Introduction

The ionizing radiation exposure to humans arises from natural and artificial radionuclides. This exposure could be internal or external. External exposure arises mainly from natural radiation due to cosmic rays and terrestrial radioactivity. Terrestrial radionuclides are the primordial radionuclides, or radionuclides with half-lives comparable to the age of Earth, mainly 238U, 232Th, and their decay products as well as 40K (Khan et al. 2012). They are present at trace levels in all ground formations (Awudu et al. 2012), and their occurrence is the main contributor, about 85%, in the dose received by living organisms due to natural radiation. The concentration of primordial radionuclides in soil varies from region to another across the world depending on local geographical and geological factors (UNSCEAR 2000). Their distribution in soil depends on the radionuclides distribution in rocks from which they are originated and on the processes through which the soil is concentrated (Song et al. 2012). Man-made or anthropogenic radionuclides have been released to the environment by different nuclear activities. In the period 1950s–1960’, they were produced from nuclear weapons testing as global and local fallout, and then they were emitted from nuclear accidents, such as Chernobyl and Fukushima accidents. Artificial radionuclides may be transported with the radioactive plume for long distances across boundaries away from the accident zone. They could be deposited by fallout or wet deposition over soil. For this reason, the determination of natural and artificial radionuclides in soils has gained great interest during the last decades in different countries, and many studies were carried out (Al-Masri et al. 2006; Ababaneh et al. 2012; Ademola et al. 2014; Aközcan 2014; Bai et al. 2017; Dolhańczuk-Śródka 2012; Guiditto et al. 2015; Cho et al. 2014; Khan et al. 2012) in order to assess public exposure from terrestrial radiation, to establish radiation maps and baseline data necessary for the assessment of any radiation emergency, and for the detection and evaluation phases, and consequently, to take appropriate response.

In Lebanon, in the framework of a National Environmental Radiation Survey Program, many studies were carried out to determine the radioactivity levels in different compartment of the Lebanese environment (El Samad et al. 2007, 2013, 2017). These were the milestone for the public dose assessment of the Lebanese Population.

This study aims to determine the activity concentration of natural and artificial radionuclides in the Lebanese soil at Mount Lebanon province, deduce the surface activity concentration, and establish the radioactive maps of 238U, 232Th, 226Ra, 137Cs, and 40K in the studied area, as well as to assess the external public exposure and radiation hazards due. This could be achieved through the estimation of total gamma dose rate, annual effective dose, radium equivalent, and external hazard index.

Materials and methods

Studied area

The Mount Lebanon province extends along Lebanon at a distance of 170 km between the Mediterranean coast and the western mountains chain till the highest mountain peak, 3088 m altitude. It surrounds the capital Beirut from three sides, south, east, and north. The studied area receives by substantial amount of precipitation and snow distributed over the winter season. It is characterized by oak and pine forests.

Sampling and sample preparation

A total of 150 samples were collected from different layers of soil down to 25 cm depth, at 87 uncultivated locations, distributed along the Mount Lebanon province. Sample gathering and measurements were carried out in the year 2015. Sampling locations are shown in Fig. 1. The coordinates of sampling sites were obtained based on the global positioning system. The collected samples were grinded, sieved, and homogenized. An aliquot from each sample was dried at 80 °C for 24 h and dry/wet ratio was calculated. Samples were prepared in the adopted counting geometry for gamma spectroscopy measurement (El Samad et al. 2013).

Fig. 1
figure 1

Map of Mount Lebanon province with sampling locations

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Gamma spectrometry analysis

Gamma spectrometry analysis was performed using three sets of gamma spectrometers with p-type coaxial high purity germanium detectors of relative efficiency 30, 40, and 50%. The laboratory is accredited by Hellenic Accreditation System ESYD, Certificate No. 788 dated January 20, 2012, according to ISO/IEC 17025 standards for calibration and testing laboratories. Energy calibration was carried out routinely by counting a standard multigamma sand sample (EG-ML-1195-11-1) from Eckert and Zeigler of 500 ml volume in polyethylene container. This is done with a tolerance 0.1 keV. The efficiency calibration was carried using mixed gamma—ray standard solution R8/31/32 of 500 ml volume from GE Healthcare Limited, and then efficiency curves were plotted for various compositions, densities, and geometries using EFFTRAN software taking in consideration attenuation and absorption factor (Vidmar et al. 2010). The estimated efficiency uncertainty is 3%. To test the performance of the detector in function of time, 152Eu point source is counted periodically; the resolution (FWHM) and the full energy peak efficiency were checked at low and high energies. To subtract background radiation from samples spectra, an empty counting container was measured periodically. The sample counting time ranged from 130,000 to 173,000 s. The spectra were analyzed offline using the Genie 2000 software from Canberra Version V3.1b. Uranium-238 and Thorium-232 were determined from the gamma line of their daughters 234mPa and 228Ac, respectively. Radium-226 was determined from its gamma line at 186.2 keV after correction for 235U interference (Völgyesi et al. 2014). As well, 137Cs and 40K were determined from their corresponding gamma lines 661.7 and 1460.8 keV, respectively. The surface activity concentration per unit area (Bq m−2) was then calculated. The inverse distance weighted (IDW) method was used to interpolate and predict unmeasured surfaces over investigated areas. Using this geometric method, the value of the variable at the unsampled locations is estimated based on the data from surrounding locations. The weights for IDW are proportional to the square distance between the prediction points and the observation points (Kravchenko and Bullock 1999; Yao et al. 2013).

Results and discussion

Radioactivity concentration in soil

The ranges of activity concentration and the average values for natural radionuclides and 137Cs in the analyzed samples are presented in Table 1. The average values were found to be 37 Bq kg−1 for 238U, 24 Bq kg−1 for 232Th, 46 Bq kg−1 for 226Ra, and 206 Bq kg−1 for 40K. Values were comparable to those reported in other Lebanese province (El Samad et al. 2013). The average activity concentration of 238U and 232Th was comparable to the worldwide average values, 35 and 30 Bq kg−1, respectively, while that of 232Th was lower than values reported in USA, 35 Bq kg−1 (NCRP No. 45 1975) and in Europe, 34 Bq kg−1 (Trevisi et al. 2012). The average value of 226Ra was found to be slightly higher than the worldwide value and European value 35 and 36 Bq kg−1, respectively, while it was found comparable to US value, 40 Bq kg−1 (NCRP No. 45 1975). The value obtained for 40K was found to be half of the world average values stated in UNSCEAR (2000), 400 Bq kg−1 and lower than that reported in USA and Europe, 370 and 483 Bq kg−1, respectively. The variation in the activity concentrations of natural radionuclides in soils in different locations across the world differs depending on soil type, geochemical composition, geological, and geographical conditions of the studied areas (Mir and Rather 2015). The average activity concentration of the 137Cs was 23 Bq kg−1. The main cause to the presence of 137Cs in the Lebanese soil could be the deposition by fallout during earlier nuclear weapons testing in 1960s and Chernobyl accident in 1986. The 134Cs was not detected in the analyzed samples, due to its short half-life. The content of 137Cs was comparable to previous studies carried out in other Lebanese province (El Samad et al. 2013). However, it was lower than values reported in some European countries such as Poland and Greece (Dolhańczuk-Śródka 2012; Kioupi et al. 2015), comparable to values determined in some areas in turkey (Karataşh et al. 2016), and it was found higher than the content detected in Ghana and Taiwan, and in other Arab countries and Middle Eastern countries (Ababaneh et al. 2012; Aközcan 2014; Bai et al. 2017; Canbazoğlu et al. 2013; Darko et al. 2015; Al Mugren 2015; Taqi et al. 2016; Tsai et al. 2008). These variations could be attributed to many factors, mainly the soil texture and the weather conditions such as the rainfall during and after the radioactive cloud passage (El Samad et al. 2007). Table 2 represents a comparison between activity concentration of natural and artificial radionuclides determined in this study and previous data determined at other Lebanese area and those reported by other countries.

Table 1 Ranges of determined activity concentrations (Bq kg−1), average values and ranges of calculated surface activity concentrations (kBq m−2)
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Table 2 Comparison of the average activity concentrations determined in the analyzed samples with values determined at North Lebanon and values reported by other countries
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Vertical distribution

For vertical distribution studies of natural radionuclides and 137Cs, seven locations were selected, four samples at four depths (0–3, 3–8, 8–15, and 15–25 cm) were collected from each location. In order to study the variation between the data obtained for 238U, 232Th, 226Ra, and 40K at different depths, an analysis of variance test, one-way ANOVA was applied. The applied statistical test showed that there is no significant variation between activity concentrations at various soil layers as p > 0.05 and F < Fcrit. This confirms with other studies carried out in other countries (Ramzaev et al. 2006; Belivermiş et al. 2010; Belivermiş 2012). Concerning the 137Cs, theoretically data should show exponential pattern specially that the soil at the studied area is characterized by low carbonate content and high amounts clays minerals and organic matters that played an important role in maintaining the cesium at the top layer (El Samad et al. 2007; Belivermiş 2012). However, the vertical distribution of 137Cs fits exponential function from layer 3–8 to 15–25 cm, with the exception of the value obtained at the first layer. This could be attributed to the environmental conditions, and to the soil erosion and degradation that have affected and disturbed the composition of the top layer over years, specially that sampling was carried out 3 decades after Chernobyl accident. This caused an underestimation of the activity concentration of 137Cs by 40–50%. The vertical distribution of 137Cs activity concentration at the studied locations was presented in Fig. 2.

Fig. 2
figure 2

Vertical distribution of 137Cs for the studied locations

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Radiation maps

The ranges of surface activity concentrations for 238U, 232Th, 226Ra, 40K, and 137Cs are presented in Table 1. Values were comparable to those calculated in other Lebanese province (El Samad et al. 2013). These surface activity concentrations were used to establish radioactive maps for Mount Lebanon province for natural and anthropogenic radionuclides. Maps for 238U, 232Th, 226Ra, 40K, and 137Cs are represented in Figs. 3, 4, 5, 6, and 7, respectively.

Fig. 3
figure 3

Activity concentration per surface area for 238U

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Fig. 4
figure 4

Activity concentration per surface area for 232Th

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Fig. 5
figure 5

Activity concentration per surface area for 226Ra

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Fig. 6
figure 6

Activity concentration per surface area for 40K

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Fig. 7
figure 7

Activity concentration per surface area for 137Cs

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Assessment of radiological hazards

Absorbed dose rate and annual effective dose

In order to assess the radiation risk, the total absorbed dose rates D (nGy h−1) in air at 1 m above ground level due to the presence of natural and artificial radionuclides were estimated taking into account their activity concentrations and the corresponding dose rates per unit activity concentrations (UNSCEAR 2000; Kurnaz et al. 2011; Ademola et al. 2014). Values varied up to 91 nGy h−1 with an average of 40 nGy h−1, where 99% of this dose is due to the natural radiation. The average value was found to be lower than the world average 57 nGy h−1 and the average value estimated in other Lebanese province 55 nGy h−1 (El Samad et al. 2013) and comparable to value reported in USA, 47 nGy h−1.

The annual effective doses were calculated in order to assess the biological hazard to which public in the sampling locations are exposed. This was achieved using the conversion coefficient of 0.7 Sv Gy−1 from absorbed dose in air to effective dose received by an adult, and 0.2 for the outdoor occupancy factor. Values varied between 5 and 112 µSv year−1 with an average of 49 µSv year−1, lower than the value calculated in North Lebanon province 69 µSv year−1, as well as lower than the world average 70 µSv year−1 and values reported in Japan and USA, 400 and 210 µSv year−1 respectively.

The total absorbed dose rate and the annual effective dose were comparable to those reported in Jordan, Kuwait, Saudi Arabia, and Ghana (Ababaneh et al. 2012; Alazemi et al. 2016; Darko et al. 2015; Al Mugren 2015). Values were higher than those estimated in Poland and some area in Turkey (Canbazoğlu et al. 2013; Dolhańczuk-Śródka 2012) and lower than those in Kücük and Mersin in Turkey, Nigeria, China, and Taiwan (Ademola et al. 2014; Aközcan 2014; Bai et al. 2017; Karataşh et al. 2016; Tsai et al. 2008).

Radium equivalent (Raeq)

In order to assess the radiation hazards caused by gamma ray of specific radionuclides of 226Ra, 232Th, and 40K, the most widely used hazard index, radium equivalent Raeq in Bq kg−1 was calculated at all sampling locations. This was carried out using the following equation (Beretka and Mathew 1985)

$$Ra_{\text{eq}} = \, C_{\text{Ra}} + 1.43C_{\text{Th}} + 0.077C_{\text{K}}$$
(1)

where CRa, CTh, CK are the activity concentrations of 226Ra, 232Th, and 40K in Bq kg−1, respectively. Values were found lower than the recommended value 370 Bq kg−1 (UNSCEAR 2000; Baykara et al. 2011; Aközcan 2014). They ranged from 13 to 304 Bq kg−1 with an average of 95 Bq kg−1.

The average value of Raeq was found comparable to that calculated in Taiwan (Tsai et al. 2008), higher than that in Kuwait and Saudi Arabia (Alazemi et al. 2016; Al Mugren 2015), and lower than that in Kücük-Turkey, Nigeria, and China (Ademola et al. 2014; Aközcan 2014; Bai et al. 2017).

External hazard index (H ex)

The external hazard index was calculated for all analyzed samples as follow (Beretka and Mathew 1985; Sroor et al. 2002).

$$H_{\text{ex}} = C_{\text{Ra}} /370 \, + C_{\text{Th}} /259 \, + C_{\text{K}} /4810$$
(2)

All results were found less than the maximum value 1 mSv year−1 which corresponds to the upper limit of Raeq (370 Bq kg−1). Values ranged from 0.03 to 0.8 mSv year−1 with an average value of 0.3 mSv year−1.

Table 3 shows a comparison between radiation hazards parameters calculated in this study and those reported in other countries.

Table 3 Comparison of radiation hazards parameters with values determined at North Lebanon and values reported in other studies
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Conclusion

A total of 150 samples were collected from 87 locations distributed uniformly at the Mount Lebanon province. Gamma analysis was performed, and activity concentrations of natural and artificial radionuclides were determined. Results were comparable to those determined in other Lebanese province. Radioactive maps for the studied area were established. The radiological hazards parameters, including absorbed gamma dose rate, total annual effective dose, radium equivalent, and external hazard index, were calculated. All results were within the limit of international recommended values. The data obtained are the basis of a national radioactivity baseline level and could be used as a reference for future studies on radionuclide distribution in the country.