Spatial distribution of radon contamination in hot springs water and its cancer and non-cancer risks in the Hunza-Nagar valley, Pakistan

Radon (222Rn) is a ubiquitous radioactive gas and could threaten human life due to its potential for cancer and non-cancer risks. This study examined the measurement of 222Rn concentration and associated health risks in the hot springs of Hunza-Nagar valley. For this purpose, the hot springs water of Hunza and Nagar districts and the background sites were analyzed for 222Rn concentration using the RAD7 detector (Durridge Company, USA). The average concentrations of 222Rn were 46.1 ± 0.94, 65.3 ± 0.45, and 5.47 ± 0.25 Bq/L in the Hunza district, Nagar district, and background sites, respectively. Results showed that 222Rn concentrations of hot springs water were multifold higher than the background sites. 222Rn concentrations for hot springs water in Hunza-Nagar valley had surpassed the maximum contamination level set by the US environmental protection agency (USEPA). Humans’ annual mean exposure dose rates of various age groups were calculated for the estimated lifetime cancer risk (ELCR) and non-cancer risks. The total annual mean exposure doses from 222Rn in water (EwTotal) values were (187 ± 3.80, 265 ± 1.84, and 22.2 ± 1.02 μSv/a) for infants (143 ± 2.92, 203 ± 1.40, and 17.0 ± 0.78 μSv/a) children, and (138 ± 2.80, 196 ± 1.35, and 16.4 ± 0.76 μSv/a) adults in the Hunza district, Nagar district, and background, respectively. Among the age groups of humans, infants showed a higher risk than others. Results showed that hot springs water consumption surpassed the world health organization threshold of 100 μSv/y for chronic or non-cancer and USEPA 0.1 × 10–3 for ELCR risks. The concentration of 222Rn showed a positive correlation (> 0.68) with hot springs' water temperature and pH suggesting a common origin.


Introduction
Radon ( 222 Rn) is a ubiquitous radioactive gas with odorless, tasteless, and colorless features (Rey et al., 2022;Tanner et al., 1964). The 222 Rn is generated in the earth's crust as a byproduct of the radioactive decay of radionuclides (Somlai et al., 2007). There are three naturally occurring isotopes of radon: 222 Rn, thoron ( 220 Rn), and actinon ( 219 Rn). The 222 Rn isotope is the most stable and abundant, with a half-life of 3.83 days (Strydom et al., 2021). Living beings, especially humans are exposed to continuous natural radiation (about 90%). The sources of this natural radiation are extra-terrestrial (cosmic rays) and terrestrial (radionuclides in earth's crust) components of the universe (Kansal & Mehra, 2015;Rani et al., 2021).
Among the terrestrial sources, only 222 Rn contributions are more than 50% of the total dose absorbed by humans (Rani et al., 2021;UNSCEAR, 2000). Owing to its abundance and high flux, 222 Rn has become one of the major concerns for the global scientific community Khan et al., 2021;Tade et al., 2021).
Radon is grouped as carcinogenic and is the second leading cause of lung cancer due to its health risks (Yoon et al., 2016). 222 Rn, when consumed/ ingested or inhaled, decays, and releases energy that can disrupt the DNA of cells in the lungs and stomach, increasing the risk of cancer (Li et al., 2006). The presence of 222 Rn in drinking water resulted in lung cancer (89%) and stomach cancer (11%) due to its ingestion and inhalation (NRC, 1999). Ahmad et al. (2020) report that 222 Rn has been involved in around 21,100 and 1100 lung cancer deaths per year in the USA and the United Kingdom, respectively. Furthermore, consuming tainted water poses a significant danger of stomach disorders. 222 Rn in water could have disastrous effects on humans, including gastrointestinal problems. Therefore, the maximum contamination level (MCL) for 222 Rn in drinking water has been set at 11.1 Bq/L (300 pCi/l) by the US Environmental Protection Agency (USEPA, 1999).
Radon in the earth's crust diffuses into the soil and groundwater regularly (Pant et al., 2020;Prasad et al., 2008). Various environmental conditions of the earth's crust could result in a buildup of a higher quantity of 222 Rn in groundwater (NRC, 1999;Ullah et al., 2022). Groundwater is an essential drinking water supply, with around one-third of all human activity depending on it (Zhang, 2021). Groundwater is found in its purest form in limited aquifers, as the countless human inputs have rendered it hazardous for residential and industrial use (Abbas et al., 2021). Due to its presence beneath the land crust, groundwater quality is the most vulnerable to trace element pollution (Mallongi et al., 2022) and 222 Rn contamination (Haroon & Muhammad, 2022;. Deep groundwater travels through the geothermal gradient to the land surface via fractures and crevices along with the active fault as thermal/hot springs or geysers. Hot springs water is enriched with 222 Rn concentrations due to passing through bedrocks having rich mineralogy of radioactive minerals under higher pressure and temperature (Sola et al., 2013;Ullah et al., 2022).
The concentration measurement of 222 Rn and associated health risks in hot springs water have been carried out in various countries, i.e., West Java, Indonesia (Nugraha et al., 2021), Yunnan, China (Qiao et al., 2022), Bageshwar, India (Kumar et al., 2022), and Yalova basin, Turkey (Tabar & Yakut, 2014). The area hosting granitic and metamorphic bedrocks had maximum radionuclides and higher 222 Rn concentrations (Knutsson & Olofsson, 2002). The 222 Rn concentrations are influenced by the geochemical properties and geological structures (faults, shears, and thrusts) in the vicinity (Choubey et al., 1999;Ullah et al., 2022). The Kohistan-Ladakh Island Arc divides two main tectonic plates, with the Main Karakorum Thrust (MKT) to the north on the Eurasian plate and the Main Mantle Thrust (MMT) to the south on the Indian plate (IP) (Coward et al., 1987;Kazmi & Jan, 1997). Such geology creates a porous and accessible channel for 222 Rn to migrate from deeper sources and into the near-surface water (Khattak et al., 2014). The study area is a tectonically active fault zone hosting hot springs (Ullah et al., 2022). Yet, little research has been conducted on the concentration of 222 Rn in hot springs and associated health risks in Pakistan. The hot springs in the Hunza-Nagar valley are well-known throughout the country for their healing powers. Several baths and collection sites have been set up for people who have traveled from all over the country in the hopes of treating their infirmity. Consequently, it is essential to investigate the 222 Rn level in hot springs water of the target area. Therefore, this study aimed to examine 222 Rn concentrations of hot springs water and develop its spatial distribution maps in the Hunza-Nagar valley. The concentrations of 222 Rn in the hot springs water were used to evaluate potential cancer and non-cancer risk assessment.

Study area
Hunza-Nagar is a hilly valley in Pakistan's Gilgit-Baltistan province in the northern region. It is located on the Hunza River's bank, with Ishkoman to the northwest, Shigar to the southeast and the north, Afghanistan's Wakhan Corridor, and China's Xinjiang province to the northeast. The size of Hunza-Nagar valley is 11660 km 2 , and the coordinates are 35°34ʹ0ʺ-37°01ʹ0ʺ N and 74°02ʹ0ʺ-75°48ʹ0ʺ E. The elevation of Hunza-Nagar valley is almost 8000 feet above the sea. Generally, this area has three regions upper, central, and Lower Hunza. The climate in Hunza-Nagar valley is immensely varied. The annual mean temperature is 16.5 °C with a mean yearly rainfall of 154 mm (Muhammad & Ahmad, 2020).

Geology
Hunza-Nagar valley host several sutures and thrust zones in northern Pakistan. The Kohistan-Ladakh Island Arc divides two main tectonic plates, with the MKT to the north on the Eurasian plate and the MMT to the south on the IP. Hunza-Nagar valley comprises Kohistan batholith in the south, the Eurasian plate in the north, and tectonically active faults along with hot springs (Coward et al., 1987;Kazmi & Jan, 1997). The contact and underthrusting of the IP below the Eurasian plate probably cause geothermal activity in northern parts of Pakistan, as indicated by natural hot springs. In the Hunza-Nagar localities, several hot springs were found along the vicinity of MKT (Yousafzai et al., 2010). Such geology creates a porous and accessible channel for the excess 222 Rn to flow from deeper sources into the water table and appreciate the contamination of 222 Rn gas into the groundwater (Khattak et al., 2014;Ullah et al., 2022).

Sampling procedure
Hot springs water was collected in the district Hunza (n = 4) and district Nagar (n = 3) in October 2021 ( Fig. 1). Polyethylene terephthalate (PET) bottles were used to collect water samples from hot springs. Compared to the high-density polyethylene bottles, PET bottles showed less 222 Rn loss during storage (Jobbágy et al., 2019;Leaney & Herczeg, 2006). Prewashed 250 ml glass bottles specifically intended for 222 Rn measurement with nitric acid (HNO 3 , 15%) were filled and ensured that no air was trapped. Background sites spring water was analyzed out of the Hunza-Nagar valley near Gilgit city (at least 80 km away) from the hot spring (Ullah et al., 2022).

Water analyses
The concentrations of 222 Rn in hot springs waters were measured in the field using a RAD7 detector (Durridge Company, USA) linked to an H 2 O accessory. The sampling bottles (250 ml) are attached to the RAD7 sensor in the setup, and the test begins using the chosen protocols (Wat-250). For 5 min, the RAD7's internal air pump runs, recirculating a closed air loop through the water sample and expelling 222 Rn from the water into the RAD7. The system begins counting the 222 Rn activity concentration of the sample after reaching equilibrium between water, air, and its progeny connected to the detector. The identical procedure repeats itself 5 min later and is completed in 30 min. A multiparameter analyzer (CONSORT 6030, Belgium) was used to measure water temperature and pH in field.

Annual mean exposure doses
The annual mean exposure doses of human noncancer risk for 222 Rn can be classified into two categories, i.e., inhalation and ingestion pathways. The annual mean exposure doses from ingestion are determined mainly by the amount of water taken by a person at a specific time. Moreover, 222 Rn present in drinking water can escape into the indoor air during showering and other domestic usages, increasing the potential risks for the inhalation pathway. Waterborne 222 Rn risk poses a more significant threat to public health than all other pollutants (Vitz, 1991). The annual mean exposure doses from 222 Rn in water for inhalation (EwInh) and ingestion (EwIng) were computed using the parameter as mentioned (UNSCEAR, 2000): where C RnW : 222 Rn concentration in water (Bq/l), F: Equilibrium factor between 222 Rn and its progenies (0.4), O: Average annual indoor occupancy time per individual (7000 h/a), DCF: Dose conversion factor for 222 Rn exposure (9 nSv/Bq/h/m 3 ), Cw: Average annual water consumption for infants, children, and adults are 0.6, 0.8, and 1.3 L, R: Ratio of 222 Rn in the air to that in water (10 -4 ). EDC: Effective dose coefficient for ingestion (3.5 nSv/Bq) (Ezzulddin & Mansour, 2020;Haroon & Muhammad, 2022). The total annual mean exposure doses from 222 Rn in water (EwTotal) of humans can be calculated by the sum of the annual mean exposure doses for inhalation and ingestion as follows: Radon excess lifetime cancer risk (ELCR) was computed using the adapted equation (4) (Haroon & Muhammad, 2022;Valentin, 2007) where H: The mean effective dose, DL: The average duration of life (70 years), and RF: The fatal cancer risk per Sievert (5.5 10 -2 /Sv).

Statistical analysis
Data calculations were carried out in MS Excel, and graphical presentations were plotted in a Sigma Plot (software 12.5, Systate Inc.) interpolation (inverse distance weightage) techniques of Arc GIS software ver. 10.4.

Radon concentration
The results of 222 Rn mean concentrations and spatial distribution of hot springs water in the Hunza district, Nagar district, and background sites of Hunza-Nagar Valley were 46.1 ± 0.94, 65.3 ± 0.45, and 5.47 ± 0.25 Bq/L (Fig. 2a,b). Results showed that the average concentration of 222 Rn in hot springs water of both Hunza and Nagar districts had surpassed the MCL (11.1 Bq/L) set by the USEPA (1999). However, 222 Rn concentrations of background spring were found within the MCL values. Among hot springs water, the 222 Rn concentrations were higher in the Nagar district than those in the Hunza district (Table 1 and Fig. 2a,b). Higher concentrations of 222 Rn in hot springs water of Nagar district could be attributed to local lithology, geochemical properties, and geological structures in the vicinity (Ullah et al., 2022). Higher 222 Rn concentrations in the hot springs water could be attributed to bedrocks (Knutsson & Olofsson, 2002), geochemical properties, and geological structures (faults, shears, and thrusts) in the vicinity (Choubey et al., 1999;Ullah et al., 2022). The 222 Rn concentrations in hot springs water were much higher than the values of Tabar and Yakut (2014) in Turkey and lower than that of Tata Pani, Gilgit, Pakistan Ullah et al. (2022), as shown in Table 2. The 222 Rn concentrations of this study were much higher than those reported by Shakoor et al. (2022) for groundwater in district Bannu and Khan et al. (2022) for district Karak, Pakistan.
The link between 222 Rn concentrations and seismic activity has been evidenced by various studies (Erees et al., 2007;Yalim et al., 2012;Zmazek et al., 2006). In addition, a transitional or rotational landslide is characterized by a mass movement in the presence of a clear weak zone separating the slide material   et al., 1998;Ullah et al., 2022). The results showed that 222 Rn concentration in hot springs water of the Hunza-Nagar valley had surpassed the MCL set by (USEPA, 1999). A higher 222 Rn concentration in water than MCL could pose several health issues, including cancer and non-cancer, to the local human population via inhalation and ingestion of this gas (Cothern, 2014;Martins et al., 2019).

Annual mean exposure doses
The annual mean exposure doses for non-cancer of various age groups were calculated for consumption of 222 Rn concentration in the Hunza-Nagar valley. Infants' EwIng values were (70.7 ± 1.44, 100 ± 0.70, and 8.38 ± 0.40 μSv/a), children (26.9 ± 0.55, 38.3 ± 0.26, and 3.19 ± 0.15 μSv/a), and adults (21.9 ± 0.44, 31.0 ± 0.22, and 2.59 ± 0.12 μSv/a) in the Hunza district, Nagar district, and background, respectively. The EwInh values were 116 ± 2.36, 165 ± 1.14, and 13.8 ± 0.64 μSv/a in Hunza district, Nagar district, and background, respectively. Similarly, the EwTotal values were for infants (187 ± 3.80, 265 ± 1.84, and 22.2 ± 1.02 μSv/a), children (143 ± 2.92, 203 ± 1.40, and 17.0 ± 0.78 μSv/a), and adults (138 ± 2.80, 196 ± 1.35, and 16.4 ± 0.76 μSv/a) in the Hunza district, Nagar district, and background, respectively (Fig. 3 abc). Among the studied sites, Nagar district hot springs water showed a higher risk than other sites. The maximum risk of the Nagar district was attributed to a higher 222 Rn concentration in hot spring water. These results showed maximum dose rates for infants compared to other age groups of the human population and prone to non-cancer risk. These higher non-cancer risk values of infants in the Hunza-Nagar valley through 222 Rn consumption could be due to their higher vulnerability. Among the human exposure routes, inhalation showed potential for non-cancer risks than that ingestion. Higher risks for infants via inhalation routes were found in support of a previous study for groundwater in the Mirpur district of Azad Jammu & Kashmir (Haroon & Muhammad, 2022). The EwTotal values in the present study were noted to be higher than Haroon and Muhammad (2022) and lower than those of Ullah et al. (2022), as shown in Table 3.
The WHO recommends a safe annual mean exposure dose limit of 100 μSv/y for drinking water. If the dose is less than (or equal to) 100 μSv/y, the water is safe to drink, and no further action is required; however, if the dose surpasses this limit, then remedial steps are necessary, such as heating or allowing in open space to reduce its level (Haroon & Muhammad, 2022;Ullah et al., 2022). The total annual mean exposure doses for all age groups of hot springs in the Hunza-Nagar valley surpassed the recommended range of safe levels set by WHO and the European Union (EU) council.
The average values of ELCR in the Hunza-Nagar valley and spatial distribution were summarized (Fig. 4 ab). Results showed that the sampling hot springs' ELCR values were higher than the US EPA threshold (0.1 × 10 -3 ) for both the Hunza and Nagar districts. However, the background sites were within these limits. Results showed that ELCR values of Hunza-Nagar districts were multifold higher than background sites. Further, it noted that the Nagar district showed higher ELCR values than the Hunza Fig. 3 Annual mean exposure dose rates: EwIng is the mean effective dose of 222 Rn ingestion, EwInh is the mean effective dose of 222 Rn inhalation, and EwTotal is the total mean effective doses of 222 Rn in the Hunza-Nagar valley, northern Pakistan; a. infants, b. children, and c. adults district. Higher ELCR values of the Nagar district were due to its maximum 222 Rn concentration in water of the hot springs that were attributed to bedrock geology in the vicinity. These ELCR values were higher than in a previous study for Mirpur district, Pakistan (Haroon & Muhammad, 2022).

Water physical characteristics and their correlations
Water characteristics such as temperature and pH values of hot springs in the Hunza-Nagar valley were tabulated (Table 1). Water pH values in the hot springs of Hunza district ranged from 7.8 to 8.0 and Nagar district 7.7-8.0. Similarly, temperature values ranged from 46 to 51 °C in the hot springs of the Hunza district, while 68-70 °C in the Nagar district (Table 1). The water pH of hot springs did not show great variation compared to the background. However, the temperature in hot springs water was multifold higher than in the background sites. Among hot springs, the water temperature of Nagar district was observed to be more elevated than that of Hunza district. The higher temperature of hot springs water could be attributed to the geothermal gradient and abrasion along the fault zones. The higher temperature of hot springs water compared to the background was found in support of a previous study (Ullah et al., 2022). Radon concentrations in the Hunza-Nagar valley hot springs were plotted as functions of physical water characteristics, including pH and temperature (Fig. 5). Results showed a strong positive correlation in the water of hot springs for 222 Rn concentration and that pH and temperature values. The positive correlation of 222 Rn values with that of temperature does not agree with the law of gas dissolution in liquids. A possible reason could be the pressure force on 222 Rn dissolution in fluids at higher depths below ground. This study was consistent with a previous study that 222 Rn solubility increases with an increasing temperature of hot springs (Ullah et al., 2022). Positive correlations of temperature and pH with 222 Rn suggested a common source from deep earth along with the fault zone. While the adjusted R 2 suggested a higher correlation with pH, the dispersion in the data is also evident. This could be explained by the different geologic factors such as lithology, geochemistry, geological structure, and geothermal processes that influence 222 Rn concentration in hot springs. These may include permeability of the strata that may vary among the sampling points. The heat in water of hot springs could be generated from the abrasions/friction between the Eurasian plate and Kohistan-Ladakh Island Arc (Ahmad et al., 2001;Bakht, 2000;Hochstein & Regenauer-Lieb, 1998;Javed et al., 2012). However, this process may be executed in differential manner having different magnitudes along the fault zone, hence generating different levels of heat signatures. As temperature is the dominant factor that influences 222 Rn solubility in water, different levels of 222 Rn concentration at the same pH may be observed owing to the differences in temperature in the study area.

Conclusion
This study concluded that the 222 Rn concentration of hot springs water in Hunza-Nagar valley had surpassed the MCL set by the USEPA. The annual mean exposure doses for various pathways showed that inhalation is the primary route for 222 Rn consumption across multiple age groups of humans. The annual mean exposure doses of humans for different age groups showed that infants were at higher risks than other age groups. Results showed that hot springs water consumption in Hunza-Nagar valley surpassed the WHO and USEPA threshold limits for non-cancer and cancer risks, respectively. The concentration of 222 Rn revealed a positive correlation (> 0.68) with the temperature and pH of hot springs water in the Hunza-Nagar valley. This association of 222 Rn concentration with temperature and pH could be attributed to bedrock geology, lithology, and geological structure in the vicinity.