Introduction

The natural ionizing radiation exposure of people is a continuing and inescapable feature of life on the earth (UNSCEAR 2008). There are two main natural radiation sources: cosmogenic and terrestrial radionuclides. Terrestrial radionuclides, which are found in air, soil, rocks, water, and food, contain the decay products in the radioactive series of uranium–radium (238U–226Ra), thorium (232Th), and actinium (235U) as well as radioactive potassium (40K). 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). The anthropogenic radionuclides can be produced and/or released in medical facilities; military-based nuclear weapons; nuclear accidents at nuclear reactors; and discharges of radioactive waste of nuclear fuel from the nuclear installations. In consequence of radioactive fallout from the Chernobyl nuclear reactor accident occurred in 1986, Thrace and Black Sea regions of Turkey were contaminated with long-lived anthropogenic radionuclide such as radiocesium (137Cs, T1/2 = 30.07 y) and radiostrontium (90Sr, T1/2 = 28 y) (Yıldız et al. 2014).

Gamma radiation emitted from terrestrial and anthropogenic radionuclides is the main external exposure to people. The annual dose received by people is about 2.8 mSv of which over 85 % is from natural radiation sources with about half coming from radon (222Rn) decay products (UNSCEAR 2008). Inhalation of 222Rn, over a period of time, has been the second leading cause of lung cancer after tobacco smoking (Chen 2005). Therefore, determination of the distribution of these radionuclides is essential 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 (Turhan et al. 2012). Recently, determination of distributions of the terrestrial and anthropogenic radionuclides and measurements of radon concentrations in soil is of great interest for many researchers in the literature (Aközcan 2014; Yıldız et al. 2014; Mutuk et al. 2014; Cho et al. 2014; Kunovska et al. 2013; Uğur et al. 2013; Canbazoğlu et al. 2013; Zubair et al. 2012; Tufan and Bostanci 2012; Manohar et al. 2013; Dhawal et al. 2013; Merešová et al. 2012; Dimovska et al. 2012; Mujahid and Hussain 2010).

The aim of the study was to (1) measure the activity concentrations of 226Ra, 232Th, 40K, and 137Cs in the soil samples collected from Samsun counties; (2) calculate the activity concentration of 222Rn in soil and air; and (3) estimate the dose rates caused by external exposure from terrestrial radionuclides in soil samples and inhalation of radon in air to evaluate radiological hazardous of soil samples.

Materials and methods

Sampling and sample preparation for activity measurements

The study area is in the region between 35°23′41.00″E–36°58′ 23.45″E and 40°54′45.00″N–41°38′05.12″N. A total of 115 soil samples were collected from different undisturbed sites located in the boundaries of Alacam (10 samples), Asarcik (5 samples), Ayvacik (5 samples), Bafra (10 samples), Carsamba (10 samples), Havza (10 samples), Kavak (10 samples), Ladik (10 samples), Ondokuzmayis (10 samples), Salipazari (5 samples), Tekkekoy (5 samples), Terme (5 samples), Vezirkopru (10 samples), and Yakakent (10 samples) which are counties of Samsun city in the Black Sea region of Turkey. Figure 1 shows the sampling locations in the study area. The geographical positions of soil sample locations are given Table 1. The exact positions of soil samples were recorded using global positioning system (GPS). Soil samples were collected from a depth of about 15–20 cm at 115 different stations. The soil samples were placed in plastic bags and labeled. Then, the samples were dried in an oven at a temperature of 105 °C for 24 h. The homogenized samples were placed in plastic containers, weighed, and hermetically sealed. Before starting the gamma-ray spectrometric measurements, the sealed samples were stored for 1 month to reach radioactive equilibrium of the 226Ra, 232Th and their decay products.

Fig. 1
figure 1

Map of Samsun showing soil sample locations

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Table 1 Geographical positions of soil sample locations
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Gamma-ray spectrometric measurements

The activity concentrations of 226Ra, 232Th, 40K, and 137Cs in the soil samples were measured by means of scintillation gamma-ray spectrometer with 7.62 × 7.62 cm NaI (Tl) detector (Ortec-905-4) enclosed in the tabletop lead shield to reduce the background radiation. Energy resolution of the detector is 46 keV at 661.8 keV 137Cs. RGU-1, RGTh-1, and RGK-1 reference materials certified by IAEA were used for efficiency calibration of the system. Each soil sample was measured for an accumulating time of 80,000 s in the same source-to-detector geometry as used for the determination of the efficiency calibration. Gamma-ray background measurements were taken under the same conditions of sample measurements. Spectrum obtained for each soil sample was analyzed using Ortec Sicitivision software. The gamma-ray photopeak of the 1764.5 keV from 214Bi was used to measure the activity concentration of 226Ra, while the gamma-ray photopeak of the 2614.4 keV from 208Tl was used to measure the activity concentration of 232Th. The activity concentration of 40K and 137Cs was determined using their own gamma-ray line at 1460.8 and 661.7 keV, respectively.

Results and discussion

Activity concentrations of radionuclides

The activity concentrations of 226Ra, 232Th, 40K, and 137Cs in the soil samples were found by the following equation (Debertin and Helmer 1988):

$$ A = \frac{N}{\varepsilon \cdot P \cdot M} $$
(1)

where A is the activity concentration of a radionuclide (Bq kg−1), N is the net count rate of a specific gamma-ray emission (counts per second), ε is the absolute efficiency, P is the emission probability, and M is the mass of the sample (kg). The activity concentrations of 226Ra, 232Th, 40K, and 137Cs measured in the 115 soil samples from fourteen counties of Samsun are presented in Table 2. As can be clearly seen from Table 2, that the activity concentrations of 226Ra, 232Th, and 40K measured in the soil samples collected from Samsun’s counties vary considerably depending on their geological structure and geochemical composition. Geological formations seen in the study area contain sandstone, marl, tuff, basalt and agglomerates, gray-blue marls, clay, siltstone, pebblestone, gypsum, andesite–basalt-type volcanic, small amounts of limestone, gravel, and silt (Akinci et al. 2011). The lowest activity concentration of 226Ra, 232Th, and 40K was measured in the soil samples from Asarcik and Ayvacik, while the highest activity concentration of 226Ra, 232Th, and 40K was measured in the soil samples from Bafra, Asarcik, and Tekkekoy, respectively. 137Cs was not observed in the soil samples from Asarcik, Havza, and Yakakent. The average value of the minimum detectable activity (MDA) for 137Cs was found as 0.4 Bq kg−1. The activity concentrations of 137Cs measured in the 33 soil samples varied from <MDA to 31.5 Bq kg−1. As known, the main source of artificial radionuclides especially 137Cs and 90Sr in Black Sea region of Turkey is the fallout from the Chernobyl accident in 1986. The existence of 137Cs measured in the soil samples from some counties indicates that the effect of Chernobyl accident has been continued.

Table 2 Activity concentrations of 226Ra, 232Th, 40K, and 137Cs measured in the soil samples
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The statistical data for the activity concentrations of 226Ra, 232Th, and 40K measured in the soil samples are given in Table 3. As can be seen from Table 3, the activity concentrations of 226Ra, 232Th, and 40K varied from 5.9 to 87.3 Bq kg−1 with an average of 38.1 ± 1.4 Bq kg−1, 18.2 to 71.3 Bq kg−1 with an average of 39.3 ± 0.8 Bq kg−1 and 110.9 to 717.5 Bq kg−1 with an average of 375.3 ± 10.2 Bq kg−1, respectively. Comparison of the average activity concentrations of 226Ra, 232Th, and 40K in the soil samples from Samsun’s counties with the available data from other countries and earth‘s crust average values is given in Table 4. It can be seen that the average activity concentrations of 232Th and 40K are lower than the average values measured in the earth’s crust, while the average activity concentration of 226Ra is slightly higher than the average value measured in the earth’s crust. The average activity concentration of 226Ra is lower than the average values of 226Ra measured in Algeria, Armenia Bulgaria, and Croatia, while the average activity concentration of 232Th is higher than the average values of 232Th measured in countries given in Table 4, except Croatia and Iran. The average activity concentration of 40K is lower than the average values of 40K measured in Bulgaria, Croatia, Greece, India, and Iran.

Table 3 Statistical data for the activity concentrations of 226Ra, 232Th, and 40K (in Bq kg−1) measured in the soil samples and 222Rn (in Bq m−3) estimated for soil and air
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Table 4 Comparison of the activity concentrations of 226Ra, 232Th, and 40K in soil samples with the available data from other countries
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The frequency distributions of the activity concentrations of 226Ra, 232Th, and 40K are presented in Fig. 2. It is observed from Table 3 and Fig. 2 that the frequency distributions of 232Th and 40K are approximately normal distribution, while the frequency distribution of 226Ra is log-normal. The range of 30–60 Bq kg−1 measured for 226Ra and 232Th includes 83 and 95 % of the total soil samples, respectively. The activity concentrations of 40K measured in 70 % of the total soil samples are between 240 and 440 Bq kg−1.

Fig. 2
figure 2

Frequency distributions of 226Ra, 232Th, and 40K in the soil samples

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Radon concentration in soil and air

As known, radon sources are distributed in rocks and soils. The concentration of radon in soil gas (C Soil in Bq m−3) in the absence of radon transport is as follows (UNSCEAR 2000):

$$ C_{\text{Soil}} = \frac{{A_{\text{Ra}} \cdot f \cdot \rho \cdot \left( {1 - \varepsilon } \right)}}{\varepsilon } $$
(2)

where A Ra is the activity concentration of 226Ra measured for the soil samples, f is the emanation factor (0.2), ρ is the density of soil (1800 kg m−3), and ε is the total porosity (0.3).

The concentration of 222Rn in the air (C Air in Bq m−3) was estimated by the below equation:

$$ C_{\text{Air}} = C_{\text{Soil}} \sqrt {\frac{{d_{\text{Soil}} }}{{D_{\text{Air}} }}} $$
(3)

where C Soil is the concentration of 222Rn, d Soil 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 statistical data for the C Soil and C Air estimated for the soil samples are given in the fifth and last column of Table 3, respectively. As can be seen from Table 3, the average value of the C Soil and C Air was estimated as 32 ± 1 kBq m−3 and 101 ± 4 Bq m−3, respectively.

Assessment of radiological hazards

Dose rates in outdoor

The external absorbed gamma dose rate in outdoor air at 1 m above ground level (D Out in nGy h−1) was evaluated using data and formulae provided by the UNSCEAR report (UNSCEAR 2008).

$$ D_{\text{Out}} = 0.462 \cdot A_{\text{Ra}} + 0.604 \cdot A_{\text{Th}} + 0.0417 \cdot A_{\text{K}} $$
(4)

where A Ra, A Th, and A K are the activity concentrations of 226Ra, 232Th, and 40K in Bq kg−1, respectively. The corresponding effective dose rate (E Ext in μSv y−1) was estimated from the equation (UNSCEAR 2008):

$$ E_{\text{Ext}} = D_{\text{Out}} \cdot {\text{DCF}}_{\text{Ext}} \cdot T \cdot 10^{ - 3} $$
(5)

where D Out is the external absorbed gamma dose rate given in Eq. 4, DCFExt is the conversion factor (0.7 Sv Gy−1) from absorbed gamma dose in air to effective dose received by adults (UNSCEAR 2008), and T is annual exposure time of 1760 h y−1, implying that 20 % of time is spent outdoors. The effective dose rate coming from inhalation of radon gas (E Inh in μSv y−1) was estimated from the equation (UNSCEAR 2008):

$$ E_{\text{Inh}} = C_{\text{Air}} \cdot {\text{EF}} \cdot {\text{DCF}}{}_{\text{Inh}} \cdot T \cdot 10^{ - 3} $$
(6)

where C Air is the concentration of 222Rn in the air given in Eq. 3, EF is equilibrium factor (0.6 for outdoors), DCFInh is the conversion factor for radon of 9 nSv (Bq h m−3)−1, and T is annual exposure time of 1760 h y−1, implying that 20 % of time is spent outdoors.

The statistical data of the D out evaluated for the soil samples are given in the first column of Table 5. As can be seen from Table 5, that the average value of D out is slightly lower than the world average outdoor absorbed gamma dose rate of 59 nGy h−1 (UNSCEAR 2008). The statistical data of E Ext and E Inh are given in the second and third column of Table 5. The values of E Ext ranged from 35 to 116 μSv y−1 with an average value of 70 μSv y−1 which is equal to the world average of 70 μSv y−1 (UNSCEAR 2008). The values of E Inh ranged from 149 to 2195 μSv y−1 with an average value of 958 μSv y−1 which is lower than the UNSCEAR recommended radon dose (UNSCEAR 2008).

Table 5 Statistical data for the outdoor absorbed gamma dose rate (D), the corresponding effective dose rate (E ext), and the effective dose rate due to inhalation of radon (E inh)
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Activity concentration index

Available soil has been used for the construction of dwellings in Samsun city. The activity concentration index (I) has been used for practical controlling purposes. Activity concentration index taking into consideration more than one radionuclide contributions to the radiation dose was estimated for building as follows (EC 1999):

$$ I = \frac{{A_{\text{Ra}} }}{{300{\text{ Bq kg}}^{ - 1} }} + \frac{{A_{\text{Th}} }}{{200{\text{ Bq kg}}^{ - 1} }} + \frac{{A_{\text{K}} }}{{3000{\text{ Bq kg}}^{ - 1} }} $$
(7)

where A Ra, A Th, and A K are the activity concentrations of 226Ra, 232Th, and 40K, respectively, in Bq kg−1. For materials used in bulk amounts, e.g., soil, concrete I ≤ 1 corresponds to an annual effective dose less than or equal to 1 mSv, while I ≤ 0.5 corresponds to an annual effective dose less than or equal to 0.3 mSv (EC 1999). From Table 5, the values of I estimated for the soil samples ranged from 0.2 to 0.7 with an average value of 0.4 which is about two times lower than the upper limit of 1.0 corresponding to an effective dose 1 mSv.

Conclusions

In the present study, the activity concentrations of 226Ra, 232Th, 40K, and 137Cs in the 115 soil samples collected from Samsun’s counties (Alacam, Asarcik, Ayvacik, Bafra, Carsamba, Havza, Kavak, Ladik, Ondokuzmayis, Salipazari, Tekkekoy, Terme, Vezirkopru, and Yakakent) were measured using by gamma-ray spectrometry with NaI (Tl) detector. The data obtained in the study can contribute to assess possible changes in environmental radioactivity caused by nuclear activities in the future because Samsun city is so close to the nuclear power facility which is going to be built in Sinop city soon. The average activity concentrations of 232Th and 40K are lower than the average values of the earth’s crust, while the average activity concentration of 226Ra is slightly higher than the average value of the earth’s crust. 137Cs activity concentration was due to Chernobyl accident. The average concentrations of radon in soil gas and air were estimated as 32 ± 1 kBq m−3 and 101 ± 4 Bq m−3, respectively. For each soil sample, the outdoor absorbed gamma dose rate, the annual effective doses, and the activity concentration index were estimated to evaluate the possible radiological hazard. The estimated average value of the outdoor absorbed gamma dose rate is slightly lower than the world average outdoor absorbed gamma dose rate. The average annual effective dose from external exposure is equal to the world average, while the average annual effective dose from inhalation of radon gas is lower than the UNSCEAR recommended radon dose. The results of the activity concentration index show that all soil samples meet the exemption annual dose criterion of 1 mSv.