2011 International Nuclear Atlantic Conference - INAC 2011 Belo Horizonte,MG, Brazil, October 24-28, 2011 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 978-85-99141-04-5

DISTRIBUTION OF GAS RADON CONCENTRATION IN THE METROPOLITAN REGION OF BELO HORIZONTE, BRAZIL AND CORRELATIONS WITH LITOLOGIES AND PEDOLOGIES

Evelise G. Lara1, Zildete Rocha2, Talita de O. Santos1, Ronaldo Araujo Miguel2, Aimoré Dutra Neto2, Maria Ângela de B. C. Menezes2 and Arno Heeren de Oliveira1

1 Departamento de Engenharia Nuclear - Escola de Engenharia Universidade Federal de Minas Gerais Av. Presidente Antônio Carlos, 6.627 - Pampulha 31270-901 Belo Horizonte, MG [email protected] [email protected] [email protected]

2 Centro de Desenvolvimento da Tecnologia Nuclear (CDTN / CNEN - MG) Av. Presidente Antônio Carlos, 6.627 - Pampulha 31270-901 Belo Horizonte, MG [email protected] [email protected] [email protected] [email protected]

ABSTRACT

The assessment of data from a previous study of radon in indoor air of about 500 built "slab-on-grade” dwellings in the Metropolitan Area of Belo Horizonte, RMBH, Brazil, showed some statistical correlations with litologies and pedologies. Forty-five measurements of radon concentration in soil gases were performed at selected points according to population density, gamma radiation background and measurements results of the mentioned indoor radon concentrations study. This paper presents the distribution of radon concentration in soil gas in the same area, where it ranges from 7 - 93 kBq.m-3, highlighting the areas of high radon concentration in soil gas, which suggests occurrence of radon prone areas. In this regard, this work looking for correlation of radon concentrations in soil gas with litologies and pedologies. The measurements were performed by using AlphaGUARD PQ2000 PRO SAPHYMO GmbH. The results of a radon survey in this areas has indicated that in the region radon prone areas may occur. Radon concentration measurements in soil gas are also being carried out to obtain higher statistical significance data for correlations and for comparison between equipment and probes from different systems suppliers like RTM 1688 of SARAD GmbH and RAD7 Electronic Radon Detector of DURRIDGE Company Inc. Thus, this research will represent the initial contribution to establish classification criteria, for radon in soil gas in a tropical climate similar to the Swedish criteria established by Åkerblom.

1. INTRODUCTION

Human beings are naturally exposed to the effects of ionizing radiation, and these exposures arising from cosmogenic and primordials radionuclides. Once dispersed into the environment, radionuclides can be incorporated by the living organism through the food chain, whose intake concentrate radioactive nuclides increases the internal exposure, and inhalation of airborne particles. Radioactive elements that contribute mostly to natural radioactivity are natural decay series radionuclides of 238U, 235U and 232Th and to a lesser extent, the 40K. Radon is a chemically inert gas from the decay of 226Ra, has a period of half-life of 3.82 days emitting alpha particles whose energy is about 5.49 MeV. Thus, the emission of ionizing radiation by rock and soil depends on its content of uranium, thorium, potassium and radium elements, since the levels of radon depend mostly on the concentration of radium. It was evaluated the soil radon level on the assumption that 1 Bq.kg-1 radium is equivalent to 1.700 Bq.m-3 radon concentration in soil [6], showing this dependence. Another important feature is the fact of owning a mobile capability of carrying information over long distances, both in the soil and in the . Indoor radon has received considerable attention in recent decades due to its potential effect of causing lung cancer if deposited in the upper respiratory tract when inhaled. These characteristics are important aspects in the context of public health and important information to geological studies. According Al-Azmi&Karunakara apud LeBrecque (2002), measurements of radon concentration in soil gas help to develop studies for prospecting faults and precursors of earthquakes [2,8].

It is possible to be the first basic contribution to establish criteria based on radon in soil gas and on soil permeability that would identify zones of high radon availability that contribute to indoor accumulation [10]. In Sweden, the limit value for radon concentrations in dwellings is 400 Bq.m-3, however, of a total of about 1.6 million houses in Sweden, it is estimated that some 40000 houses (2.5%) have radon concentrations exceeding this limit [4]. According to the study, for most of these houses, the soil gas radon is the source of the high indoor radon concentration. Thus, Sweden has established criteria for radon hazard assessment, that consider just the radon concentrations in soil gas, less than 10 kBq.m-3 as “low risk” and require no special construction, the range 10 - 50 kBq.m-3 is classified as “normal risk” and requires “radon protective” construction, and values greater than 50 kBq.m-3 are considered “high risk” and require “radon safe” construction [5].

The Metropolitan Region of Belo Horizonte (RMBH), Minas Gerais, Brazil, has 34 municipalities, covering an area of approximately 9.460 km², equivalent to 1.6% of the total area of Minas Gerais [15]. Studies have shown that the variation in concentrations of radon gas within homes in the RMBH is largely due the local geological setting [13, 14]. The part of RMBH is located over a granite embasement area, the granite gneissic Complex, suggesting the passive existence of radon prone areas. Most bedrock of this area consists of Archaean rocks of the granitic gneissic Complex and metasedimentary sequences of the great unity of the Precambrian Iron Quadrangle of Minas Gerais, Brazil. The granitic rocks have high of natural radionuclides concentrations in general, us that of 235U and 232Th series [7].

The objective of this paper is to look for knowing the distribution and also the concentration of soil radon gas in the Metropolitan Region of Belo Horizonte with the geological structure of the soil of this region _ corresponding litologies and pedologies.

INAC 2011, Belo Horizonte, MG, Brazil.

2. PARAMETERS RELATED TO THE RADON CONCENTRATION IN SOIL GAS

The distribution and concentration of radon in soil gas is influenced by physical, chemical, geological and climatic factors, among these factors, the 226Ra concentrations and soil permeability, are considered the most important ones [12].

The determination of radon entry rates into pore spaces, also called emenation rate, and its exhalation to the atmosphere depends on the radon concentrations in soil, on the distribution and concentrations of the radium radionuclide in the bedrock and on the permeability of the soil as well. Certain generalizations can be made about the radium concentrations in several types of bedrocks, but there are very large ranges within each type. In general, granites have relatively high radium contents, sedimentary and metamorphic rocks intermediate contents, and basalts and most lime stones low contents, although there are exceptions to this rule [15]. Due to the diversity of radium concentrations in the soil, it becomes difficult to generalize their characterization. Radium transfers more readily to vegetation than the parent uranium radionuclides, and the emanation from is more effective than from soil minerals. The effective permeabilities of rocks and are also highly variable, being related to degree of weathering, porosity, moisture content, and the presence of cracks or fissures [15].

Although geological substrates potentially fertile to the release of radon should have a basic feature, enriched in uranium and thorium and fractured, these are not, however, enough conditions, especially in areas with hot and humid climate, where important pedogenetics residuals cover the rocks. Other crucial properties of the ground for secondary porosity are: type and thickness of soils and their distribution space, regime and geometry of the aquifer table. In this context, rocks with the same levels in normal radionuclide uranium and thorium can be transformed into an appreciable radon suppliers material [1].

3. MATERIALS AND METHODS

3.1. The Choice of Study Area

The surveyed area includes the industrial complex of the state of Minas Gerais, which has about five million inhabitants; the highest percentages of population are concentrated in the cities of Belo Horizonte, Contagem, Betim and Ribeirão das Neves, and counts with 75.7% of the total population of this region [16].

Most part of the inhabitants lives over a granitic gneissic Complex. Studies show that the granite has high concentrations of natural radionuclides, especially 238U and 232Th and the radionuclides of their respective natural decay series [7]. An indoor air radon survey in dwellings of the RMBH has presented a high radon concentration average value and a great number of results above the action levels suggested by international agencies, such as the United States Environmental Protection Agency (USEPA) and the International Commission of Radiological Protection (ICRP), which are 148 and 200 Bq.m-3, respectively. The high average concentration together with the great number of values above the USEPA and ICRP

INAC 2011, Belo Horizonte, MG, Brazil. actions level suggest a relevant contribution of the local geological setting specially the and the litology. The above features, the RMBH shows an interesting area of study about radon concentrations indoor [13, 14].

Forty-five measurements of radon concentration in soil gases were performed at selected points according to population density, gamma radiation background and measurements of indoor radon concentrations in dwellings in the region.

3.2. Detection Equipment and Procedure

The radon concentrations in soil gases were determined by using a radon in soil gas probe and the AlphaGUARD detector, model PQ2000 PRO, both of SAPHYMO GmbH, Germany. The AlphaGUARD detector incorporates a pulse-counting ionization chamber. Through the geometry of the chamber and the signal evaluation, this radon monitor is suitable for the continuous monitoring of radon concentrations between 2 Bq.m-³ and 2000 kBq.m-³. For the determination of radon in soil gases, the AlphaGUARD is used in continuous flow mode at intervals of 1 minute. In this case, the technique consisted of positioning the probe, a metal rod into the soil to a depth of about 70 cm. The soil gas with radon is sucked by a pump that transports it through a capillary tube into the detector ionization chamber in an open circuit. With the circuit presented in Figure 1, the pump is turned on for 5 minutes, then it is turned off. The counting continues for another 10 minutes. The soil gas in his way to the detector is directed through a water-stop filter in order to absorb his water content and also through a progeny filter before passing through the air pump. By interactions, in the electric field, the alpha particle of radon and its progeny ionize the air volume in detector ionization chamber, generating electrical current that will cause a pulse. After establishing the equilibrium between radon and its progeny, the resulting electrical current is directly proportional to the radon concentration in the soil gas.

Figure 1. Experimental arrangement for radon measurement in soil gas.

INAC 2011, Belo Horizonte, MG, Brazil.

4. RESULTS AND DISCUSSION

The forty-five measuring points results sorted by litology or by pedology are graphically presented in the Figure 2. Table 1 presents the mean concentrations of radon gas sorted by litology and pedology. Relating to the litology and to the pedology, the distribution of forty- five measured points is presented in the Figures 3 and 4. Just two kinds of litology (hierarchy) have presented radon concentration in soil gas in the high risk range according to the Swedish Criteria, namely the Complex and the Group. Complex is the embasement of the most density population area that includes the city of Belo Horizonte. The Group corresponds to an area more related to the Iron Quadrangle of Minas Gerais. Although few points have been measured in this preliminary work, the highest mean value for radon in soil gas of the Iron Latosol pedology area suggests that the soil gas radon concentration may the main radon contributor to the higher in indoor air radon concentration in the area. A general observation of the results shows that according to Swedish Criteria, 11% of the radon in soil gas measurements points were above the level of 50 kBq.m-3, the corresponding soils are classified as "High Risk Range", where the constructions require protective measures against radon.

Table 1. Means results of measurements

PEDOLOGY LITOLOGY Mean Mean Mean Number Number of [²²²Rn] in [²²²Rn] in Mean Standard Soil Group Standard Hierarchy of Measures Soil Soil Deviation Deviation Measures (kBq.m-³) (kBq.m-³) Rock Outcrop - - - Complex 26 25.4 3.7 Cambie Soil 5 30.0 12.1 Body 2 19.5 12.4 Iron Latosol 6 44.4 10.4 Formation 4 37.4 5.6 Yellow Red Latosol - - - Group 9 30.4 8.1 Dark Red Latosol - - - Member 4 20.3 2.9 Litosoils - - - Unit - - - Red Yellow Podzoil 27 26.8 3.6 Dark Red Podzoil 7 29.0 5.0 Alluvial Soil - - -

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Radon concentration in soil gas - Pedology Radon concentration in soil gas - Litology

Åkerblom Criteria 100 Åkerblom Criteria Åkerblom 100 90 Åkerblom Criteria 90 Criteria 80 80 70 “High 70 “High 60 risk” 60 risk” 50 50 40 40 “Normal “Normal 30 risk” 30 risk” 20 20

10 10 “Low “Low ²²²Rn concentration0 insoil gas(kBq.m-³) ²²²Rn concentration in soil gas (kBq.m-³) risk” 0 risk” 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 Measurement number Hierarchy Measurement number Cambie Soil Iron Latosol Red Yellow Dark Red Complex Body Formation Group Member Podzoil Podzoil

Figure 2. Radon concentrations in soil gas - Pedologies, Litologies and Åkerblom Criteria.

INAC 2011, Belo Horizonte, MG, Brazil.

Figure 3. Pedology of the RMBH.

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Figure 4. Litology of the RMBH

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3. CONCLUSIONS

This work examined the relationship between the radon concentrations in soil gas with the litologies and pedologies of the region. This preliminary study pointed out that in some areas where the is highly variable over short distance; it is particularly important performing a larger number of measurements in order to acquire better knowing about the effects of the local geological settings. Until now the results are indicating that the pedology “Iron Latosol” presents the highest values for radon in soil gas and that the litology classified by the hierarchies “Group”, “Formation” and "Complex" presented the highest mean values, but only the litologies "Group” and "Complex" have presented results above the level of 50 kBq.m-3. According to Swedish Criteria, these soils may be classified as "High Risk Soil", corresponding to areas where the constructions may require protective measures against radon, even for a tropical country. Due to the small number of measured points all the conclusions above are preliminary ones. The research work is planned to continue by doing about more 80 measuring points in next year as indicated by white points in the Figures 3 and 4. It is also planed to continue the work by analyzing the permeability by using the equipment RADON-JOK of RADON V.O.S., density and chemical composition looking for finding the relationship between these parameters and the concentrations of radon in soil gas looking for a better comprehension of the indoor radon origin in the RMBH. The work will be also continued by using radon equipment and probes from different systems suppliers like RAD7 Electronic Radon Detector of DURRIDGE Company Inc. and RTM 1688 of SARAD GmbH to compare their performances and to get statistically more significant values.

ACKNOWLEDGMENTS

The authors would like to thank CDTN, especially to the technician Antônio Francisco Dias who helped a lot in doing the field studies and many devices, and UFMG for their scientific and technical support, all the authors and contributors. Special thanks also to the Geographer Ricardo Scott Varela Malta for the assistance in GIS. Thanks to CAPES as well, for the financial support for this study.

REFERENCES

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