222Rn and 220Rn levels of Mansa and Muktsar district of Punjab, India

Introduction

²²²Rn is a radioactive inert gas that is a decay product of radium in the naturally occurring uranium series. The measurement of ²²²Rn in the environment is of great interest due to its alpha emitting nature. A certain fraction of the ²²²Rn escapes into the air, where in the outdoors; it is quickly diluted and is of no further concern. However, in confined spaces such as homes and office buildings, ²²²Rn can accumulate to harmful levels. The main source of indoor ²²²Rn and ²²⁰Rn levels are the building material, soil and tap water. ²²²Rn monitoring in soil involves either measuring ²²²Rn in soil or measuring the ²²²Rn flux from a soil, but the former measurement is more easy and quick by using the active technique of ²²²Rn monitoring (Ruckerbauer and Winkler, 2001). ²²²Rn is responsible for about half of the radiation dose received by the general population (UNSCEAR, 1994). The inhalation of ²²²Rn and its progeny contributes more than 50% of the total dose from natural sources (UNSCEAR, 2000). A high value of the ²²²Rn concentration in the particular geological area can be health hazard and will be the cause of lung Cancer for the residents of that area (Sevc et al., 1976; Khan, 2000). The ²²²Rn from water contributes to the total inhalation risk associated with ²²²Rn in indoor air. The high values of ²²²Rn concentration in drinking water also lead to significant risk of stomach and gastrointestinal Cancer (Zhuo et al., 2001; Kendal and Smith, 2002).

As reported in earlier studies the concentration of Th, U, Pb, Cr, Ni, F and SO₄ are higher than the permissible limits in soil of South western Punjab (Kochhar et al., 2006; Mehra, 2009), so a study has been carried out in the Mansa and Muktsar area to make assessment of ²²²Rn exposure for screening purpose and for investigating the geographical variation of the ²²²Rn concentration as well as for the health-related hazards of the locality if any. The main objective of this work is to assess the indoor ²²²Rn and ²²⁰Rn Concentrations, soil gas concentration, ²²²Rn concentration in water and the average annual effective dose to the population.

Geology

The scattered outcrops of the Aravali-Delhi Subgroup occur at Tosham (Haryana) just south of the study area i.e., Mansa and Muktsar districts as shown in Figure 1. The soil in the study area falls in the arid and moisture regime. The soils associated with alluvial planes shows better indurations and mature development of soil profile. They are composed of different layers of clay, sticky clay and fine to coarse grained micaceous sandstone (Kochhar et al., 2006).

FIGURE 1

Figure 1. Map of studied area in Punjab.

Experimental Setup

An active technique RAD-7 in different modes has been used to measure the ²²²Rn and ²²⁰Rn concentration in air, water and soil of 35 locations in vicinity of Mansa and Muktsar districts of Punjab, India.

²²²Rn and ²²⁰Rn Measurement in Air

The measurement of ²²²Rn and ²²⁰Rn concentration in indoor air has been taken using RAD7 air accessories for continues 48 h protocol. EPA test protocols have been used for operating the RAD7 in indoor air (USEPA, 1993). The doors and windows of the houses were closed for at least 12 h before the measurement of ²²²Rn and ²²⁰Rn in air. The detector has an ability to distinguish alpha particles from ²¹⁸Po and ²¹⁴Po with energies of 6.0 and 7.9 MeV. RAD7 can measure the ²²²Rn concentration > 0.4 Bq m⁻³ and < 750,000 Bq m⁻³.

²²²Rn Measurement in Water

The sampling locations of studied area have been chosen with great care and an attempt has been made to cover most of the area of study region. The in situ measurement of water samples was made using RAD- H₂O accessories. The RAD- H₂O uses a standard pre-calibrated degassing system and preset protocols, built into the RAD-7 which gives direct reading of ²²²Rn concentration in water. A 250 ml vial has been used to collect the samples of water from various locations. A Wat– 250 protocol and grab mode with 5 min cycle and four recycles have been used to run the instrument for the estimation of ²²²Rn in Water. The water samples have been collected from each village by varying the depth of ground water, surface water and tap water.

Soil Gas Measurement

For analysis of ²²²Rn concentration in soil gas the measurements were done in same 35 locations and four measurements were done at each location. The pilot rod has been hammered into the ground to the depth required for sampling. Once the rod has been successfully driven to the required depth, the pilot rod has been removed for the penetration of drive rod along with probe more easily to sampling site. The drive rod has been positioned inside the probe and continuously hammered the rods together into the sampling site. When drive rod along with probe has come to be required depth then removed the drive rod from the sampling site and connects the probe with RAD7 soil accessories for sucking the soil gas from deep soil. The soil gas sucked through the tube pipe into the measuring instrument for 5 min. The gab mode and sniff protocol were used for the soil gas sampling on the measuring instrument at each site. The measurement of ²²²Rn and ²²⁰Rn concentration in indoor air has been taken using RAD7 air accessories for continues 48 h protocol. EPA test protocols have been used for operating the RAD7 in indoor air (USEPA, 1993).

Theoretical Formalism

The ingestion, inhalation and annual effective dose have been calculated by Equations (1) and (2) (UNSCEAR, 2000).

$\begin{array}{cc}{E}_{WIg}(mSv{a}^{-1})={\text{C}}_{Rn\text{W}}\times {\text{C}}_{\text{w}}\times (\text{EDC})& (1)\end{array}$

Where E_WIg is the effective dose for ingestion, C_RnW is ²²²Rn concentration in water (kBq m⁻³) and C_w is weighted estimate of water consumption (60 la⁻¹), respectively and EDC is the effective dose coefficient for ingestion 3.5 nSv Bq⁻¹.

$\begin{array}{cc}{E}_{WIh}({\text{mSva}}^{-1})={\text{C}}_{Rn\text{W}}\times {\text{R}}_{\text{aW}}\times \text{ F}(\text{Rn})\times \text{ O }\times (\text{DCFR})& (2)\end{array}$

where E_WIh is the effective dose for inhalation, R_aW is the ratio of ²²²Rn in air to ²²²Rn in tap water (10⁻⁴), F(Rn) is the equilibrium factor (0.4) between ²²²Rn and its decay products, O is the average indoor occupancy time per person (7000 h y⁻¹) and DCFR is the dose conversion factor for ²²²Rn exposure 9 nSv h⁻¹ (Bq m⁻³)⁻¹. The average annual effective dose for indoor ²²²Rn is calculated by Equations (3) and (4) using parameters introduced in report by UNSCEAR (2008).

$\begin{array}{cc}AEDR({\text{mSva}}^{-1})={}^{222}{\text{Rn}}_{\text{air}}\times \text{ F}(Rn)\times \text{ O }\times (\text{DCFR})& (3)\end{array}$

Where AEDR (mSva⁻¹) is annual the effective dose for ²²²Rn, ²²²Rn_air is the indoor ²²²Rn concentration (Bq m⁻³) and remaining factors have been explained above.

$\begin{array}{cc}AEDT({\text{mSva}}^{-1})={}^{220}R{n}_{\text{air}}\times \text{ F}(\text{Th})\times \text{ O }\times (\text{DCFT})& (4)\end{array}$

Where AEDT(mSva⁻¹) is the annual effective dose for ²²⁰Rn, ²²⁰Rn_air is the indoor ²²⁰Rn concentration (Bq m⁻³), DCFR is the dose conversion factor for ²²²Rn exposure 40 nSv h⁻¹(Bq m⁻³)⁻¹ and F(Th) is the equilibrium factor (0.1) between ²²⁰Rn and its decay products UNSCEAR (2008).

The daughter concentration of ²²²Rn and ²²⁰Rn in terms of Potential Alpha Energy Concentration (PAEC) in mWL has been calculated using Equation (5).

$\begin{array}{cc}R{n}_{air}or{}^{220}R{n}_{air}=\frac{PAEC(WL)\times 3700}{F(Rn)orF(Th)}& (5)\end{array}$

Results and Discussion

Table 1 summarizes the results of measurement of ²²²Rn concentration in collected water samples of 35 villages. The ²²²Rn concentration in water samples of studied area varies from 0.4 ± 0.2 Bq l⁻¹ to 16.7 ± 2.8 Bq l⁻¹ with an average value of 4.7 Bq l⁻¹, which is lower than the recommended value of USEPA (1991). The USEPA recommended the maximum contamination level (MCL) for ²²²Rn concentration in water as 11 Bql⁻¹ (USEPA, 1991). The measured values of ²²²Rn in water of studied area are well within the safe limit of 4–40 Bq l⁻¹ (UNSCEAR, 2008).

TABLE 1

Table 1. Average ²²²Rn concentration data and annual effective dose of different water sources in Mansa and Muktsar Districts of Punjab.

The ²²²Rn concentration in water is lower as compared to higher value of uranium content in the nearby region of the studied area (Kochhar et al., 2006). It may be because of the solubility of ²²²Rn in water is relatively low and with its short radio-active half life much of it will decay before it has opportunity of release from the ground water. When ²²²Rn-containing groundwater reaches the surface by natural or man-made forces, the ²²²Rn will inevitably be out gassed into the atmosphere. In some villages, the samples of ground water have been collected from different depth of ground water and tap water. It has been observed that ²²²Rn concentration in water has increased with depth of groundwater (Figure 2). The level of ²²²Rn concentration in tab water has been found to be lower than the level of ²²²Rn concentration in ground water. Tab water is actually stored ground water and ²²²Rn gas escapes out when it stored. This may be due to the surface water typically containing very low concentrations of ²²²Rn, with activities below 4 Bq l⁻¹ (Hopke et al., 2000). However, concentrations of ²²²Rn in ground water may be high, depending on the aquifer or hydrogeology of the region. In most of the cases the ground water concentration is higher than the tap water. The increase in the value of ²²²Rn concentration in water with depth has been reported by many researchers as given in Table 2.

FIGURE 2

Figure 2. Variation of radon concentration in water with depth.

TABLE 2

Table 2. The comparison of mean ²²²Rn concentrations in indoor air, water, and soil gas samples from different countries.

The ²²²Rn in the drinking water is the main source of the radiation doses for stomach (Ingestion dose) and lungs (Inhalation dose). The ingestion and inhalation doses in the studied area vary from 0.1 μSva⁻¹ to 3.6 μSv a⁻¹ and 1.0 μSva⁻¹ to 42.8 μSv a⁻¹ and are less the worldwide average annual of Inhalation (1.26 mSv a⁻¹) and Ingestion (0.29 mSv a⁻¹) recommended by UNSCEAR (2008). The total average annual effective dose for ²²²Rn in water is 13 μSva⁻¹. The annual effective dose of 0.1 mSv a⁻¹ is the safe limit of drinking water from three radioisotopes viz. ²²²Rn, ³H, and ⁴⁰K recommended by European Commission and world health organization (European Commission, 1998; WHO, 2004).

It can be seen from Table 3 that the average Value of indoor ²²²Rn and²²⁰Rn concentration in the vicinity of these districts are 47 ± 21 Bq m⁻³ and 39 ± 19 Bq m⁻³, respectively. These values are within the range of intervention level (200–300 Bq m⁻³) recommended by International commission on Radiological Protection (ICRP, 2009). The present average indoor ²²²Rn values are lower in comparison to the values reported in USA, China, Brazil, Kenya and Uttrakhand region of India but greater than Muree region of Pakistan as reported in Table 2.

TABLE 3

Table 3. Statistical Parameters of ²²²Rn and ²²⁰Rn concentration in indoor air samples, ²²²Rn concentration in soil samples.

The ratio of PAEC of ²²⁰Rn to that of ²²²Rn is in the range of 0.06–0.46. Stranden and Dixon have reported measurements on a variety of underground mines and enclosures in Norway and UK (Dixon et al., 1985; Stranden, 1985). The estimated ratios of PAEC (²²⁰Rn)/PAEC (²²²Rn) are usually within the range of 0.1–1.0. The AEDR (mSv a⁻¹) and AEDT (mSv a⁻¹) in the studied area are varied from 0.4 mSv a⁻¹ to 2.4 mSv a⁻¹ and 0.3 mSv a⁻¹ to 2.3 mSv a⁻¹, respectively. In all the villages studied, the average AEDR (mSv a⁻¹) was 1.24 mSva⁻¹ which is slightly less than the worldwide average annual dose (1.26 mSv a⁻¹) recommended by UNSCEAR (2008). The ²²²Rn concentration in soil of studied area has been varied from 3.2 kBq m⁻³ to 17.2 kBq m⁻³. Figure 3 shows the correlation coefficient for measured ²²²Rn concentration in air (Rn_air) and soil (Rn_soil). It has been found that a positive correlation (R² = 0.57) exits between ²²²Rn concentration in air and soil gas. The value of ²²²Rn concentration in soil is lower than the values reported in USA, Islamabad region of Pakistan and almost equivalent to china as given in Table 2. Using student's t–distribution with 95% confidence limit as shown in Equation (6).

$\begin{array}{cc}\text{m}\pm \frac{t_{0.975}(\sigma )}{\sqrt{n-1}}& (6)\end{array}$

FIGURE 3

Figure 3. Correlation of radon concentration in air and soil.

Where σ is the standard deviation, m is arithmetic mean and (n - 1) is degree of freedom. The calculated 95% confidence limits for ²²²Rn and ²²⁰Rn in indoor air is 33.3–4.2 Bq m⁻³ and 48.9–54.9 Bq m⁻³, respectively. The calculated 95% confidence limits for ²²²Rn in soil and water are 6.8–8.7 kBq m⁻³ and 3.8–5.6 Bq l⁻¹, respectively.

Conclusion

It has been seen that ²²²Rn concentration in water samples are well below the recommended value. The total average annual effective dose for ²²²Rn in water is lower than reference level (0.1 mSv a⁻¹) recommended by EC and WHO organization (European Commission, 1998; WHO, 2004). It is observed that there is a positive correlation between ²²²Rn concentration in air and soil, so the soil of the study area has significant contribution to the indoor ²²²Rn concentration. The average Indoor values of ²²²Rn and ²²⁰Rn concentration in the studied areas are greater than the world average value of 40 Bq m⁻³ (UNSCEAR, 2000), but the values are in general within the range of recommended action level 200–300 Bq m⁻³ (ICRP, 2009). The estimated ratios of PAEC (²²⁰Rn)/PAEC (²²²Rn) are usually within the range of 0.1–1.0. The average annual effective dose in indoor air received by the residents of the study area is lower than the recommended action level of 1.26 mSv a⁻¹ (UNSCEAR, 2008).

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The authors are grateful to the Director of NIT, Jalandhar for their necessary support and facility in carrying out the experimental work and Special thanks to BRNS for their financial support under project 2011/36/28 BRNS.

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