Introduction

Soil is one of the essential natural resources available in upper layer of the earth crust. It consists of organic matter, mineral particles, air, organisms and water [1, 2]. Soil is derived from the parent rocks. The activity of radionuclides in soil mainly depends on activity of radionuclides present in the parent rock, types of rock, interaction of rock with water and meteorological parameters and chemical changes. Physiochemical properties of soil play a vital role in the distribution, concentration and behavior of radionuclides in soil [3,4,5]. People living in granite or mineralized sand areas receive more terrestrial radiation compared to other areas. The public living or working at high altitude receive more cosmic radiation [6,7,8]. Whether it is natural or artificial radionuclides, they are accessible for uptake by animals and plants and enter the human food chain [9]. When radionuclides enter the atmosphere, they undergo decay and get deposited on the surface of the earth by dry or wet deposition within relatively short periods. They initially get deposited on the top surface of the soil, but rapidly diffuse into the first few centimeters of the soil [10]. The activity of radionuclides in food grains also depends on the activity of radionuclides present on the top surface of the soil. Hence, assessment of radiological risk due to the radionuclides present in soil is very important.

The area of present study is around Kolar Gold Fields (KGF), Kolar located at latitude 13°07′48″N and longitude 78°13′48″E. The second deepest underground mines Bharath Gold Mine Limited (BGML) is located in the area of study. Geologically, the study area belongs to hornblende gneiss, closepet granite, quartzite, champion gneiss, metabasalt, dolerite dyke and grey granite. The metabasalt overlying the champion gneiss is composed of massive schistose and granular varieties of amphibolite showing primary volcanic structure such as pillows, variolites and amygdales. Post Dharwar intrusive present in the study area are granite, dolerite and quartz veins. Granite occurs as plutons, tongues and apophyses within the peninsular gneiss.

The key economic mineral deposit in the study area is gold. KGF is known to contain 26 numbers of gold bearing quartz lodes and the bulk of the metal has come from the main Champion quartz lode system having a strike length of 8 km and worked to a maximum vertical depth of about 3209 m. More than 800 tonne of gold has been produced from the mining. Major portion of the study area consists of granite and hornblende gneiss. The types of soil in study area are red sandy/loamy and lateritic. Geological map of the study area with selected places is as shown in Fig. 1. Kolar district is called the land of silk, milk, and gold.This study is important to provide baseline datafor the general public of the study area from the health point view. Furthermore, no research work on measurement of radionuclides concentration in the soil has been carried out in Kolar district till day.

Fig. 1
figure 1

Geological map of Kolar District with location numbers

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Materials and methods

Collection and preparation of soil sample

The soil samples were collected at 30 different places of the study area. The places, which were free from surface runoff during heavy rain, were carefully selected. An area of ~ 0.5 m2 was cleared of vegetation and roots. The marked spot was dug up to a depth of 15 cm and ~ 2 kg soil was collected at each place. Finally, the samples were mixed thoroughly and extraneous materials like plants, debris, big pieces of stones and pebbles were removed and the samples were transferred to a porcelain dish [11]. These samples were kept in an oven for overnight at 110 °C to remove water content in the samples. Then the samples were sieved through 200 µm mesh. About 300 g of each sample was taken and packed in a 300 ml plastic container and left for at least 40 days before taking readings to attain radioactive equilibrium between 226Ra, 222Rn and their progeny.

Activity concentration of radionuclide

The gamma ray spectrometric procedure is used to determine the activity concentration of 226Ra, 232Th and 40K in the collected soil samples at different places of the study area [12]. The detector used is Hyper Pure Germanium (HPGe) detector (41% efficiency n-type detector, Canberra Industries, Inc., Meriden, CT, USA) coupled to a DSA-1000 with 16 K Multi Channel Analyzer. The detector was enclosed in a 0.1-m thick graded lead shield (Model 747, Canberra, USA). The detector has an in-built 2002 CSL FET cooled charge sensitive pre-amplifier. As discussed above, the output pulses from the detector are amplified using this pre-amplifier. The signal from the pre-amplifier is then fed to a computer controlled Digital spectrum analyser (DSA −1000) unit which is plugged to a PC. This DSA-1000 unit consists of linear amplifier, high voltage supply (0–5000 V), Analogue to Digital convertor (ADC), and a 16 K Multi Channel Analyser (MCA). This MCA is one of the most advanced systems with the option to analyse both the background and sample spectra, simultaneously. The MCA has built in Analogue to digital converter capable of producing highly accurate data transformation and categorisation. Each channel in the MCA is defined as an arbitrary interval of time, ranging from 10 ms to 1 s per channel.

Activity of 226Ra, 232Th and 40K was determined from photo peaks of 214Bi (609.3, 1129.3 and 1764.5 keV), 228Ac (911.2 keV) and 1460.8 keV, respectively, after subtracting the background counts and applying Compton corrections. The detector efficiency calibration was performed using the IAEA quality assurance reference materials: RG U-238, RG Th-232 and RG K-1. The minimum detection levels (MDL) for the detecting system used in this study were 0.9, 1.2, and 4.0 Bq kg−1, respectively, for 226Ra, 232Th, and 40K, for a counting time of 60,000 s. The activity of radionuclides (in Bq kg−1) is calculated by using the relation [13]

$$A \left( {{\text{Bq}}\;{\text{kg}}^{ - 1} } \right) = \frac{{\left( {s \pm \sigma } \right) \times 100 \times 1000 \times 100}}{EWa}$$
(1)

where A is the activity of radionuclide (Bq kg−1), s is the net counts per second under the photo peak of intensity, σ is the standard deviation of s, E is the counting (%) efficiency, a is the gamma abundance (%) of the radionuclide’s and W is the mass of the sample (kg).

Ambient gamma radiation level

Ambient gamma radiation levels in the outdoor atmosphere at all the places of soil samples have been collected and measured using scintillometer [Type SM 141D, ECIL]. The detector is a thallium activated sodium iodide crystal optically coupled to a photomultiplier. All the readings have been taken at 1 m above the ground level. At each place 5–6 readings have been taken, and then the average of all these readings has been taken. By using the conversion factor, exposure rate (µR h−1) is converted into absorbed dose rate (nGy h−1) 1 µR h−1 = 8.7 nGy h−1 [14].

Results and discussion

Distribution of radionuclides in soil samples

The average activity concentration of naturally occurring radionuclides such as 226Ra, 232Th and 40K in soil samples collected at places around KGF are measured using HPGe detector. The obtained values are summarized in the Table 1. From the Table 1, the average activity concentration of 226Ra, 232Th and 40K are varied from 15.7 ± 2.5 to 41.9 ± 1.8, 37.0 ± 1.5 to 92.8 ± 3.6, and 481 ± 4 to 1125 ± 11 Bq kg−1 with an average value of 27.3 ± 1.8, 63.1 ± 2.5 and 818 ± 6 Bq kg−1, respectively. The study shows that activity of 226Ra and 232Th is relatively lower than that of 40K. Activity concentration of thorium is higher than that of radium at all locations [15]. The ratio of 232Th and 226Ra varies between 1.6 and 3.2 with an average value of 2.4. This ratio gives the sign of relative presence of uranium and thorium. The activity concentration of radium is lower than thorium at all places. This is due to pegmatite introduced in rocks and mineral composition [15].

Table 1 The average activity of 226Ra, 232Th and 40K in soil samples with 232Th/226Ra ratio at different places of the study area
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From the Table 2, max concentration of radionuclides in soil samples have been observed at places such as Kolar, Nelawanki, Yalduru and Srinivaspur. This is due to types of rock from which the soil has been originated. The rocks in these locations are granite. Activity of radionuclides is more in granite rocks when compared to other types of rocks [16] Slightly lesser concentration of 226Ra and 232Th in soil samples have been observed at the locations of Narasapura, Masthi, Sugaturu when compared to Nelawanki, Yalduru, Srinivaspur and Kolar. This is due to geology of these locations that consists of hornblende gneiss and amphibolites. These consist of less activity of radionuclides compared to granitic rocks. The minimum concentration of 226Ra and 232Th in soil samples has been observed at the locations of Andersonpet, BEML Nagar, Marikuppam. This is because of local geology. These locations consist of metabasalt, metagabbro, quartzite and Schist, and these have less activity concentration of radionuclides compared to granitic rocks [17].

Table 2 Comparison of activity of radionuclides from soil samples collected from the study area with the values obtained at different regions of the world
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The present study reveals that the average activity concentration of 226Ra is found less, and activity concentration of 232Th and 40K are found higher than the world average values of 32, 45 and 420 Bq kg−1 and Indian average values of 29, 64 and 400 Bq kg−1, respectively [10]. The activity concentration of primordial radionuclides such as 226Ra, 232Th and 40K in soil samples of different environment are compared with the values obtained in the study area and has been summarized in the Table 2.

The correlation between the activity concentrations of 226Ra and 232Th are shown in Fig. 2. This shows that there is a positive correlation between activity concentrations of 226Ra and 232Th with correlation coefficient 0.65. The correlation between 40K and 226Ra and 40K and 232Th are shown in Figs. 3 and 4. These correlations show positive values with the coefficient values of 0.49 and 0.45, respectively. The correlation shows the property of the soil in retaining these radionuclides under changing weather conditions. It indicates that the individual result for any one of the radionuclide concentration in every pair is a good interpreter of other concentration.

Fig. 2
figure 2

Correlation between 226Ra and 232Th concentration of the soil samples

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Fig. 3
figure 3

Correlation between 226Ra and 40K concentration of the soil samples

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Fig. 4
figure 4

Correlation between 232Th and 40K concentration of the soil samples

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Radium equivalent activity (Raeq)

In India, local soil is used as building material directly or indirectly. Raeq is calculated due to the presence of radionuclides 226Ra, 232Th, and 40K in collected samples using their activity concentration using the relation [18, 19]

$${\text{Ra}}_{\text{eq}} \left( {{\text{Bq}}\;{\text{kg}}^{ - 1} } \right) = A_{{{\text{Ra}} }} + 1.43A_{\text{Th}} + 0.07 A_{\text{K}}$$
(2)

where A Ra, A Th, and A K, are the measured activity concentrations of 226Ra, 232Th, and 40K in Bq kg−1, respectively and the values are summarized in Table 3. The values vary between 102.3 and 242.7 Bq kg−1 with an average value of 174.8 Bq kg−1, less than the criterion limit of 370 Bq kg−1 [18]. This shows that the collected soil samples of study area does not create any radiological hazard when it is used as building material for construction. Correlation between activity concentration of 232Th and Raeq with the coefficient value 0.918 has been observed (Fig. 5). This shows that radium equivalent activity is mainly due to concentration of 232Th in soil samples.

Table 3 Radium equivalent activity, absorbed dose (calculated and measured) and annual effective dose of soil samples at different places of the study area
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Fig. 5
figure 5

Correlation between 232Th and Raeq activity concentration of the soil samples

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Hazard indices

The ultimate use of the measured activity concentrations was to assess the radiological hazards. Radiological hazard indices such as alpha index, gamma index, internal hazard index and external hazard index values have also been calculated for the soil samples from the study area.

Alpha index (I α)

The alpha index (Iα) in Bq kg−1 gives the excess alpha radiation due to radon inhalation originating from building materials, such as soil, which is defined as [20]

$$I_{\upalpha } = \frac{{A_{\text{Th}} }}{200}$$
(3)

where A Ra is the activity concentration of 226Ra in Bq kg−1. The recommended maximum concentration of 226Ra is 200 Bq kg−1, which gives I α is equal to unity.

Alpha index (I α) values of all the collected samples in the area of study vary from 0.08 to 0.21 with an average of 0.14 which is less than the recommended maximum value of 1. This shows that the radon exhalation from soil samples would cause indoor concentration less than 200 Bq m−3.

Gamma index (I γ)

As per the European Commission [21], gamma index (Iγ) defined by the following relation [21]. It has been introduced to account for the combined impact of 226Ra, 232Th and 40K as radiological hazard associated with material.

$$I_{\upgamma } = \frac{{A_{\text{Ra}} }}{300} + \frac{{A_{\text{Th}} }}{200} + \frac{{A_{\text{K}} }}{3000}$$
(4)

where A Ra, A Th and A K are the specific activities of 226Ra, 232Th and 40K respectively in any material. Materials having I γ ≤ 2 will make an increase of 0.3 mSv in the annual effective dose. Whereas 2 < I γ ≤ 6 correspond to an increase of 1 mSv year−1 [20, 21]. Gamma index (I γ) values in soil samples of the area of study vary from 0.398 to 0.930 with an average value of 0.679, within the recommended safe limit of 1 as per European Commission [21].

Internal hazard index (H in)

The internal exposure to radon and its daughter products is controlled by the internal hazard index (H in) and is given by the relation [21]

$$H_{\text{in}} = \frac{{A_{\text{Ra}} }}{185} + \frac{{A_{\text{Th}} }}{259} + \frac{{A_{\text{K}} }}{4810}$$
(5)

where A Ra, A Th, and A K, represent the measured activity concentration of 226Ra, 232Th, and 40K in Bq kg−1, respectively. For safe use of soil H in should be < 1 [22]. Internal hazard index (H in) values of all the collected samples range from 0.328 to 0.783 with an average value of 0.561. All the values are found to be less than the maximum limit of 1 as per European Commission [21].

External hazard index (H ex)

The external hazard index (H ex) evaluates the indoor radiation dose rate due to the external exposure to gamma radiation from the natural radionuclides, which is given by the relation [10]

$$H_{\text{ex}} = \frac{{A_{\text{Ra}} }}{370} + \frac{{A_{\text{Th}} }}{259} + \frac{{A_{\text{K}} }}{4810}$$
(6)

where A Ra, A Th, and A K, represent the measured activity concentration of 226Ra, 232Th, and 40K in Bq kg−1, respectively. The maximum value of H ex is unity. External hazard index (H ex) values of all samples vary from 0.285 to 0.674 with an average value of 0.488 less than the recommended value. This shows that, soil from these locations is safe and can be used a building material for construction without posing any considerable radiological threat to public.

Absorbed dose rate (D)

The indoor absorbed gamma dose rate in air (D in nGy h−1) at a height of ~ 1 m above ground level due to individual natural radionuclides 226Ra, 232Th and 40K in soil samples has been calculated by using equation [21];

$$D_{\text{in}} \left( {{\text{nGy}}\; {\text{h}}^{ - 1} } \right) = 0.92 A_{{{\text{Ra}} }} + 1.1 A_{\text{Th}} + 0.084 A_{\text{K}}$$
(7)

where 0.92, 1.1 and 0.084 are the activity to indoor dose rate conversion factors in nGy h−1 per Bq kg−1 for 226Ra, 232Th and 40K, respectively. The indoor absorbed dose lies between 95.6 and 224.1 nGy h−1 with an average value of 163.3 nGy h−1 (Table 3).

The outdoor absorbed gamma dose rate (D out nGy h−1) in air has been calculated using the equation;

$$D_{\text{out}} \left( {{\text{nGy h}}^{ - 1} } \right) = 0.462 A_{\text{Ra }} + 0.604 A_{\text{Th}} + 0.0417A_{\text{K}}$$
(8)

where A Ra, A Th, and A K, are the measured activity concentration of 226Ra, 232Th, and 40K in Bq kg−1, respectively. 0.462, 0.604 and 0.0417 are corresponding dose coefficients. The outdoor absorbed dose lies between 49.7 and 116.6 nGy h−1 with an average value of 84.9 nGy h−1 (Table 3), which is higher than the population weighted average value for outdoor 59 nGy h−1 [10]. Outdoor absorbed gamma dose with 226Ra, 232Th and 40K concentration of the soil samples shows good correlation coefficients of 0.789, 0.886 and 0.755, respectively. The measured value of absorbed gamma dose from scintillometer varies from 72.5 to 166.7 nGy h−1 with an average value of 115.3 nGy h−1 (Table 3). Figure 6 shows positive correlation between radiation dose measured from scintillometer and the calculated dose from activity concentration of radionuclides in the soil samples with correlation coefficient of 0.756.

Fig. 6
figure 6

Correlation between gamma absorbed dose measured values using scintillometer with calculated values using activity of radionuclides in soil

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Annual effective dose

Indoor and outdoor annual effective dose rate (E in and E out in µSv year−1) are estimated from calculated indoor and outdoor absorbed gamma dose (D in and D out in nGy h−1) by using the equations;

$$E_{\text{in}} \left( {{\text{mSv year}}^{ - 1} } \right) = D_{\text{in}} \times 8760 \times {\text{OF }} \times {\text{CF}}$$
(9)
$$E_{\text{out}} \left( {{\text{mSv year}}^{ - 1} } \right) = D_{\text{out}} \times 8760 \times {\text{OF }} \times {\text{CF}}$$
(10)

where D in and D out are the indoor and outdoor absorbed dose respectively in nGy h−1, OF is the Occupancy Factor and it is 0.2 for outdoor and 0.8 for indoor. CF is the Conversion Factor from absorbed dose rate in air to effective dose received by the person (0.7 × 10−6 Sv year−1). The time spent in the indoor and outdoor is 8760 × 0.8 and 8760 × 0.2 hy−1, respectively. The values are presented in Table 3. The indoor annual effective dose value varies between 468.8 and 1099.2 µSv year−1 with an average value of 801.0 µSv year−1 and the outdoor values lie between 60.9 and 142.6 µSv year−1 with an average value of 104.1 µSv year−1 which is higher than the world average value of 70.0 µSv year−1 [10]. Total dose values vary between 529.7 and 1241.7 µSv year−1 with an average value of 905.1 µSv year−1. This average value is less than the recommended safe limit of 1000 µSv year−1 by ICRP for the individual members of the public [23]. A comparative study of average outdoor annual effective dose rate obtained from the present work has been made with the different areas of the world, and is mentioned in Table 4.

Table 4 Comparison of average outdoor annual effective dose rate from present study with the values obtained at other parts of the world
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Conclusion

The present study reveals that activity of thorium is more than that of radium in soil samples in and around of Kolar Gold Field, Kolar District. It has been observed that soil originated from granitic region has higher activity of radionuclides than the soil originated from gneiss, metabasalt, metagabbro, amphibolites. The average activity concentrations of 226Ra, 232Th and 40K in the soil samples were 27.3, 63.1 and 818 Bq kg−1, respectively. The values of activity concentration of 232Th and 40K in soil samples of the study area are higher than Indian and Global average values. The radium equivalent activity (174 Bq kg−1) is found less than the recommended safe value (370 Bq kg−1). The absorbed dose rate is calculated from activity of radionuclides and found greater than the world average value. The measured dose rate (84.9 nGy h−1) and calculated absorbed dose rate (115.3 nGy h−1) are exceeding the world average (59 nGy h−1). The calculated values of hazard indices were less than unity.