Introduction

The inevitable ionizing radiation exposure to human being mainly stems from natural and anthropogenic radioactive sources. Natural radiation exposure is caused by cosmogenic and terrestrial radionuclides. Terrestrial radionuclides contain the radioactive series of uranium–radium (238U–226Ra), thorium (232Th) and radioactive potassium (40K) in the earth’s crust. The average annual dose to human being is 2.8 mSv of which over 85 % is from natural radiation sources with about half coming from radon (222Rn) decay products (UNSCEAR 2008). Soil is the most important source of the terrestrial radionuclides whose activity concentrations depend primarily on the geological and geochemical conditions of each region in the world (UNSCEAR 2008). Major anthropogenic radionuclide sources which contribute to the radionuclide contamination of the environment contain nuclear weapon tests, nuclear power plants, commercial fuel reprocessing and geological repository of high level nuclear wastes. The majority of the global fallout radionuclides resulted from atmospheric nuclear weapon tests conducted from 1945 to 1963 and nuclear accidents. Among different fallout radionuclides, 137Cs (half-life 30.1 years) is the most prominent isotope detected by its gamma energy (661.8 keV) on the earth’s surface. During the Chernobyl nuclear accident in 1986 about 3.8 × 1016 Bq of 137Cs was released to the atmosphere and carried across the international boundaries (Mohapatra et al. 2015; UNSCEAR 1988). Also the nuclear accident in Fukusima plays a major role in fallout release of 137Cs. Gamma radiation emitted from terrestrial and fallout radionuclides is the main external exposure to human beings. Hence determination of the terrestrial and fallout radioactivity levels in various environmental samples such as soil, plants, foods, rocks, etc. is very important for evaluation of public exposure, storage reference data records on radionuclides for producing a radiation map of the country and ascertaining possible changes in environmental radioactivity due to anthropogenic activities (Turhan et al. 2012).

Soil which contains the major sources of natural radionuclides (238U, 232Th, 226Ra, 222Rn, 210Pb, 40K etc.) and fallout radionuclides (137,134Cs, 89,90Sr, 3H, etc.) is a direct radiation source in the environment (Cho et al. 2014; Lee et al. 1995). Hence, it is critically important to assess the amount of radioactivity in the soil because of its major influence on the terrestrial ecosystem and humans (Cho et al. 2014). Moreover, the baseline concentration is needed when examining incidents in nuclear power plant facilities to determine the characteristics and behavior of the radiation environment, and to conduct research on the geological features and soil (Jun et al. 1990; Velasco et al. 2012; Cho et al. 2014). Recently, many studies related to measurement of the activity concentrations of the terrestrial and fallout radionuclides were published in the literature (Srilatha et al. 2015; Santawamaitre et al. 2014; Sivakumar 2014; Dusane et al. 2014; Aközcan 2014; Miller and Voutchkov 2014; Ajmal et al. 2014; Rajeshwari et al. 2014; Kunovska et al. 2013; Manohar et al. 2013; Dhawal et al. 2013; Khan et al. 2012). However, there are few studies related to the activity concentrations of terrestrial and fallout radionuclides in environmental samples collected from Mersin province in literature (Özmen et al. 2014; Kurt and Berker 2014).

Mersin province is near the Mediterranean Sea in southern of Turkey. It is located at the Çukurova region and situated between 32 56′–35 11′E and 37 26′–36 01′N (Fig. 1). Mersin has a population of 1,727,255 as of 2014 and occupies 15,853 km2. Mersin is one of Turkey’s most important cities with its economy determined by agriculture, trade, tourism and industry. Akkuyu nuclear power plant (NPP) having 4 × 1200 MW VVER units to be the first nuclear power plant in Turkey will be built in Büyükeceli which is one of the districts of Mersin (Özmen et al. 2014). The aim of the study is to determine the activity concentrations of 226Ra, 232Th, 222Rn, 40K and 137Cs in soil samples collected from Mersin and Akkuyu nuclear power plant (NPP) region. The measurements of the activity concentrations of 226Ra, 232Th, 40K and 137Cs were performed by high-resolution gamma-ray spectrometer with a hyper pure germanium (HPGe) detector. An evaluation of the radiation hazards for human beings due to the natural and fallout radioactivity arising from soil samples was determined in terms of the absorbed gamma dose rate in outdoor air (DRout), annual effective dose rate from external exposure (E Ext), annual effective dose rate from inhalation of radon (E Inh) and lifetime cancer risk (LTCR).

Fig. 1
figure 1

Geological map of Mersin province (Duran 2014)

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Materials and methods

Sample processing and activity measurements

Surface soil (0–5 cm) samples were collected randomly from 32 undisturbed sites in Mersin province and around Akkuyu NPP region (Fig. 2). The surface soil samples were properly coded according to the location of the sampling site (Table 1). The soil samples were dried in a temperature-controlled furnace at 105 °C for 24 h to remove moisture. After homogenization, samples were sieved, placed in plastic containers, weighed and hermetically sealed. Before starting the gamma spectrometric measurements, the sealed samples were stored for 4 weeks to reach radioactive equilibrium of the 226Ra, 232Th and their decay products.

Fig. 2
figure 2

Location of sampling in Mersin province and Akkuyu NPP region

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Table 1 Positions of the soil samples locations in Mersin and Akkuyu NPP region
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Activity measurements were performed by a high-resolution gamma-ray spectrometer at the Gülten Günel Nuclear Physics Research laboratory in Physics Department of Çukurova University. The gamma-ray spectrometer is equipped with a coaxial p-type HPGe detector (GX5020) with a relative efficiency of 50 %. The HPGe detector’s energy resolution is 2.0 keV at 1332.5 keV. For gamma-ray shielding, a front opening split-top shield was used to reduce the background. The detector was interfaced to the digital spectrum analyzer (DSA-1000), which was a full-featured 16K channel multichannel analyzer on advanced digital signal processing (DSP) techniques. DSA-1000 operates through Genie-2000 gamma spectroscopy software including peak searching, peak evaluation, energy/efficiency calculation mode, nuclide identification (Uğur et al. 2013). Each soil sample was placed on the top of the detector and counted for 24 h. Background measurements were taken under the same conditions of sample measurements and subtracted in order to get net counts for the sample. The HPGe detector was calibrated for energy and efficiency using reference materials RGU-1 (U-ore), RGTh-1 (Th-ore) and RGK-1 (K2SO4) supplies by International Atomic Energy Authority (IAEA). The activity concentration of 226Ra was derived from the average of the activities of the gamma-ray line of 609.3 keV from 214Bi and 351.9 keV from 214Pb, while the gamma-ray lines of 911.2 keV from 228Ac and 583.2 keV from 208Tl were used to determine the activity concentration of 232Th. The activity concentrations of 40K and 137Cs were measured from 1460.8 and 661.6 keV direct gamma-ray lines, respectively.

The combined standard uncertainty of the activity concentration is calculated by the next formula:

$$ \Delta A = A \times \sqrt {\left( {\frac{{\Delta C_{\text{R}} }}{{C{}_{\text{R}}}}} \right)^{2} + \left( {\frac{\Delta I}{I}} \right)^{2} + \left( {\frac{\Delta \varepsilon }{\varepsilon }} \right)^{2} + \left( {\frac{\Delta M}{M}} \right)^{2} } $$
(1)

where A and ΔA is the activity concentration and its uncertainty; C R and ΔC R is the count rate and its uncertainty; I and ΔI is the gamma emission probability and its uncertainty, ε and Δε is the absolute efficiency of the detector and its uncertainty; M and ΔM is the mass and its uncertainty.

The minimum detectable activity (MDA) of the gamma-ray measurement system at 95 % confidence level was calculated using the following formula:

$$ {\text{MDA}} = \frac{4.66 \times \sqrt B }{\varepsilon \times I \times T \times M} $$
(2)

where B is the background counts, ε is the absolute efficiency of the detector, I is the gamma emission probability and T is the counting time (s) and M is the mass of the sample (kg). The average value of the MDA for 226Ra, 232Th, 40K and 137Cs was found as 0.3, 0.4, 5.4 and 1.8 Bq kg−1, respectively.

Results and discussion

Radioactivity measurement

Thirty-two surface soil samples collected from the study area were analyzed for terrestrial and fallout radionuclides using the gamma-ray spectrometer with the HPGe detector. The activity concentrations of 226Ra, 232Th, 40K and 137Cs measured in the soil samples are given in Table 2. The average concentrations of 226Ra, 232Th, 40K and 137Cs were measured as 27.1 ± 1.9 Bq kg−1 (range 14.1 ± 0.7–65.4 ± 2.9 Bq kg−1), 34.3 ± 1.7 Bq kg−1 (range 12.0 ± 0.8–51.7 ± 2.1 Bq kg−1), 370.5 ± 13.8 Bq kg−1 (range 172.2 ± 15.8–511.1 ± 37.8 Bq kg−1) and 18.6 ± 3.6 Bq kg−1 (range <MDA to 86.2 ± 1.4 Bq kg−1), respectively. The highest value of 226Ra, 232Th and 40K was observed at the location of Gülnar (S17), Delikkaya (S19) and Akkuyu (S27), while the lowest value of 226Ra, 232Th and 40K was found at the location of Mediterranean Municipal (S1), Ismet Inonu Boulevard (S2) and Kayabaşı (S22), respectively. The world average value of 226Ra, 232Th and 40K is 33, 45 and 420 Bq kg−1, respectively (UNSCEAR 2008). The average activity concentrations of 226Ra, 232Th and 40K measured in the soil samples are lower than the world average values because the significant part of the province is formed by the limestone related to the geological process (Duran 2014). It is known that the activity concentration levels of terrestrial radionuclides are related to the types of rock from which the soils originate (UNSCEAR 2000). In general, basalts and most limestones have relatively low radium contents (UNSCEAR 2000). The measured average activity of 226Ra, 232Th and 40K were compared with the values reported in different parts of Turkey and other countries as shown in Table 3. As seen in Table 3, the natural radionuclide concentrations were comparable with the other reported values. It can be seen from Table 2 that the activity concentrations of 137Cs were below the lower the MDA measured for six soil samples (S1, S8, S11, S14, S15 and S22). The activity concentrations of 137Cs varied from MDA (<1.8 Bq kg−1) to 86.2 ± 1.4 Bq kg−1 with an average of 18.6 ± 3.6 Bq kg−1. The highest value of 137Cs was observed at location of Akkuyu (S29).

Table 2 The activity concentrations of terrestrial and fallout radionuclides in the soil samples
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Table 3 Comparison of the activity of 226Ra, 232Th and 40K in soil samples with those reported for different regions of Turkey and other countries
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Estimation of radon concentration in soil and air

The concentration of radon in soil gas (A Rnsoil in Bq m−3) in the absence of radon transport is as follows (UNSCEAR 2000):

$$ A_{\text{Rnsoil}} = \frac{{A_{\text{Ra}} \times f \times \rho \times \left( {1 - \varepsilon } \right)}}{\varepsilon } $$
(3)

where A Ra is the activity concentration of 226Ra measured for the soil samples, f is the emanation factor (0.21), ρ is the density of soil (1800 kg m−3) and ε is the total porosity (0.3).

The concentration of radon in the air (A Rnair in Bq m−3) was estimated by the below equation:

$$ A_{\text{Rnair}} = A_{\text{Rnsoil}} \sqrt {\frac{{d_{\text{Soil}} }}{{D_{\text{Air}} }}} $$
(4)

where A Rnsoil is the concentration of 222Rn in the soil given in Eq. (3), dSoil is the diffusion rate constant of 222Rn in the soil (0.5 × 10−4 m2 s−1) and D Air is the diffusion rate constant of 222Rn in the air (5 m2 s−1). The activity concentrations of 222Rn estimated for the soil samples and air are given in Table 4. The values of A Rnsoil and A Rnair varied from 12.4 to 57.7 kBq m−3 with an average of 23.9 kBq m−3 and 39–182 Bq m−3 with an average of 76 Bq m−3, respectively. Results show that 75 % of the A Rnair values are lower than the reference level of 100 Bq m−3 recommended by WHO (2009).

Table 4 The activity concentrations of 222Rn estimated for soil samples and air
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Evaluation of the radiation hazards

Absorbed gamma dose rate in outdoor air (DRout)

The external terrestrial gamma dose rate in outdoor air at 1 m height from the ground in each sampling locations was estimated using data and formulae provided by the UNSCEAR report (2008).

$$ {\text{DR}}_{\text{Out}} \left( {{\text{nGy h}}^{ - 1} } \right) = 0.462 \times A_{\text{Ra}} + 0.604 \times A_{\text{Th}} + 0.0417\times A_{K} + 0.1243 \times A_{\text{Cs}} $$
(5)

where A Ra, A Th, A K and A Cs are the activity concentrations of 226Ra, 232Th, 40K and 137Cs in Bq kg−1, respectively. The estimated values of DRout are given in the second column of Table 5. The values of DRout varied from 26 to 81 nGy h−1 with an average of 51 nGy h−1. The minimum and maximum value of DRout was estimated for the samples of S1 (Mediterranean Municipal) and the sample of S17 (Gülnar), respectively. The average percentage contribution of the natural and fallout radionuclides to the outdoor absorbed gamma dose rate is shown in Fig. 3. The average value of DRout is 14 % lower than the world average outdoor absorbed gamma dose rate of 59 nGy/h (UNSCEAR 2008).

Table 5 Outdoor absorbed gamma dose rate, annual effective dose rates and lifetime cancer risk
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Fig. 3
figure 3

Percentage contribution of natural and fallout radionuclides in the soil samples to the outdoor absorbed gamma dose rate

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Annual effective dose rates due to external exposure and inhalation of radon

The annual effective dose rate due to external exposure (E Ext) was estimated from outdoor external gamma radiation dose rate (DRout) taking into account the conversion factor for adults (0.7 Sv Gy−1) and the outdoor occupancy (0.2) implying that 20 % of time is spent outdoors. The E Ext was estimated using the following equation proposed by the UNSCEAR report (1982).

$$ E_{\text{Ext}} = {\text{DR}}_{\text{Out}} \times 0.7 \times 8766 \times 0.2 \times 10^{ - 3} $$
(6)

where DROut is the outdoor gamma absorbed gamma dose rate given in Eq. (5).

Annual effective dose rates due to inhalation of radon (E Inh)

The annual effective dose rate (E Inh) coming from inhalation of radon gas was estimated taking into account the equilibrium factor (0.6 for outdoors), the conversion factor for radon (9 nSv h−1 per Bq m−3) and the outdoor occupancy (0.2) implying that 20 % of time is spent outdoors (UNSCEAR 2008).

$$ E_{\text{Inh}} = A_{\text{Rnair}} \times 0.6 \times 9 \times 8766 \times 0.2 \times 10^{ - 3} $$
(7)

where A Rnair is the concentration of 222Rn in the air given in Eq. (4).

The estimated values of E Ext and E Inh are given in the third and fourth column of Table 5. The values of E Ext varied from 31 to 100 μSv year−1 with an average of 62 μSv year−1 which is lower than the world average of 70 μSv year−1 (UNSCEAR 2008). The values of E Inh varied from 372 to 1727 μSv year−1 with an average of 715 μSv year−1 which is lower than the UNSCEAR (2008) recommended radon dose.

Lifetime cancer risk (LTCR)

The LTCR caused by the annual effective dose rate due to external exposure (E Ext) was estimated using following equation (ICRP 1990):

$$ {\text{LTCR}} = E_{\text{Ext}} \times {\text{AL}} \times {\text{RF}} $$
(8)

where E Ext is the annual effective dose rate given in Eq. (6), AL is the average life time (70 years) and RF is the risk factor (0.05). The estimated values of LTCR are given in the fifth column of Table 5. The values of LTCR varied from 1.1 × 10−4 to 3.5 × 10−4 with an average of 2.2 × 10−4 which is less than the world average (2.9 × 10−4).

Conclusions

The activity concentrations of natural and fallout radionuclides (226Ra, 232Th, 40K and 137Cs) in the soil samples collected from Mersin and Akkuyu NPP region were determined to evaluate the possible changes in environmental radioactivity caused by nuclear activities in the future. The activity concentrations of radon in soil gas and air were estimated using the activity concentrations of 226Ra measured in the soil samples. The results of the activity measurements have shown that the average activity concentrations of natural radionuclides are lower than the world average values reported UNCEAR (2008) as the significant part of the province is formed by the limestone. As known, 137Cs is released into the environment and can be transferred by some meteorological events such as wind to thousands of kilometers. In the study the average concentration of 137Cs due to Chernobyl accident and around 500 atmospheric nuclear weapons tests conducted until 1980 was 18.6 Bq kg−1. The highest value of 137Cs was observed at the location of Akkuyu (S29).

The radiological hazard information of the average outdoor absorbed gamma dose rate, annual effective dose rate from external exposure, annual effective dose rate from inhalation of radon and lifetime cancer risk for each adult person living in the region was 51 nGy h−1, 62 μSv year−1, 715 μSv year−1 and 2.2 × 10−4, respectively. These values do not exceed the recommended values.