Benedek, A. Trinkl, T. Benesch, R. Schorn. Report on the IAEA-CU proficiency test on determination of 137 Cs and 210 Pb in spiked soil. IAEA (2006). Disponível em: Acesso em: 28 jan