Radiation Protection Dosimetry Advance Access published April 27, 2015 Radiation Protection Dosimetry (2015), pp. 1–4

doi:10.1093/rpd/ncv228

SOIL FEATURES AND INDOOR RADON CONCENTRATION PREDICTION: RADON IN SOIL GAS, PEDOLOGY, PERMEABILITY AND 226RA CONTENT E. Lara1,*, Z. Rocha2, T. O. Santos1, F. J. Rios2 and A. H. Oliveira1 1 Departamento de Engenharia Nuclear, Universidade Federal de Minas Gerais, Av. Pres. Antoˆnio Carlos, 6.627—Pampulha, Belo Horizonte 31270-901, Brazil 2 Centro de Desenvolvimento da Tecnologia Nuclear, Comissa˜o Nacional de Energia Nuclear, Av. Pres. Antoˆnio Carlos, 6.627—Pampulha, Belo Horizonte 31270-901, Brazil *Corresponding author: [email protected] This work aims at relating some physicochemical features of soils and their use as a tool for prediction of indoor radon concentrations of the Metropolitan Region of Belo Horizonte (RMBH), Minas Gerais, Brazil. The measurements of soil gas radon concentrations were performed by using an AlphaGUARD monitor. The 226Ra content analysis was performed by gamma spectrometry (high pure germanium) and permeabilities were performed by using the RADON-JOK permeameter. The GEORP indicator and soil radon index (RI) were also calculated. Approximately 53 % of the Perferric Red Latosols measurement site could be classified as ‘high risk’ (Swedish criteria). The Litholic Neosols presented the lowest radon concentration mean in soil gas. The Perferric Red Latosols presented significantly high radon concentration mean in soil gas (60.6 + 8.7 kBq m23), high indoor radon concentration, high RI, 226Ra content and GEORP. The preliminary results may indicate an influence of iron formations present very close to the Perferric Red Latosols in the retention of uranium minerals.

INTRODUCTION The soil features are important variables to predict the potential of the indoor radon, because the radon concentration in soil gas can be transported into a home through cracks in solid floors and walls below the construction level; through gaps in suspended concrete and timber floors and around service pipes; through crawl spaces, cavities in walls, construction joints and small cracks or pores in hollow-block walls. Recent studies have shown a positive correlation between geology and high indoor radon concentrations and confirmed that geologic variables such as lithology and structure are the primary parameters for determining the radon potential (RP) of the soil(1). Therefore the main source of indoor radon in most buildings is the subjacent soil gas(1 – 9). The main purpose of this work was to relate some physicochemical features of soils such as radon concentration in soil gas, pedology, permeability and 226 Ra content, and use them as a tool for prediction of indoor radon concentrations.

1991 in the Czech Republic, the RP for prospective building sites has been characterised by performing in situ measurements in the soil(11). In Sweden, the Swedish Radiation Protection Authority conducted an extensive programme of determining the indoor radon concentrations in dwellings. Thus, local risk maps were based on geological criteria, like soils rich in uranium and thorium and very permeable soils. This criterion for risk assessment known as ‘Sweden Criteria’ was established, by creating a classification based on radon concentrations in soil gas. This criterion states that soils showing radon concentrations in soil gas below 10.0 kBq m23 are considered ‘low risk’ and do not require special buildings. Radon concentrations in soil gas between 10.0 and 50.0 kBq m23 are classified as ‘normal risk’ and require protective actions in dwellings. If soils presented concentrations above 50.0 kBq m23, they are classified as ‘high risk’ and require buildings with safety criteria against radon(2). Other authors, Eisenbud and Gesell(12), state that in typical soils, the radon concentrations in soil gas ranges from 4.0 to 40.0 kBq m23.

MAPPING GEOGENIC RADON

Soil radon potential and soil radon index

Aiming at preventing the population from being exposed to high levels of radon concentration, some governmental or government-linked research centres of many countries have identified higher risk areas through mapping geogenic radon risk. These maps serve as a management tool for authorities assisting them in making decisions on priority areas(10). Since

The soil RP allows to one estimate the relationship between the radon soil gas and permeability, and also the lithologies that will have the highest potential for indoor radon accumulation to exceed the indoor guideline limit (13). The RP is a dimensionless value calculated from measurements of radon concentrations in soil gas to a depth of 0.8 m and the permeability of

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E. LARA ET AL.

this soil, in accordance with Equation (1). Thus, the higher the RP value, the greater the potential for radon to migrate through the soil to dwellings is RP ¼

C  C0 ; log ðPÞ þ log ðP0 Þ

ð1Þ

where C is the radon concentration in soil gas (kBq m23), P is the permeability of the soil (m2) and C0 and P0 are 1.0 kBq m23 and 1.0`  10210 m2, respectively. Soil radon index (RI) of a building site is an index that indicates the level of risk of radon release from the bedrock, surface material and/or soil and depends on the RP determination. The RI is classified in low, medium and high risk. Therefore, the RP is the value expressing the soil RI. If RP is ,10, the RI is low; if 10  RP , 35, the RI is medium; and if RP is 35, the RI is high. This methodology for risk assessment of indoor radon concentration is employed in the Czech Republic(1). Geological radon potential The Geological Radon Potential (GEORP) indicator is established based on the percentage of dwellings located in a given area which has indoor radon concentrations that exceed action levels set by regulators. Talbot et al. classify soils by its radon geological potential in three types: ‘low’ to GEORP values below of 5 %; ‘moderate’ to GEORP between 5 and 10 % and ‘high’ to GEORP above 10 %. This work used data related to the indoor radon concentrations in dwellings of Metropolitan Region of Belo Horizonte (RMBH), Minas Gerais, Brazil. The GEORP indicator was calculated based on the percentage of dwellings that exceeds the first action level of the United States Environmental Protection Agency, which is 148.0 Bq m23(14). Therefore, each pedology estimated the percentage of dwellings with indoor radon concentrations above this action limit were estimated for each pedology in the RMBH. Thus, the GEORP values for each pedology were obtained. MATERIALS AND METHODS

assigned to the concentration of the radon soil gas used for each pedology. About 150 measurements of radon concentration in soil gas, 90 measurements of permeability and 150 determinations of 226Ra activity concentrations were performed. The number of samples for each pedology is given in Table 1. Radon concentration in soil gas and permeability The radon concentrations in soil gas were performed by using the AlphaGUARD detector (PQ2000 PRO), by SAPHYMO GmbH, Germany. For this purpose, the AlphaGUARD is used for the measurement in situ by using a metallic probe inserted into the ground at a depth of 0.8 m. The soil permeability was performed for the same area using the RADON-JOK equipment, by RADON V.O.S, Czech Republic. Indoor radon concentration The indoor radon concentrations inside dwellings were performed by using Eletret Ion Chamber—E-PERM electret, Rad Elec, Inc. Measurements were performed by SST (S—short-term chamber and ST—short-term Eletrect) option in the closed condition, for at least 2 d, according to the protocol established by US EPA displayed in tabs ‘Protocols for Radon and Radon Decay Product Measurements in Dwellings’(18). 226

Ra content

The soil samples were collected from RMBH 0.8 m deep and, after crushed, the samples were put in 0.5 l sealed Marinelli’s for 4 weeks to achieve the radioactive secular equilibrium. Then, the samples were analysed by gamma spectrometry with a high pure germanium (HPGe) detector of Canberra, model GC1519, with a closed-end coaxial geometry and 15 % to relative efficiency of NaI. The specifications of the detector are: relative efficiency at 1.33 MeV 60Co is 18.7 %, full width at half maximum resolution of 1.78 keV, full width tenth maximum resolution of 3.29 keV both at 1.33 MeV 60 Co, and the peak-to-Compton ratio is 44.7:1 to 60Co.

Geological aspects of the study area—RMBH The RMBH (Metropolitan Region of Belo Horizonte) located in the central-western portion of Minas Gerais, Brazil, is comprised 34 cities, covering an area of 9461 km2. It is inserted into the large geological unit called Craton Sa˜o Francisco, tectonically stable since the end of the Paleoproterozoic and whose southern boundary is in the Iron Quadrangle(15 – 17). Radon concentration and sampling distribution The field sites in the RMBH were chosen and subdivided by pedology. The arithmetic mean of the measurement points was calculated, and this value was

RESULTS AND DISCUSSIONS Radon concentration in soil gas Each sampling point was assigned to a pedology type according to the mapping of pedology by Brazilian System of Soil Classification—SiBCS, described in Manual of Technical Pedology(19). Thus, all the pedologies of RMBH were distributed in classes, and as Red Ultisol, Yellow Red Ultisol, Haplic Cambisol, Red Latosol, Perferric Red Latosol, Yellow Red Latosol and Litholic Neosol. The overall mean of radon concentrations is 26.5 + 2.2 kBq m23, in agreement with the range of mean concentration

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High 19.0 Low 7.5 (10212 –10211) 12.4 + 2.5 21 13.6 + 3.0 101.1 + 11.1

5

Moderate 5.0 15.0 Medium 20 23.7 + 12 80.8 + 7.0

4

21.7 + 0.5

(10212 –10211)

High High 37.5 26.5 13.0 Medium 49.0 High 16 68 14.1 + 3.2 60.6 + 8.7 137 + 11.5 130 + 17

4 15

15.3 + 6.1 50.3 + 13.0

(10211) (10214 –10211)

High 16.0 17.0 Medium 63 21.7 + 6.4 98.1 + 9.3

15

18.1 + 3.4

(10214 –10211)

High High 18.0 16.5 18.5 Medium 16.0 Medium (10212 –10211) (10214 –10211) 21.6 + 0.3 23.7 + 3.4 14 65 60 252 28.1 + 3.4 24.0 + 2.2 142.2 + 23 113.2 + 8.4

Red Ultisol Yellow Red Ultisol Haplic Cambisol Red Latosol Perferric Red Latosol Yellow Red Latosol Litholic Neosol

RI RP Range of soil permeability (K ) (m2) [226Ra] in soil (Bq kg21) N (soil) N (air) Arithmetic mean [222Rn] Arithmetic mean [222Rn] in the air (Bq m23) in soil gas (kBq m23) Pedology

Table 1. Pedologies, radon concentrations in the air and soil gas, 226Ra, permeability, RP, RI and GEORP of RMBH.

GEORP (%)

GEORP classification

SOIL FEATURES AND INDOOR RADON CONCENTRATION PREDICTION

suggested by Eisenbud and Gesell(12). The activity concentrations of 226Ra ranged from 12.4 + 2.5 to 23.7 + 3.4 Bq kg21, also in agreement with the mean values for typical soils based on UNSCEAR(20). However, the mean radon concentration in soil gas ranged from 13.6 + 3.0 kBq m23 for Litholic Neosols to 60.6 + 8.7 kBq m23 for Perferric Red Latosols. The 226 Ra activity concentration showed values of 12.4 + 2.5 Bq kg21 for Litholic Neosols and 50.3 + 13.0 Bq kg21 for Perferric Red Latosols. Therefore, the highest values of radon concentration in soil gas and 226Ra content were found in Perferric Red Latosols. Indoor radon concentration, GEORP and RI Santos(18) reported a mean of 108.6 + 2.7 Bq m23 for indoor radon concentration RMBH dwellings. In this previous study, the overall GEORP for RMBH was calculated to be 18.8 %, i.e. of the 500 dwellings measured, 94 presented indoor radon concentrations above of 148.0 Bq m23 (US EPA action limit). The Perferric Red Latosol presented high indoor radon concentration and highest RI, which can be attributed to the influence of the permeability on the indoor radon concentration. This pedology also presented highest 226Ra content in soils and high GEORP (26.5 %), suggesting a positive influence of radon concentration in soil gas in indoor concentrations. The preliminary results may indicate an influence of iron formations present very close to the Perferric Red Latosols in the retention of uranium minerals. It is known that the adsorption capacity of uranium on the mineral surfaces is high in iron oxides(21, 22). Since uranium may stay adsorbed by the iron oxides, it may be inferred that such regions may present elevated indoor radon concentrations, although it depends also on other characteristics, such as quality and state of the floor, soil permeability and building materials. Although the Red Latosol also exhibits elevated values, these will not be considered because it is missing a greater improvement in the number of measurements. The GEORP of all soils (except Yellow Red Latosol) showed percentages above 10 % of the US EPA action limit, considered ‘high’ potential. The results can be seen in Table 1. CONCLUSIONS Overall for the soils of RMBH Approximately 17 % of the overall measurements performed in the RMBH presented radon concentrations in soil gas above 40.0 kBq m23, high reference value suggested by Eisenbud and Gesell(12). According to Swedish classification criteria, 70 % of the measurements showed radon concentrations in soil gas between 10.0 and 50.0 kBq m23 considered ‘normal risk’, and 13 % presented concentrations greater than 50.0 kBq m23, classified as ‘high risk’ areas. Page 3 of 4

E. LARA ET AL.

Approximately 40 % of the soils presented high permeability (10211 m2). By pedologies The Perferric Red Latosols presented significant radon concentrations in soil gas. The range was 13.6–124.4 kBq m23. Approximately 53 % of the measurement site can be classified as ‘high risk’, according to the Swedish classification criteria. It is noteworthy that Haplic Cambisol, Yellow Red Latosol and Litholic Neosol have presented radon concentrations in indoor air and in soil gas below the overall means (108.6 + 2.7 Bq m23 and 26.5 + 2.2 kBq m23, respectively). The Litholic Neosol presented the lowest mean radon concentration in soil gas, 13.6 + 3.0 kBq m23. On the other hand, this pedology showed low RI and high GEORP (19 %). Similarly, the Red Latosol also showed low radon in soil gas concentration, medium RI and highest GEORP (37.5 %). These facts may be attributed to the dwellings condition, such as quality and state of the floors, soil permeability, ventilation conditions and building materials. However, it is suggested a greater number of measurements to draw conclusions. The Perferric Red Latosol also presented high indoor radon concentration and highest RI, which can be attributed to the influence of the permeability on the indoor radon concentration. This soil group also presented highest 226Ra content and high GEORP (26.5 %), suggesting a positive influence of the radon concentration in soil gas in indoor concentrations. The preliminary results may indicate an influence of iron formations present very close to the Perferric Red Latosols in the retention of uranium minerals. The GEORP of all soils (except Yellow Red Latosol) had percentages above 10 % of the level of action US EPA. The determination of radon concentration in soil gas proved to be a good indicator for predicting the GEORP for RMBH. Similarly, the RI is also a good indicator of the potential risk of radon, because it considers soil permeability and radon concentration in soil as primary variables. FUNDING The authors are thankful to Coordination for the Improvement of Higher Level - or Education Personnel (CAPES) for financial support. REFERENCES 1. Neznal, M., Neznal, M., Matolin, I. B., Barnet, I. and Miksova, J. The new method for assessing the radon risk of building sites. Czech Geological Survey Special Papers 16, pp. 1– 48 (2004). ˚ kerblom, G. Investigations and mapping of radon risk 2. A areas. Geology Environ. Plan. 2, 96–106 (1987).

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Soil features and indoor radon concentration prediction: radon in soil gas, pedology, permeability and 226Ra content.

This work aims at relating some physicochemical features of soils and their use as a tool for prediction of indoor radon concentrations of the Metropo...
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