Journal of Environmental Radioactivity xxx (2016) 1e5
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
A study on the correlation between soil radon potential and average indoor radon potential in Canadian cities Jing Chen a, *, Ken L. Ford b a b
Radiation Protection Bureau, Health Canada, 775 Brookfield Road, Ottawa, K1A 1C1, Canada Geological Survey of Canada, Natural Resources Canada, 601 Booth Street, Ottawa, K1A 0E8, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 October 2015 Received in revised form 23 December 2015 Accepted 27 January 2016 Available online xxx
Exposure to indoor radon is identified as the main source of natural radiation exposure to the population. Since radon in homes originates mainly from soil gas radon, it is of public interest to study the correlation between radon in soil and radon indoors in different geographic locations. From 2007 to 2010, a total of 1070 sites were surveyed for soil gas radon and soil permeability. Among the sites surveyed, 430 sites were in 14 cities where indoor radon information is available from residential radon and thoron surveys conducted in recent years. It is observed that indoor radon potential (percentage of homes above 200 Bq m3; range from 1.5% to 42%) correlates reasonably well with soil radon potential (SRP: an index proportional to soil gas radon concentration and soil permeability; average SRP ranged from 8 to 26). In five cities where in-situ soil permeability was measured at more than 20 sites, a strong correlation (R2 ¼ 0.68 for linear regression and R2 ¼ 0.81 for non-linear regression) was observed between indoor radon potential and soil radon potential. This summary report shows that soil gas radon measurement is a practical and useful predictor of indoor radon potential in a geographic area, and may be useful for making decisions around prioritizing activities to manage population exposure and future land-use planning. Crown Copyright © 2016 Published by Elsevier Ltd. All rights reserved.
Keywords: Radon-222 Indoor radon Soil radon
1. Introduction Radon (222Rn) is a naturally occurring radioactive gas generated by the decay of uranium bearing minerals in rocks and soils. A certain fraction of the radon escapes from the ground into the air. In the open air, radon is diluted to low concentrations and is not considered a health concern. However, radon that enters an enclosed space, such as a family house or a school building, can sometimes accumulate to concentrations above the Canadian Action Level (200 Bq m3). Soil gas radon contributing to the indoor environment has been identified as the main source of natural radiation exposure to the population (UNSCEAR, 2006). Radon has been identified as the second leading cause of lung cancer after tobacco smoking (WHO, 2009). Soil gas radon measurement is a useful tool for the assessment of environmental radon potential and for the prediction of potential indoor radon concentrations in a geographic area, as demonstrated by many studies in various geographic locations (Keller et al., 1992;
* Corresponding author. E-mail address: [email protected] (J. Chen).
Mose et al., 1992; Neznal et al., 1996; Akerblom and Mellander, 1997; Vaupotic et al., 2002; Sundal et al., 2004; Reimer and Szarzi, 2005; Neznal et al., 2006; Kemski et al., 2006, 2009; Chen et al., 2009a; Minda et al., 2009; Barnet, 2012; Cinelli et al., 2015; Lara et al., 2015). Various studies have shown that in addition to radon levels in the soil, the soil permeability is another important factor in determining the radon potential of a site or an area, because higher permeability enables the increased migration of soil gas radon from soil into houses. For example, in a German case study in 1988e1989, Keller et al. found that high radon levels in the soil cause high indoor concentrations, if there are easy pathways into houses, and positive radon anomalies in the soil gas coincide with the locations of houses showing the highest concentrations (Keller et al., 1992). In northern Virginia and southern Maryland, Mose et al. attempted to predict which geographic areas should be associated with a high percentage of homes with unusually high indoor radon levels based on estimates of soil radon and soil permeability for geological units (Mose et al., 1992). They concluded that predictions of indoor radon based on estimates of home site soil radon and soil permeability are very useful. Similar conclusions were obtained in a recent study in Brazil (Lara et al.,
http://dx.doi.org/10.1016/j.jenvrad.2016.01.018 0265-931X/Crown Copyright © 2016 Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Chen, J., Ford, K.L., A study on the correlation between soil radon potential and average indoor radon potential in Canadian cities, Journal of Environmental Radioactivity (2016), http://dx.doi.org/10.1016/j.jenvrad.2016.01.018
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J. Chen, K.L. Ford / Journal of Environmental Radioactivity xxx (2016) 1e5
2. Methods
areas. The soil survey sites were areas of about 10 10 m2 in lowtraffic areas of community parks and away from roads. For each site, generally two permeability and five soil gas radon measurements were performed with four probes at each corner and one probe in the centre of the 10 10 m2 survey area. The soil radon potential (SRP) index is defined as, SRP ¼ (C C0) / (log(P) þ log(P0)) where C is the radon concentration of the soil gas in kBq m3, and P is the soil permeability in m2. C0 and P0 are set to be 1 kBq m3 and 1$1010 m2, respectively (Neznal et al., 1996, 2006). The SRP is proportional to the radon concentration in soil. It depends also on soil gas permeability; the more permeable the soil, the higher is the SRP. Due to the extremely wide range of soil gas permeability over several orders of magnitude, the SRP is adjusted with the logarithm of soil permeability.
2.1. Soil radon and permeability measurements
2.2. Indoor radon measurements
Soil gas radon concentration was determined by measuring the radioactivity of soil gas samples taken from a depth of 80 cm below the ground surface (Radon VOS, 2007a). Because the boulder-rich nature of glacial tills made it difficult to reach the target depth at some sites, the sample depth was occasionally reduced to 60 cm. Samples of soil gas were collected using a 150 ml syringe. The soil gas samples were then introduced into ionisation chambers for measurement. Background control measurements of the ionization chambers were performed before sampling soil radon. An ionisation chamber was only used when its background reading was below 0.7 kBq m3, as instructed by the manufacturer (Radon VOS, 2007a). Soil radon concentrations measured below 1 kBq m3 were excluded since any potential leakage during the soil gas sampling could result in a lower radon concentration. Therefore, the lowest measured value of soil gas radon was excluded in the calculation of the average radon concentration for a survey site. The permeability of the soil is influenced by natural soil moisture, density, effective porosity, as well as other parameters. Although absolute soil moisture measurements were not conducted in the surveys, the influence of soil moisture was indirectly measured with the permeability. In situ permeability measurements were performed with RADON-JOK, an instrument manufactured in the Czech Republic (Radon VOS, 2007b), at a depth of 60e80 cm below the ground surface. The measurement is based on air withdrawal by means of negative pressure. The soil gas is pumped under constant pressure through a probe (the same probe as used for soil radon collecting) with a constant surface, an active area created in the head of the probe at 80-cm below the ground. The soil gas permeability was calculated based on Darcy's equation (Koorevaar et al., 1987). For sites having very low permeability, the in situ soil gas permeability measurement could potentially take hours to complete. In these cases, a limit value of 2$1014 m2 was assumed. Long-term monitoring of soil radon variations was conducted at two reference sites in Ottawa (Chen et al., 2009a), from thawing of the ground in the late spring to 1 day before the first snowfall in early winter. Results showed that during the normal field survey period from June to September in Canada, a single field survey with multiple measurements of soil gas radon concentrations at a depth of 60e80 cm could characterise the soil gas radon level of a site within a deviation of ±30%. All soil radon surveys summarised here were conducted from June to September. All surveys followed well established protocol of the National Soil Radon Project, a Canadian add-on project to the North American Soil Geochemical Landscapes Project (GSC, 2009). Surveys were conducted on a dry day with a clear sky at least the previous evening. Within a city, soil gas radon and soil permeability measurements were conducted in community parks within residential
In order to assess thoron (220Rn) contribution to indoor radon and thoron exposure combined, simultaneous radon and thoron measurements were first conducted in Ottawa, Winnipeg, Halifax and Fredericton (Chen et al., 2008a, 2009b, 2011). In 2014, a nationwide survey of residential radon and thoron concentrations was completed in 33 cities (accounting for approximately 70% of the Canadian population), with a sample size of roughly 4000 homes (Chen et al., 2015). In order for the result to be indicative of the average annual radon and thoron exposure, all tests were conducted for a period of at least three months in the typical heating season that runs from October to April. A passive integrated radonethoron discriminative detector developed at the National Institute of Radiological Sciences in Japan (commercially known as RADUET) was used in these surveys. These results serve as the main data source of indoor radon concentrations in this summary report. To gain a better understanding of radon concentrations in homes across Canada, a national residential radon survey was launched in April 2007. The survey used alpha track detectors, and conducted long-term (3-months or longer) radon measurements in roughly 14,000 homes in 121 health regions (administrative areas defined by the provincial ministries of health) across Canada (Health Canada, 2012). Results of this survey were added to the results from the above-mentioned city-based radon/thoron surveys when health region boundaries approximately matched the boundaries of cities considered here. All surveys confirmed that radon was present in all homes in varying concentrations. The indoor radon concentrations varied widely in a geographic area. Even though it is hard to predict indoor radon concentration for any given house in a community, the average characteristics of indoor radon distribution should be representative for a geographic area. All surveys confirmed that indoor radon concentrations in a community followed log-normal distribution. A log-normal distribution is a two-parameter distribution characterised with the geometric mean (GM) and geometric standard deviation (GSD). Indoor radon potential of a geographic area or a city should be a quantity representing the characteristics of the two-parameter distribution of indoor radon concentrations in that area. Therefore, the percentage of homes above the Canadian Action Level of 200 Bq m3 was chosen as the indicator of indoor radon potential for an area.
2015) when using soil features to predict indoor radon concentration. Barnet (2012) compared two different methods (real and calculated) for evaluating the relationship of soil gas radon and probability of indoor radon exceeding 200 Bq m3 based on data sets from the Czech Republic, the results showed minor differences. Since radon in soil is believed to be the main source of radon in Canadian homes, it is of public interest to study the correlation between radon in soil and radon indoors in different geographic locations. Based on measurement data collected from various soil and indoor radon surveys in the past 7 years, a summary report on the correlation between soil radon potential and indoor radon potential in Canadian cities is presented here.
3. Results and discussion From 2007 to 2010, a total of 1070 sites were surveyed for soil gas radon (Chen et al., 2008b, 2009c, 2012; Ford and Chen, 2008; Goodwin et al., 2009, 2010; Ford et al., 2010, 2015; Friske et al., 2012, 2013), as shown in Fig. 1. Among the sites surveyed, 476 sites were in cities where data could be correlated with indoor
Please cite this article in press as: Chen, J., Ford, K.L., A study on the correlation between soil radon potential and average indoor radon potential in Canadian cities, Journal of Environmental Radioactivity (2016), http://dx.doi.org/10.1016/j.jenvrad.2016.01.018
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3
Fig. 1. Activities of soil gas radon survey in 2007e2010. Survey sites are marked in brown. Black dots represent locations of the cities surveyed. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
radon measurements (represented as black dots in Fig. 1; a few cities with less than 7 sites surveyed were not considered for further analysis). In order to provide semi-quantitative estimations of permeability for those sites where in-situ soil permeability measurements
were not performed due to limited time available for a field trip, estimations of the relative (low, moderate, high) permeability were made for more than 1400 probes where in-situ direct soil permeability measurements were conducted. The subjective estimation was based on the relative resistance encountered during collection
Table 1 Summary of indoor radon potentials and soil radon potentials in 14 cities. City
# Homes surveyed
% Homes > 200Bq/m3
Soil survey# sites
Average soil radon kBq/m3
Average SRP
SRP range
Calgary Edmonton Fredericton Gatineau Halifax Kingston Kitchener-Cambridge-Waterloo London al Montre Ottawa Regina Toronto Windsor Winnipeg
185 170 157 62 167 196 203 190 172 159 190 196 289 283
13% 7.6% 17% 13% 29% 17% 2.4% 2.6% 12% 10% 42% 1.5% 13% 28%
7 10 20 36 61 26 9 12 76 42 20 74 11 26
13 23 25 17 36 11 17 28 28 22 33 23 36 35
11 20 18 12 26 14 8 16 20 12 23 12 21 21
(0, (0, (0, (0, (0, (0, (0, (0, (0, (0, (0, (0, (0, (0,
35) 42) 110) 61) 223) 35) 18) 75) 64) 61) 70) 44) 79) 84)
Please cite this article in press as: Chen, J., Ford, K.L., A study on the correlation between soil radon potential and average indoor radon potential in Canadian cities, Journal of Environmental Radioactivity (2016), http://dx.doi.org/10.1016/j.jenvrad.2016.01.018
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J. Chen, K.L. Ford / Journal of Environmental Radioactivity xxx (2016) 1e5
Fig. 2. Average soil radon potential index as a function of percentage of homes above 200 Bq/m3 in 14 Canadian cities.
of soil gas sample using a 150 mL syringe, i.e. drawing soil gas to the full amount of the syringe, releasing the syringe and recording how far the syringe pulled back. If the syringe pulled back slightly (not below 100 mL), the permeability was considered high. If the syringe pulled back between 50 and 100 mL, the permeability was considered moderate. For a pullback to less than 50 mL or if it was difficult to draw the full amount of 150 mL, the permeability was considered low. From 840 subjective estimations of “high” permeability, the average permeability from the direct in-situ permeability measurements at the same probes was 7.0 1012 m2. Averages were 5.1 1013 m2 and 3.8 1014 m2 from 290 records of moderate and low permeability, respectively. These estimates were used in the SRP calculations for those sites where in-situ soil permeability measurements were not performed. The SRP for a city is the average of SRPs from all sites surveyed within the city. The first detailed soil radon survey was conducted in Ottawa (Chen et al., 2009b) where soil gas radon and permeability measurements were performed at all five probes for 32 sites across the five most populated communities in the city. Indoor radon measurements were available for 167 Ottawa homes. Among those five most populated communities in the city of Ottawa, the indoor radon potential (the percentage of homes above 200 Bq m3) varied from as low as 1% in one community to as high as 29 in another. The
overall average indoor radon potential for the city of Ottawa was 11%. Similar to wide distribution of indoor radon concentrations, the average soil gas radon concentrations varied significantly from site to site within the city, and ranged from 2.8 kBq m3 to 65 kBq m3. The overall average SRP indexes for the five communities varied from 5.4 to 16. The study demonstrated that the average SRP in a community area correlates reasonably well (R2 ¼ 0.98) with the indoor radon potential in that community. In the 2010 soil radon survey in 4 cities (Chen et al., 2012), the SRP index was determined to be 20 ± 16, 12 ± 11, 8 ± 9 and 12 ± 10 for Montreal, Gatineau, Kingston and Toronto, respectively. The average soil radon potentials and indoor radon potentials for Canadian cities are summarised in Table 1. Fig. 2 showed a positive association (R2 ¼ 0.46) between SRP and percentage of homes above 200 Bq m3 in 14 Canadian cities studied here. For 5 cities (Ottawa, Halifax, Fredericton, Winnipeg and Regina) where detailed in-situ soil permeability were measured in more than 20 sites per city, a strong correlation (R2 ¼ 0.68 for linear regression and R2 ¼ 0.81 for non-linear regression) was found (Fig. 3). If, instead of SRP, the average soil radon concentration in these 5 cities is considered, the non-linear regression with the percentage of homes above 200 Bq/m3 decreases slightly to R2 ¼ 0.78. This indicates that radon in soil is the main source of radon in homes, and that soil permeability also plays an important
Fig. 3. Average soil radon potential index as a function of percentage of homes above 200 Bq m3 in 5 cities where soil radon surveys were carried out by professional teams in more than 20 sites per city (solid line: linear regression; dashed line: non-linear regression).
Please cite this article in press as: Chen, J., Ford, K.L., A study on the correlation between soil radon potential and average indoor radon potential in Canadian cities, Journal of Environmental Radioactivity (2016), http://dx.doi.org/10.1016/j.jenvrad.2016.01.018
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role in soil radon potential and, ultimately, in indoor radon potential. 4. Conclusions Soil radon survey was conducted in 14 Canadian cities where information of indoor radon distribution was available. Among the 14 cities, indoor radon potential (percentage of homes above 200 Bq m3) varied from 1.5% to 42% while the SRP index ranged from 8 to 26. Generally speaking, indoor radon potential correlates reasonably well with soil radon potential index. For 5 cities (Ottawa, Halifax, Fredericton, Winnipeg and Regina) where detailed in-situ soil permeability was measured in more than 20 sites per city, a strong correlation was found. The results show that SRP can serve as a useful indicator for the average indoor radon potential in a geographic area. The strong positive associations between the estimated soil radon potential and the percentage of homes above the guideline of 200 Bq m3 demonstrate that SRP can be an effective predictor of areas prone to elevated indoor radon concentrations. This includes areas with little or no residential development, thereby informing future land-use planning and construction requirements. SRP is relatively easy to measure, and can therefore be used as a practical indicator to help public health officials better identify areas or regions with elevated radon potential and to prioritize efforts to manage population exposure. This being said, however, no area should be excluded from indoor radon testing, even if the estimated SRP is low, because indoor radon levels can never be adequately predicted prior to home construction. References Akerblom, G., Mellander, H., 1997. Geology of radon. In: Radon Measurements by Etched Track Detectors. World Scientific Publishing, Singapore, New Jersey, London, Hong Kong, pp. 29e49. Barnet, I., 2012. Indoor radon probability calculated from the Czech soil gas radon data in a grid net for the European geogenic radon map construction: test of feasibility. Environ. Earth Sci. 66, 1149e1153. Chen, J., Tokonami, S., Sorimachi, A., Takahashi, H., Falcomer, R., 2008a. Results of simultaneous radon and thoron tests in Ottawa. Radiat. Prot. Dosim. 130, 253e256. Chen, J., Ly, J., Bergman, L., Wierdsma, J., Klassen, R.A., 2008b. Variation of soil radon concentrations in southern Ontario. Radiat. Prot. Dosim. 131, 385e389. Chen, J., Falcomer, R., Ly, J., Wierdsma, J., Bergman, L., 2009a. Long-term monitoring of soil gas radon and permeability at two reference sites. Radiat. Prot. Dosim. 131, 503e508. Chen, J., Schroth, E., Fife, I., MacKelay, E., Tokonami, S., Sorimachi, A., 2009b. Simultaneous 222Rn and 220Rn measurements in Winnipeg, Canada. Radiat. Prot. Dosim. 134, 75e78. Chen, J., Falcomer, R., Bergman, L., Wierdsma, J., Ly, J., 2009c. Correlation of soil radon and permeability with indoor radon potential. Radiat. Prot. Dosim. 136, 56e60. Chen, J., Moir, D., Pronk, T., Goodwin, T., Janik, M., Tokonami, S., 2011. An update on thoron exposure in Canada with simultaneous 222Rn and 220Rn measurements in Fredericton and Halifax. Radiat. Prot. Dosim. 147, 541e547. Chen, J., Moir, D., MacLellan, K., Leigh, E., Nunez, D., Murphy, S., Ford, K., 2012. Soil radon measurements in the Canadian cities. Radiat. Prot. Dosim. 151, 172e174. Chen, J., Bergman, L., Falcomer, R., Whyte, J., 2015. Results of simultaneous radon and thoron measurements in 33 metropolitan areas of Canada. Radiat. Prot. Dosim. 163, 210e216. Cinelli, G., Tositti, L., Capaccioni, B., Brattich, E., Mostacci, D., 2015. Soil gas radon assessment and development of a radon risk map in Bolsena, Central Italy. Environ. Geochem Health 37, 305e319. Ford, K., Chen, J., 2008. Studies of Soil Gas Radon and Natural Radioactivity Related to the Soil Geochemical Landscapes Project. Available online at: https://gsa. confex.com/gsa/responses/2008AM/345.pdf.
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Ford, K., Chen, J., Harvey, B., 2010. Studies of soil gas radon and natural radioactivity related to Canada's national radon program. In: 10th International Workshop on the Geological Aspects of Radon Risk Mapping. September 22e25 2010, Prague. Available online at: http://www.radon.eu/workshop2010/pres/ford.pdf. Ford, K., Harvey, B., Whyte, J., Chen, J., 2015. Predicting Geographic Variations in Indoor Radon Potential across South-western Ontario Using Geoscience Data. Geological Survey of Canada Open File: 7795. 145. http://ftp2.cits.rncan.gc.ca/ pub/geott/ess_pubs/296/296420/of_7795.pdf. Friske, P.W.B., Ford, K.L., McNeil, R.J., 2012. Soil Geochemical, Mineralogical, Radon and Gamma Ray Spectrometric Data from the 2007 North American Soil Geochemical Landscapes Project in New Brunswick, Nova Scotia, and Prince Edward Island. http://dx.doi.org/10.4095/290237. Geological Survey of Canada, Open File 6433, 1 CD-ROM. Friske, P.W.B., Ford, K.L., McNeil, R.J., 2013. Soil Geochemical, Radon and Gamma Ray Spectrometric Data from the 2008 and 2009 North American Soil Geochemical Landscapes Project Field Surveys. http://dx.doi.org/10.4095/292514. Open File 7334. Geological Survey of Canada, 2009. North American Soil Geochemical Landscapes Project: Database of GRTS Sample Sites with Notes on the Sampling Design and Site Selection Procedure. Available at: http://geogratis.gc.ca/api/en/nrcanrncan/ess-sst/d44b35d3-50f6-5705-9f83-0370a4e8a820.html. Goodwin, T.A., Ford, K.L., Fiske, P.W.B., Machsaac, E.M., 2009. Radon Soil Gas in Nova Scotia. Report ME 2009-1. Nova Scotia Department of Natural Resources, pp. 25e34 [ISN:22230]. Goodwin, T.A., O'Brien, K.E., Ford, K.L., Fiske, P.W.B., 2010. Radon Soil Gas in the Halifax Regional Municipality. Report ME 2010-1. Nova Scotia Department of Natural Resources, pp. 29e34 [ISN:22563]. Health Canada, 2012. Cross-Canada Survey of Radon Concentrations in Homes. Available at: http://www.hc-sc.gc.ca/ewh-semt/alt_formats/pdf/radiation/ radon/survey-sondage-eng.pdf. Keller, G., Schneiders, H., Schuetz, M., Siehl, A., Stamm, R., 1992. Indoor radon correlated with soil and subsoil radon potential e a case study. Environ. Geol. Water Sci. 19, 113e119. Kemski, J., Klingel, R., Siehl, A., Valdivia-Manchego, M., 2006. Radon risk prediction in Germany based on gridded geological maps and soil gas measurements. In: Barnet, I., Neznal, M., Pacherova, P. (Eds.), Proceedings of the 8th International Workshop on the Geological Aspects of Radon Risk Mapping, pp. 139e156 (Prague). Kemski, J., Klingel, R., Siehl, A., Valdivia-Manchego, M., 2009. From radon hazard to risk prediction-based on geological maps, soil gas and indoor measurements in Germany. Environ. Geol. 56, 1269e1279. Koorevaar, P., Menelik, G., Dirksen, C., 1987. Elements of Soil Physics. Developments in Soil Science, Vol. 13. Elsevier. Lara, E., Rocha, Z., Santos, T.O., Rios, F.J., Oliveira, A.H., 2015. Soil features and indoor radon concentration prediction : radon in soil gas, pedology, permeability and 226Ra content. Radiat. Prot. Dosim. 167, 126e129. Minda, M., Toth, G., Horvath, I., Barnet, I., Hamori, K., Toth, E., 2009. Indoor radon mapping and its relation to geology in Hungary. Environ. Geol. 57, 601e609. Mose, D.G., Mushrush, G.W., Chrosniak, C.E., 1992. Soil radon, permeability, and indoor radon prediction. Environ. Geol. Water Sci. 19, 91e96. Neznal, M., Neznal, M., Smarda, J., 1996. Assessment of radon potential of soils e a five years experience. Environ. Int. 22, 819e828. Neznal, M., Neznal, M., Matolin, M., Barnet, I., Miksova, J., 2006. The New Method for Assessing the Radon Risk of Building Sites. Project report. State Office for Nuclear Safety, Prague. Radon VOS, 2007a. Soil Radon Monitoring System RM-2. User Manual. Prague. Available on. http://www.radon.eu/rm2.html. Radon VOS, 2007b. Equipment for In Situ Permeability Measurements. RADON-JOK Manual. Prague. Available on. http://www.radon-vos.cz/?lang¼en&lmenu¼en_ measuring&page¼en_measuring_jok. Reimer, G.M., Szarzi, S.L., 2005. Indoor radon risk potential of Hawaii. J. Radioanal. Nucl. Chem. 264, 365e369. Sundal, A.V., Henriksen, H., Lauritzen, S.E., Soldal, O., Strand, T., Valen, V., 2004. Geological and geochemical factors affecting radon concentrations in dwellings located on permeable glacial sedimentsda case study from Kinsarvik, Norway. Environ. Geol. 45, 843e858. United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), 2006. Effects of ionizing radiation. In: Sources-to-effects Assessment for Radon in Homes and Workplaces, I. ISBN 978-92-1-142263-4. United Nations, New York. Vaupotic, J., Andjelov, M., Kobal, I., 2002. Relationship between radon concentrations in indoor air and in soil gas. Environ. Geol. 42, 583e587. World Health Organization (WHO), 2009. WHO Handbook on Indoor Radon. ISBN 978-92-4-154767-3. Available at: http://whqlibdoc.who.int/publications/2009/ 9789241547673_eng.pdf.
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Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
A study on the correlation between soil radon potential and average indoor radon potential in Canadian cities Jing Chen a, *, Ken L. Ford b a b
Radiation Protection Bureau, Health Canada, 775 Brookfield Road, Ottawa, K1A 1C1, Canada Geological Survey of Canada, Natural Resources Canada, 601 Booth Street, Ottawa, K1A 0E8, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 October 2015 Received in revised form 23 December 2015 Accepted 27 January 2016 Available online xxx
Exposure to indoor radon is identified as the main source of natural radiation exposure to the population. Since radon in homes originates mainly from soil gas radon, it is of public interest to study the correlation between radon in soil and radon indoors in different geographic locations. From 2007 to 2010, a total of 1070 sites were surveyed for soil gas radon and soil permeability. Among the sites surveyed, 430 sites were in 14 cities where indoor radon information is available from residential radon and thoron surveys conducted in recent years. It is observed that indoor radon potential (percentage of homes above 200 Bq m3; range from 1.5% to 42%) correlates reasonably well with soil radon potential (SRP: an index proportional to soil gas radon concentration and soil permeability; average SRP ranged from 8 to 26). In five cities where in-situ soil permeability was measured at more than 20 sites, a strong correlation (R2 ¼ 0.68 for linear regression and R2 ¼ 0.81 for non-linear regression) was observed between indoor radon potential and soil radon potential. This summary report shows that soil gas radon measurement is a practical and useful predictor of indoor radon potential in a geographic area, and may be useful for making decisions around prioritizing activities to manage population exposure and future land-use planning. Crown Copyright © 2016 Published by Elsevier Ltd. All rights reserved.
Keywords: Radon-222 Indoor radon Soil radon
1. Introduction Radon (222Rn) is a naturally occurring radioactive gas generated by the decay of uranium bearing minerals in rocks and soils. A certain fraction of the radon escapes from the ground into the air. In the open air, radon is diluted to low concentrations and is not considered a health concern. However, radon that enters an enclosed space, such as a family house or a school building, can sometimes accumulate to concentrations above the Canadian Action Level (200 Bq m3). Soil gas radon contributing to the indoor environment has been identified as the main source of natural radiation exposure to the population (UNSCEAR, 2006). Radon has been identified as the second leading cause of lung cancer after tobacco smoking (WHO, 2009). Soil gas radon measurement is a useful tool for the assessment of environmental radon potential and for the prediction of potential indoor radon concentrations in a geographic area, as demonstrated by many studies in various geographic locations (Keller et al., 1992;
* Corresponding author. E-mail address: [email protected] (J. Chen).
Mose et al., 1992; Neznal et al., 1996; Akerblom and Mellander, 1997; Vaupotic et al., 2002; Sundal et al., 2004; Reimer and Szarzi, 2005; Neznal et al., 2006; Kemski et al., 2006, 2009; Chen et al., 2009a; Minda et al., 2009; Barnet, 2012; Cinelli et al., 2015; Lara et al., 2015). Various studies have shown that in addition to radon levels in the soil, the soil permeability is another important factor in determining the radon potential of a site or an area, because higher permeability enables the increased migration of soil gas radon from soil into houses. For example, in a German case study in 1988e1989, Keller et al. found that high radon levels in the soil cause high indoor concentrations, if there are easy pathways into houses, and positive radon anomalies in the soil gas coincide with the locations of houses showing the highest concentrations (Keller et al., 1992). In northern Virginia and southern Maryland, Mose et al. attempted to predict which geographic areas should be associated with a high percentage of homes with unusually high indoor radon levels based on estimates of soil radon and soil permeability for geological units (Mose et al., 1992). They concluded that predictions of indoor radon based on estimates of home site soil radon and soil permeability are very useful. Similar conclusions were obtained in a recent study in Brazil (Lara et al.,
http://dx.doi.org/10.1016/j.jenvrad.2016.01.018 0265-931X/Crown Copyright © 2016 Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Chen, J., Ford, K.L., A study on the correlation between soil radon potential and average indoor radon potential in Canadian cities, Journal of Environmental Radioactivity (2016), http://dx.doi.org/10.1016/j.jenvrad.2016.01.018
2
J. Chen, K.L. Ford / Journal of Environmental Radioactivity xxx (2016) 1e5
2. Methods
areas. The soil survey sites were areas of about 10 10 m2 in lowtraffic areas of community parks and away from roads. For each site, generally two permeability and five soil gas radon measurements were performed with four probes at each corner and one probe in the centre of the 10 10 m2 survey area. The soil radon potential (SRP) index is defined as, SRP ¼ (C C0) / (log(P) þ log(P0)) where C is the radon concentration of the soil gas in kBq m3, and P is the soil permeability in m2. C0 and P0 are set to be 1 kBq m3 and 1$1010 m2, respectively (Neznal et al., 1996, 2006). The SRP is proportional to the radon concentration in soil. It depends also on soil gas permeability; the more permeable the soil, the higher is the SRP. Due to the extremely wide range of soil gas permeability over several orders of magnitude, the SRP is adjusted with the logarithm of soil permeability.
2.1. Soil radon and permeability measurements
2.2. Indoor radon measurements
Soil gas radon concentration was determined by measuring the radioactivity of soil gas samples taken from a depth of 80 cm below the ground surface (Radon VOS, 2007a). Because the boulder-rich nature of glacial tills made it difficult to reach the target depth at some sites, the sample depth was occasionally reduced to 60 cm. Samples of soil gas were collected using a 150 ml syringe. The soil gas samples were then introduced into ionisation chambers for measurement. Background control measurements of the ionization chambers were performed before sampling soil radon. An ionisation chamber was only used when its background reading was below 0.7 kBq m3, as instructed by the manufacturer (Radon VOS, 2007a). Soil radon concentrations measured below 1 kBq m3 were excluded since any potential leakage during the soil gas sampling could result in a lower radon concentration. Therefore, the lowest measured value of soil gas radon was excluded in the calculation of the average radon concentration for a survey site. The permeability of the soil is influenced by natural soil moisture, density, effective porosity, as well as other parameters. Although absolute soil moisture measurements were not conducted in the surveys, the influence of soil moisture was indirectly measured with the permeability. In situ permeability measurements were performed with RADON-JOK, an instrument manufactured in the Czech Republic (Radon VOS, 2007b), at a depth of 60e80 cm below the ground surface. The measurement is based on air withdrawal by means of negative pressure. The soil gas is pumped under constant pressure through a probe (the same probe as used for soil radon collecting) with a constant surface, an active area created in the head of the probe at 80-cm below the ground. The soil gas permeability was calculated based on Darcy's equation (Koorevaar et al., 1987). For sites having very low permeability, the in situ soil gas permeability measurement could potentially take hours to complete. In these cases, a limit value of 2$1014 m2 was assumed. Long-term monitoring of soil radon variations was conducted at two reference sites in Ottawa (Chen et al., 2009a), from thawing of the ground in the late spring to 1 day before the first snowfall in early winter. Results showed that during the normal field survey period from June to September in Canada, a single field survey with multiple measurements of soil gas radon concentrations at a depth of 60e80 cm could characterise the soil gas radon level of a site within a deviation of ±30%. All soil radon surveys summarised here were conducted from June to September. All surveys followed well established protocol of the National Soil Radon Project, a Canadian add-on project to the North American Soil Geochemical Landscapes Project (GSC, 2009). Surveys were conducted on a dry day with a clear sky at least the previous evening. Within a city, soil gas radon and soil permeability measurements were conducted in community parks within residential
In order to assess thoron (220Rn) contribution to indoor radon and thoron exposure combined, simultaneous radon and thoron measurements were first conducted in Ottawa, Winnipeg, Halifax and Fredericton (Chen et al., 2008a, 2009b, 2011). In 2014, a nationwide survey of residential radon and thoron concentrations was completed in 33 cities (accounting for approximately 70% of the Canadian population), with a sample size of roughly 4000 homes (Chen et al., 2015). In order for the result to be indicative of the average annual radon and thoron exposure, all tests were conducted for a period of at least three months in the typical heating season that runs from October to April. A passive integrated radonethoron discriminative detector developed at the National Institute of Radiological Sciences in Japan (commercially known as RADUET) was used in these surveys. These results serve as the main data source of indoor radon concentrations in this summary report. To gain a better understanding of radon concentrations in homes across Canada, a national residential radon survey was launched in April 2007. The survey used alpha track detectors, and conducted long-term (3-months or longer) radon measurements in roughly 14,000 homes in 121 health regions (administrative areas defined by the provincial ministries of health) across Canada (Health Canada, 2012). Results of this survey were added to the results from the above-mentioned city-based radon/thoron surveys when health region boundaries approximately matched the boundaries of cities considered here. All surveys confirmed that radon was present in all homes in varying concentrations. The indoor radon concentrations varied widely in a geographic area. Even though it is hard to predict indoor radon concentration for any given house in a community, the average characteristics of indoor radon distribution should be representative for a geographic area. All surveys confirmed that indoor radon concentrations in a community followed log-normal distribution. A log-normal distribution is a two-parameter distribution characterised with the geometric mean (GM) and geometric standard deviation (GSD). Indoor radon potential of a geographic area or a city should be a quantity representing the characteristics of the two-parameter distribution of indoor radon concentrations in that area. Therefore, the percentage of homes above the Canadian Action Level of 200 Bq m3 was chosen as the indicator of indoor radon potential for an area.
2015) when using soil features to predict indoor radon concentration. Barnet (2012) compared two different methods (real and calculated) for evaluating the relationship of soil gas radon and probability of indoor radon exceeding 200 Bq m3 based on data sets from the Czech Republic, the results showed minor differences. Since radon in soil is believed to be the main source of radon in Canadian homes, it is of public interest to study the correlation between radon in soil and radon indoors in different geographic locations. Based on measurement data collected from various soil and indoor radon surveys in the past 7 years, a summary report on the correlation between soil radon potential and indoor radon potential in Canadian cities is presented here.
3. Results and discussion From 2007 to 2010, a total of 1070 sites were surveyed for soil gas radon (Chen et al., 2008b, 2009c, 2012; Ford and Chen, 2008; Goodwin et al., 2009, 2010; Ford et al., 2010, 2015; Friske et al., 2012, 2013), as shown in Fig. 1. Among the sites surveyed, 476 sites were in cities where data could be correlated with indoor
Please cite this article in press as: Chen, J., Ford, K.L., A study on the correlation between soil radon potential and average indoor radon potential in Canadian cities, Journal of Environmental Radioactivity (2016), http://dx.doi.org/10.1016/j.jenvrad.2016.01.018
J. Chen, K.L. Ford / Journal of Environmental Radioactivity xxx (2016) 1e5
3
Fig. 1. Activities of soil gas radon survey in 2007e2010. Survey sites are marked in brown. Black dots represent locations of the cities surveyed. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
radon measurements (represented as black dots in Fig. 1; a few cities with less than 7 sites surveyed were not considered for further analysis). In order to provide semi-quantitative estimations of permeability for those sites where in-situ soil permeability measurements
were not performed due to limited time available for a field trip, estimations of the relative (low, moderate, high) permeability were made for more than 1400 probes where in-situ direct soil permeability measurements were conducted. The subjective estimation was based on the relative resistance encountered during collection
Table 1 Summary of indoor radon potentials and soil radon potentials in 14 cities. City
# Homes surveyed
% Homes > 200Bq/m3
Soil survey# sites
Average soil radon kBq/m3
Average SRP
SRP range
Calgary Edmonton Fredericton Gatineau Halifax Kingston Kitchener-Cambridge-Waterloo London al Montre Ottawa Regina Toronto Windsor Winnipeg
185 170 157 62 167 196 203 190 172 159 190 196 289 283
13% 7.6% 17% 13% 29% 17% 2.4% 2.6% 12% 10% 42% 1.5% 13% 28%
7 10 20 36 61 26 9 12 76 42 20 74 11 26
13 23 25 17 36 11 17 28 28 22 33 23 36 35
11 20 18 12 26 14 8 16 20 12 23 12 21 21
(0, (0, (0, (0, (0, (0, (0, (0, (0, (0, (0, (0, (0, (0,
35) 42) 110) 61) 223) 35) 18) 75) 64) 61) 70) 44) 79) 84)
Please cite this article in press as: Chen, J., Ford, K.L., A study on the correlation between soil radon potential and average indoor radon potential in Canadian cities, Journal of Environmental Radioactivity (2016), http://dx.doi.org/10.1016/j.jenvrad.2016.01.018
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J. Chen, K.L. Ford / Journal of Environmental Radioactivity xxx (2016) 1e5
Fig. 2. Average soil radon potential index as a function of percentage of homes above 200 Bq/m3 in 14 Canadian cities.
of soil gas sample using a 150 mL syringe, i.e. drawing soil gas to the full amount of the syringe, releasing the syringe and recording how far the syringe pulled back. If the syringe pulled back slightly (not below 100 mL), the permeability was considered high. If the syringe pulled back between 50 and 100 mL, the permeability was considered moderate. For a pullback to less than 50 mL or if it was difficult to draw the full amount of 150 mL, the permeability was considered low. From 840 subjective estimations of “high” permeability, the average permeability from the direct in-situ permeability measurements at the same probes was 7.0 1012 m2. Averages were 5.1 1013 m2 and 3.8 1014 m2 from 290 records of moderate and low permeability, respectively. These estimates were used in the SRP calculations for those sites where in-situ soil permeability measurements were not performed. The SRP for a city is the average of SRPs from all sites surveyed within the city. The first detailed soil radon survey was conducted in Ottawa (Chen et al., 2009b) where soil gas radon and permeability measurements were performed at all five probes for 32 sites across the five most populated communities in the city. Indoor radon measurements were available for 167 Ottawa homes. Among those five most populated communities in the city of Ottawa, the indoor radon potential (the percentage of homes above 200 Bq m3) varied from as low as 1% in one community to as high as 29 in another. The
overall average indoor radon potential for the city of Ottawa was 11%. Similar to wide distribution of indoor radon concentrations, the average soil gas radon concentrations varied significantly from site to site within the city, and ranged from 2.8 kBq m3 to 65 kBq m3. The overall average SRP indexes for the five communities varied from 5.4 to 16. The study demonstrated that the average SRP in a community area correlates reasonably well (R2 ¼ 0.98) with the indoor radon potential in that community. In the 2010 soil radon survey in 4 cities (Chen et al., 2012), the SRP index was determined to be 20 ± 16, 12 ± 11, 8 ± 9 and 12 ± 10 for Montreal, Gatineau, Kingston and Toronto, respectively. The average soil radon potentials and indoor radon potentials for Canadian cities are summarised in Table 1. Fig. 2 showed a positive association (R2 ¼ 0.46) between SRP and percentage of homes above 200 Bq m3 in 14 Canadian cities studied here. For 5 cities (Ottawa, Halifax, Fredericton, Winnipeg and Regina) where detailed in-situ soil permeability were measured in more than 20 sites per city, a strong correlation (R2 ¼ 0.68 for linear regression and R2 ¼ 0.81 for non-linear regression) was found (Fig. 3). If, instead of SRP, the average soil radon concentration in these 5 cities is considered, the non-linear regression with the percentage of homes above 200 Bq/m3 decreases slightly to R2 ¼ 0.78. This indicates that radon in soil is the main source of radon in homes, and that soil permeability also plays an important
Fig. 3. Average soil radon potential index as a function of percentage of homes above 200 Bq m3 in 5 cities where soil radon surveys were carried out by professional teams in more than 20 sites per city (solid line: linear regression; dashed line: non-linear regression).
Please cite this article in press as: Chen, J., Ford, K.L., A study on the correlation between soil radon potential and average indoor radon potential in Canadian cities, Journal of Environmental Radioactivity (2016), http://dx.doi.org/10.1016/j.jenvrad.2016.01.018
J. Chen, K.L. Ford / Journal of Environmental Radioactivity xxx (2016) 1e5
role in soil radon potential and, ultimately, in indoor radon potential. 4. Conclusions Soil radon survey was conducted in 14 Canadian cities where information of indoor radon distribution was available. Among the 14 cities, indoor radon potential (percentage of homes above 200 Bq m3) varied from 1.5% to 42% while the SRP index ranged from 8 to 26. Generally speaking, indoor radon potential correlates reasonably well with soil radon potential index. For 5 cities (Ottawa, Halifax, Fredericton, Winnipeg and Regina) where detailed in-situ soil permeability was measured in more than 20 sites per city, a strong correlation was found. The results show that SRP can serve as a useful indicator for the average indoor radon potential in a geographic area. The strong positive associations between the estimated soil radon potential and the percentage of homes above the guideline of 200 Bq m3 demonstrate that SRP can be an effective predictor of areas prone to elevated indoor radon concentrations. This includes areas with little or no residential development, thereby informing future land-use planning and construction requirements. SRP is relatively easy to measure, and can therefore be used as a practical indicator to help public health officials better identify areas or regions with elevated radon potential and to prioritize efforts to manage population exposure. This being said, however, no area should be excluded from indoor radon testing, even if the estimated SRP is low, because indoor radon levels can never be adequately predicted prior to home construction. References Akerblom, G., Mellander, H., 1997. Geology of radon. In: Radon Measurements by Etched Track Detectors. World Scientific Publishing, Singapore, New Jersey, London, Hong Kong, pp. 29e49. Barnet, I., 2012. Indoor radon probability calculated from the Czech soil gas radon data in a grid net for the European geogenic radon map construction: test of feasibility. Environ. Earth Sci. 66, 1149e1153. Chen, J., Tokonami, S., Sorimachi, A., Takahashi, H., Falcomer, R., 2008a. Results of simultaneous radon and thoron tests in Ottawa. Radiat. Prot. Dosim. 130, 253e256. Chen, J., Ly, J., Bergman, L., Wierdsma, J., Klassen, R.A., 2008b. Variation of soil radon concentrations in southern Ontario. Radiat. Prot. Dosim. 131, 385e389. Chen, J., Falcomer, R., Ly, J., Wierdsma, J., Bergman, L., 2009a. Long-term monitoring of soil gas radon and permeability at two reference sites. Radiat. Prot. Dosim. 131, 503e508. Chen, J., Schroth, E., Fife, I., MacKelay, E., Tokonami, S., Sorimachi, A., 2009b. Simultaneous 222Rn and 220Rn measurements in Winnipeg, Canada. Radiat. Prot. Dosim. 134, 75e78. Chen, J., Falcomer, R., Bergman, L., Wierdsma, J., Ly, J., 2009c. Correlation of soil radon and permeability with indoor radon potential. Radiat. Prot. Dosim. 136, 56e60. Chen, J., Moir, D., Pronk, T., Goodwin, T., Janik, M., Tokonami, S., 2011. An update on thoron exposure in Canada with simultaneous 222Rn and 220Rn measurements in Fredericton and Halifax. Radiat. Prot. Dosim. 147, 541e547. Chen, J., Moir, D., MacLellan, K., Leigh, E., Nunez, D., Murphy, S., Ford, K., 2012. Soil radon measurements in the Canadian cities. Radiat. Prot. Dosim. 151, 172e174. Chen, J., Bergman, L., Falcomer, R., Whyte, J., 2015. Results of simultaneous radon and thoron measurements in 33 metropolitan areas of Canada. Radiat. Prot. Dosim. 163, 210e216. Cinelli, G., Tositti, L., Capaccioni, B., Brattich, E., Mostacci, D., 2015. Soil gas radon assessment and development of a radon risk map in Bolsena, Central Italy. Environ. Geochem Health 37, 305e319. Ford, K., Chen, J., 2008. Studies of Soil Gas Radon and Natural Radioactivity Related to the Soil Geochemical Landscapes Project. Available online at: https://gsa. confex.com/gsa/responses/2008AM/345.pdf.
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