Geographic Analysis of Indoor Radon and Soil Gas Radon Distributions in Ottawa

Transcription

1 Geographic Analysis of Indoor Radon and Soil Gas Radon Distributions in Ottawa Jing Chen *, Renato Falcomer, Lauren Bergman, Jessica Wierdsma, Jim Ly Radiation Protection Bureau, Health Canada, 775 Brookfield Road, Ottawa, Ontario K1A 1C1, Canada. Abstract. Indoor radon measurements were performed in 167 private homes across the five most populated communities in the city of Ottawa. Soil gas radon and soil permeability measurements were conducted in community parks within residential areas. A total of 32 sites were surveyed in the five most populated districts. All measurement points were geo-coded. Geographic analysis was performed for both indoor radon and soil gas radon concentrations. The geographic analysis helped visualize the correlations between soil gas radon concentration and indoor radon hazard. The results demonstrated that soil gas radon measurement is a useful tool for identifying radon rich areas, especially for identifying indoor radon potential in future construction areas before dwellings are built. KEYWORDS: Radon, soil gas, GIS analysis. 1. Introduction Radon is a naturally occurring radioactive gas generated by the decay of uranium-bearing minerals in rocks and soils. With a half-life less than 4 days, radon is normally dissipated harmlessly in the outdoor air. However, in some confined spaces, such as homes, it can accumulate to harmful levels [1]. Radon is identified as the second leading cause of lung cancer after tobacco smoking. Pooled analyses of radon studies in Europe, North America and China have confirmed that radon in homes contributes substantially to the occurrence of lung cancers world-wide [2]. In Canada, concern about indoor radon levels began in the mid-1970 s. Health Canada conducted a cross-canada radon survey in 14,000 homes in 18 cities [3]. However, Ottawa was not included in the past cross-canada radon survey. It is of public interests to measure radon concentrations in Ottawa homes. Soil gas radon is considered to be the main source of radon in the indoor environment. Soil gas radon measurement has been identified as an additional tool for the assessment of radon risk potential in different areas [4-11]. In addition to radon levels in the soil, the permeability of soils is another important factor in determining the radon risk potential of a site. Higher permeability enables the increased migration of soil gas and radon from the soil into the building. In the development of a radon risk map for Canada, a national soil radon survey was initiated in 2007 as an add-on Canadian component to the North American Geochemical Landscapes Project. In this study, indoor radon measurements were performed in private homes across the five most populated communities in the city of Ottawa. Soil gas radon and soil gas permeability measurements were conducted in community parks within those residential areas. Geographic data analysis was then carried out to demonstrate that soil gas radon measurement adjusted with soil gas permeability is a useful tool for identifying radon rich areas. 2. Radon Measurements The city of Ottawa is the fourth largest city in Canada with an area of 2,760 square kilometres. It consists of 9 districts as shown in Fig 1. The most populated five districts from the east to the west are: Cumberland, Gloucester, Ottawa, Nepean, and Kanata. * Presenting author, jing_chen@hc-sc.gc.ca 1

2 Figure 1: Map of the city of Ottawa with its districts. 2.1 Indoor Radon Measurements Indoor radon measurements were performed in 167 private homes across the five most populated areas. The types of houses that were tested for radon were single detached houses, town homes, and row units. Electret ion chambers (E-PERMs produced by Rad Elec Inc.) were used in this informal survey. Shortterm electrets with standard chambers were deployed in the lowest occupied areas of homes for 2 to 7 days. The seasonal correction was performed based on the correction factors given in a previous publication [12]. After correction for seasonal changes, the estimated annual average radon concentrations were obtained. They range from 19 Bq/m 3 to 1460 Bq/m 3. Figure 2: Results of indoor radon measurements in 5 districts of Ottawa: percentages of homes above Canadian radon guideline 200 Bq/m 3. 2

3 Geo-coding was performed for all 167 addresses. Results were grouped at district level based on geographic locations. For each district, distribution characteristics of annual radon concentrations were estimated. Based on the geometric means and geometric standard deviations determined for each district, percentages of homes above Canadian radon guideline 200 Bq/m 3 were calculated. Results of the percentages of homes above 200 Bq/m 3 in the five most populated districts are summarized in Fig Soil Radon Measurements Soil gas radon and soil permeability measurements were conducted in community parks within residential areas. A total of 32 sites were surveyed in the five most populated districts. The soil survey sites were areas of about 10 x 10 m 2 in the centre of community parks and away from roads. A minimum of one permeability and three soil gas radon concentrations was taken. In most sites, permeability and soil gas radon were measured at five different locations, with four probes at each corner and one in the centre of the 10 x 10 m 2 survey area. Soil gas radon was determined by measuring the radioactivity of soil gas samples extracted from 80 cm depth below ground surface. Soil gas samples were collected using small-diameter hollow steel probes with a free, sharpened lower end (a lost tip) combined with a syringe. Soil gas samples of 150 ml in a syringe were introduced into ionizing chambers for measurement of radioactivity. Soil gas permeability was measured 80 cm below ground surface with the use of a RADON-JOK manufactured by Radon v.o.s. in Czech Republic [13]. Results of soil gas radon measurements are summarized in Fig. 3. Figure 3: Results of soil gas radon measurements in 5 districts of Ottawa. Soil gas radon concentration and soil permeability are the two important factors in the determination of radon availability from soil to air. To characterize radon availability from soil to air, soil radon potential (SRP) index is defined: C C0 SRP = (1) log( P) + log( P0 ) 3

4 where C is the radon concentration in soil gas in units of kbq/m 3, and P is the soil permeability in units of m 2. C 0 and P 0 are set to 1 kbq/m 3 and 1x10-10 m 2, respectively. The SRP index was successfully implemented in the Czech Republic for building site radon risk assessment [14]. SRP indexes were calculated based on Eq (1) for each measurement point on each site. Among the 32 sites, the SRP indexes vary from 0.5 to 32. GPS locations were recorded for all 32 sites. The sites were then grouped at district level. For each district, average SRP indexes were calculated. Results of average SRP indexes for the 5 districts in Ottawa are presented in Fig 4. Figure 4: Results of Soil Radon Potential (SRP) index in 5 districts of Ottawa. 3. Discussion and Conclusion The soil radon potential, an index representing the combined effect of soil radon concentration and soil permeability, correlates better with the indoor radon potential characterized by the percentage of homes above 200 Bq/m 3 than the soil gas radon concentration alone. One can see from Figs 2 and 4 that the radon potential is lower in the east community, medium in the centre and south, and higher in the west part of Ottawa. Radon potentials differ from the east to the west region of Ottawa depending mainly on the composition of the local bedrock or soil, the soil moisture and permeability. The geographic analysis helped visualize the correlations between soil gas radon concentration adjusted with soil gas permeability and indoor radon hazard. The results demonstrated that soil gas radon measurement is a useful tool for identifying radon rich areas, especially for identifying radon rich areas in future construction areas before dwellings are built. However, it should be emphasized that the SRP index determined from soil radon and permeability measurements can only serve as a tool for the prediction of average potential indoor radon levels in a geographic area. It is important to be aware that the only way to determine the actual concentration of radon in a home is by a direct indoor radon measurement. 4

5 REFERENCES [1] CANADA MORTGAGE AND HOUSING CORPORATION, HEALTH CANADA, Radon A guide for Canadian homeowners. ISBN (2007), [2] THE WORLD HEALTH POGANIZATION, Fact Sheet No.291: Radon and cancer (2005), [3] LETOURNEAU, E.G., et al., Design and interpretation of large surveys for indoor exposure to radon daughters. Radiat. Prot. Dosim. 7, 303 (1984). [4] AKERBLOM, G., MELLANDER, H., Geology of Radon. In: Radon Measurements by Etched Track Detectors World Scientific Publishing, Singapore New Jersey London Hong Kong. (1997). [5] DEHANDSCHUTTER, B., Detailed-scale radon mapping in radon-prone municipalities using combined indoor and soil gas measurements (Proc. 8 th International Workshop on the Geological Aspects of Radon Risk Mapping, Prague, 2006), 79 (2006). [6] GARAVAGLIA, M., et al., Radon measurements in soil of Friuli Venezia Giulia (North-East Italy), (Proc. 8 th International Workshop on the Geological Aspects of Radon Risk Mapping, Prague, 2006), 107 (2006). [7] KEMSKI, J., et al., Valdivia-Manchego M. Radon risk prediction in Germany based on gridded geological maps and soil gas measurements (Proc. 8 th International Workshop on the Geological Aspects of Radon Risk Mapping, Prague, 2006), 139 (2006). [8] MOSE, D.G., et al., Soil radon, permeability, and indoor radon prediction. Environ. Geol. Water Sci. 19, 91 (1992). [9] NEZNAL, M., et al., Assessment of radon potential of soils a five years experience. Environ. Int. 22, 819 (1996). [10] PETERSELL, V., et al., Radon risk map of Estonia. Explanatory text to the radon risk map set of Estonia at scale of 1: SSI Report 2005:16, SGU Dnr /2002. Tallinn Stockholm (2005). [11] SEGOVIA, N., et al., Environmental radon studies in Mexico. Environ. Geochem. Health 29, 143 (2007). [12] CHEN, J., Estimate of annual average radon concentration in the normal living area from shortterm tests. Health Physics 85, 740 (2003). [13] RADON V.O.S., Equipment for in situ permeability measurements, RADON-JOK manual. Prague (2007). [14] NEZNAL, M., et al., The new method for assessing the radon risk of building sites. Project report. State Office for Nuclear Safety, Prague (2006). 5