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This study intends to shed light on the radiological quality of potable water in one of the most important hill stations, Darjeeling, and its surrounding foothill areas of West Bengal, India, which has not yet been explored. A radon concentration measuring study has been carried out in 62 water samples collected from both natural springs (SW) and tube wells (TW) in the above mentioned area using an AlphaGUARD radon monitor. The measured maximum and minimum radon levels of drinking water samples collected from natural springs are 13.04 Bq/l and 0.43 Bq/l respectively while those for tube wells are 71.02 Bq/l and 1.02 Bq/l respectively. The average radon activities in the two water sources separately and together are 2.88 ± 0.41 and 25.67 ± 3.89 and 10.96 ± 1.97 Bq/l respectively. This study reveals that the average radon concentrations of water samples from both sources in this area are much below the reference level of 100 Bq/l as prescribed by the WHO and EU Commission. The evaluated radon activities are used to determine inhalation, ingestion and then total annual effective dose (AED). The average total AEDs for the two types of water sources separately and together (SW + TW) are 7.86 ± 1.11, 70.07 ± 10.64 and 29.93 ± 5.37 μSv/y respectively. The evaluated average total AEDs for the two types of drinking water sources separately and together are also much below the reference limit of 100 μSv/y prescribed by the above mentioned two agencies. It suggests that the drinking water of this region is radiologically safe so far as water-borne radon hazards are concerned.

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  • There has been no such measurement in the study area before.

  • Radon level measurement study was done for both natural springs and tube wells drinking water.

  • The study area is in a famous tourism spot and hence of national and international importance.

  • Annual effective dose due to radon exposure is evaluated.

  • Help protect the local people from radiological hazards due to water dissolved radon.

222Rn, AlphaGUARD, annual effective dose, Darjeeling, drinking water
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Radiological quality of potable drinking water is currently of great concern to numerous agencies worldwide as its consumption can transport different radioactive pollutants to the human body causing fatal ailments like cancer. Many times it is reported that ground water can contain different natural radioactive sources such as uranium (238U), thorium (232Th), radium (226R), and radon (222Rn) (Fonollosa et al. 2016; Kasić et al. 2016; Ahmad et al. 2018). The above mentioned point and non-point radioactive sources can be highly detrimental to different organisms directly or indirectly connected to human life (UNSCEAR 1982; Singh et al. 2011; Naskar et al. 2017, 2022; Opoku-Ntim et al. 2019). Among these radon-222 (222Rn), which is earth's only naturally produced radioactive gas, is mostly present in ground water (Deb et al. 2017; Naskar et al. 2017, 2018; Ahmad et al. 2018; Opoku-Ntim et al. 2019). It is continuously generated from radium-226 (226Ra), one of the decay products of natural uranium-238 (238U). It is omnipresent in soil and rocks in the earth's crust and hence may contaminate underground water aquifers (WHO 2009, 2011) as it is readily water soluble. There are two possible ways in which the potential radiological health risks are caused by water-borne radon: firstly through direct ingestion of enriched dissolved radon in potable water (Ramola et al. 1997; Kim et al. 2016; Deb et al. 2017; Naskar et al. 2018, 2022; Opoku-Ntim et al. 2019) and secondly through inhalation of the emanated radon from water (Ramola et al. 1997; Kim et al. 2016; Deb et al. 2017; Naskar et al. 2018; Opoku-Ntim et al. 2019). Radon gas produces energetic alpha particles during its natural decay to its progenies (Kendall & Smith 2002). These energetic alpha particles (∼ 5 MeV) deposit their energies to the nearby tissues or organs over a very short distance leading to DNA damage (Kendall & Smith 2002; Somlai et al. 2007; WHO 2009; Ahmad et al. 2018). Because of its colorless, odorless and tasteless properties people used to take it unknowingly both through ingestion and inhalation and as a consequence may suffer from different radiological health risks. To reduce the hazardous effects of radon on human health it is important to monitor radon level in drinking water so as to take necessary measures if the level is found beyond the reference level as prescribed by different international or local bodies. Darjeeling hill station in West Bengal, India is an internationally famous tourism centre. The absence of an effective drinking water supply system and a growing population due to urbanization and a seasonal population due to tourism are the main causes of water crisis in this study area. Water from different natural springs is used for drinking as well as for fulfilling different basic needs. It is not surprising to detect radon and its decay products in the natural spring water as it might come in contact with different types of rocks containing varying amounts of uranium or radium. In India, R. C. Ramola et al. and V. M. Choubey et al. performed radon level measurement in natural spring water from hilly regions of Garhwal Himalaya and the north-central part of the Kumaun Lesser Himalaya in Northern India respectively. The reported maximum radon activities were 880 Bq/l and 887 Bq/l respectively.

Globally, many studies have been conducted in different countries in recent years to estimate the radon concentration in natural spring water samples (Ramola et al. 1997; Choubey et al. 2000; Abdallah et al. 2007; Ródenas et al. 2008; Kozłowska et al. 2009; Yousuf et al. 2009; Fonollosa et al. 2016; Kasić et al. 2016; Erdogan et al. 2017; Doğan et al. 2018; Khan et al. 2019). However, no such study for assessing the radiological quality of drinking water has been performed so far in Darjeeling area. Radon level measurement study for both natural springs (SW) and tube wells (TW) drinking water in Darjeeling and its foot hill regions has been carried out using AlphaGUARD, a globally accepted radon monitor. In this investigation water samples are collected once only from each sampling site. The main aim of this study is to measure radon concentration of the maximum possible number of potable drinking water samples covering the largest possible area in the study area. This study will generate base line data of radon in drinking water and thus will protect the local people from radiological hazards from water.

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Study area

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Darjeeling is the northernmost district of the state of West Bengal, India at an elevation of 2,045 m. Geographically Darjeeling is squeezed between the Nepal to the west and Bhutan to the east. The overlying deep-seated unaltered sedimentary rocks and the regional domain of gneisses was observed and traced by Dr Hooker (Hooker 1850). The Sub-Himalaya is composed of Siwalik formations of the Tertiary age and a large amount of Siwalik deposition can be seen along the Teesta River. North of the Siwaliks is the coal-bearing lower Gondwana formations. The present form of the Darjeeling foothill areas was arranged after the final upheaval of the Himalayan orogeny and consists of almost horizontal layers of undeveloped sand, silt, pebbles and gravels (Gansser 1964). Water samples are collected from 62 sites in and around Darjeeling and its foothill areas. The study area and sample collection sites are indicated in Figure 1(a) and 1(b) respectively.
Figure 1
(a) Map showing our study area. (b) Water sample collection sites.
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(a) Map showing our study area. (b) Water sample collection sites.

Figure 1
(a) Map showing our study area. (b) Water sample collection sites.
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(a) Map showing our study area. (b) Water sample collection sites.

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Sample collection strategy

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The peoples of the Darjeeling foothill areas mostly collect their drinking water from local tube wells. Whereas in the absence of tube wells the peoples of the Darjeeling hill areas use natural spring water as alternative source of water for drinking as well as for other domestic uses. So for the evaluation of radon level in drinking water of these two study areas, strategies have been set to collect the maximum possible number of water samples both from natural springs and tube wells at various places of Darjeeling hill areas and foothill areas respectively maintaining a gap of about 5 kilometres between two water sampling points for each type of water source. A total of 62 drinking water samples were collected, among which 22 samples were from tube wells and remaining 40 water samples were from natural springs. In this investigation water samples are collected once only from each sampling site so that the maximum possible number of water samples can be collected.

Quality control assurances during sampling

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While collecting tube well water samples each tube well was pumped manually for 5 minutes so that the partially degassed water gets pumped out and then fresh ground water samples were collected in pre-levelled clean plastic bottles (250 ml) for the study (Naskar et al. 2018, 2022). Care had been taken to collect the water without agitation to fill the containers up to the brim and seal them tightly, without leaving any space between the liquid surface and the lid to ensure that no radon could come out of the water. pH of each sample was measured in-situ by holding one pH meter within another container filled with the same water. The latitude (Lat.) and longitude (Long.) of each sample collection site were determined with a GPS meter. For minimum degassing the collected water samples were carried immediately to a temporary laboratory setup near the sample collection sites.

Measurement method

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222Rn activities of the samples were measured within 4–5 hours of collection using an instant radon monitor AlphaGUARD PQ 2000 PRO, primarily an ionization chamber, manufactured by Genitron Instruments, Germany, together with its accessories AlphaPUMP and AquaKIT. Its measurement range is 2–2 × 10 6 Bq/m3 with instrumental calibration error of 3% (Genitron 1997). The experimental set up is depicted in Figure 2.
Figure 2
Schematic diagram of experimental set up for 222Rn concentration measurements of the water samples.
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Schematic diagram of experimental set up for 222Rn concentration measurements of the water samples.

Figure 2
Schematic diagram of experimental set up for 222Rn concentration measurements of the water samples.
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Schematic diagram of experimental set up for 222Rn concentration measurements of the water samples.

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The instruments were connected by tubes to operate in a closed gas cycle, in which radon was expelled from the water sample placed in the degassing vessel by using the pump. AlphaGUARD was operated in 1 minute flow mode and the pumping rate of AlphaPUMP was adjusted at 0.3 L/min. The pump was kept on for 10 minutes, during which, after every one minute, radon concentration in air was recorded using Data Expert software provided with the instrument. Then the pump was switched off, but AlphaGUARD remained on for another 20 minutes so that radon concentration measurement was continued. The value of radon concentration given by AlphaGUARD is not the correct radon concentration in the sample because the radon gas expelled from the sample was diluted by air within the closed gas cycle of the set-up, and some radon remained dissolved in the sample. The actual radon concentration is obtained by using the following relation as mentioned in the AlphaGUARD manual (Genitron 1997).
(1)
where,

  • Cwater = 222Rn concentration in water sample (Bq/l)

  • Cair = 222Rn concentration (Bq/m3) in the measuring set-up (indicated by AlphaGUARD)

  • C0 = 222Rn concentration in the measuring set-up before sample injection (Bq/m3)

  • Vsystem = interior volume of the measurement set-up (ml)

  • Vsample = volume of water sample (ml)

  • k = 222Rn diffusion coefficient

Under the measurement conditions, relation 1 could be simplified to:
(2)

With Vsystem=1,102 ml, Vsample = 100 ml, k = 0.26, C0 = 4 in Bq/m3

Quality control assurances during sample analysis

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In the present study, the collected water samples were analyzed as soon as possible after the collection without any appreciable delay and hence decay correction was not applied. Within a temperature range of 10 °C to 30 °C the value of diffusion coefficient (k) is 0.26 as provided by the manufacturer (Genitron 1997). Before each sampling the degassing tube was dried well with tissue paper and then an active coal filter cartridge (equivalent air volume of 500 L) was used in the closed setup along with AlphaPUMP to remove resting radon in the measuring setup until the radon level was less than 5 Bq/m3.

Estimation of annual effective dose (AED) due to water-borne radon

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People receive radiation dose due to ingestion of dissolved radon in drinking water. Radon gas can escape from water into indoor air due to different domestic activities such as showering, washing clothes and utensils and other daily domestic activities which can also result in an inhalation dose.

The total annual effective dose is the sum of the annual doses from ingestion and inhalation of water-borne radon. The doses can be calculated by using the following relations given by UNSCEAR (1982, 2000):
(3)
where,
Inhalation dose:
(4)
Ingestion dose:
(5)
where:

The temperature of the study area generally remains low (average temperature close to 12 °C) (IMD 2014; Sarkar et al. 2017). So, water is not consumed directly, usually consumed after heating when radon is readily lost. Therefore, we have taken the direct annual consumption of drinking water of 60 litre as proposed in UNSCEAR (2000) report.

EDC: Effective dose coefficient for ingestion (3.5 nSv/Bq) (UNSCEAR 1982, 2000; Naskar et al. 2022)

Acceptable water-borne 222Rn activity in drinking water

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Radon and its numerous decay products contribute a significant radiation dose to the general population and thus can induce carcinogenesis in different organs (EPA 1991; Gorchev & Ozolins 2011; Naskar et al. 2022). According to the World Health Organization (WHO), radon and its decay products are significant causes of lung cancer, after cigarette smoking (Åkerblom 1999; Kendall & Smith 2002; WHO 2009, 2011; Kim et al. 2016). Realizing radon's potential to cause fatality, different international agencies as well as different countries have introduced their own reference level of radon concentration in drinking water. United States Environmental Protection Agency (USEPA) has recommended maximum contaminant level (MCL) for radon in drinking water as 11.1 Bq/l (USEPA 1991; Opoku-Ntim et al. 2019). In their recent report (2018) the United States Environmental Protection Agency has proposed that the upper level of radon in drinking water should not exceed 148 Bq/l if drinking water suppliers are able to reduce the indoor air radon as prescribed by USEPA enhanced indoor air programs. Otherwise, through proper action the radon level in drinking water should be brought down below the MCL of 11.1 Bq/l (USEPA 2018). A more conservative guidance or reference level of 100 Bq/l has been suggested by WHO (WHO 2009, 2011; Opoku-Ntim et al. 2019). Similar reference level of 100 Bq/l has been recommended by the European Commission (EC 2013). The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has suggested a value of radon concentration in water for human consumption of 4–40 Bq/l (UNSCEAR 2000). So far no such limit exists in India. So we have followed the prescription of WHO as well as the European Commission in this regard as has been done by many other workers in this field.

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The measured water-borne average radon concentration of all the studied water samples (natural springs and tube wells) is 10.96 ± 1.97 Bq/l. Among drinking water samples collected from natural springs, the measured maximum and minimum radon level are 13.04 Bq/l (SW38) and 0.43 Bq/l (SW25) whereas the measured maximum and minimum radon level in the drinking water collected from tube wells are 71.02 Bq/l (TW10) and 1.02 Bq/l (TW7) respectively. Considering different sources of drinking water separately, the average radon activity for water samples collected from natural springs and tube wells are 2.88 ± 0.41 Bq/l and 25.67 ± 3.89 Bq/l respectively. Measured average radon concentration of the tube well water samples is 8.9 times higher than that of the natural spring samples. Water samples from natural springs suffer much agitation due to the free flow of water from the springhead which causes escape of radon gas from water. Moreover, it was drizzling at the time of sample collection. Average radon activity of the spring water samples may have reduced due to the mixing of rain water with the samples. In contrast, ground water from tube wells is mostly unaffected by the above mentioned factors which is the root cause of increased radon level compared to that of natural spring water. Figure 3 shows the drinking water radon activity profile for natural springs (upper panel) and tube wells (lower panel).
Figure 3
Radon concentration profile of the natural spring and tube well water samples.
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Radon concentration profile of the natural spring and tube well water samples.

Figure 3
Radon concentration profile of the natural spring and tube well water samples.
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Radon concentration profile of the natural spring and tube well water samples.

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No water sample is found to have water-borne radon activity above the WHO and European Commission prescribed reference level of 100 Bq/l while 71% (44 out of 62) of the water samples have radon concentration even below the USEPA recommended MCL of 11.1 Bq/l. The result shows that Darjeeling hill and foothill areas possess radiologically safer drinking water than Garhwal Himalaya, northern India regions where about 16% (13 out of 83) and 43% (22 out of 51) of drinking water samples were seen to have higher than 50 Bq/l radon activity as reported by R. C. Ramola et al. and V.M. Choubey et al. respectively whereas only 3% of samples (2 out of 62) in this case are seen to have concentration higher than 50 Bq/l.

Frequency distribution of the measured radon activities for both types of water sources has been shown in Figure 4. It shows a highly skewed distribution having a long tail extending up to 72 Bq/l with 58.3% of the data in the 0–4 Bq/l interval. It is revealed from the study that radon level in the water samples collected from natural springs and tube wells from Darjeeling hill and foothill areas is on the low side even in the global context. Worldwide study on measurement of radon level in natural spring water is shown in Table 1.
Table 1

National and international study on measurement of radon level in drinking water of natural springs

Sampling siteActivity range (Bq/l)Average activity (Bq/l)No. of samplesReferences
Garhwal Himalaya, northern India 27–840 - 83 Ramola et al. (1997)  
North-central part of the Kumaun Lesser Himalaya (Garhwal Himalaya) 0.40–887.00 125.36 (calculated)  51 Choubey et al. (2000)  
Lebanon 0.46–49.60 12.78 (calculated) 15 Abdallah et al. (2007)  
Spain <4.00–1,868.0 30.60 82 Ródenas et al. (2008)  
Mt. Etna, Italy 1.40–12.70 5.20 (calculated) 3 Kozłowska et al. (2009)  
Nenevah governorate of north region in Iraq 0.14–0.37 0.21 7 Yousuf et al. (2009)  
South of Catalonia 1.40–105.00 20.30 (calculated) 15 Fonollosa et al. (2016)  
Tuzla area, Bosnia and Herzegovina 0.21–1.61 0.60 6 Kasić et al. (2016)  
Seydisehir, Turkey 1.85–99.27 23.50 (calculated) 7 Erdogan et al. (2017) 
Istanbul, Turkey 1.60–14.00 6.30 (calculated) 21 Doğan et al. (2018) 
Muzaffarabad, Pakistan Administered Kashmir 0.24–34.36 10.16 60 Khan et al. (2019) 
Darjeeling hill and foothill areas of West Bengal, India 0.40–13.00 2.90 40 This study 
Sampling siteActivity range (Bq/l)Average activity (Bq/l)No. of samplesReferences
Garhwal Himalaya, northern India 27–840 - 83 Ramola et al. (1997)  
North-central part of the Kumaun Lesser Himalaya (Garhwal Himalaya) 0.40–887.00 125.36 (calculated)  51 Choubey et al. (2000)  
Lebanon 0.46–49.60 12.78 (calculated) 15 Abdallah et al. (2007)  
Spain <4.00–1,868.0 30.60 82 Ródenas et al. (2008)  
Mt. Etna, Italy 1.40–12.70 5.20 (calculated) 3 Kozłowska et al. (2009)  
Nenevah governorate of north region in Iraq 0.14–0.37 0.21 7 Yousuf et al. (2009)  
South of Catalonia 1.40–105.00 20.30 (calculated) 15 Fonollosa et al. (2016)  
Tuzla area, Bosnia and Herzegovina 0.21–1.61 0.60 6 Kasić et al. (2016)  
Seydisehir, Turkey 1.85–99.27 23.50 (calculated) 7 Erdogan et al. (2017) 
Istanbul, Turkey 1.60–14.00 6.30 (calculated) 21 Doğan et al. (2018) 
Muzaffarabad, Pakistan Administered Kashmir 0.24–34.36 10.16 60 Khan et al. (2019) 
Darjeeling hill and foothill areas of West Bengal, India 0.40–13.00 2.90 40 This study 
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Figure 4
Frequency distribution of radon activity of the water samples.
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Frequency distribution of radon activity of the water samples.

Figure 4
Frequency distribution of radon activity of the water samples.
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Frequency distribution of radon activity of the water samples.

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Water samples with pH values between 6.5 and 8.5 are usually considered good quality water (Kassim 2011; WHO 2011). The pH value for SW and TW water samples under consideration ranges from 6.7 (SW38) to 8.7 (SW14) and 6.8 (TW14) to 9.5 (TW6) respectively with an average of 7.85 ± 0.51; 10% and 4.55% of the natural spring and tube well water samples respectively are found outside this range. To have a quantitative estimation of the relationship between them, the Pearson's correlation coefficients (also called Pearson's R) (Steele 2004) has been calculated. The correlation coefficients for SW and TW samples are evaluated as −0.32 and −0.8 respectively and considering all the samples together the evaluated correlation coefficient factor (R2) is −0.22 suggesting negative correlation between the quantities. Different researchers reported either positive (Kassim 2011) or negative (Kassim 2011; Naskar et al. 2022) or even null (Kasić et al. 2016) correlation coefficient between water radon activity and pH of specified water samples around the world. Baeza et al. (1995) and Lauria & Godoy (2002) reported that radium dissolves in water with low pH and has high positive correlation between them. The possibility of presence of high radon activity in water with low pH is obvious as radon is a direct decay product of radium. The maximum radon activity among SW and TW water samples are found to be 13.04 (SW38) and 71.02 (TW10) Bq/l with pH value 6.7 and 7.9 respectively. The minimum radon activity for SW and TW water samples are found to be 0.43 (SW25) and 1.02 (TW7) Bq/l with pH value 8.4 and 8.2 respectively. For both types of water samples the maximum and minimum radon activity occurs for water with low and high pH respectively. It can be seen from the lower panel of Figure 5 that the radon activity of TW water samples gradually increases initially with low pH (till 7.9) and has a positive correlation of 0.34 (excluding four TW samples with pH above 8). But while including the remaining four TW water samples (pH ≥8 with low radon activity) the correlation coefficient reduces to a negative value of 0.8. 222Rn concentration and the pH of the water samples are plotted in Figure 5 to study any relation between them.
Figure 5
Variation of radon concentration and pH of both types of water samples.
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Variation of radon concentration and pH of both types of water samples.

Figure 5
Variation of radon concentration and pH of both types of water samples.
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Variation of radon concentration and pH of both types of water samples.

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The evaluated water-borne radon activities are used to determine inhalation and ingestion doses with equations 4 and 5. Finally total AEDs are determined using equation 3. Inhalation, ingestion and total AED doses for SW samples vary from 1.07 ± 0.9 to 32.85 ± 6.40, 0.09 ± 0.08 to 2.74 ± 0.53 and 1.16 ± 0.98 to 35.59 ± 6.95 μSv/y respectively with mean value of 7.25 ± 1.03, 0.6 ± 0.08 and 7.86 ± 1.11 μSv/y respectively. Those for TW samples vary from 2.57 ± 0.88 to 178.97 ± 36.59, 0.21 ± 0.07 to 14.91 ± 3.05 and 2.78 ± 0.95 to 193.88 ± 39.64 μSv/y respectively with mean value of 64.68 ± 9.81, 5.39 ± 0.82 and 70.70 ± 10.64 μSv/y respectively. Considering all the water sources together the mean total AED is found to be 29.93 ± 5.37 μSv/y. So the evaluated average total AED for the two types of drinking water sources separately and all together also are much below the reference limit of 100 μSv/y prescribed by the WHO (2009, 2011) and European Commission (EC 2013). However, the total AED for six (9.68%) TW water samples, as shown in Table 2, are found to exceed the reference limit of the two agencies. To avoid the possible health hazards due to water-borne radon the local users should avoid those six TWs if possible. Otherwise, they should not drink water directly from those six TWs. To reduce the radon activity the collected drinking water should be either boiled or stirred; even storing after the collection will serve the purpose. Boiling or stirring or storing of water samples should be done in open places or places with good ventilation to avoid inhalation dose. In this regard it may be mentioned that water regulatory authorities in India have not set yet any reference limit for dissolved radon in drinking water. The summary of total annual effective dose due to inhalation and ingestion of water-borne radon in the study areas for both types of water sources are simultaneously presented in Figure 6.
Table 2

Details of the TW water samples exceeding total AED of 100 μSv/y

Sample CodeLatitude (°N)Longitude (°E)LocationMean 222Rn activity (Bq/l)Total AED (μSv/y)
TW1 26.68992 88.46281 PCRA Colony, Siliguri, West Bengal 734004 40.08 109.42 
TW2 26.67245 88.47452 Sahudangi Rd, Binnaguri, West Bengal 734004 45.12 123.18 
TW9 26.87092 88.74899 VPCX +9JC, Mal Bazar, West Bengal 735221 52.10 142.23 
TW10 26.88121 88.79468 VQJV + JVR, Chalsa, West Bengal 735223 71.02 193.88 
TW17 26.9023 88.80484 Chalsa- Hatkhola- Matelli Rd, Kilkote Tea Garden, West Bengal 735223 49.30 134.58 
TW22 26.88226 88.48878 NH 17, Mongpong, Mangpong Forest, West Bengal 734005 45.35 123.80 
Sample CodeLatitude (°N)Longitude (°E)LocationMean 222Rn activity (Bq/l)Total AED (μSv/y)
TW1 26.68992 88.46281 PCRA Colony, Siliguri, West Bengal 734004 40.08 109.42 
TW2 26.67245 88.47452 Sahudangi Rd, Binnaguri, West Bengal 734004 45.12 123.18 
TW9 26.87092 88.74899 VPCX +9JC, Mal Bazar, West Bengal 735221 52.10 142.23 
TW10 26.88121 88.79468 VQJV + JVR, Chalsa, West Bengal 735223 71.02 193.88 
TW17 26.9023 88.80484 Chalsa- Hatkhola- Matelli Rd, Kilkote Tea Garden, West Bengal 735223 49.30 134.58 
TW22 26.88226 88.48878 NH 17, Mongpong, Mangpong Forest, West Bengal 734005 45.35 123.80 
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Figure 6
Summary of total annual effective dose for both types of water sources.
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Summary of total annual effective dose for both types of water sources.

Figure 6
Summary of total annual effective dose for both types of water sources.
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Summary of total annual effective dose for both types of water sources.

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Radon concentrations of water samples from different tube wells as well as natural spring sources of the study area are well below the WHO and European Commission proposed reference level of 100 Bq/l. The measured results show that the average radon concentration of the tube well water samples is 8.9 times higher than that of the natural spring samples. Correlation study between 222Rn concentrations and pH values suggests that the acidic water samples have higher 222Rn concentrations than the alkaline water samples of this study area. The evaluated average total AEDs for the two types of drinking water samples are much below the reference limit of 100 μSv/y prescribed by the WHO and European Commission. However, it is observed that the total AEDs of 9.68% of the collected water samples exceed the prescribed dose limit. From this study we may conclude that drinking water of Darjeeling hill and foothill regions is safe so far as water-borne radon hazards are concerned.

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We thankfully acknowledge the financial support given under RUSA 2.0 program of JU and DST, Govt. of India. We are also thankful to the local people for their assistance and cooperation during sample collection.

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All authors contributed to the study conception and design. Material preparation and sample collection were performed by Mahasin Gazi, Arindam Kumar Naskar, Mitali Mondal and Argha Deb and data analysis was performed by Mahasin Gazi and Arindam Kumar Naskar. The first draft of the manuscript was written by Mahasin Gazi and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

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The financial support has been given under Rashtriya Uchchatar Shiksha Abhiyan 2.0 (RUSA 2.0) program of Jadavpur University and DST FIST project of the department of Physics, Jadavpur University.

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Data cannot be made publicly available; readers should contact the corresponding author for details.

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The authors declare there is no conflict.

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