THESIS PROPOSAL

DEGREE PROGRAMME: M.Sc.

FIELD OF SPECIALIZATIOB: Environmental Geology

SUPERVISOR and COMMITTEE: Supervisors - Daniel Rainham - Dalhousie University / David Risk - Saint Francis Xavier

University

Committee Member - Anne-Marie Ryan - Dalhousie University

TITLE OF PROPOSAL: Radon Soil Gas within Halifax Regional Municipality, Nova Scotia

KEY WORDS radon soil gas, soil permeability, surficial geology, gas transport, soil columns, HRM, indoor

radon, GIS, till, environmental health

LIST INNOVATIONS or EXPECTED SIGNIFICANT OUTCOMES:

Create an indoor 222

Rn health risk potential map of Halifax.

Develop new techniques to clearly define the spatial risk distribution of 222

Rn.

Quantify 222

Rn production in the tills, rather than the bedrock as the primary source of

radon gas at the ground surface.

Develop a scientific methodology for 222

Rn soil gas production/transport characterization

that can be applied globally, with implications for human health.

SUMMARY OF PROPOSED RESEARCH:

Radon gas is a human health risk, as long-term exposure to high radon concentrations through

inhalation is the second leading cause of lung cancer after smoking (WHO 2005). High (> 200

Bq m3) radon soil gas levels are typically associated with granites and slates (Je 1998); Goodwin

et al. (2008b) observed measurable quantities of radon in all 72 tested soils (till) samples across

Nova Scotia. In particular, radon in Halifax Regional Municipality (HRM) has been previously

identified as a potential human health risk (White et al. 2008). Previous 2-dimensional numerical

modelling has demonstrated that if the "C" soil horizon (till) has > 1 m thickness above the

bedrock, the radon produced from the bedrock source alone will not be transported to the surface

before it decays (O'Brien 2010). As radon soil gas is the main source of radon in buildings,

understanding the movement and transport of this gas is key to mitigating this potential

geohazard for human health. The results of this modelling suggested that surficial geology is

likely the controlling factor on radon soil gas transport in HRM, but this hypothesis has yet to be

tested.

Three overall goals are proposed for this study. (1) Determine the control on the 222

Rn

concentration observed at the soil surface - soil permeability, or radon production rate. (2)

Determine the primary source of 222

Rn production detected at the soil surface - the surficial

geology, or the underlying bedrock. (3) Identify spatial trends (within and between surficial

units) given indoor radon readings and field soil gas readings.

Using a RAD7 radon gas measurement instrument, soil gas samples will be measured from three

different controlled soil column experiments. The first soil column will measure laboratory

radon-222 concentration profiles from 0.8 m depth up to the surface, with a granite bedrock

source and overlying inert soils. The second will measure laboratory radon-222 concentration

profiles from 0.8 m depth up to the surface, with a granite bedrock source combined with the

corresponding Beaver River Till (BRT) surficial facies. The third column is the control with only

BRT. This BRT granite till, the highest radon unit, will be used in this study as it has previously

been documented as the 'worst case' radon exposure till in HRM (Goodwin et al. 2010). The

combined data from these three columns will determine the amount of radon being produced

from the bedrock, as well as the amount produced from the till. GIS analysis of existing

industrial datasets of radon in indoor air measurements will be used to establish spatial trends

and variability seen within the surficial geology units of HRM. We hypothesize that indoor air

concentrations are not strictly controlled by bedrock 222

Rn production. Radon in indoor air may

also be strongly correlated spatially to the till 222

Rn production and permeability. Overall, the

outcomes of this study will provide new techniques to more clearly define radon risk

distribution, and a better understanding of the driving factors on this carcinogenic geohazard.

TIMETABLE:

Statement of Problem

Long-term exposure to high radon gas concentrations through inhalation is a human health risk.

Understanding the processes affecting the travel of radon soil gas to the surface will advance

understanding of the driving factors on 222

Rn production and transport. This study aims to test

the following hypotheses: 1) The controlling factor on the concentration of radon present at the

soil surface is due to permeability of the soils through which the gas passes, and not the

production of the radon itself. 2) The production of radon measured at the soil surface is

primarily derived by the overlying surficial geology, and not predominantly from the underlying

bedrock. This will be particularly significant for regions where houses are built on a till cover or

drumlin.

Background

Introduction 222

Rn is a naturally occurring, invisible radioactive gas (half-life of 3.82 days) that is present in

measurable quantities in all till and soil types in Nova Scotia (Goodwin et al. 2008b). It is a 238

U-

series daughter product, and decays to 218

Po, releasing a potentially harmful alpha particle. High

radon soil gas values are typically associated with granites and black shales (Je 1998). Radon is a

human health risk, as long-term exposure to high radon concentrations through inhalation is the

second leading cause of lung cancer after smoking (WHO 2005). Radon soil gas (where the soil

= glacial till), and radon in indoor air has been studied worldwide, where it was recognized as an

important and significant area of research.

Importance of Permeability/Bedrock Contribution Diffusivity is the main soil quality component influencing the transport of gas (Ball et al. 1999).

Being an inert noble gas, radon can be produced in, or diffuse into, the interstitial pores spaces of

the till. It diffuses through the soil to the surface before decaying, allowing for the controlled

measurement of radon from the bedrock or the till. Permeability plays an important role in the

expression of soil gas, as it is a proxy for the diffusion of gas transport. The most recent way soil

permeability has been measured, particularly across North America, has been using the Radon-

JOK portable permeability instrument (as described in Friske et al. 2010); these were among the

first permeability readings taken at a province-wide scale within Nova Scotia (Goodwin et al.

2010).

Soil radon concentration and soil gas permeability are the two most important constituents

required to predict the radon risk potential of a region (Neznal et al. 2004). This radon risk

describes the potential for high levels of carcinogenic radon gas to be detected in indoor air,

making it a more susceptible region for human health. The soil radon potential (SRP) index helps

quantify the radon gas/ permeability relationship (Neznal et al. 2004). The higher the SRP value,

the greater the potential for radon to migrate through the till and enter a home to levels that

exceed the national guideline of 200 Bq m-3

(Health Canada 2009). There is currently no radon

soil gas guideline, therefore it is studied in terms of the potential for indoor radon accumulation

and exposure. The SRP index is defined as:

where C is the radon soil gas concentration for a field sample site in kBq m-3

, and P is the soil

permeability of the field site in m2. C0 and P0 are set constants, respectively, 1 kBq m

-3 and

1x10-10

m2.

Previous National Work There have traditionally been three major groups of instrumentation for radon soil gas

measurements (as described by Papastefanou 2002): i) Instantaneous - The methods surrounding

the instantaneous 'grab' sampling focus on scintillation cells, such as the Lucas cell, or

scintillation flasks. The activity ranges of these cells are not optimal for column experiments as

they are restricted to a kBq range of concentrations. ii) Integrating - There are two main passive

integrating instruments: the alpha track detector (ATD), and the electret ion chamber (EIC). The

ATD is not a preferred instrumentation, as it requires a lengthy 1-2 week field exposure: the EIC

is currently a popular tool for indoor radon measurements (Chen et al. 2009b). iii) Continuous -

These instruments have the ability to provide real-time 222

Rn measurements, as sampling and

analysis occur simultaneously. Continuous measurement methodology is dominated by

radon/thoron monitors, such as the Durridge RAD7. The range of measurement from Bq to kBq

scale for this instrument far exceeds the other gas measurement tools, and does so with a

sensitivity of 0.01 Bq/m3.

An indoor radon risk potential map of Canada has identified central Canada (Winnipeg) and

Atlantic Canada (Nova Scotia) as high risk areas where homes exceed the national indoor radon

guideline (Chen et al. 2008a, 2009a). This was the first radon map generated to identify regions

of high-exposure risk within Canada, and was compiled using field radon gas readings, and

indoor measurements.

Previous Work within Nova Scotia Previous Work within Nova Scotia Soil gas testing completed across the province of Nova Scotia

showed measurable quantities of radon in all 72 field sites, with a density of 1 sample every 800

km2, following the NAGLP methodology (Goodwin et al. 2008b). Recent results determine that,

within HRM, the granite facies of the BRT had the highest average radon concentration of 48.5

kBq m-3

, followed by the slate facies of the BRT with 36.1.4 kBq m-3

, the metasandstone facies

of the BRT with 22.4 kBq m-3

, and finally the Lawrencetown Till returning the lowest average

radon of 19.3 kBq m-3

(Goodwin et al. 2010). The BRT granite facies (as defined by Stea and

Fowler 1981; MacDonald and Horne 1987) has been consistently measured as HRM's "indoor

radon high-risk" unit, and should be further studied. Approximately 40% of the HRM study area

is covered by BRT granite facies till, therefore it is crucial to understand the 222

Rn soil gas

characteristics inherent to this potentially hazardous till unit.

O'Reilly et al. (2010) is currently developing a geographic information systems (GIS) based map

showcasing the potential for radon in indoor air in Nova Scotia. He is combining airborne

radiometrics, bedrock geology, and surficial geology to give each 250 m centered grid cell a

radon potential score. This map is anticipated be published within the year, and would be an

excellent reference to correlate with a potential map created in this study.

Need for further work

Within HRMs bedrock and surficial geology units, radon has been previously identified as a high

potential health risk for radon in indoor air (White et al. 2008). Finding the controlling factor for

radon generation and transport is crucial in assessing risk potential of homes built on bedrock

versus homes built on till. 2D numerical soil gas modelling showed that unless the till is less than

1 m thick above the bedrock, the radon produced from a bedrock source alone will not reach the

surface before it decays (O'Brien 2010, O'Brien and Goodwin 2011). This has implications for

building constructions (building a home on certain till types, versus bedrock), outdoor recreation

(soccer fields with imported granite till), and particularly remediation techniques worldwide. The

extent to which bedrock or soils play a role in emitting problematic radon in homes and schools

is still an outstanding issue (O'Brien and Goodwin 2011).

OBJECTIVES

Long Term Objectives

1. Determine what controls 222

Rn concentration observed at the soil surface - soil

permeability, or radon production rate.

2. Determine the primary source of 222

Rn production detected at the soil surface - the

surficial geology, or the underlying bedrock.

3. Identify spatial trends (within and among surficial units) given indoor radon readings and

field soil gas readings.

Short Term Objectives

1. At field locations, measure 222

Rn productions of the six major till units of HRM

[(Lawrencetown Till, and Beaver River Till (metasandstone facies, slate facies, and

granite facies, subdivided based on the cooling history into primitive, middle, and

evolved facies)].

2. In a laboratory setting, measure 222

Rn concentration profiles from 0.8 m depth to the

column surface, from a granite bedrock source overlain by inert field soils.

3. In a laboratory setting, measure 222

Rn concentration profiles from 0.8 m depth to the

column surface, from a granite bedrock source and the overlaying Beaver River Till.

4. In a laboratory setting, measure 222

Rn concentration profiles from 0.8 m depth to the

column surface, from the Beaver River Till granite facies till.

5. Construct a radon potential map of HRM, from compiled indoor radon gas readings and

field soil gas readings.

METHODOLOGY

The RAD7 will be the radon gas instrument used in all gas measurement stages of the methods,

as it is accurate over a large range of concentration values (kBq m-3

to Bq m-3

). It is also an

extremely portable and durable machine which is required for field use. In order to correlate with

previous field instrumentation used in the Friske et al. (2010) study (Radon-JOK portable

permeability instrument, IK-250 sampling ionization chambers, RM-2 portable soil radon

monitoring system), a sensitivity of 0.1 kBq m3 is needed with a 2 sigma precision. The RAD7

concentrations are given with 2 sigma precision, and have a sensitivity of 0.1 Bq m3, which far

exceeds the requirements.

In each of the respective methods, every till sample that is collected will be characterized. Pebble

count, clast size, clast density, clast composition, grain size, degree of sorting, degree of

rounding, and clay content will be observed and record in the field. Clay content may also be

determined quantitatively using sediment characterization labs at the Nova Scotia Agriculture

College, pending permission. Also, general observations will be made about the thickness of the

overlying soil horizons and the depth to the 'C' horizon, when the pits are dug. All these

characterizations will be important later on, in helping to explain the potential variability seen

within units in the results. There are three overall methods:

Method 1 Native

222Rn emission levels of the six main till types ("C" horizon soils) in HRM will be tested

to ensure the results are comparable to past field work (Goodwin et al. 2010). They consist of

Lawrencetown Till (defined by Stea and Fowler 1981), and BRT, which is subdivided into three

mapable units: slate facies; metasandstone facies; and granite facies. The granite facies have

been further subdivided on the basis of their lithology into a monzogranite till, a coarse grained

leucomonzogranite till, and a fine grained leucomonzogranite till (MacDonald and Horne 1987).

The till samples will be characterized as one of three types, based on their pebble lithology. An

unpacked sample of each 'C' horizon soil (till) type will be collected in 500 mL glass jars with

rubber sealing collars; after a 5 minute equilibration period, radon concentrations will be

measured using the RAD7. There will be (tentatively) 10 replicates of the 6 respective surficial

units, for a total of 60 measurements. The unit with the highest previously-documented

variability (fine grained leucomonzogranite facies; Goodwin et al. 2010) will be used to

determine the number of samples before the average concentration converges, and using this

number of samples for the remaining units. This method eliminates influences such as

permeability variability and synchronous bedrock production seen in the field, as the unpacked

surficial geology samples will ensure that permeability is not a limiting factor on the

concentration. Removed from potential bedrock sources in the field, these readings will be able

to give an accurate average radon level produced in the till, and with that, the potential indoor

exposure risk for human health. The granite-derived till (highest radon in the field) is expected to

have the highest radon concentration in the jars, confirming that it will be used in the rest of the

proposed work, as it represents the highest potential hazardous unit.

Method 2 Soil (till) columns will be constructed to host the collected till samples taken from the field. The

columns will be built using a clear polyvinyl chloride (PVC) cylinder pipe, with holes of 5 mm

diameter drilled at 10 cm intervals along one side of each column. They will be rinsed twice with

sterile distilled H20, and placed to dry under sterile flow hoods. Each column will be packed

manually to a diffusivity of roughly 10-7

m (or permeability of 4x10-12

m2, as per model and field

conditions), and allowed to equilibrate. Radon soil gas concentrations will be measured with a

syringe through the drilled holes at 10 cm intervals (which are plugged when measurements are

not being acquired). In the laboratory, tills will then be wetted: 42 mm of water will be added

uniformly across the top of the column (95 mm is a monthly average in the summer for Halifax;

3 mm a day for two weeks). The gas collection will be repeated after a two week percolation and

equilibration period to determine if radon moves slower through water-filled pore spaces than

air-filled (representative of Nova Scotia's rainy climate). The methods for each column

experiment are summarized in Table 1. The column apparatus' can be seen in Figure 1.

Soil Column A- Granite Bedrock and Inert Soils O'Brien and Goodwin (2011) used 2D modelling to demonstrate that the production of radon

from only the bedrock was not enough to be detected at the surface, when the overlying tills were

> 1 m thick, and therefore the tills must have been emitting significant levels of gas. This was

based on a constant 222

Rn production rate from bedrock, and the permeability effect of the

overlying till - this will be tested in first soil column A. The inert soils used (from sands sampled

by Goodwin in 2009 that contained almost negligible radon at 0.6 m depth; < 5 kBq m-3

) are not

radon producing, therefore, the concentration profile of radon flux through a till with a

characterized permeability can be measured from a bedrock source alone. Due to the time

sensitive nature of till collection, and gas measurement, only one till column apparatus can be set

up at a time, therefore, it will take one month to run all 6 replicates.

Soil Column B - BRT Granite bedrock and overlying granite till The second laboratory soil column apparatus will be used to quantify the amount of radon the

granite soils produce through depth. By comparing the measurements from the two column set-

ups (A and B), the difference should roughly represent the concentration of 222

Rn emitted solely

from the tills. These tests will determine whether potential radon highs at the surface are

associated with the local surficial geology, or the underlying bedrock geology of HRM, in order

to complete the third objective. Should the results at 60 cm not fall within the range of values

previously documented (O'Brien and Goodwin, 2011), permeability and moisture will be

adjusted to reach field conditions. Soil column B measurements should take roughly one month,

due to the time-sensitive soil collection, column experiments, and gas measurement processes.

Soil Column C - BRT Granite facies till only The final laboratory soil column is the control profile; there will be no bedrock production

source in the bottom. This column will act as a reference for the till productions determined from

the A and B column difference. It will take a month to complete all replicates and measure the 222

Rn concentration profile.

Table 1: Summarized methods.

Figure 1 Simplified column apparatus' for columns A, B, and C respectively.

Method 3

Previously collected indoor radon measurements (from two major testing companies in HRM)

will be compared to radon soil gas measurements from the surficial geology units within HRM,

to see if potential indoor risk trends seen between units are similar to the previous field results

(O'Brien and Goodwin 2011, Goodwin et al. 2010). Indoor data databases will be acquired from

the two major radon testing companies within Halifax: Radon Atlantic Consultants and Maritime

Testing. The goal is to construct an indoor radon exposure potential map of HRM using a GIS

approach. Geospatial analysis software will combine geocoded indoor radon concentrations with

soil gas measurements, and the surficial geology map of HRM to determine 222

Rn variability in

each till facies. By assessing the variability of indoor radon concentrations with sampled

geological measurements, spatial trends can be established, and basic map patterns can be

interpreted. This methodology is similar to the on-going work by O'Reilly et al. (2010) who is

creating a map of Nova Scotia showing the potential for indoor radon concentrations to exceed

the guideline of 200 Bq m3. However, this study's proposed work will incorporate indoor

222Rn

readings, an element missing in the indoor potential map (O'Reilly et al. 2010). Ideally, the final

map results will be comparable to the future final O'Reilly map, to see if indoor data is a reliable

and accurate source of exposure risk determination, and will satisfy the third long-term goal. The

raster based approach (described in O'Reilly et al. 2010), eliminates the need for the

confidentiality of homeowners, as the grid scale will be larger than houses. If enough indoor

measurements are not collected [at least 10 per till unit to compare statistically with the Goodwin

(2010) field measurements], the limitations of drawing conclusions on a restricted sample set

will be further expanded upon.

ANTICIPATED RESULTS AND SIGNIFICANCE

Understanding the dominant processes affecting the transport of radon in HRM tills will further

remediation techniques to protect against 222

Rn infiltration and collection, ultimately decreasing

the risk of exposure and potential for lung cancer, and other respiratory diseases. The laboratory

radon gas concentration at 60 cm depth are expected to be, on average, 51.0 kBq m-3

, and have a

range of 18.6 kBq m-3

to 154.6 kBq m-3

. These were the average measured field concentrations

of the fine grained leucomonzogranite till (Goodwin 2010), therefore, a laboratory result within

this range would mean that the results from this study can be comparable to field concentrations.

The concentration of radon gas is expected to decrease rapidly towards the top of the column as

ambient air mixes in; measured concentrations should be in Bq m-3

by the top 20 cm. Comparing

column A and B results, the radon gas concentrations measured near the column surface due to

the granite till are expected to be more significant than the concentrations with only a bedrock

source at depth.

These expected results would be the first within Nova Scotia to re-create field conditions to show

that production and transport of 222

Rn in the till is overall a more important control on the

potentially dangerous indoor radon risk than 238

U concentration from the bedrock. This will have

important implications for houses that are built on bedrock, with a thin overlying till cover. The

outcomes of this study can potentially impact building policies, mitigation techniques, testing in

tills, and home inspections. This new information will aid in understanding the extent, impact,

and sources of radon gas that effect human health worldwide.

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