Introduction

Human beings have been constantly exposed to ionizing radiation since the formation of the universe. Natural radiation originates from long-lived radioactive nuclei present in nature since the Earth's existence. Due to the development of nuclear technology in the past century, the presence of certain nuclear waste in the environment determines the artificial radiation dose levels worldwide. Individuals are exposed to ionizing gamma radiation emitted from natural radionuclides such as 232Th, 226Ra, and their decay products, as well as 40K. These radionuclides and their progenies are major contributors to ionizing radiation in soil and rocks [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18].

Artificial radionuclides are released into the environment for various reasons such as nuclear weapon tests, nuclear power plant accidents, radioisotope production and disposal of nuclear pharmaceutical waste. Following the nuclear power plant incidents at Fukushima Daiichi (2011), Chernobyl (1986), and Three-Mile Island (1979), significant amounts of artificial radionuclides were released into the environment. As a result of these accidents, artificial radionuclides were dispersed in both marine and terrestrial ecosystems. Given the relative proximity of the western region of Turkey to the Chernobyl site, individuals living in this region have faced significant effects of nuclear fallout. Numerous investigations have been conducted since 1986 to ascertain the extent of radionuclides present both in our nation and in neighboring countries of Chernobyl [19,20,21,22,23,24,25,26]. One of the important radionuclides released into the terrestrial environment as a result of radioactive fallout is 137Cs. With a half-life of 30 years, 137Cs pose a significant health hazard.

It is important to determine the concentrations of these radionuclides due to the harmful effects on human health caused by natural and artificial radiation. Particularly, the determination of artificial radiation dose levels serves as a reference source to detect potential changes arising from industrial, nuclear, and other human activities.

Istanbul is one of the most important cities in Turkey and the world due to its high population density and its economically important position. This study aims to determine the levels of natural and artificial radiation doses in the soil of the coastal areas near the Marmara Sea on the European side of Istanbul, which has a high population density. In this study, the artificial radiation dose level in the soil of Istanbul, which was affected by the Chernobyl nuclear accident in 1986, will be determined. The results obtained in this study will serve as a reference in the literature for any possible nuclear threat in the future (e.g., the Zaporizhzhia Nuclear Power Plant). Zaporizhzhia Nuclear Power Plant is the largest nuclear power plant in Europe and Ukraine. There are studies in the literature that summarize the events that occurred at the Zaporizhia Nuclear Power Plant during the military conflict between the Russian Federation and Ukraine [27,28,29]. Depending on environmental conditions, radiation leakage from the power plant may pose a health threat that may affect other countries [27, 30].

In this study, soil samples were collected from the Buyukcekmece and Silivri districts in Istanbul and the Marmara Ereglisi district in Tekirdag. Activity concentrations were determined using the ɤ-ray spectroscopy system with HPGe detector. The gamma dose rate (D), radium equivalent activity (Raeq), excess lifetime cancer risk (ELCR), and annual effective dose equivalent (AEDE) were computed as radiological hazard parameters. The obtained radiological hazard parameters were compared with the values reported in the literature.

Study area

The study area covers Silivri and Büyükçekmece, two districts of Istanbul, and Marmara Ereğlisi in Tekirdağ province, which is adjacent to these districts. In the study area, three districts located in the western part of Istanbul and with coastlines along the Sea of Marmara were selected. A total of 20 soil samples were collected from various locations, including 6 samples from Büyükçekmece, 6 samples from Silivri, and 8 samples from Marmara Ereğlisi (Fig. 1). The locations of the soil samples collected in the study area are shown in Table 1. In Table 1, sampling points S1 to S6 belong to Büyükçekmece, points S7 to S12 belong to Silivri, and points S13 to S20 belong to Marmara Ereğlisi. The distance between the collected samples varies between 3 and 6 km. The average distance between samples is approximately 4 km. The distance from the study area to the Zaporizhzhia NPP is approximately 850 km, and the distance to the Chernobyl NPP is approximately 1150 km (Fig. 1).

Fig. 1
figure 1

Sample sites in İstanbul and Tekirdağ were determined along the borders of the districts to the Marmara Sea, Zaporizhzhia and Chernobyl Nuclear Power Plant

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Table 1 Geographical coordinates of the samples
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Materials and methods

At each sampling point, four separate subsamples were taken from a 1 m2 area to obtain approximately 1 kg of soil sample. Soil samples were collected from a depth ranging between 0 and 6 cm. Each soil sample was sieved through a 0.1-mm mesh to remove stone, weed, grass and other impurities. The soil samples were dried in an oven at 105 °C for 2 days and placed inside 250 ml polyethylene containers. To achieve secular equilibrium between 226Ra and its daughters/decay products, soil samples were stored in the containers for 40 days.

Activity concentrations were obtained using a gamma-ray spectrometry with a high-purity germanium (HPGe) detector (ORTEC GEM70P4-95, USA) in Kirklareli University Central Research Laboratory. The detector has a resolution of 2.0 keV and 70% relative efficiency for 1.332 meV gamma energy of 60Co.

The energy and efficiency calibrations were made using a 250 ml multi-nuclide standard source (Isotope Product Laboratories, Eckert&Ziegler, Berlin, Germany) containing various radionuclides (241Am, 109Cd, 57Co, 123mTe, 51Cr, 113Sn, 85Sr, 137Cs, 88Y and 60Co) whose energy peaks vary between 80 and 2500 keV. The activity concentrations of 226Ra were measured from gamma-ray lines of 214Pb (351.9 keV) and 214Bi (609.3 keV). To evaluate the activity concentrations of 232Th, gamma-ray lines of 228Ac (911.1 keV) and 208TI (583.1 keV) were used. The activity of 40K was determined from the 1460 keV peak.

To assess the radiological risks due to radionuclides, radiological parameters such as absorbed gamma dose rate (D), radium equivalent activity (Raeq), annual effective dose equivalent (AEDE), excess lifetime cancer risk (ELCR), external hazard index (Hex), internal hazard index (Hin) and gamma representative level index (Iγ) estimated.

The radiation dose contribution resulting from the calculated radionuclides in the soil samples exhibits a non-uniform distribution. Radium equivalent activity serves as a singular parameter to compare the activities of varying concentrations of 226Ra, 232Th, and 40K. Equation 1 was utilized to calculate the radium equivalent activity [31].

$${{\text{Ra}}}_{{\text{eq}}}\left(\mathrm{Bq }{{\text{kg}}}^{-1}\right)={\text{C}}+0.077{{\text{C}}}_{{\text{K}}}+1.43{{\text{C}}}_{{\text{Th}}}$$
(1)

where CTh, CRa, and CK denote the specific activities of 232Th, 226Ra, and 40K (Bq kg−1), respectively.

To determine the absorbed gamma dose rates in air at a height of 1 m above the ground surface for the uniformly distributed natural radionuclides (226Ra, 232Th, and 40K), Eq. (2) from UNSCEAR (2000) was employed [32]:

$$\mathrm{D }\left(\mathrm{nGy }{{\text{h}}}^{-1}\right)=0.462{{\text{C}}}_{{\text{Ra}}}+0.0417{{\text{C}}}_{{\text{K}}}+0.604{{\text{C}}}_{{\text{Th}}}$$
(2)

In equation, D represents the absorbed gamma dose rate, and CTh, CRa, and CK denote the specific activities of 232Th, 226Ra, and 40K (Bq kg−1), respectively.

Equation 3 was employed for the calculation of AEDE. In the equation; 8760: hours per a year, 0.2: the outdoor occupancy factor, and 0.7: is the dose convention factor (Sv Gy−1). The equation was multiplied by 10−3 to convert to μSv [32].

$$\mathrm{AEDE }\left(\mathrm{\mu Sv }{{\text{y}}}^{-1}\right)=\mathrm{D }\left(\mathrm{nGy }{{\text{h}}}^{-1}\right)\times 0.7 \left({{\text{SvGy}}}^{-1}\right)\times 0.2 \times 8760 \left(\mathrm{h }{{\text{y}}}^{-1}\right) \times {10}^{-3}$$
(3)

Excess lifetime cancer risk (ELCR) refers to the probability that an individual will develop cancer during his or her lifetime when exposed to a given dose of radiation. The calculation of the risk of developing cancer over a person's entire life (Eq. 4) involves multiplying the parameters of annual effective dose (AEDE in µSv y−1), average life expectancy (DL = 70 years), and the risk factor (RF = 5.10–2 Sv−1). The risk factor is defined by the International Commission on Radiological Protection (ICRP, 1990) as the probability of fatal cancer occurrence in stochastic effects (Eq. 4) [33].

$${\text{ELCR}}=\mathrm{AEDE }\left(\mathrm{\mu Sv }{{\text{y}}}^{-1}\right)\times \mathrm{DL }\left({\text{y}}\right)\times {\text{RF}}\left({{\text{Sv}}}^{-1}\right)$$
(4)

External Hazard Index (Hex) is a radiological parameter used to evaluate the potential radiation hazard to individuals as a result of external exposure to gamma radiation released by natural radionuclides in the environment. Hex takes into account the gamma dose rate near a specific location and helps estimate potential health risks associated with exposure to external radiation.

The Internal Hazard Index (Hin) is another radiological index that evaluates the potential radiation hazard from inhaling or ingesting radioactive materials, especially those containing uranium and thorium decay products. Hin is critical in evaluating the risk of internal radiation exposure due to the presence of radioactive particles in the environment. It takes into account the radioactive decay of inhaled or ingested materials that can cause irradiation of internal organs and tissues.

The Gamma Representative Level Index (Iγ) is an indicator used to characterize the typical or average gamma radiation level within a specific area or region. This index provides information on the current gamma radiation environment and is important in risk assessment. It represents the expected gamma radiation dose from natural radionuclides and helps determine the level of radiological safety at a particular location.

Hex, Hin, and Iγ were calculated using the following equations (Eqs. 5–7) [34, 35].

$${H}_{ex}=\frac{{C}_{Th}}{259}+\frac{{C}_{Ra}}{370}+\frac{{C}_{K}}{4810}$$
(5)
$${H}_{in}=\frac{{C}_{Th}}{259}+\frac{{C}_{Ra}}{185}+\frac{{C}_{K}}{4810}$$
(6)
$${I}_{\gamma }=\frac{{C}_{Th}}{100}+\frac{{C}_{Ra}}{150}+\frac{{C}_{K}}{1500}$$
(7)

Results and discussion

In the Büyükçekmece district, 226Ra concentration was calculated to be lowest at the S1 station with a value of 21 ± 2 Bq kg−1 and highest at the S4 station with a value of 28 ± 2 Bq kg−1. The average concentration was determined as 25 ± 2 Bq kg−1. In Silivri, 226Ra concentration varied between 20 ± 1 Bq kg−1 at the S7 station and 34 ± 1 Bq kg−1 at the S9 station, and the average concentration was found to be 27 ± 2 Bq kg−1. In Marmara Ereğlisi, the lowest 226Ra concentration was found as 21 ± 1 Bq kg−1 at the S17 station, and the highest 226Ra concentration was 34 ± 4 Bq kg−1. The average concentration in this region was determined as 26 ± 2 Bq kg−1. The average 226Ra concentration is 26 ± 3 Bq kg−1 (Fig. 3). The average activity concentration of 226Ra in the present study is less than the world average of 35 Bq kg−1 [32]. The lowest 232Th concentration in Büyükçekmece was determined as 20 ± 1 Bq kg−1 at the S3 station. The highest 232Th concentration was obtained as 37 ± 2 Bq kg−1 at the station S4. The mean 232Th concentration was calculated as 27 ± 2 Bq kg−1. In Silivri, the 232Th concentration varied between 23 ± 0.3 Bq kg−1 at station S10 and 40 ± 2 Bq kg−1 at the station S8, and the average was found to be 33 ± 2 Bq kg−1. The activity concentration of 232Th in Marmara Ereğlisi varied between 19 ± 1 and 36 ± 1 Bq kg−1 at the S17 station, and the average was found to be 26 ± 2 Bq kg−1. The average 232Th concentration in all soil samples was determined to be 30 ± 3 Bq kg−1 (Fig. 3).

It was determined that the 40K concentration in Büyükçekmece was at its minimum level at the S2 station with a value of 442 ± 6 Bq kg−1, and reached its maximum at the S4 station with a value of 586 ± 10 Bq kg−1. The mean concentration was calculated as 500 ± 9 Bq kg−1. In the Silivri region, the 40K concentration varied between 135 ± 10 Bq kg−1 at station S7 and 700 ± 9 Bq kg−1 at station S11, with an average concentration of 539 ± 11 Bq kg−1. The activity concentration of 40K in Marmara Ereglisi varied between 122 ± 8 Bq kg−1 at station S18 and 803 ± 6 Bq kg−1, and the average was found to be 572 ± 10 Bq kg−1. The average 40K concentration across all soil samples was determined as 540 ± 10 Bq kg−1 (Fig. 2) and this value is higher than the world average of 400 Bq kg−1 [32].

Fig. 2
figure 2

Radioactivity concentration for 40K, 226Ra and 232Th according to sampling sites

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Investigation of natural radiation levels at designated sampling points revealed differences within the recorded data set. The values of Raeq varied from 72 ± 2 Bq kg−1 at sampling point S7 to 134 ± 2 Bq kg−1 at sampling point S11, with a mean of 110 ± 3 Bq kg−1 (Fig. 3 and Table 2). All estimated Raeq values are lower than the recommended permissible limit of 370 Bq kg−1 [32]. The calculated values of D ranged from 33 ± 2 (S7) to 64 ± 2 nGy h−1 (S11) (Fig. 4 and Table 2). The average absorbed gamma dose rate (52 ± 3 nGy h−1) was lower than the world average (57 nGy h−1). AEDE ranged from 40 ± 1 to 79 ± 1 μSv y−1. The average of AEDE was found as 64 ± 2 μSv y−1 (Fig. 5 and Table 2). The average value of AEDE is found lower than the world average (70 μSv y−1).

Fig. 3
figure 3

Radium equivalent activity (Raeq), according to sampling sites

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Table 2 Radiological risk parameters (Raeq, D, AEDE and ELCR) due to natural radionuclides in soils
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Fig. 4
figure 4

Absorbed gamma dose rate (D) according to sampling sites

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Fig. 5
figure 5

Annual effective dose equivalent (AEDE) according to sampling sites

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The lowest ELRC value was 0.14 × 10–3 at sampling point S7, whereas the highest was 0.27 × 10–3 at S11 (Fig. 6 and Table 2). The average of ELCR was found to be 0.23 × 10–3 which was lower than the world average value of 0.29 × 10−3. In addition, the data related to the external hazard index (Hex) displayed a similar variance, with the lowest value of 0.19 at S7 and the highest of 0.36 at S11, with an average of 0.30. The internal hazard index (Hin) data showcased fluctuations, ranging from 0.25 at the minimum in S7 to 0.44 at the maximum in S11, with an overall average of 0.37. The gamma representative level index values ranged from 0.52 at the lowest point in S7 to 1.01 at the highest point in S11, with an average of 0.83 (Fig. 7). All values were found less than unity and thus the radiological hazard to be negligible.

Fig. 6
figure 6

Excess lifetime cancer risk (ELCR) according to sampling sites

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Fig. 7
figure 7

External hazard index (Hex), internal hazard index (Hin) and gamma representative level index (Iɣ) according to sampling sites

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The distribution of natural radionuclides in soil is affected by various factors, including the geological and geographical characteristics of the region and the degree of fertilizer application in agricultural areas [32, 36]. Activity concentration levels are typically elevated in salt rocks, granite, and phosphorus-enriched soils compared to sedimentary rock formations [37]. Current studies suggest that soils with a slips-debris composition, containing abundant raw materials, minerals, lower clay content and reduced organic matter, tend to show high mean activity concentrations of 232Th and 226Ra [37, 38]. The application of artificial fertilizers, especially phosphate-based fertilizers, to increase productivity in agricultural areas increases the 40K and 232Th activity concentrations in the soil [39, 40]. These studies highlight the complex character of natural radionuclide distribution in soil and the numerous factors that contribute to its configuration.

Heterogeneity in radionuclide concentration distribution underscores the complex nature of natural radiation and the need for a comprehensive evaluation of radiological consequences and associated potential health risks. Analyzing and interpreting these findings is important for understanding the complex interaction between natural radioactivity and human exposure, and laying the foundations for effective radiation protection strategies.

137Cs activity concentrations were found below 0.5 Bq kg−1 at 10 sampling points. 137Cs activity concentration values were determined between 0.5 and 1 Bq kg−1 at 8 sampling points (Fig. 8). The highest 137Cs activity concentration was observed at sample point S7 (2 ± 0.1 Bq kg−1). The average 137Cs activity concentration was found as 1 ± 0.2 Bq kg−1 in Büyükçekmece, 0.60 Bq kg−1 in Silivri, 0.4 ± 0.3 Bq kg−1 in Marmara Ereğlisi. The overall average radiation concentration for the entire study area was 0.6 ± 0.1 Bq kg−1. The distribution of 137Cs radionuclide is influenced by various factors. High 137Cs activity concentration levels can be explained by collected areas rich in organic matter (e.g. inorganic fertilizers) [41]. Activity concentration levels indicate that the studied areas are contaminated with 137Cs and radiation pollution continues due to the Chernobyl accident.

Fig. 8
figure 8

Artificial radioactivity concentration for 137Cs, according to sampling sites

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Extensive investigation of natural radioactivity levels in various regions has revealed significant differences in the concentration distributions of natural radionuclides. The data presented exemplify the complex variations observed in different geographical contexts, highlighting the need for localized studies for a comprehensive understanding of natural radioactivity patterns. Table 3 presents the results of natural radioactivity studies conducted in various countries. The data in this table provides a comparative overview of the levels of natural radioactivity found in soil samples from different regions. This comparative analysis contributes to a broader understanding of the distribution of natural radioactivity and helps evaluate the regional differences observed in this study.

Table 3 Comparison of obtained average natural radioactivity concentrations and the average radiological parameters with other studies
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A notable difference emerges when comparing the 40K activity concentration in various regions. In our study area, an average concentration of 540 ± 29 Bq kg−1 was found for 40K, which is close to the accepted averages in the world. The Thrace region in Turkey has a higher 40K activity concentration compared to other study regions [51]. Owo in Ondo State, Nigeria, exhibits a significantly higher concentration of 1190 Bq kg−1, while Songkhla /Thailand demonstrates a lower concentration of 213 Bq kg−1. Observed values highlight the importance of region-specific evaluations [42, 44]. It was observed that 226Ra activity concentrations varied in the studies conducted in the world. In a study conducted in South India, a concentration of 23 Bq kg−1 was found for 226Ra, while this value was found to be 129 Bq kg−1 in Hungary. The observed differences are due to the complex interplay of geological and environmental factors influencing radionuclide concentrations [49, 50].

Comparing the data in this study with data from various regions including Thailand, Greece, Argentina, Algeria, and Hungary further highlights the diverse nature of natural radioactivity. Each region displays unique concentration patterns for 40K, 226Ra, and 232Th, emphasizing the necessity of region-specific studies to capture the full spectrum of variations. Due to regional inconsistencies observed in radioactivity values, factors related to the geological structure of the region need to be investigated. Differences observed in radionuclide distributions in small geographical regions strengthen the necessity of region-specific and comprehensive research.

In conclusion, this study contributes to the understanding of global natural radioactivity by highlighting the significant disparities in radionuclide concentrations. The data highlight the importance of regional assessments and the need for a comprehensive understanding of natural radioactivity in different geographical contexts. Localized studies are crucial to reveal the complex interplay of factors affecting radionuclide concentrations and provide valuable information about the diverse patterns observed worldwide.

Studies conducted in the world regarding the determination of the 137Cs artificial radionuclide concentration are given in Table 4. 137Cs activity levels ranged from 0.25 to 2.3 Bq kg−1 in Egypt [53]. In a study conducted in Georgia, values ranging from 4 to 33 Bq kg−1 were determined [54]. In another study in Georgia, 137Cs activity concentration ranged from 0 to 53 Bq kg−1 [55]. In a study conducted by Karakelle and colleagues in Turkey in 2002, it was revealed that 137Cs levels varied between 2 and 25 Bq kg−1 [56]. In the present study, 137Cs activity levels ranged from 0.1 to 2 Bq kg−1, with an average of 0.6 Bq kg−1. The average absorbed gamma dose rate (52 nGy h−1) and radium equivalent activity (110 Bq kg−1) in the present study were found lower than Nigeria, China, Macedonia, India, and the Thrace region of Turkey. These findings highlight the importance of regional assessments and localized studies to comprehensively understand artificial radioactivity levels in different geographical contexts.

Table 4 Comparison of anthropogenic radionuclide (137Cs) concentration with soil samples in Turkey and other countries
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Statistical analysis

Statistical analysis was performed using the statistics software package SPSS version 25.0. The frequency distributions for activity concentrations of 226Ra, 232Th, 40K and 137Cs in soil samples were analyzed to estimate the probability distributions and the histograms are given in Fig. 9. The frequency distribution graphs of 226Ra, 232Th, and 40K show that these radionuclides were distributed in a normal distribution. However, the distribution of 137Cs demonstrated some degree of multi-modality. Skewness gives information about whether the frequency distribution is symmetrical or asymmetrical [59]. The skewness coefficients of 226Ra, 232Th, 40K and 137Cs were found to be 0.178, − 0.196, − 1.192 and 1.285, respectively. A skewness value close to zero indicates that the data set is closer to normal distribution [60]. The fact that the skewness value of 226Ra is close to zero indicates that it has an almost normal distribution. In this study, the positive value of the skewness coefficient of 137Cs indicates that the distribution of 137Cs is asymmetric with the right tail longer than the left tail. Negative skewness coefficients of 232Th and 40K indicate distributions are with an asymmetric tail extending towards values that less positive concerning the mean. The kurtosis values of 226Ra, 232Th, 40K and 137Cs were found to be − 0.808, − 0.837, 2.033 and 2.601, respectively. The distributions associated with 226Ra and 232Th have negative kurtosis values, indicating flat distributions. A positive kurtosis coefficient of 137Cs suggests that relatively peaked distribution [61].

Fig. 9
figure 9

Frequency distributions of a. 226Ra, b. 232Th, c. 40K, and d. 137Cs

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Correlation analysis was performed to determine the strength of the relationship between variables. Linear Pearson correlation coefficients between all variables for soil samples are given in Table 5. As seen in the table, variables are positively correlated except correlation between 137Cs and radiological variables. Raeq, D, AEDE, ELCR, Hex, Hin and Iɤ have positive correlation coefficients with 226Ra, 232Th and 40K which indicates that these radiological parameters exist due to the high concentration of radionuclides. A poor degree of correlation was observed between 137Cs and radiological hazard parameters indicating that the activity concentration of 137Cs is not significantly responsible for radiological hazards. Linear Pearson correlation coefficient analysis suggested that the radioactivity in the study area was due to the activity concentration of natural radionuclides and their high contribution to radiological parameters.

Table 5 The Pearson correlation coefficient matrix between variables
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The principal component analysis was obtained using the varimax rotation method with Kaiser Normalization. A graphical representation of components is given in Fig. 10. Obtained components are listed in Table 6. Component 1 accounts for 50.68% of the total variance and is mainly characterized by positive loading of concentrations of 226Ra (0.105), 232Th (0.155), 40K (0.936) and all radiological parameters (≥ 0.539). This indicates that the total level of radioactivity in the study area is due to natural radionuclides. Component 2 accounts for 39.28% of the total variance. The principal component analysis is in good agreement with Pearson correlation analysis.

Fig. 10
figure 10

Graphical representation of component 1 (50.68%) and component 2 (39.28%)

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Table 6 Rotated factor loadings of variables
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Conclusion

In conclusion, this study focused on assessing the current levels of natural and artificial radioactivity in soil samples obtained from the Büyükçekmece and Silivri districts of Istanbul, as well as the Marmara Ereğlisi district in Tekirdağ. The investigation aimed to provide insight into the potential impact of nuclear leakage, particularly in light of concerns related to facilities such as the Zaporizhzhia Nuclear Power Station.

A total of twenty soil samples were meticulously collected from the designated study areas. The concentrations of radioisotopes 226Ra, 232Th, 40K, and 137Cs were measured using a HPGe detector. Average concentrations were determined as 26 ± 2 Bq kg−1 for 226Ra, 30 ± 2 Bq kg−1 for 232Th, 540 ± 29 Bq kg−1 for 40K, and 0.6 ± 0.1 Bq kg−1 for 137Cs.

Furthermore, radium equivalent activity (110 ± 6 Bq kg−1), absorbed dose (52 ± 4 nGy h−1), and lifetime cancer risk (0.23 × 10–3) were calculated using radioactivity concentrations in soil samples. These findings contribute valuable data to the ongoing discourse about environmental radioactivity, particularly potential nuclear incidents. The results highlight the importance of continuous monitoring and taking proactive measures to ensure the radiological safety of the studied regions.