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Home All issues Volume 59 / No 4 (October - December 2024) Radioprotection, 59 4 (2024) 317-326 Full HTML
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Issue
Radioprotection
Volume 59, Number 4, October - December 2024
Page(s) 317 - 326
DOI https://doi.org/10.1051/radiopro/2024028
Published online 13 December 2024

© E. Srinivasa et al., Published by EDP Sciences, 2024

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1 Introduction

Radionuclides such as 238U, 226Ra, 232Th, and 40K that are present in air, water, food, and soil are called natural radionuclides (UNSCEAR, 2000; Jabbar et al., 2010). The radioactivity exhibited by these radionuclides present in the environmental matrix is called environmental radioactivity. The radiation produced due to natural radionuclides present in the environmental matrix is called natural background radiation. The most important radiation sources to which all individuals are exposed are the ionizing radiation such as 𝛼, β, and γ radiations arising from natural radionuclides in the earth’s crust and the cosmic ray interaction on the earth’s atmosphere (UNSCEAR, 2000; Lilley, 2013). Man-made radiation arising from, consumer products, diagnostic X-rays, nuclear medicine procedures, radiation oncology, fuel cycle facility, industrial radiography, nuclear power plants, nuclear accidents, military waste, the fallout of radionuclides from nuclear bomb explosions, industrial waste, etc. (Eisenbud and Gesell, 1997). Human beings were exposed to two types of radiation, which are called internal exposure and external exposure. Internal exposure is due to radionuclides taken into the body through ingestion of food materials, etc., and due to inhalation of 222Rn, 220Rn, and their progenies present in the atmosphere. External exposures are due to radioactive nuclides present in varying amounts in the earth’s crust, atmosphere, and building materials (Eisenbud and Gesell, 1997; NCRP, 1975; Srinivasa et al., 2022). Nearly 80% of the total radiation dose received by the public is from natural environmental radiation, out of which the concentration of radionuclides in soil, rock, building materials and water are the main contributors (Sannappa et al., 2003). According to UNSCEAR, the total world average annual natural radiation exposure from all sources is 2.4 mSvy−1 (UNSCEAR, 2000). The average levels of radiation exposure due to the medical uses of radiation in developed countries is equivalent to approximately 50% of the global average level of natural exposure (UNSCEAR, 2000). The knowledge of the distribution of radionuclides and radiation levels in the environment is very important in assessing the effects due to radiation exposures on human beings (Ramola and Negi, 2004; Srinivasa et al., 2015; ICRP, 1993). The study of natural background radiation and terrestrial radioactive nuclides is very important to assess radiation hazards in a particular area. The activity concentration of radionuclides depends on geology and the types of rocks and soil present in that area. In this regard, an attempt has been made for the first time in the present study area to estimate the activity concentration of primordial radionuclides in the soil samples and to estimate the radiological hazards.

1.1 Geology of the study area

The Hassan district lies between 12° 13′ and 13° 33′ north latitude and 75° 33′ and 76° 38′ east longitude. The geological map of the study area as shown in Figure 1. Geologically, Hassan district forms a part of Precambrian terrain. The major litho units are granitic gneiss, granulites, granites, associated pegmatite rocks and metamorphic rocks, steatite, Dharwarian schists and dyke rocks. The most important rock formation of the district is Nuggehalli and Holenarasipura Schist belt. These two schist belts are the host of number of mineral deposits like Chromites, Titaniferous magnetite, Chalcopyrite, Kaolin, Asbestos, Quartz, Granites, Gabbroid, Ultramafic igneous rocks, Dolerites, Diorites, Bauxite, magnesite, green granites (Dykes), Garnets, micas, Kyanite, columbite and tantalite etc. The important types of soil observed in the Hassan district are red sandy soil, red silty-clayey soil, mixed soil and black soil (Srinivasa et al., 2015, 2019).

thumbnail Fig. 1

Geological and lithological map of Hassan district, Karnataka state, India.

2 Materials and methods

2.1 Collection and preparation of soil samples

The soil samples were collected from 40 randomly selected undisturbed locations in the Hassan district and its surrounding areas to determine the activity concentration of natural radionuclides in the soil using standard procedures (Srinivasa et al., 2019; Volchok and de Planque, 1983). The samples were dried with sunlight to remove the moisture and then powdered and sieved through 125 μm size mesh. The samples were poured into porcelain dish and then dried in an oven at 110 °C for the overnight. The samples were then poured into plastic container of 250 ml capacity. The containers were firmly sealed internally and externally using proper adhesive tapes and weighed. The samples were kept for at least 30 days to attain secular equilibrium between parent and daughters of uranium and thorium series (Srinivasa et al., 2019; Srilatha et al., 2015).

2.2 Estimation of 238U, 226Ra, 232Th, and 40K in soil samples

The activity concentration of 238U, 226Ra, 232Th, and 40K in soil samples was determined by gamma ray spectrometry employing an n-type low background HPGe detector using GENIE-2000 spectrum analysis software at the laboratory of the University Science Instrumentation Centre, Mangalore University, for the present study. HPGe n-type detector having 48% efficiency coupled to 16 K MCA was used. It was enclosed by lead shield of thickness 10 cm (model GR 4021, Canberra, USA). Using area under the photo peaks of 214Pb (351.9 keV) to estimate 238U, 214Bi (609.3, 1129.3 and 1764.5 keV) to estimate 226Ra, one photo peak of 228Ac (911.2 KeV) and two photo peaks of 208Tl (583.1 and 2614.5 KeV) to estimate 232Th, and (1460.8 keV) to estimate 40K activity concentrations respectively. The standard procedure was followed to calibrate the instrument. Compton corrections were applied, background counts are subtracted, and counting was made for 60,000 s. The activity of radionuclides (Bq kg−1) was calculated using the following formula (IAEA/RCA, 1989).

Activity (Bq kg−1)=(S±σ)×100×1000×100EWA(1)

where s is the net counts per second under the photo peak of intensity, σ is the standard deviation of s, E is the counting (%), and A is the gamma abundance (%) of the radionuclides and W is the mass of the sample in grams.

3 Results and discussion

3.1 Distribution of radionuclides in soil samples

The average activity concentrations of the primordial radionuclides, such as 238U, 226Ra, 232Th, and 40K, present in the soil samples collected from the study region have been calculated using equation (1), and the results are given in Table 1. The activity concentrations of 238U, 226Ra, 232Th, and 40K in the soil samples under investigation vary from 16 ± 1 to 72 ± 2, 13 ± 1 to 64 ± 2, 19 ± 1 to 185 ± 2, and 197 ± 9 to 1214 ± 22 Bq kg−1, with an average value of 42 ± 2, 39 ± 1, 58 ± 2, and 592 ± 15 Bq kg−1, respectively. It is observed that the activity concentration of 238U and 226Ra are same in almost all samples, which confirms the radioactive equilibrium between them.

The average 238U, 226Ra, 232Th, and 40K activity concentrations and gamma absorbed dose rates in soil samples are found to be higher in some locations; Arsikere, Bageshpura, Javgal, Malnad Engineering College, MMIL (Katihally), Gorur, Holenarsipura, Arehally, Padavalahippe, and Devanur. The activity concentrations of 238U, 226Ra, 232Th, and 40K in the soil samples largely depend on the types of rock from which, the soil is originating and the types of formation and transfer processes that are involved (Srinivasa et al., 2019; Ahmed, 2005; Srilatha et al., 2015). The types of rocks in these locations are granite, pegmatite, titaniferous magnetite, and other associated rocks, which have higher concentrations of 238U, 226Ra, 232Th, and 40K than other rock types, and hence higher activity in soil samples was also observed (Srilatha et al., 2015; Rangaswamy et al., 2016). A slightly higher activity concentration of 238U, 226Ra, 232Th, and 40K and a gamma absorbed dose rate from these radionuclides were observed in rock and soil samples collected from the locations Ramanathapura, Magge, Byrapura, Halebeedu, Yagachi, Thagadur, Nuggehally, Gorur, the Master Control Facility (MCF), Nonavinakere, Chiknayakanahally, Kadur, and Devanur. This is due to the fact that these regions were attributed to gneiss, altered grey granites, titaniferous magnetite, and pegmatite rocks (Srinivasa et al., 2015, 2019). In Nuggehally, Thagadur, Byrapura, and Channarayapatna, which contain slightly less concentration of 238U, 226Ra, 232Th, and 40K compared to pink and grey granites (Srilatha et al., 2015).

The lower average activity concentrations of 238U, 226Ra, 232Th, 40K, and gamma absorbed dose rates in soil samples are found at some locations in Palya, Kundur, Konanur, Doddamagge, Vijay nagar, Sakaleshpura, Anemahal, Konanuru, Belur, Hagare and Hethur. The lower activity concentration in these locations is due to fact that the rock samples mainly consist of ultramafic schist, gneiss, dolerite (green colour) rocks, and sedimentary rocks, which contain lower concentrations of 238U, 226Ra, 232Th, and 40K in these regions (Megumi et al., 1988; Rafique et al., 2014). The results show that the average activity concentration of 40K in soil samples in the study area has a higher activity concentration than the global average value of 400 Bq kg−1. In some locations, the activity concentration of 232Th is higher than 238U and 226Ra due to intrusion of pegmatite rocks into the granitic rocks (Ningappa et al., 2008).

The activity concentration values of 238U, 226Ra, 232Th, and 40K obtained from the study area were higher than the values reported for Indian soils and for normal background areas, respectively (UNSCEAR, 2008). According to the UNSCEAR 2008 report, the Indian average activity concentrations of 238U, 226Ra, 232Th, and 40K in soil samples are 31, 29, 64 and 400 Bq kg−1, respectively (UNSCEAR, 2000, 2008). It is clear from this study that, except for 232Th, the average activities of 238U, 226Ra, and 40K values are slightly higher than the Indian average. The world average activity concentration values of 238U, 226Ra, 232Th, and 40K for soil are 35, 32, 45, and 420 Bq kg−1, respectively (UNSCEAR, 2000). The data shows that the average activity concentration values of 238U, 226Ra, 232Th, and 40K in soil samples from the study area were found to be slightly higher than the global average values (UNSCEAR, 2000). The average activity concentration values of 238U, 226Ra, 232Th, and 40K as obtained in the study area were compared with the values reported in different parts of Karnataka shown in Table 2 (UNSCEAR, 2000; Srinivasa et al., 2022; Ningappa et al., 2008; Narayana et al., 2001; Prasad et al., 2008; Shivakumara et al., 2014; Sannappa et al., 2010; Karunakara et al., 2001; Yashodhara et al., 2011; Suresh et al., 2022; Kerur et al., 2011; Srilatha et al., 2015; Umesh Reddy et al., 2017).

Table 1

Average activity concentration of 238U, 226Ra, 232Th, and 40K in Hassan district soil samples.

Table 2

Comparison of the average activity concentration of 238U, 226Ra, 232Th, and 40K recorded in literature for different region soil samples.

3.2 Frequency distribution graphs for radionuclides in soil

Figures 2a–2c represent the frequency distribution graphs of 226Ra, 232Th, and 40K measured by using HPGe in soil samples of the study area, respectively. The frequency distribution graph of 226Ra in the study area is shown in Figure 2a. The results reveal that about 57.5% of the soil samples were below 40 Bq kg−1, and the remaining 42.5% of the samples showed concentrations above 40 Bq kg−1. The frequency distribution of activity of 232Th in the study area is shown in Figure 2b. The results reveal that about 37.5% of the soil samples were below the 40 Bq kg−1, and the remaining 62.5% of the samples showed concentrations above 40 Bq kg−1. The frequency distribution of activity of 40K in the study area is shown in Figure 2 (c). The results reveal that, only 22.5% of the soil samples were lower than 400 Bq kg−1, and the remaining 77.5% of the samples showed concentrations above 400 Bq kg−1.

thumbnail Fig. 2

(a) Frequency distribution of 226Ra in soil. (b) Frequency distribution of 232Th in soil. (c) Frequency distribution of 40K in soil.

3.3 Radiological characterization

Radium-equivalent activity (Raeq) can be used to calculate the radiological effect of radionuclides because of internal and external exposure. Gamma absorbed dose rate (GADR) (D), AEDE, internal and external hazard indices (Hin and Hext), and gamma index (Iγ), in all these calculations, CRa, CTh, and CK are the activity concentrations of 226Ra, 232Th, and 40K in Bq kg−1 respectively. Various radiological hazard indices were calculated using the standard formulas and are listed in Table 3. The average values of radium equivalent activity, gamma absorbed dose rate (GADR) (D), AEDE, internal and external hazard indices (Hin and Hext), and gamma index (Iγ) are summarised in Table 4.

The Raeq values in the soil samples range from 68 to 395 Bq kg−1, with a mean value of 168 Bq kg−1. The mean Raeq value of the present study is less than the criterion limit of 370 Bq.kg−1 (Beretka and Mathew, 1985), and as such, this soil does not pose any radiological hazard when used for construction purposes.

The estimated gamma absorbed dose rate due to the primordial radionuclides in soil samples varies from 31 to 176 nGy h−1, with an average value of 78 nGyh−1, which is higher than the average value of global primordial radiation of 44 nGy h−1 for soil and the all-India average value of 69 nGy h−1 (Srinivasa et al., 2015). The annual equivalent effective dose rate in soil is found to vary from 0.19 to 1.08 mSv y−1, with an average of 0.49 mSv y−1. The average annual effective dose is higher than the average value for India of 0.0843 mSv y−1 (Nambi et al., 1986) and the world average value of 0.0725 mSv y−1 (Srinivasa et al., 2015).

External gamma dose rates in air were measured at the same locations where soil samples were taken using a portable GM tube-based environmental radiation dosimeter (ER-709, manufactured by Nucleonix Systems Pvt. Ltd.) in Hyderabad, India. Table 4 also displays the results of these measurements.

The measured values of indoor gamma absorbed dose rates were found in the range of 80 ± 5 to 391 ± 6 nGy h−1, with an average value of 200 ± 7 nGy h−1. This value is nearly two times higher than the world average of 84 nGy h−1 (Srinivasa et al., 2015). Similarly, outdoor gamma absorbed dose rates were found in the range of 63 ± 4 to 356 ± 11 nGy h−1, with an average value of 166 ± 6 nGy h−1. This value is nearly three times higher than the global average of 59 nGy h−1. The total annual effective dose for adults living in the study area ranges from 0.47 to 2.35 mSv y−1, with a mean of 1.18 mSv y−1, Nambi et al. found dose levels in India to be (0.44 mSv y−1) (Nambi et al., 1986).

The correlation between the annual effective dose measured by the dosimeter and the activity of the soil sample is shown in Figure 3. It is clear from Figure 3 that there is a good positive correlation between the radiation dose measured by the portable radiation survey meter and the estimated dose from the HPGe detector, with a correlation coefficient of 0.73. A comparison between the measured and estimated doses showed very good agreement between the two. This good correlation confirms the meter’s accuracy and assists the researcher in quickly determining the dose at any location. There is little difference between the estimated and measured values of the absorbed dose because it is only the estimated dose in the soil, and this measurement does not include the dose due to rocks, building materials and the cosmic ray component. The ratio of indoor to outdoor gamma absorbed dose rates varies from 0.98 to 2.23, with a median value of 1.21. The ratio of indoor to outdoor absorbed doses of gamma radiation has been found to be approximately 1.2 in areas with normal background radiation in India (Nambi et al., 1986).

For safe use of the material in housing construction, Hex and Hin should be less than 1 so that the radiation risk is negligible. The calculated internal (Hin) and external (Hex) radiation hazard index values of the soil samples range from 0.24 to 1.27 with an average value of 0.57 and 0.18 to 1.07 with an average value of 0.46. The average values of external and internal health risk indices due to the presence of natural radionuclides in soil samples are less than unity, which is highly acceptable and within the safe limit in the studied area (Srinivasa et al., 2015; Farai and Vincent, 2006; Mahur et al., 2008; Beretka and Mathew, 1985; Akkurt et al., 2009; Nambi et al., 1986; UNSCEAR, 1993). Therefore, these areas may not pose a radiological risk to residents due to the harmful effects of ionizing radiation from natural radionuclides in the soil.

The estimated gamma index (Iγ) values of the soil samples vary from 0.25 to 1.42, with an average of 0.63. The value of Iγ must be less than 1 to keep the radiation hazard negligible for the general population. Except for the soil samples collected from Mysore Mineral Industries Limited (MMIL) of Hassan city, the calculated Iγ values are well below the criterion limit of unity as per European Commission of Radiation Protection reports (European Commission, 1999).

Table 3

Various radiological hazard indices calculating formulae.

Table 4

Average gamma absorbed dose, annual effective dose, and health hazard indices of soil samples.

thumbnail Fig. 3

Correlation between measured and estimated annual effective dose rate in soil.

3.4 Variation of activity of radionuclides in soil with depth and grain size

To study the variation of activity concentration with different depths, soil samples were collected at the same location from different depths, viz. 0–15, 15–30, and 30–45 cm. The activity of radionuclides with different depth profile is shown in Figure 4. Various processes such as weathering, water action, water transport, chemical and geochemical reactions, among others, cause the uneven distribution of natural radionuclide activity in soil (Volchok and de Planque, 1983). The highest activity was found in the topsoil, and activity values decrease as depth increases.

The activity concentration also varies with the grain size of the soil matrix. To quantify variation, soil samples were collected at a depth of 15 cm, crushed, and sieved through 150 μm, 250 μm, 400 μm, and 500 μm sieves to obtain different soil grain sizes. The activity of radionuclides with different grain sizes in the soil samples is shown in Figure 5. The lowest activity was observed for coarser particles (400–500 µm). The activity increases in the next fraction (400–250 µm and 250–150 µm). The highest activity of all three radionuclides was found to be in the (<150 µm) particle size fraction. This means that the activity is inversely proportional to the grain size. Higher activity was associated with the finest particle size. This is due to the weathering of minerals and a high degree of absorption (Kerur et al., 2010).

thumbnail Fig. 4

Distribution of radionuclides as a function of depth profile.
thumbnail Fig. 5

Distribution of radionuclides as a function of grain size.

4 Conclusion

From the study, it was observed that the average activity concentration of 238U, 226Ra, 232Th, and 40K in soil samples was found to be slightly higher than the global average value. The activity concentration of primordial radionuclides increases with a decrease in grain size. The activity concentration of 226Ra and 232Th decreases with an increase in depth. However, 40K activity concentration is independent of depth. The average radium equivalent activity values due to radionuclides present in the soil were found to be less than the maximum criterion limit of 370 Bq kg−1, and the average values of radiological hazard indices are well within the safe limit (≤1). Finally, it is concluded that the radiation emitted from the radionuclides present in the soil samples of the study area does not pose any radiological health hazard to the public in and around the study area.

Funding

There is no funding to conduct this work.

Conflicts of interest

The author declares that they don’t have competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability statement

All data generated or analysed during this study are included in this paper.

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Cite this article as: Srinivasa E, Rangaswamy DR, Suresh S. 2024. Natural radioactivity and radiological hazards assessment in soil samples of Hassan district, Karnataka State, India. Radioprotection 59(4): 317–326

All Tables

Table 1

Average activity concentration of 238U, 226Ra, 232Th, and 40K in Hassan district soil samples.

In the text
Table 2

Comparison of the average activity concentration of 238U, 226Ra, 232Th, and 40K recorded in literature for different region soil samples.

In the text
Table 3

Various radiological hazard indices calculating formulae.

In the text
Table 4

Average gamma absorbed dose, annual effective dose, and health hazard indices of soil samples.

In the text

All Figures

thumbnail Fig. 1

Geological and lithological map of Hassan district, Karnataka state, India.
In the text
thumbnail Fig. 2

(a) Frequency distribution of 226Ra in soil. (b) Frequency distribution of 232Th in soil. (c) Frequency distribution of 40K in soil.
In the text
thumbnail Fig. 3

Correlation between measured and estimated annual effective dose rate in soil.
In the text
thumbnail Fig. 4

Distribution of radionuclides as a function of depth profile.
In the text
thumbnail Fig. 5

Distribution of radionuclides as a function of grain size.
In the text

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