2. 1983; Hess et al., 1985; Michel and Jordana, 1987; Longtin, 1988)
indicate that high radon activity in groundwater occurs in
uranium-bearing granite aquifers, in magmatic and metamorphic
bedrock aquifers, or when groundwater is present close to fault
zones. In the United States, main 222
Rn-prone areas include the
Appalachian Mountains, the Rocky Mountains and the Basin and
Range (National Research Council, 1999). In Canada, 222
Rn-prone
areas are located in terrains favorable to the emanation of radon,
such as magmatic and metamorphic rocks of the Canadian Pre-
cambrian shield and their products of alteration (Tilsey et al.,
1993).
In Canada, the harsh climate and the limited ventilation of
homes during the winter season enhance the risk for an increased
concentration of indoor radon. Voluntary radon surveys in houses
were carried out regularly since 1970 (Health Canada, 2009). The
latest recommended action level is 200 Becquerel by air cubic
meter (Bq/m3
) for indoor radon (Health Canada, 2007). Extended
surveys for soil gas radon in major Canadian urban areas are briefly
summarized by Chen et al. (2012). The Geological Survey of Canada
carried out detailed studies of radon in ground water since 1980 in
Saskatchewan, Ontario and Prince Edwards Island (e.g., Dyck, 1979,
1980).
Levesque et al. (1995) carried out a detailed survey for indoor
radon in one thousands homes of the Province of Quebec and
showed that the Gaspe Peninsula and the Appalachian Mountains
regions had the highest222
Rn activities, with averages of 120.8 and
59.7 Bq/m3
respectively. These data were recently updated by
Drolet et al. (2013) to define potential radon emission level maps
in Quebec. Local surveys in homes of the Mount Oka and Mont-
Saint-Hilaire areas, two magmatic intrusions located close to
Montreal, showed high 222
Rn activities (18e10,500 Bq/m3
, geo-
metric mean of 1245 Bq/m3
in Mount Oka (Savard et al., 1998), and
20e1432 Bq/m3
, geometric mean of 69 Bq/m3
in Mont-Saint-
Hilaire (Dessau et al., 2005)). These high values are due to the
high U contents in these rocks (Murphy et al., 1978; Savard et al.,
1998).
Canadian guidelines for radon in groundwater recommend a
maximum activity of 2000 Bq/L. This value is much higher than the
limits recommended in other countries (100 Bq/L is the suggested
value by the World Health Organization, 2008). Roireau and
Zikovsky (1989) carried out a radon survey in 50 drinking water
wells mainly located in a radius of less than 100 km around Mon-
treal and in the north of the “Laurentides” region, where Protero-
zoic metamorphic rocks of the Grenville Province outcrop. They
measured 222
Rn activities between 0.2 and 439 Bq/L with a mean of
16.9 Bq/L (because the authors did not report all the data, the
median or geometric mean cannot be calculated and reported
here), and conclude that the measured activities, similar to those
found in other countries, did not represent a real hazard to water
users. Chah and Zikovsky (1989) carried out 32 radon measure-
ments mostly at the same sites as Roireau and Zikovsky (1989).
They measured 222
Rn activities between 1.6 and 23.1 Bq/L, except
for water samples from the Mount Oka area where the 222
Rn ac-
tivities ranged from 48 to 627 Bq/L.
Although numerous radon surveys were carried out in the
Province of Quebec, most studies do not provide a spatial analysis of
the data. This information is important for public health issues. It is
also essential when using 222
Rn as a short-term groundwater
tracer. This study reports the results of an extensive survey for
222
Rn in groundwater from local municipal and domestic wells in
southern Quebec (Pinti et al., 2013a). The spatial distribution of
222
Rn activity in groundwater in relation to the geology and hy-
drogeology of the study area and the possible health hazards that
222
Rn levels could generate in the most populated area of the
Province of Quebec are discussed.
2. Local geology and hydrogeology
The study area covers 15,435 km2
in the St. Lawrence Lowlands,
a relatively flat plain between Montreal and Quebec City, delimited
by the Appalachian Mountains to the southeast and by the north
shore of the St. Lawrence River to the northwest (Fig. 1). This area
covers totally or partially four administrative regions: Monteregie,
Centre-du-Quebec, Chaudiere-Appalaches and Mauricie. Three
geological provinces are represented: the Cambrian-Ordovician St.
Lawrence Platform; the Ordovician-Devonian Appalachian Moun-
tains; and the Cretaceous magmatic intrusions of the Monteregian
Hills (Fig. 1).
The St. Lawrence Platform corresponds to a Cambrian-Lower
Ordovician siliciclastic and carbonate platform having a
maximum thickness of ca. 1200 m, overlain by a minimum of ca.
1800 m of Middle-Late Ordovician foreland carbonate-clastic de-
posits (Lavoie, 2008). Geographically, it corresponds to the flat area
between Montreal and Quebec City called the St. Lawrence Low-
lands. The Ordovician geological units of the St. Lawrence Platform
represent a regional fractured rock aquifer tapped by the sampled
water wells. The St. Lawrence Platform units of interest in this study
are (Fig. 1): (1) the Utica Shale, a fine-grained limy mudstone with
organic content between 1 and 1.5% targeted for potential shale gas
production (Lavoie et al., 2013). (2) A turbiditic thick succession (up
to 3800 m; Globensky, 1987) of mudstones with subordinate
alternating sandstone and siltstone of the Lorraine Group. This
group is the most exposed unit of the St. Lawrence Platform. (3)
Finally, the Queenston Group, consisting of post-orogenic shale,
sandstone and conglomerate (Globensky, 1987).
The terrains outcropping on the Appalachian Mountains corre-
spond to imbricated thrust sheets composed of Cambrian red
shales (Shefford Group), followed by alternated dolomitic or calcitic
schists, quartzites and phyllades (Bennett, Oak Hill and Caldwell
Groups). Ordovician terrains are mainly formed by pyritic or
graphitic slates (Stanbridge, Melbourne and Bulstrode formations)
to end with black shales alternating with siltstone, sandstone and
arkosic greywacke of the Magog Group (Globensky, 1993).
A nearly continuous till sheet, overlaid by discontinuous patches
of unconsolidated Quaternary sediments made of marine and
lacustrine silt and clay of the Champlain Sea (11,200e9800 yrs) and
sands deposited during marine regression cover the Cambrian-
Ordovician sequence of the St. Lawrence Platform (Lamothe, 1989).
The units of the St. Lawrence Platform and the Appalachian
Mountains are intruded by nine alkaline Cretaceous magmatic in-
trusions, named the Monteregian Hills, one of them, Mont-Saint-
Hilaire is located in the study area and composed of gabbro, py-
roxenite and a few syenite (Eby, 1984).
Most exposed strata in the St. Lawrence Lowlands are sub-
horizontal, but are locally affected by mesoscopic open folding
such as the Chambly-Fortierville Syncline (Fig. 1). Major structural
features in the study area include the Yamaska Fault, one of the
many regional normal faults cutting through the basementeplat-
form succession and the Logan's Line (Fig.1) a thrust fault that mark
the transition between the St. Lawrence Platform and the Appala-
chian Mountains.
The regional aquifers are located in the fractured bedrock of
moderate hydraulic conductivity (~10À6
to 10À5
m/s). They are
semi-confined or confined. In general, wells in the fractured
bedrock aquifers yield enough water to supply single-family
dwellings and, in a few areas, small- to medium-size municipal-
ities (Carrier et al., 2013; Larocque et al., 2013a,b). Medium- to high-
yield aquifers are found in coarse-grained surficial sediments, such
as the Quaternary glacio-fluvial or fluvial sediments. In areas close
to the St. Lawrence River, most residential wells tap groundwater
from the rock aquifer whereas municipal wells tap groundwater
D.L. Pinti et al. / Journal of Environmental Radioactivity 136 (2014) 206e217 207
3. from either the rock or granular aquifers. The main groundwater
flow directions in the bedrock aquifers are SEeNW and follow the
general topography, with recharge occurring mostly in the Appa-
lachian Mountains and discharge to the St. Lawrence River or its
main tributaries. Local recharge areas are also distributed in the
Lowlands, particularly where the impermeable Champlain Sea
clays are absent (Carrier et al., 2013; Larocque et al., 2013a,b;
Leblanc et al., 2013).
Groundwater chemistry shows the occurrence of low-salinity
water, dominantly of CaeMgeHCO3 type close to the recharge
areas of the Appalachian Mountains. This water evolves into a Na-
HCO3 type downstream, probably caused by ion exchange (e.g.,
Cloutier et al., 2006), with groundwater electric conductivity
ranging from 88 to 4466 mS/cm in the study area. Saline ground-
water (conductivity from 717 to 31,500 mS/cm) is found in a 10 km
wide zone bordering the St. Lawrence River close to the Chambly-
Fortierville syncline (Larocque et al., 2013a,b). This saline ground-
water becomes clearly brackish in a 2200 km2
area to the north of
the Monteregie Est basin where salinity reaches 5 g/L locally
(Beaudry, 2013). 3
H-3
He ages of the freshwater end-member indi-
cate sub-modern water (2e35 yrs old; Pinti et al., 2013b) in the
Becancour area, while uncorrected 14
C activities of the brackish
end-member suggest ages of 3000e14,000 years (Beaudry, 2013)
corresponding to the Champlain Sea invasion and seawater trap-
ping within the clayey Holocene units.
3. Sampling and analytical methods
A total of 198 wells (167 domestic/municipality supply wells, 23
drilled observation wells and 8 piezometers) were selected, mostly
on the south shore of the St. Lawrence River, with a few located to
the north of it (Fig.1). A total of 165 wells tap groundwater from the
basal Ordovician fractured bedrock aquifer system whereas 33
wells tap groundwater from sandy Quaternary aquifers. Well
depths range from 20 to 225 m. Most of these wells have also been
studied as a part of recent groundwater characterization projects
Fig. 1. Geological map of the study area with the location of the sampled wells. Positions of main faults and the Chambly-Fortierville Syncline are also drawn. Redrawn from
Sejourne et al. (2013).
D.L. Pinti et al. / Journal of Environmental Radioactivity 136 (2014) 206e217208
4. funded by the Quebec Ministry of Environment (Programme
d'acquisition de connaissances des eaux souterraines: acronym
PACES). These projects involve sampling and analyses for solute ion
contents, trace elements, stable isotopes of water (dD and d18
O) and
groundwater residence time indicators using 14
C and 3
H/3
He
methods (Larocque et al., 2013a,b; Carrier et al., 2013; Pinti et al.,
2013b).
In the current study, groundwater was collected at the obser-
vation wells using a submersible pump with speed control (Redi-
Flo2®
), which prevents significant water degassing. In municipal
wells, the water was collected directly at the wellhead and in do-
mestic wells at the closest water faucet from the well, taking pre-
cautions to avoid intermediate reservoirs where the water could
degas. Observation wells and domestic wells were purged a volume
equivalent to three times the water volume in the well bore and the
water was sampled once the water physico-chemical parameter
shad stabilized (pH, T and electric conductivity). Municipal wells,
which are continuously pumping, were briefly purged prior to
sampling. Radon was collected using 250 cm3
glass bottles by
inserting a PVC tube to the bottom and filling with water at con-
stant and low flow to avoid degassing. Once filled, the bottle was
rapidly sealed with a plastic cap and examined visually for the
presence of any air bubble. This is a well-tested field method for
sampling radon from water with minimal loss of radon using glass
or PET bottles (Leaney and Herczeg, 2006). The bottles were labeled
with the time of sampling in order to correct the measured 222
Rn
activities of the time lap between field collection and laboratory
analyses.
The sealed and bubble-free bottles were sent to the radon lab-
oratory at Universite du Quebec a Montreal every 2e3 days to
measure the 222
Rn activity using a Hidex SL-300 liquid alpha
scintillometer. Scintillometer efficiency was first calibrated using
international standards. Sampling and analysis protocols were
optimized and analytical tests were performed to obtain the best
possible accuracy and precision of 222
Rn activity in a continuous
interval from 0.5 to 35 Bq/L. Details of the instrument calibration
are reported in Lefebvre et al. (2013).
Sample preparation (radon concentration) before analysis can
be performed following two distinct methods: the direct method
(DM) and the extraction method (EM). In the DM method, a
cocktail of water and di-isopropyl naphthalene (DIN)-based
scintillant is directly prepared in the vial that is inserted in the
scintillometer. Counting is performed directly on this mixture
after homogenization, but errors on DM measures are generally
high (3e10% for 222
Rn activity ranging from 2 to 20 Bq/L; Leaney
and Herczeg, 2006). This method is typically used for water ex-
pected to exceed 2.5 Bq/L in 222
Rn activity. The extraction method,
used for low 222
Rn activity samples, requires more preparation but
loss of radon during manipulation is reduced drastically and
detection level is lowered to 0.003 Bq/L with an average error of
3% (Leaney and Herczeg, 2006). EM manipulation is as follows:
20e50 cm3
of scintillant are mixed with the entire volume of
sampled water (250 cm3
). Approximately 10 cm3
of air are intro-
duced into the glass bottle to facilitate emulsion between water
and the scintillant. The cocktail is mixed for 4e5 min and left to
stand until phases separate. A volume 8 cm3
of scintillant was
then extracted and added to the counting glass vials. During
calibration of the method, mean background noise values were
smaller than 0.12 Bq/L with an average error of 4% for analysis
(Lefebvre et al., 2013).
Radon activity results were interpolated with Kriging using
ESRI®
ArcMap™ 10.1 to create radon distribution maps. The ordi-
nary kriging method with “prediction” output and constant “trend”
was the most reliable from the statistical point of view to represent
the prediction map of the radon activity. The model used for the
kriging is the “stable” type with no assumed anisotropy and does
not include a nugget effect.
4. Results
Radon activities measured in the groundwater wells are re-
ported in Table A1 in the Appendix. Measured 222
Rn activities vary
from 0.2 to 310 Bq/L, in the same range reported by Roireau and
Zikovsky (1989) (see also Fig. 3 for comparison). The histograms
shown in Fig. 2 represent the frequency distributions of 222
Rn
measured in all groundwater samples and separately for wells
located in the Appalachian Mountains and in the St. Lawrence
Platform, i.e. the two main geological provinces in the study area.
Average 222
Rn activity in the entire study area is 25.2 Bq/L while the
wells from the Appalachian Mountains shows a higher average of
31.3 Bq/L compared to an average of 16.1 Bq/Lfor the wells located
in the St. Lawrence Platform.
The distribution of 222
Rn activities is lognormal (Fig. 2) as usu-
ally observed in radon surveys (Zikovsky and Chah, 1990) and,
generally, for element distributions in the geosphere (Ahrens,
1965). Median 222
Rn activities are 8.6 Bq/L for the entire dataset,
5.2 Bq/L for wells from the St. Lawrence Platform and 11.9 Bq/L for
wells in the Appalachian Mountains. These values are higher than
the median values of 2.3 and 3.3 Bq/L reported by Roireau and
Zikovsky (1989) and Chah and Zikovsky (1989), respectively. This
difference could be related to the size of the dataset and the region
covered (198 wells against 50 and 32 wells for Roireau and
Zikovsky, 1989 and Chah and Zikovsky, 1989, respectively). Sam-
pling a larger number of wells increases the probability of tapping
into “anomalous” areas, therefore influencing the median value.
The range of 222
Rn activities measured in this study is also
similar to that measured in Saskatchewan, Canada by Dyck (1979)
and in the United States by Hess et al. (1985) (Fig. 3), though
higher activities have been measured in the nearby State of Maine,
mainly in U-rich granitic terrains (Brutsaert et al.,1981). High radon
activities are characteristics of Nordic countries such as Finland and
Fig. 2. Frequency distribution histograms of the 222
Rn activity measured in wells (a)
from the entire study area; and those located in (b) the St. Lawrence Platform; and (c)
the Appalachian Mountains.
D.L. Pinti et al. / Journal of Environmental Radioactivity 136 (2014) 206e217 209
5. Norway (Fig. 4), because of the large outcrops of granitic terrains
belonging to the Fennoscandia Precambrian Shield (e.g., Banks
et al., 1998; Morland et al., 1998; Vesterbacka et al., 2005). It was
not possible to illustrate the most complete survey ever carried out
on Fig. 3 (30,957 wells sampled in Sweden; Knutsson and Olofsson,
2002) because no data tables are reported in this work, but 222
Rn
activities as high as 57,000 Bq/L have been measured in U-rich
granitic terrains in this study.
Fig. 4 presents the radon distribution prediction map in the
study area. Results show that higher 222
Rn activities are recorded in
two areas, the Mont-Saint-Hilaire and the Appalachian Mountains.
222
Rn activity reaches 233.4 Bq/L in Mont-Saint-Hilaire, a gabbroic
intrusion of the Cretaceous Monteregian Hills that is already known
for high indoor radon anomalies (20e1432 Bq/m3
; Dessau et al.,
2005). The Appalachian Mountains are also known for high in-
door radon activities (58 Bq/m3
; Levesque et al., 1995). 222
Rn in this
region varies from 0.3 to 310 Bq/L. A few restricted areas with
relatively high 222
Rn activities (30e97 Bq/L) also occur near the St.
Lawrence River. The larger anomaly surrounds the town of Sorel in
the northern part of the Monteregie Est region (Fig. 4), and corre-
sponds to the 2200 km2
area where brackish groundwater has been
found in the fractured rock aquifer confined by thick marine clay
(Beaudry, 2013).
5. Discussion
5.1. Expected radon activity in water
Both the descriptive statistical treatment and the spatial dis-
tribution of 222
Rn activities (Figs. 2 and 4) clearly show that 222
Rn
activities recorded in wells located in the St. Lawrence Platform are
lower than those recorded in wells located in the Appalachian
Mountains. The reason could be simply related to the difference in
lithology. Wells in the St. Lawrence Platform collect groundwater
from fractured carbonate-shale formations whereas groundwater
from the Appalachian Mountains is in contact with graphitic and
black shales and metasediments (slates, schists and phyllades)
which are expected to contain more uranium than the sedimentary
rocks belonging to the St. Lawrence Platform.
The concentration of U needed to produce the measured 222
Rn
activities can be estimated by simple calculation. The 222
Rn activity
in water derives from the radioactive decay of 226
Ra (a daughter
element of 238
U) adsorbed on the solid grains or contained in
minerals (Cecil and Green, 2000). Thus the 222
Rn activity in water
can be estimated from the U content of the rock, and assuming
secular equilibrium between 226Ra in the solid phase and 222
Rn in
the water phase (25 days; Andrews and Lee, 1979), following the
equation modified from, e.g. Bonotto and Caprioglio (2002):
½222
RnŠwater ¼ 12:469$r$ð1 À f=fÞ$ARn$½UŠrock (1)
where [222
Rn]water is the radon activity in the liquid phase calcu-
lated in Bq/L; r is the bulk density of rock (g/cm3
); (1 À f/f) is the
void ratio (unit less); ARn is the emanation coefficient of radon (unit
less) and [U] the concentration of U in the rock (mg/kg). The
emanation coefficient is the fraction of the total activity of 222
Rn
produced by 226
Ra decay that escapes from the rock to the pores of
the medium. The constant 12.496 is the 238
U disintegration rate
recalculated to take into account the correct units to convert the
final [222
Rn]water to Bq/L (Bonotto and Caprioglio, 2002).
The average aquifer porosity and the radon emanation coeffi-
cient are the two parameters that are the most difficult to evaluate
and they are source of the larger uncertainties. Based on pumping
tests, Larocque et al. (2013a) suggest porosities of 1e5% for the
fractured Ordovician aquifers around Becancour (Fig.1). Nowamooz
et al. (2013) report porosities for the Lorraine shales (the most
widespread terrain in the St. Lawrence Lowlands) and the Utica
shales of 3.5 ± 1%. Ryder (2008) reports porosities of the Queenston
shale of 3e4%. These porosity values are on the order of those
measured in Cambro-Ordovician lower sandstone and carbonate
unit cores of the St. Lawrence Platform, which vary between
1.22 ± 0.24% and 6.22 ± 0.18% (Tran Ngoc et al., 2014). Cambrio-
Ordovician metasedimentary series of the Appalachian Mountains
constitute aquifers of poor permeability (Simard and Des Rosiers,
1979) with porosities equivalent to those of the fractured lithol-
ogies of the St. Lawrence Platform, or lower. To cover the entire
porosity range encountered in the study area, we assume a porosity
(f) from 1 to 5%. The emanation coefficient of 222
Rn depends on
lithology, temperature and moisture content (e.g., Ball et al., 1991;
Nazaroff, 1992). Characteristic emanation coefficients for silty and
clayey rocks, which closely relate to the terrains outcropping in the
study area, range between 11 and 31% (Markkanen and Arvela,
1992).
Using the above values of porosity and emanation coefficients,
and assuming secular equilibrium, U contents from 0.2 to 2.2 mg/kg
for the rocks of the St. Lawrence Platform and from 0.3 to 2.9 mg/kg
for the rocks of the Appalachian Mountains are needed to produce
the 222
Rn activities recorded in our water samples. Calculated U
contents are in agreement with those estimated by Pinti et al.
(2011) for the carbonate-silty facies of the St. Lawrence Platform
(0.4e2.0 mg/kg of U) and for the gabbro-pyroxenite-diorite in-
trusions of the Monteregian Hills such as Mont-Saint-Hilaire
(0.3e2.5 mg/kg U) (Pinti et al., 2011).
The U contents in the slate and schistose terrains of the Appa-
lachian Mountains are not well known but certainly reach or even
exceed the values estimated above for the rocks of the St. Lawrence
Platform. Uranium enrichment in these terrains is clearly evidenced
by the occurrence of several U mineralizations along the Appala-
chians piedmont and highlands, often at the contact with granitic
bodies, as observed in Vermont State, USA (Dahlkamp, 2010). A few
U anomalies have been discovered by the Canadian Johns-Manville
Company in 1968 in the area of Thetford Mines, which is just a few
Fig. 3. Range of 222
Rn activity recorded in groundwater in this study compared to
those measured in Quebec, Canada, the USA and other countries worldwide (when
available, the number of samples is shown on the bar corresponding to each study).
References are: Dyck (1979) (Canada); Brutsaert et al. (1981) (Maine); Hess et al. (1985)
(USA); Graves (1989) (United Kingdom and Italy); Roireau and Zikovsky (1989)
(Quebec [1]); Chah and Zikovsky (1989) (Quebec [2]); Soto et al. (1995) (Spain);
Freyer et al. (1997) (Germany); Kulich et al. (1997) (Sweden); Morland et al. (1998)
(Norwegian, Quaternary); Banks et al. (1998) (Norwegian, bedrock); Alabdula'aly
(1999) (Saudi Arabia); Han et al. (2004) (Taiwan); Vesterbacka et al. (2005) (Finland).
D.L. Pinti et al. / Journal of Environmental Radioactivity 136 (2014) 206e217210
6. tens of km SE of the highest 222
Rn activity that we recorded in the
Appalachian Mountains (Figs. 1 and 4).
The general distribution of 222
Rn activity in the study area ap-
pears to be closely related to the geology with a clear NEeSW
oriented anomalous region corresponding to the Appalachian
Mountains and the isolated spot of the Mont-Saint-Hilaire
magmatic intrusion (Fig. 4). Other local anomalies, such as those
observed close to the St. Lawrence River could be explained by the
local geology and hydrogeology.
5.2. Radon distribution: relation with lithology and water
chemistry
Fig. 5 shows the frequency distribution of the 222
Rn activity for
the three most represented rock aquifer types in the area, i.e. the
carbonate or silty shales and mudstones occurring mainly in the St.
Lawrence Lowlands and some areasin the Appalachians Mountains,
and the slate and schist-phyllades occurring in the Appalachian
Mountains. The larger 222
Rn anomalies have been detected in
groundwater from schist-phyllade aquifers rather than in slate and
shales. This behavior could be related to an increased U content in
these rocks though there is no known evidence of increasing U
Fig. 4. Interpolated spatial distribution of the 222
Rn activity in the study area. The traces of geological unit boundaries and structural features from Fig. 1 are shown in background,
including Oak Hill formation for the Appalachian Mountains (dotted area).
Fig. 5. Probabilistic density distribution of 222
Rn activities measured in groundwater
flowing in: (a) shale, (b) schist-phyllade and (c) slate.
D.L. Pinti et al. / Journal of Environmental Radioactivity 136 (2014) 206e217 211
7. content with the metamorphic grade (e.g., Dostal and Capedri,
1978). Most of the highest 222
Rn activities in groundwater were
observed in Oak Hill Group (222
Rn average activity of 18.6 Bq/L and
median of 52 Bq/L) (Fig. 4), which is composed of phyllades,
quartzites and dolomites.
If the 222
Rn activity is plotted against the main groundwater
types in the region (Fig. 6), i.e. CaeMgeHCO3, Na-HCO3 and NaeCl
(Carrier et al., 2013; Larocque et al., 2013a), the NaeCl type waters
show more frequent anomalies in 222
Rn, although not in absolute
intensities. Cloutier et al. (2006; 2010) proposed a general model to
explain the evolution of groundwater chemistry in a sedimentary
rock aquifer located North of the St. Lawrence River. At recharge,
freshwater flowing in unconfined aquifers acquires Ca-Mg-HCO3
chemical character by interacting with dolostone and limestone
units of the St. Lawrence Platform. This water evolves toward a Na-
HCO3 type by Ca2þ
eNaþ
ion exchange, where Ca2þ
water exchanges
with Naþ
mineral in semi-confined to confined aquifers. Finally, close
to the St. Lawrence River, groundwater evolves to a NaeCl type with
salinity probably derived from exchange of freshwater with pore
seawater trapped into the Champlain Sea clays or the fractured rock
aquifers, especially in areas confined by thick marine clay that have
limited recharge (Beaudry et al., 2011). Under neutral/alkaline
conditions, 226
Ra, the parent element of 222
Rn is rapidly adsorbed
on the surface of clay minerals (e.g., Ames et al., 1983; Pickler et al.,
2012; Wood et al., 2004; Vinson et al., 2013). The proximity of this
sorbed 226
Ra to the active groundwater flow system allows the
release of its decay progeny 222
Rn directly to the water phase
(Wood et al., 2004). This process could explain why we observe
patchy 222
Rn anomalies close to the St. Lawrence River (Fig. 4),
where fractured aquifers are often confined by a thick marine clay
layers.
5.3. Evaluating health risks from groundwater radon measurements
The 222
Rn dissolved in groundwater constitutes a potential risk
for human health. Inhalation of radon progeny accounts for about
89% of the risk associated with domestic water use, with almost 11%
resulting from direct ingestion of radon in drinking water (USEPA,
1999).
The estimation of the increment of airborne 222
Rn in a dwelling,
arising from the use of water that contains dissolved radon is a
complex problem. It involves the solubility of radon in water, the
amount of water used in the dwelling, the volume of the dwelling,
and the ventilation rate. The increase in the indoor radon concen-
tration due to radon release from indoor water use is described by
the transfer coefficient T (unit less):
T ¼
DCa
Cw
(2)
where (DCa) is the average increase of the indoor radon concen-
tration that results from using water having an average radon
concentration of CW. The accepted value of T is 1 ± 0.2 Â 10À4
(National Research Council, 1999). This means that a given activity
of 222
Rn in groundwater adds approximately 1/10,000 as much to
the radon concentration in the air. As an example, the geometric
mean of the 222
Rn activity measured in this study is 10 Bq/L (or
10,000 Bq/m3
), which means that water could contribute to the
total indoor radon by 1 Bq/m3
.
The study by Levesque et al. (1995) on indoor radon carried out
roughly in the same area as the current study (Monteregie,
Lanaudiere, Chaudiere-Appalaches) reports a geometric mean for
indoor 222
Rn concentrations of 42.1 to 59.7 Bq/m3
in the building
basement and from 15.8 to 26.5 Bq/m3
for the ground floor. This
means that 222
Rn from groundwater (median of 8.6 Bq/L all over
the study area) might contribute from 1.4 to 5.4% of the total indoor
radon.
Contribution to indoor radon can be estimated in areas were the
222
Rn activity in groundwater exceeds the 200 Bq/L. At Mont-Saint-
Hilaire, the maximum 222
Rn activity measured is 233,400 Bq/m3
of
water (233.4 Bq/L), which means that the contribution of 222
Rn
dissolved in water to indoor radon could amount to 23.4 Bq/m3
. A
survey carried out in 40 homes at Mont-Saint-Hilairein 2001
showed that in 77% of the houses the 222
Rn activity in air was lower
than 150 Bq/m3
while for the 20% of them the 222
Rn activity ranged
from 150 to 800 Bq/m3
, and only one house showed a very high
value (1432 Bq/m3
) (Dessau et al., 2005). The indoor 222
Rn average
activity per home was calculated to be 178 Bq/m3
. This means that
radon from groundwater could contribute for 2e3% in the houses
showing the highest indoor 222
Rn activities (800e1432 Bq/m3
).
However in houses having indoor radon contents of 150e178 Bq/
m3
or less, the contribution from groundwater radon could be as
much as 13e15%. Although not dramatic, this contribution is not
negligible.
The other radon risk for human health is its direct ingestion by
drinking water (USEPA, 1999). The annual effective dose (AED in
micro-Sievert per year or mSv/yr) derived from ingestion of radon
and its adsorption on the stomach wall can be calculated following
the simple equation:
AEDðmSV=yrÞ ¼ 222
Rn activityðBq=LÞ Â dose coefficientðSv=BqÞ
 annual water consumptionðL=yrÞ Â 1  106
(3)
The dose coefficient, more commonly known as the “equivalent
dose to stomach per unit activity of 222
Rn ingested” has been
calculated by several authors and ranges between 1.6 Â 10À9
(Harley and Robbins, 1994) to 3.0 Â 10À7
Sv/Bq (Crawford-Brown,
1989). The average accepted value of the dose coefficient for
adults is 3.5 Â10À9
Sv/Bq (National Research Council, 1999).
Assuming an average drinking water consumption of 730 L/year
(World Health Organization, 2002) and a median 222
Rn activity
value of 8.6 Bq/L for the entire study area (Fig. 2), we can estimate
atotal annual dose of 22.5 mSv/yr through water ingestion. This
dose corresponds only to 1.3% of the total annual dose received by
Canadians (1769 mSv/yr; Grasty and LaMarre, 2004). However, in
areas where 222
Rn activity in groundwater is high, such as Mont-
Fig. 6. Probabilistic density distribution of 222
Rn activities measured in (a) Na-HCO3,
(b) NaCl and (c) CaeMgeHCO3 groundwater types.
D.L. Pinti et al. / Journal of Environmental Radioactivity 136 (2014) 206e217212
8. Saint-Hilaire (222
Rn ¼ 233.4 Bq/L; Fig. 4 and Table A1) or in the
Appalachian piedmont (222
Rn ¼ 310 Bq/L; Fig. 4), the dose received
by direct ingestion of radon ranges from 596 to 793 mSv/yr, i.e. an
additional 34e45% of the mean annual dose. This dose becomes
more important for small children for which the effective dose
coefficient is assumed to be 1 Â10À8
Sv/Bq (National Research
Council, 1999). The added dose by drinking water could then
reach 96e128% of the average radioactive annual dose.
6. Conclusions
Measurements of 222
Rn activity in groundwater from 198 wells
between Montreal and Quebec City provided the first distribution
map of 222
Rn activity in the shallow exploited aquifers of the most
populated region of the Province of Quebec (Fig. 4). Results show
that 90% of the wells have 222
Rn activities lower than 100 Bq/L, the
maximum concentration limit accepted by the World Health
Organization (2008), a few wells spanning between 100 and 310
Bq/L. These anomalous higher values occur in groundwater flowing
close to the magmatic intrusion of Mont-Saint-Hilaire and in
metasedimentary aquifers of the Appalachian Mountains. At Mont-
Saint-Hilaire and in the Appalachian Mountains, 15% of the indoor
radon and 45% of the total radioactive annual dose receives
contribution from the groundwater radon.
The spatial distribution of 222
Rn in the region seems to be
related to the lithological characteristics of the terrain in the study
area. The next step would be to improve sampling resolution, in
particular in the Appalachian Mountains, where the higher
anomalies are concentrated (Fig. 4). This is the topic of a com-
plementary project aimed at measuring the main physico-
chemical parameters of groundwater along with 222
Rn activities
in an additional 120 wells in this area, therefore providing more
than 200 wells in a smaller area of 3400 km2
. This resolution will
be helpful to better understand the role of watererock in-
teractions and water mixing in the spatial distribution of radon
and to facilitate the use of 222
Rn as a tracer of groundwater in
surface water.
Acknowledgments
We thank the handling editor, Dr. Paul Martin and two anon-
ymous reviewers for their fruitful comments. We wish to thank
the municipalities and private citizens who allowed collection of
water samples for this study. Ch^atelaine Beaudry and Xavier Malet
(INRS), Antoine Armandine Les landes (Universite Rennes 1), Mina
Ibrahim (Concordia University) and Christine Boucher (GEOTOP)
are thanked for their help during sampling. This research was
funded by the Quebec Ministry of Environment (Ministere du
Developpement durable, de l'Environnement, de la Faune et des
Parcs) (study EES E3-9) and the Quebec Province Research Funds
(FRQ-NT, Fonds de Recherche Quebec e Nature et Technologie)
through the program Initiatives Strategiques pour l'Innovation
(project no. 163607).
Appendix A
Table A1
Radon measurements in the study area.
Sample name X coordinates Y coordinates TC TDS mg/L pH 222
Rn Bq/L ± Geological Province Lithology Aquifer Chemistry AED mSv/yr
BEC002 À72.079 46.300 9.50 395 7.43 3.7 0.1 SL Platform Slate Fractured Ca-HCO3 9
BEC005 À72.078 46.181 9.40 569 7.34 11.5 0.5 Appalachians e Fractured Na-HCO3 29
BEC007 À72.081 46.400 8.80 252 9.24 11.0 0.4 SL Platform Shale Fractured Na-HCO3 28
BEC009 À72.007 46.251 9.90 120 7.82 1.7 0.1 Appalachians e Fractured Ca-HCO3 4
BEC010 À71.932 46.269 10.40 124 8.08 1.5 0.1 Appalachians e Fractured Ca-HCO3 4
BEC014 À71.984 46.446 10.30 640 8.82 4.1 0.2 SL Platform Shale Fractured Na-HCO3 10
BEC015 À71.924 46.528 9.20 172 9.19 19.6 0.8 SL Platform Shale Fractured Na-HCO3 50
BEC016 À71.931 46.487 8.60 202 8.63 3.3 0.1 SL Platform Shale Fractured Na-HCO3 8
BEC021 À71.951 46.323 11.10 206 8.14 7.0 0.3 Appalachians Schists Fractured Ca-HCO3 18
BEC028 À72.058 46.113 12.90 335 8.11 12.1 0.5 Appalachians e Fractured Na-HCO3 31
BEC029 À71.996 46.170 10.10 203 7.81 2.7 0.1 Appalachians e Fractured Ca-HCO3 7
BEC030 À71.936 46.163 10.50 227 8.87 110.6 4.4 Appalachians e Fractured Na-HCO3 283
BEC031 À71.920 46.205 8.50 198 8.37 50.3 2.0 Appalachians e Fractured Na-HCO3 128
BEC032 À71.857 46.166 9.40 382 6.79 17.9 0.7 Appalachians Slate Fractured Ca-HCO3 46
BEC034 À71.850 46.241 9.50 126 8.02 4.1 0.2 Appalachians e Fractured Ca-HCO3 10
BEC036 À71.789 46.209 12.50 430 7.22 13.8 0.6 Appalachians Schists Fractured Ca-HCO3 35
BEC056 À72.248 46.250 10.60 553 8.23 5.0 0.2 SL Platform Shale Fractured Na-HCO3 13
BEC100 À72.073 46.260 10.40 128 8.97 32.1 1.3 Appalachians e Fractured Na-HCO3 82
BEC101 À72.168 46.276 9.90 571 9.22 4.5 0.2 SL Platform Shale Fractured Na-HCO3 12
BEC102 À72.029 46.489 8.10 283 7.74 42.6 1.7 SL Platform Shale Fractured Ca-HCO3 109
BEC103 À72.320 46.148 11.10 351 7.28 3.3 0.1 SL Platform Slate Fractured Ca-HCO3 8
BEC104 À72.257 46.105 8.97 89 8.60 11.9 0.5 Appalachians e Fractured Ca-HCO3 30
BEC105 À72.001 46.364 10.40 158 6.20 5.7 0.2 SL Platform Slate Granular Ca-HCO3 15
BEC106 À72.227 46.368 9.00 44 6.59 3.3 0.1 SL Platform Shale Granular Ca-HCO3 8
BEC107 À72.247 46.215 11.00 131 8.27 5.2 0.2 SL Platform Shale Fractured Ca-HCO3 13
BEC108 À72.410 46.223 8.50 448 7.91 9.5 0.4 SL Platform Shale Granular Na-HCO3 24
BEC109 À72.151 46.344 9.80 42 5.53 4.0 0.2 SL Platform Shale Granular Na-HCO3 10
BEC110 À72.501 46.185 8.70 236 8.17 10.5 0.4 SL Platform Shale Fractured Ca-HCO3 27
BEC111 À72.217 46.238 8.60 194 7.34 5.0 0.2 SL Platform Shale Granular Ca-HCO3 13
BEC114 À71.663 46.296 8.00 180 7.48 14.4 0.6 Appalachians e Granular Ca-HCO3 37
BEC117 À72.108 46.544 7.50 70 7.00 5.2 0.2 SL Platform Shale Granular Ca-HCO3 13
BEC119 À72.052 46.509 11.40 205 7.69 18.7 0.7 SL Platform Shale Fractured Na-HCO3 48
BEC120 À72.486 46.191 9.70 315 8.36 7.3 0.3 SL Platform Shale Fractured Na-HCO3 19
BEC121 À72.386 46.163 10.30 681 8.46 12.4 0.5 SL Platform Shale Fractured Na-HCO3 32
BEC122 À72.380 46.117 9.50 254 8.06 2.6 0.1 SL Platform Slate Fractured Na-HCO3 7
BEC124 À71.600 46.250 14.80 42 6.53 23.7 0.9 Appalachians Slate Granular Ca-HCO3-Cl 60
BEC126 À71.545 46.304 8.90 153 8.06 3.9 0.2 Appalachians Schists Fractured Ca-HCO3 10
(continued on next page)
D.L. Pinti et al. / Journal of Environmental Radioactivity 136 (2014) 206e217 213
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Table A1 (continued )
Sample name X coordinates Y coordinates TC TDS mg/L pH 222
Rn Bq/L ± Geological Province Lithology Aquifer Chemistry AED mSv/yr
ROC02 À71.754 46.296 2.89 131 9.38 1.2 0.0 Appalachians e Fractured Na-SO4 3
ROC04 À71.846 46.382 8.50 233 9.12 1.1 0.0 Appalachians Slate Fractured Na-HCO3 3
ROC05 À71.914 46.450 8.68 345 8.40 2.3 0.1 SL Platform Shale Fractured Na-HCO3 6
ROC07 À71.973 46.516 1.97 259 9.29 39.9 1.6 SL Platform Shale Fractured Na-HCO3 102
ROC09 À72.468 46.326 8.70 1444 8.14 5.9 0.2 SL Platform Shale Fractured Na-HCO3 15
RS01 À73.093 45.951 9.80 9373 6.70 47.9 1.9 SL Platform Shale Granular Na-Cl-SO4 122
SLV001 À72.426 46.492 7.10 e 5.60 4.3 0.2 SL Platform Shale Granular e 11
TR001 À72.670 46.371 8.70 e 6.21 3.2 0.1 SL Platform Shale Granular e 8
TR002 À72.602 46.424 7.80 e 6.63 3.6 0.1 SL Platform Shale Granular e 9
TR003 À72.611 46.405 7.50 e 7.15 3.9 0.2 SL Platform Shale Granular e 10
TR004 À72.533 46.376 9.60 e 6.47 3.5 0.1 SL Platform Shale Granular e 9
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