Preliminary Findings of Radon Potential Indexes in Five Canadian Cities

Radon has been identified as the second leading cause of lung cancer after tobacco smoking. Since radon in soil is believed to be the main source of radon in Canadian homes, a radon potential index determined from soil radon concentration and soil permeability can be used to describe the indoor radon potential resulting from radon in soil gas. The index increases with increasing radon concentration in soil gas and soil permeability. This study reports detailed measurements of soil gas radon concentrations and soil permeability in a total of 254 sites in five cities, Montreal, Gatineau, Ottawa, Kingston and Toronto. Average radon potential indexes were determined for each individual site of five measurement locations. The results provided additional data for the mapping of radon potentials in Canada.


Introduction
Radon 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 a health concern.In indoor environments, radon originating as soil gas is the main source of natural radiation exposure to the population.Radon has been identified as the second leading cause of lung cancer after tobacco smoking (WHO, 2005).
Based on new scientific information and a broad public consultation, the Canadian radon guideline was lowered from 800 to 200 Bqm -3 in June 2007 (Health Canada, 2007).To support the implementation of the revised guideline, a National Radon Program (NRP) was developed.A significant component of the NRP is the development of a radon database that will also include a number of targeted mapping activities.In Canada, radon maps are intended to be used mainly by governments to prioritize radon outreach and education efforts, to encourage testing and remediation where necessary, and to assist in community planning and future development.
Canadian radon maps will be based on a variety of data, such as ground uranium concentration from airborne gamma ray spectrometric surveys, radon in soil gas, soil permeability, soil geochemistry, surficial geology, bedrock geology, and indoor radon data (Chen, 2009).To assist radon mapping, measurements of soil gas radon concentration and soil permeability have been conducted in cities since 2007.Determination of soil radon potential indexes in five cities (Montreal, Gatineau, Ottawa, Kingston & Toronto) are reported here.

Methods
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, 2008).Surveys were conducted on a dry day with a clear sky at least the previous evening.Soil gas radon and soil permeability measurements were conducted in community parks within residential areas.The soil survey sites were areas of about 10x10 m 2 in low-traffic areas of community parks and away from roads.For each site, five soil gas radon measurements and at least two in-situ soil permeability measurements were performed with four probes at each corner and one in the centre of the 10x10m 2 survey area.
Soil gas radon was determined by measuring the concentration of radon in soil gas samples extracted from 80 cm depth below ground surface using the RM-2 system manufactured by Radon v.o.s. in Czech Republic (http://www.radon.eu/rm2.html).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 radon concentrations.An ionization chamber was only used when its background reading was below 0.7 kBq m -3 , as instructed by the manufacture.Soil radon concentrations measured below 1 kBq m -3 were excluded.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 also excluded in the calculation of the average radon concentration for a site.
Soil gas permeability was measured 80 cm below ground surface with the use of RADON-JOK also manufactured by Radon v.o.s. in Czech Republic (http://www.radon-vos.cz/?lang=en&lmenu=en_measuring&page=en_measuring_jok).RADON-JOK 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., 1983).For sites having very low permeability, the in-situ soil gas permeability measurement could potentially take hours to complete.For logistical reasons, in those cases, a default value of 2 •10 -14 m 2 is assumed.In the city of Ottawa, in-situ permeability measurements were conducted in all five probes per site while permeability measurements were performed at two out of five probes at each site in other cities.For those sites where only two in-situ permeability measurements were performed, the semi-quantitative estimation of permeability was applied with consideration of how hard to collect soil gas with a syringe at other three probes.For each probe where a soil gas sample was collected with a syringe, a subjective description of how hard to collect soil gas with a syringe was recorded as easy, medium or hard.This provides a semi-quantitative estimation of permeability, especially for those values between in-situ measured permeability and the default value of 2•10 -14 m 2 .Soil gas radon concentration and soil permeability are the two most important factors that affect radon flux from soil to air (Neznal et al., 2006).Since radon in soil is believed to be the main source of radon in Canadian homes, a radon potential index determined from soil radon concentration and soil permeability can be used to describe the indoor radon potential resulting from radon in soil gas.The index was also called soil radon potential (SRP) index in previous publications (Chen et al., 2008a(Chen et al., , 2008b)), that is defined as: 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 1•10 -10 m 2 , respectively.In this study, SRP indexes were calculated for each site surveyed.

Results and Discussion
For the sites where in-situ permeability measurements were performed in all five probes, an average permeability of the five measurements was assigned to that site.For the sites where only two permeability measurements were performed at each site of five probes for soil radon measurements, an average permeability of the two measurements was assigned to that site where at least one in-situ permeability measurements were successfully performed, i.e. above the default value for very low permeability (2 •10 -14 m 2 ).Some sites surveyed had very low permeability at both probes for in-situ measurement.For those sites, the semi-quantitative estimation of permeability is applied with consideration of how hard to collect soil gas with a syringe at other three probes, as given in a previous publication (Chen et al., 2011).
A total of 254 sites were surveyed with 76, 36, 42, 26 and 74 sites in Montreal, Gatineau, Ottawa, Kingston and Toronto, respectively.Soil radon concentrations varied significantly from site to site and also from probe to probe for most sites surveyed.Therefore, an average soil radon concentration was calculated for each site.With the average soil radon concentration and measured and/or assigned permeability, a soil radon potential (SRP) index was calculated to characterize each site.Summary results are given in Table 1.Since radon in soil is believed to be the main source of radon in Canadian homes, the indoor radon potential could have a strong association with the soil radon potential as given in Table 1.
Indoor radon potential for a community could be represented by the percentage of homes above the Canadian guideline value of 200 Bqm -3 in that community.It characterises the average radon level in a community, and can not be used to predict radon concentration in any individual homes.From Table 1, one can see that, within certain variations, percentages of homes above 200 Bqm -3 could be comparable in the five cities surveyed.This prediction needs to be verified by direct indoor radon measurements when detailed results of the cross Canada radon survey (Health Canada, 2011) become available.

Table 2 .
Soil radon survey results (average soil radon concentration in kBq/m 3 , average permeability in m 2 , and SRP) in 76 community parks in Montreal

Table 3 .
Soil radon survey results (average soil radon concentration in kBq/m 3 , average permeability in m 2 , and SRP) in 36 community parks in Gatineau

Table 4 .
Soil radon survey results (average soil radon concentration in kBq/m 3 , average permeability in m 2 , and SRP) in 42 community parks in Ottawa

Table 5 .
Soil radon survey results (average soil radon concentration in kBq/m 3 , average permeability in m 2 , and SRP) in 26 community parks in Kingston