Studies on Radon in soil gas and Natural …...Studies on Radon in soil gas and Natural...

INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 1, No 5, 2011

© 2011 Shashikumar T.S. et al., licensee IPA- Open access - Distributed under Creative Commons Attribution License 2.0

Research article ISSN 0976 – 4402

Received on December, 2010 Published on January 2011 786

Studies on Radon in soil gas and Natural radionuclides in soil, rock and

ground water samples around Mysore city. Shashikumar.T.S, Chandrashekara.M.S, and Paramesh.L

Department of Studies in Physics, University of Mysore, Manasagangotri, Mysore, India

[email protected]

doi:10.6088/ijessi.00105020008

ABSTRACT

The 222

Rn in soil gas were measured in 0.5m and 1m depth for different locations around

Mysore city using Solid State Nuclear Track Detectors. The concentration of radon in soil

was observed higher near Chamundi hills and Karighatta village in rainy season. These

higher concentrations may be due to higher 226

Ra concentration in the soil at these places.

The Geometric Median (GM) activity of 226

Ra, 232

Th and 40

K concentration in Mysore city

for soil and rock samples is found to be 20.3, 64.0 and 396.7 Bq kg-1

; 46.4, 68.7 and 634.9

Bq kg-1

. The activity concentration is low compared with the values of world activity

concentration. The highest 226

Ra and 222

Rn activity concentration in bore well water sample is

found to be 189.10 mBq L-1

and 434.60 Bq L-1

in Chamundi Hills.

Keywords: Radon, SSNTDs, Dosimeter, 226

R, 232

Th, 40

K, HPGe, Bubbler.

1. Introduction

Radon is a radioactive gas emitted from the radioactive decay of 226

Ra, the daughter of 238

U

(Surinder singh, 2005). The environmental conditions controlling this exposure are subject to

wide variations (Kendall, 2002). The amount of radon that emitted from the earth depends

mainly on the amounts of 226

Ra and 232

Th in the ground along with other factors, like the type

of the soil cover, porosity, etc (Harmanjit Singh, 2008). The highest contribution to the

radiation field is of natural origin; it is due to cosmic rays, the natural radionuclides in soil,

radioactivity of the ground and the radioactive decay products of radon in the air. Artificial

radioactivity emitted from nuclear power plants, industrial plants and research facilities has

smaller contribution to the overall radiation. These emissions are very low in normal

operation, although high amounts of radioactivity can be released to the environment through

accidents (Rahman et al., 2008).

One of the important factors influencing the dose assessment of human exposure to

radionuclides through the food chain in a contaminated area is the transfer factors (TFs) of

radionuclides from soil to plants (Davis et al., 1999). Such factors vary from one plant to the

other and depend, in general, on the physical and chemical properties of the soil,

environmental conditions and chemical form of the radionuclide in the soil (IAEA, 1994;

Sheppard, 1985 ). Several studies have been carried out to determine TFs for various plants in

different regions (Ciuffo et al., 2002; Velasco et al., 2004; IAEA, 1994; Al-Kharouf et al.,

2008). In general, the naturally occurring radioactivity concentration in plant increases with

that in soil in a linear fashion, especially for high soil activity concentrations (Sheppard,

1985; Blanco Rodrıguez et al., 2006; Linsalata et al., 1989; UNSCEAR, 1988).

Studies on Radon in soil gas and Natural radionuclides in soil, rock and ground water samples around Mysore

city.

787

Groundwater generally contains radionuclides, at concentrations varying over a wide range,

which depends on nature of the aquifer and chemical characteristics of the water, such as

salinity and pH (Seghour, 2009). The decay of radionuclides enters into the body through

ingestion and inhalation pathways causes internal exposure of humans to ionizing radiations.

The elevated levels of natural radionuclides in ground waters are mainly related with uranium

and thorium bearing rock minerals and soil or with uranium, radium and thorium deposits.

Therefore, the occurrence and distribution of radioactivity in water depends on the local

geological characteristics of the source, soil or rock. Other major factors that control the

occurrence and distribution of radionuclides in ground waters are the hydro-geological

conditions and the geochemistry of each radionuclide (Ajayi, 2009). The occurrence of

natural radionuclides in Ground water has been studied with the objective to assess the safety

of drinking water with respect to its radionuclide content.

The present study was carried out in Mysore city, Karnataka, India is shown in Fig (1). It

lies between 12°15″–12°25″N lat. and 76°35″– 76°45″E long., at an altitude of about 767 m

amsl. The study area was about 140 sq. km. A large water reservoir, namely Krishna Raja

Sagar (KRS), situated at the northwestern part and Chamundi Hills (1048 m amsl) located on

the southeastern part of the city are prominent features of the region. Meta-sedimentary rocks

like biotite, schist, mica schist and hornblende schist belonging to Dharwar group are seen in

the form of patches. Younger intrusions like felsite, pegmatite and granite are found in the

study area. These intrusions are known for rich concentrations of radioactive minerals.

Fig (1): Geology Map of Study Area


city.

788

2. Instrumentation

2.1 Measurement of Radon in soil gas

The Solid State Nuclear Track Detectors (SSNTDs), LR-115 Type II detectors have been

used for measurements of radon in the soil. For measurement of radon in the soil, a twin-cup

dosimeter designed and fabricated by Mayya and his group at Bhabha Atomic Research

Centre (BARC), Mumbai has been used here (Mayya et al., 1998) is as shown in the Fig (2)

(Shashikumar et al., 2008). The soil-gas radon concentration, CR, is calculated using the

following relation (Mayya et al., 1998).

DS

TmBqCR )( 3 (1)

Where, T is the track density of the film (tracks cm–2), D is the period of exposure (days) and

S is the sensitivity factor (0.021 tracks cm–2

d–1

per Bq m–3

).

Fig (2): The experimental setup for soil gas measurements

Surface level

SSNTD film

Plastic cup

Filter paper and

Membrane

PVC pipe

Top cover

Insulating

material


city.

789

2.2 Determination of 226

Ra, 232

Th and 40

K in Soil and Rock samples

The Gamma ray spectrometry (HPGe) method was employed to estimate the activity of 226

Ra, 232

Th and 40

K in the soil and rock samples collected from several locations around Mysore

city (Shashikumar et al., 2008). The gamma peak of energy 609.51 keV (emitted by 214

Bi, a

decay product of 222

Rn) with intensity 46.1% was used as proxy for the quantitative

determination of 226

Ra (IAEA, 1989).

AWE

σ)(S )(Bq.kgActivity -

10010001001 (2)

Where, S is the net counts/s under the photo peak of interest, σ the standard deviation of S, E

the counting efficiency (%), A the gamma abundance (%) of the radionuclide and W the

mass of the sample (g).


Ra by Bubbler Method

The water samples of 20 liters was collected and pre-concentrated by chemical method to

estimate the activity of 226

Ra. Pre concentrated sample of about 60 ml was transferred to the

radon bubbler is as shown in Fig (3). A vacuum pump was connected to the bubbler and air

was sucked through the sample solution to scrub it for about 5 min. This would purge the

solution of dissolved 222

Rn. The solution in the blubber was then allowed to stand for a

known period of about 21 days (about 3-5 half lives of 222

Rn) for enough radon to build up.

At the end of this period, an evacuated scintillation cell was connected to the bubbler through

the quick connector. Under the influence of the vacuum in the scintillation cell, air gets

sucked through the solution and fills the scintillation cell. In the process the air carries the

radon dissolved in the solution quantitatively. By careful manipulation of the stopcocks the

bubbling was controlled and adjusted to be uniform and steady to ensure complete transfer of

radon into the cell. The cell was kept for 3 hours or more to allow radon daughters to reach

equilibrium with radon. Then, the alpha activity was counted for a period of 1000 seconds.

The activity of 226

Ra in the sample was determined using the equation (3) (Raghavayya,

1980).

eeeEV

DmBqLRa

Tt 11

1097.6 21226

(3)

Where, D = counts above background, V = Volume of water (20 liters), E = Efficiency of the

scintillation cell (74 %), λ = decay constant for radon (2.098 x 10-6

s-1

), T = Counting delay

after sampling, t = Counting duration (s) and θ = build up time in the bubbler (s).


city.

790


Rn by Bubbler method

The samples were collected from the selected locations around Mysore City from manually

operated bore wells. About 100 mL of water sample was collected in airtight plastic bottles

with minimum disturbance. The plastic bottles were filled completely in a gentle manner, so

that zero headspace was present. Care was taken to see that no air bubbles were present inside

the container and also to avoid aeration during the sampling process, which might lead to out

gassing10

. The samples were brought to the laboratory with minimal loss of time and were

analyzed immediately. The activity concentration of 222

Rn in water was estimated by the

emanometry (Strain and Watson, 1979). In this method, about 40 to 60 mL of the water

sample was transferred into the bubbler by the vacuum transfer technique. The dissolved

radon in the water was transferred into a pre-evacuated and background counted scintillation

cell. The scintillation cell was stored for 180 minutes to allow radon to attain equilibrium

with its daughters and then it was coupled to a photomultiplier and alpha counting assembly.

The concentration was calculated using the relation given by Raghavayya (Raghavayya,

1980).

tT eeeEV

DBqLRn

1

1097.6 21222

(4)

Fig (3): Radon Bubbler (Emanometry method)


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791

Fig (4): Spectrum of soil sample collected from Chamundi Hills.

2.5 Dose due to Ingestion and Inhalation of 222

Rn

Using the measured concentration of 222

Rn in Bore well water, the effective dose for the

population of the region was estimated. The dose due to 222

Rn can be divided into two parts,

namely, the dose from ingestion and the dose from inhalation. The effective dose to the

Ingestion mainly depends upon the amount of water consumed by a human being in a day

(ICRP, 1991). The parameters for the Inhalation pathway were 222

Rn concentration in water,

air water concentration ratio of 10-4

, indoor occupancy of 7,000 hours per year and inhalation

dose coefficient applied is that for the gas. The ingestion of tap water was estimated to be 100,

75, and 50 l a-1

by infants, children, and adults. Assuming the proportion of these groups in

the population to be 0.05, 0.3, and 0.65, the weighted estimate of consumption is 60

(liter/year) (UNSCEAR, 1993). The dose due to inhalation and ingestion are calculated by the

equation (5 & 6) (UNSCEAR, 2000).

Inhalation (mSv):

222

Rn conc. (Bq L-1

) × 10-4

× 7,000 h × 0.4 × 9 nSv (Bq h m-3

)-1

(5)

Ingestion (mSv): 222

Rn conc. (Bq L-1

) × 60 L.y-1

× 10-3

m3 L

-1 × 3.5 nSv Bq

-1 (6)

2.6 Effective dose

Radon in tap water/bore well water may lead to exposures from the ingestion of drinking

water and from the inhalation of radon released to air when water is used. The contributions

to effective dose from two relatively minor pathways of exposure to radon can be added,

namely dissolution of the gases in blood with distribution throughout the body and the

presence of radon in tap water (UNSCEAR, 2000). The effective dose due to radon intake

was assessed using the measured activity of 222

Rn and dose per unit activity of radon ingested.

The mean daily intake of water is assumed minimum 1 liter per person.


city.

792

Fig (5): Spectrum of soil sample collected from Yelwala.

Fig (6): Spectrum of rock sample collected from Chamundi Hills.

MGM BRK CH KG K.R.S YL

0

100

200

300

400

500

0

100

200

300

400

500

222

Rn in Water samples

226

Ra activity in Water samples

226 R

a ac

tivit

y c

onc.

in w

ater

sam

ple

s (m

Bq L

-1)

222R

n c

onc. in

wate

r sam

ple

s (

Bq L

-1)

Locations

Fig (7): 226

Ra and 222

Rn activity in Bore well water.


city.

793

10 20 30 40 50 60 70 80

0

1

2

3

4

5

6

22

2 Rn i

n s

oil

gas

(K

Bq m

-3)

226Ra in soil samples (Bq Kg

-1)

Fig (8): Correlation between 222

Rn in soil gas and 226

Ra in soil samples.

3. Results and Discussion

The 222

Rn in soil gas were measured in 0.5m and 1m depth for different locations around

Mysore city using Solid State Nuclear Track Detectors are shown in Table(1). The seasonal

variations of radon in soil gas are found to be higher at a depth of 1m compare to at a depth

of 0.5m from the ground surface. The higher concentration of radon in soil was observed near

Chamundi hills and Karighatta village in rainy season with an average value of 11.85 kBq.m–

3 and 8.96 kBq.m

–3 at 1m depth from the ground surface. The higher values in Chamundi

Hills and Karigatta village may be due to higher 226

Ra concentration in the soil at these places.

The activity concentration of 226

Ra, 232

Th and 40

K radionuclides in soil and rock samples; its

absorbed dose rate in air are shown in Table (1) and (2). The activity concentration of 226

Ra, 232

Th and 40

K radionuclides in soil samples is found to be higher in Chamundi Hills and

Yelwala locations due to the presences of granites, which might contain a small amount of

uranium and its daughter products. The activity concentration of 226

Ra, 232

Th and 40

K

radionuclides are found to be 70.3, 156.2 and 1138.1 Bq kg-1

in Chamundi Hills and 74.2,

134.5 and 401.6 Bq kg-1

in Yelwala. The spectrum of soil samples is as shown in Fig (4) and

(5). The GM value of absorbed dose rate in air referred from concentrations of radionuclides

in soil is found to be 63.35 nGyh–1

and the GM value of measured gamma exposure level is

found to be 96.39 nGyh–1.

The median values of world activity concentrations for soil samples vary from 17 to 60 Bq

kg-1

with an average value of 35 Bq kg-1

for 226

Ra, from 11 to 64 Bq kg-1

with an average

value of 30 Bq kg-1

for 232

Th and 140 to 850 Bq kg-1

with an average value of 400 Bq kg-1

for 40

K radionuclides. The activity concentration in India varies from 7 to 81 Bq kg-1

for 226

Ra,

14 to 160 Bq kg-1

for 232

Th and 38 to 760 Bq kg-1

with average values of 29, 64 and 400 Bq

kg-1

(UNSCEAR 2000). The GM value of 226

Ra, 232

Th and 40

K activity concentration in

Mysore city for soil samples is found to be 20.3, 64.0 and 396.7 Bq kg-1

and the

concentration is low compared with the values of world activity concentration.


city.

794

Table (1): Concentration of Radon in Soil gas for different seasons and Activity of 226

Ra, 232

Th and 40

K radionuclides in soil samples and its absorbed dose rate in air with

pH and EC values around Mysore City.

The GM activity concentrations of 226

Ra, 232

Th and 40

K (Bq kg-1

) in rock samples, around

Mysore is found to be 46.4, 68.7 and 634.9 Bq kg-1

. The highest 226

Ra, 232

Th and 40

K in rock

samples was found in Chamundi Hills which is found to be 110.9, 146.0 and 820 Bq kg-1

and

the spectrum is shown in Fig (6). The rocks in this region are granite than the other parts of

the city and usually granite has more of 226

Ra. 40

K and

232Th are higher than

226Ra because

Mysore region is surrounded by pegmatite and granite rocks. The pegmatite rocks are richer

in 232

Th. The GM value of the gamma dose rates in air from rock samples is found to be

113.62 nGy h-1

. The Geometric Mean value of dose rate in air due to 226

Ra, 232

Th and 40

K (Bq

kg-1

) in rock samples is found to 78.10 nGy h-1

. Absorbed dose rate in air referred from

concentrations of radionuclides in soil and rock samples shows good correlation with the

direct gamma measurements with Correlation coefficient 0.88 & 0.77.

The activity concentration of 226

Ra and 222

Rn in water samples are shown in Table (2). The

highest 226

Ra activity concentration in bore well water sample is found to be 189.10 mBq L-1

in Chamundi Hills and lowest 226

Ra activity concentration in Karighatta which is found to be

0.28 mBq L-1

. The highest 222

Rn activity concentration of bore well water sample is found to

be 434.60 Bq L-1

in Chamundi Hills and lowest 222

Rn activity concentration in Karighatta

which is found to be 4.25 Bq L-1

. The plot of 226

Ra and 222

Rn in water samples is as shown in

Fig (7). The GM of Inhalation and Ingestion dose due to 222

Rn is found to be 0.024µSv y−1

and 1.82 µSv y−1

. The GM of effective dose due to Ingestion and Inhalation is found to be

1.84 μSv y−1

respectively. The 226

Ra concentrations in ground water samples from Mysore

city are lower compared to those in other parts of the world. Radon concentration in water

samples shows good correlation with 226

Ra activity in water with correlation coefficient of

0.99 and radon concentration in soil–gas shows good correlation with 226

Ra activity in soil

with correlation coefficient of 0.65 is as shown in Fig (8).

Loca-

tions

222Rn in soil-gas (k Bq m

-3)

Activity of Soil samples

(Bq kg-1

)

Absorbed

dose rate

in air

(nGy h-1

)

pH values

(Soil

Samples)

EC (Soil

Samples)

(ms)

Winter Summer Rainy Autumn

0.5m 1m 0.5m 1m 0.5m 1m 0.5m 1m 226

Ra 232

Th 40

K

MGM 3.33 4.27 0.24 0.40 3.20 3.78 2.11 3.35 20.3 60.4 343.0 89.95 7.67 29

BRK 3.96 4.42 0.23 0.46 3.02 4.18 0.81 2.07 11.9 50.2 492.3 94.13 8.39 244

NH 1.14 0.90 0.11 0.30 0.85 1.04 0.44 0.35 13.0 36.3 359.8 98.65 8.29 391

VN 2.53 3.11 0.35 0.32 1.34 2.54 1.38 2.31 48.2 68.7 391.9 85.43 7.77 210

C H 3.28 4.50 0.72 1.26 9.40 11.85 1.86 3.81 70.3 156.2 1138.1 159.55 7.68 68

KG 1.98 2.94 0.32 0.97 4.26 8.96 1.24 6.36 51.1 67.6 51.1 88.39 8.18 88

YL 2.22 2.48 0.48 1.57 3.63 5.44 1.39 2.06 74 .2 134.5 401.6 111.18 7.11 57

TH 0.99 1.54 0.54 0.29 2.06 2.37 0.91 0.91 11.3 32.0 1225.6 109.62 7.56 78

Median 2.38 3.03 0.34 0.43 3.11 3.98 1.31 2.19 20.3 64.0 396.7 96.39 7.72 83

SD 1.06 1.35 0.20 0.50 2.66 3.66 0.55 1.88 26.6 45.3 410.6 24.13 0.42 125


city.

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Table (2): Activity of 226

Ra, 232

Th and 40

K radionuclides in Rock samples, 226

Ra and 222

Rn in

Water samples and dose rate due to 222

Rn around Mysore City.

The measure of pH value shows the Acidity or alkalinity in the soil samples. Electrical

Conductivity (EC) measurements can vary greatly and are affected by several environmental

factors including, climate, local biota, bedrock and surficial geology, as well as human

impacts on the land. Higher the dissolved material substance in a water or soil sample, the

higher the EC will be in that material. The pH value of soil samples in present study shows

the best pH value range for soil that is 7 in KRS and Yelwala, as this is the range in which

most nutrients can be readily available and soil is good for plantation. The pH value above

7.4 in Manasagangotri, ChamundiHills, Vijayanagar, Baburayanakoppalu, Naguvanahalli

Thuruganur and Karighatta soil samples are alkaline soil and contain relatively more lime.

Alkaline soil suits lime–loving (calcicole) plants such as many alpines and many vegetables,

especially brassicas (cabbage family).

4. Conclusion

From radiation protection point of view, an attempt has been made to estimate health hazard

index due to radiation exposure in the study area. The activity of 226

Ra in soil, radon in soil-

gas, radon concentrations in the atmosphere were studied around Mysore city. The radon in

soil-gas is found to be higher at a depth of 1m compare to at a depth of 0.5m from the ground

surface. The higher radon concentrations in Chamundi Hills and Karigatta village may be due

to higher 226

Ra concentration in the soil at these places. The activity concentrations of 226

Ra, 232

Th and 40

K in soil and rock samples have been measured by gamma-ray spectrometry. The

average activity concentrations of 226

Ra, 232

Th and 40

K in soil samples is found to be slightly

low when compared with the values of world activity concentration. The 226

Ra concentrations

in Ground water samples from Mysore city are lower compared to those in other parts of the

world. Radon concentration in water samples shows good correlation with 226

Ra activity in

water with correlation coefficient of 0.99 and radon concentration in soil–gas shows good

correlation with 226

Ra activity in soil with correlation coefficient of 0.65.

Locations

Activity of Rock samples

(Bq kg-1

) Water Samples

Dose due to 222

Rn in

water samples

(µSv y−1

)

226Ra

232Th

40K

226Ra

(mBq L-1

)

222Rn

( Bq L-

1)

Inhalation Ingestion Effective

MGM -- -- -- 1.73 4.95 0.004 0.36 0.36

BRK -- -- -- 1.52 18.19 0.016 1.09 1.10

C H 110.9 146.0 820.4 189.10 434.60 0.399 33.21 33.60

KG 13.9 46.8 950.9 0.28 4.25 0.003 0.29 0.29

KRS 47.2 22.1 449.5 18.06 48.44 0.044 3.65 3.69

YL 45.7 90.7 422.8 4.62 37.18 0.033 2.55 2.58

Median 46.4 68.7 634.9 3.17 27.68 0.024 1.82 1.84

SD 40.6 54.3 265.1 75.34 169.11 0.155 12.97 13.12

http://en.mimi.hu/gardening/calcicole.html

http://en.mimi.hu/gardening/plant.html

http://en.mimi.hu/gardening/alpines.html

http://en.mimi.hu/gardening/vegetable.html

http://en.mimi.hu/gardening/brassica.html


city.

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