Study on Estimation of Inhalation and Ingestion Dose due to 222 Rn Concentration in Different Types of Ground Water of Some Taluks of Uttara Kannada District, Karnataka, India

The primary goal of this study was to determine the risk due to radon concentration in drinking water. Water is essential for most of the life on the Earth, and the quality of drinking water is a key criterion for one's health. Radon ( 222 Rn) and radium ( 226 Ra) are the radioactive elements that can be found in water. Thirty samples of drinking water were collected in Haliyal, Joida, and Mundgod taluks of Uttara Kannada district. Emanometry technique was used to determine the concentration of 222 Rn activity. The activity concentration of 222 Rn in water varies between 3.78 ± 0.38 Bq l -1 . and 135.96 ± 1.72 Bq l -1 with a mean value of 26.23 ± 0.65 Bq l -1 . The cumulative average annual effective doses for all of the samples under investigation are well below the safe limit of 100 µSv y -1 suggested by WHO.


Introduction
Water is necessary for life on this planet. The requirement for high-quality drinking water is essential for human health. Demand for pure and fresh water has risen in recent decades as a result of fast population growth, erratic rainfall, increased environmental degradation, urbanization, and industry. Water becomes contaminated as a result of increased industrial and human activities. Ground water is substantially more radioactive than surface water due to its passage through rocks and soil formations, as well as the dissolution of various chemicals, minerals, and radioactive pollutants. The concentration of 222 Rn in ground water varies depending on the hydrogeology of the location or aquifer, whereas it is very low in surface water, with concentration below 4 Bq l -1 [1][2][3][4][5][6][7]. 222 Rn is a naturally occurring alpha emitting radioactive gas that is chemically inert. This colourless, tasteless, and odourless gas is produced by the natural radioactive decay series of uranium, which is found in trace amount throughout the Earth's crust in rocks and soils. The three naturally occurring isotopes of radon are radon ( 222 Rn), thoron ( 220 Rn), and action ( 219 Rn). Among these, radon ( 222 Rn) is the most stable isotope produced by the disintegration of 238 U, accounting for approximately 99.3 % of total uranium in the Earth's crust. Thoron ( 220 Rn) is a 232 Th decay product, whereas actinon ( 219 Rn) is generated via 235 U decay [5][6][7][8]. Because of its 3.825-day half-life, the 222 Rn isotope has adequate time to persist in water and air, posing health risks such as lung cancer, stomach-colon cancer, and leukaemia [9,10]. The transmission of radon from water to indoor air and its inhalation, as well as ingestion, are the two potential health risks provided by radon-enriched drinking water. While radon is emitted from water, such as when showering or bathing, it increases the overall risk of inhalation associated with indoor radon. Although radon in drinking water is not a direct health risk, it is harmful to the environment [11]. Long-term exposure of the population to high levels of radon and its daughters causes pathological outcomes such as respiratory functioning abnormalities and lung cancer [12].
However, drinking water with a high 222 Rn level can increase the risk of stomach cancer and gastrointestinal cancer [13]. According to the Environmental Protection Agency (EPA), radon is the second most common cause of lung cancer in the United States, after smoking [14]. In the United States, 222 Rn gas in drinking water is responsible for approximately 168 cancer deaths each year, with 89 % of these deaths are due to lung cancer caused by water-borne radon and 11 % are due to stomach cancer caused by consuming radon-contaminated water [15]. The permissible maximum contamination level (MCL) for 222 Rn in water has been suggested by the USEPA as 11.1 Bq l -1 [16]. From public health perspective, understanding the activity concentration of 222 Rn in drinking water is very significant. Hence, an attempt has been made for the first time in the current study to estimate 222 Rn concentration in drinking water.

Study area
The study area Haliyal, Joida, and Mundgod taluks are located in Uttara Kannada district in Karnataka state of India, between north latitudes13° 55' 0'' to 15° 31' 01'' and east longitudes 74°0'35'' to 75°10' 23''.The geology of the study area consists of different types of rocks like matabasalt, laterite, granitic gneiss and chloriteschist as well as minerals such as iron ore, bauxite ore, magnetite and silicate. Soil types include red soil, red loamy soil and black cotton soil. The geological map of the study area is as shown in Fig. 1.

Samples collection and preparation
Total 30 drinking water samples were taken from various locations in Haliyal, Joida, and Mundgod taluks of Uttara Kannada district. From each location four samples were taken for measuring radon concentration. The water was permitted to run fully for at least 10-15 min before the sample was taken in order to obtain fresh water from the bore wells and hand pumps. Approximately 250 mL of water was collected in special plastic bottles for the calculation of radon in-water activity, ensuring minimal radon loss by degassing and without air contact [17]. The plastic bottles were completely filled in a gentle way, so that there was zero head space. It was taken care to ensure that there is no air bubble left within the container and also prevented aeration during the sampling processes, which could lead to degassing [18]. All the collected samples were analyzed within 24 h.

Determination of 222 Rn by bubbler method
The 222 Rn concentration in drinking water was determined using the emanometry technique [19,20]. The complete setup of radon bubbler is shown in Fig. 2.After collecting water samples using the aforesaid standard process, the samples were brought to the laboratory and were immediately analyzed. 70 mL of water sample was transferred into the bubbler using the vacuum transfer technique. The dissolved 222 Rn in the water was transferred to a scintillation cell that had been pre-evacuated and background counted. To allow radon to achieve equilibrium with its daughters, the scintillation cell was undisturbed for 180 min (3 h) and then it was coupled to a photomultiplier and alpha counting assembly. The concentration of radon in groundwater samples was determined using equation (1) [20].
where, D is counts above the background, V is volume of water (L), E is efficiency of the scintillation cell (74 %), λ is decay constant for radon (2.098 × 10 -6 s -1 ), T is counting delay after sampling (s), and t is counting duration (s).

Estimation of dose due to inhalation and ingestion of 222 Rn in drinking water
The public is exposed to a higher dosage of radiation from waterborne radon than from any other water pollutants [21]. The annual effective doses of radon in water for inhalation and ingestion were calculated using the parameter set out in the UNSCEAR 2000 report as (D In μSv y -1 )= A RnW C aW F I DCF (2) where D In is the effective dose for inhalation, A RnW is the radon concentration in water (Bq l -1 or kBq m -3 ), C aW is the ratio of radon in air to the radon in water (10 -4 ), F is the equilibrium factor between radon and its progenies (0.4), I is the average indoor occupancy time per individual (7000 ha -1 ) and DCF is the dose conversion factor for radon exposure [9 nSv (Bq h m -3 ) -1 ]. The ingestion dose is calculated using the following formula, (D Ig μSv y -1 )= A RnW C W EDC (3) Where D Ig is the effective dose for ingestion, C w is the weighted estimate of water consumption (60 l y -1 ) and EDC is the effective dose coefficient for ingestion (3.5 nSvBq -1 ) respectively. The dose contribution to the lungs and stomach from this source is estimated by multiplying the inhalation and ingestion doses by the appropriate tissue weighting factor for the lungs and stomach respectively.

Results and Discussion
The mean activity concentrations of 222 Rn in drinking water and annual effective dose due to inhalation and ingestion of the study area are summarized in Table 1. observed in water taken from bore well and hand pumps at Haliyal, Bhagavati, Kerosalli, Karlakatta, Nujji ,Ganadahalli, and Pala. This is due to the fact that this region is attributed with granitic gneiss at various depths and the presence of thrusts, faults and shears which provides the way for the upward movement of radon gas with higher radioactivity [22,23]. The intermediate concentration of 222 Rn in drinking water was observed in water samples collected from open wells, this may be due to the underlying bedrock system attributed by laterite and chlorite schist which contains lower activity concentration of radionuclide [22,23] and the least (minimum) activity of radon is observed in the water samples collected from ponds, rivers, spring and dams. This may be due to lesser depth and also they are well exposed and radon gas is escaped easily into the atmosphere due to atmospheric temperature variation, hence radon concentration in water is the least at some locations. The variation of 222 Rn concentration in water with the locations is shown in Fig. 3. Figure shows that, there is a wide variation of radon concentration. The geological region with high uranium/radium containing granite rock (local geology) and geohydrological conditions of the aquifer are the dependent factors for variation of 222 Rn activity in drinking water [24]. 226 Ra rich minerals present along the cracked surface putrefy and discharge 222 Rn into the groundwater. The diverse depths of water resources, as well as a different process influencing the climate and geo-hydrological nature of the locations, are further causes for the differences in 222 Rn concentration [25,26]. The concentration of 222 Rn in ground water (from bore wells and hand pumps) is higher than in surface water. This could be because, radionuclides like 238 U and 226 Ra can be found in varying amounts in rocks and soil. The greater depth of the bore wells and hand pumps is another factor for the higher 222 Rn concentration. Deeper bore wells and hand pumps allow more water to interact with an aquifer of significant thickness [27,28]. In comparison to tube well and hand pump water, tap water has lower 222 Rn content. This is due to dissolved 222 Rn being disrobed and released along the path between the water source and the water sample collection point. Surface water had the lowest 222 Rn activity concentration. Because of the atmospheric temperature variations, the surface water (river, pond, and spring) is well exposed, and the dissolved radon is easily discharged into the atmosphere.
Among 30 samples 63.3 % (19 samples) showed radon concentration exceeding the maximum contaminant level of 11.1 Bq l -1 proposed by USEPA [29]. From this study, it is observed that 6 out of 30 drinking water samples (about 20 %) have 222 Rn activity concentration levels much higher than the UNSCEAR maximum recommended level of 40 Bq l -1 [30]. The measured radon concentrations were compared to the European Commission's recommendations for public protection against radon exposure in drinking water supplies, which recommends 100 Bq l -1 as action threshold for public water supplies [31]. Only one sample was observed to exceed the action threshold of 100 Bq l -1 [32]. The variation of inhalation, ingestion and total dose with the location is shown in Fig. 5. The measured inhalation dose varied from 9.52 µSv y -1 to 342.62 µSv y -1 with a mean of 66.10 µSv y -1 . Ingestion dose received varies from 0.79 µSv y -1 to 28.55 µSv y -1 with an average value of 5.51 µSv y -1 .The quantity of radon reaching the lungs from these sources ranges from 1.14 µSv y -1 to 41.11 µSv y -1 with a geometric mean of 7.93 µSv y -1 . Thequantity of radon reaching the stomach from these sources ranges from 0.10 µSvy -1 to 3.43 µSv y -1 with a geometric mean of 0.66 µSv y -1 . The total annual dose due to inhalation and ingestion of 222 Rn ranges from 10.32 µSv y -1 to 371.17 µSv y -1 with a geometric mean of 71.61 µSv y -1 . The World Health Organization (WHO) and the EU Council recommend the determination of reference level of the annual effective dose received from drinking water consumption at 100 µSv y -1 [31][32][33].

Conclusion
Wide variations in radon concentrations in drinking water samples were found in this analysis. This is due to the heterogeneous distribution of radionuclides resulting from granite, granitic gneiss and chlorite schist of the nearby aquifers.About 36.7 % of the samples actually used by the inhabitants of the study area have radon concentrations less than the EPA recommended level of 11.1 Bq l -1 . In this study, the mean total annual effective dose due to 222 Rn in drinking water is less than 100 µSv y -1 and hence the water is appropriate for drinking and no further intervention is required.