Introduction

Low-level ionizing radiation is ubiquitous in our dwelling environment, and exposure to such radiation is unavoidable by living beings [1]. The utilization of soil, the surface material that forms the uppermost layer of earth and one of the principal substrata of life on earth, in various spheres of life is considered as the major pathway of gamma-ray exposures to human health [2]. The investigation of the dispersion of radioactive segments in the soil is imperative to comprehend the radioactivity level in any territory. Specifically, it is likewise critical to assess the radiation danger arising from the exposure of gamma-beam to the human body. Raised levels of natural radionuclides in soil and other dwelling media may introduce extra radiation doses to human bodies [3].

Prolonged exposure to an elevated level of radiation can cause acute health effects such as radiation sickness or long-term health effects such as cardiovascular disease [4]. Many studies demonstrated the non-linear effects of radiation exposure to human health using the framework of the linear no-threshold (LNT) model. The current principle of radiation exposure to the occupational workers is ‘as low as reasonably achievable (ALARA)’, however, the total dose should not exceed 50 mSvy−1 [5]. On the other hand, an amount of 2.4 mSvy−1 has been reported as the average radiation dose received by a person from natural sources of radiation including cosmic radiation, radon, and any medical radiation according to the UNSCEAR [6] report. However, according to the LNT model, there is still non-negligible uncertainty on the health effects of radiation, which may involve some carcinogenic risk even at the levels of natural background.

Measurement of radioactivity in soil, sand, and sediment samples in sea beaches in Bangladesh such as Kuakata, Cox’s Bazar, Saint Martin, and Potenga was performed by several researchers in recent decades [7,8,9,10,11]. However, there is no radioactivity data available in the Guliakhali sea beach area of Chattogram district in Bangladesh. Moreover, the present study area is located very close to the ‘ship breaking industry’, one of the most hazardous industrial activities, and is operating for several decades [12]. It is one of the largest ship breaking yards in the world and receives criticism for contaminating the coastal environment of Bangladesh. It is presumed that these ship scrapping activities may contribute Technologically Enhanced Naturally Occurring Radioactive Materials (TENORMs) in this or peripheral area. Therefore, an assessment of Naturally Occurring Radioactive Materials (NORMs) in the environmental media of the Guliakhali beach and surrounding area is required to ensure the safety of public health and furthermore for the vacationers who visit this spot reliably due to its normal excellence, sporting movement and investing relaxation energy.

The goal of this investigation is to measure the prevailing concentrations of primordial radionuclides 40K, 226Ra, 232Th in the sandy soil of the examination territory which is Guliakhali sea beach located at Chattogram district in Bangladesh, and to evaluate the radiation risk to the concerned population by calculating the radiation hazard parameters. In this way, the measured radioactivity in the sandy soil may assist us to know the radiological safety standard of the area as well as the foodstuffs produced from this area. Moreover, this study may also contribute to establish baseline data which is important for Bangladesh, because the country’s first nuclear power plant is now under construction and expected to go into operation in 2023–2024 [13].

Materials and methods

Description of the study area

The study area ‘Guliakhali sea beach’ is a muddy beach situated at the west-southern part of Sitakunda upazila at Chattogram district, Bangladesh. Due to its coastal scenic beauty and easy access from the major cities including the capital of Bangladesh, in recent times, this place is receiving greater attention from both local and regional tourists. The entire area is secured by a meager layer of topsoil. Human habitation in the area is moderate. The geographical characteristics of southeastern Bangladesh including the study area is shown in Fig. 1. The global position of this area is around 22°36′51″ NL and 91°38′58″ EL. Sitakunda upazila had a population of 335,178 (in 2018 survey), it occupies an area of 483.97 km2 [14], and currently possesses the population density of almost 1,200/km2.

Fig. 1
figure 1

The geographic location of Guliakhali Sea Beach in the south-eastern part of Bangladesh

Full size image
Fig. 2
figure 2

Sample processing flowchart

Full size image

Sample collection and preservation

Fifteen (× 3) sandy soil samples were collected randomly from fifteen different locations of Guliakhali area in September 2018. The sampling points of the study area are shown in Fig. 1. Approximately 1 kg (wet weight) of samples were collected at 0–5 cm depth from each location. The sampling points were identified using an online satellite map before starting the collection of samples. Global Positioning System (GPS) was used to record the sampling locations. The samples were collected from the node points of the square grids with a distance of ~ 50 m. Collected samples were kept in clean plastic packets/bags, marked properly, and transported to the laboratory for analysis. The samples collected were appropriately coded and standard quality control procedures were maintained according to International Atomic Energy Agency (IAEA) guidelines during sampling, sample preservation, and processing [15] which is shown in Fig. 2.

Sample preparation

After the collection, unwanted materials, for example, stones, roots, and vegetation were isolated from the soil samples. The samples were first dried under the sunlight, subsequently crushed with mortar and pestle, homogenized, screened with a test sifter of opening 425 µm. The weight of each sample was about 500 g. All the samples were packed individually into cylindrical plastic containers, sealed tightly, and kept at room temperature for 28 days to ensure that 226Ra and 232Th were in secular equilibrium with short-lived daughter products [16, 17].

Measurement system

A number of analysis systems exist which are able to determine the radionuclide contents of various types of samples and geometry. For all types of samples, gamma-ray spectrometry is generally the most effective technique to analyze the gamma-emitting radionuclides in these samples. A coaxial, high-resolution HPGe gamma-ray spectrometer coupled with associated electronics including digital spectrum analyzer (DSPEC jr 2.0) was used for measurement of radionuclides concentration in the sample. The detector was housed with a cylindrical lead shielding arrangement having a fixed bottom and a movable cover to suppress the background contribution from the surrounding environment. The energy resolution of the detector is a measure of the sharpness of a photo-peak and it determines the ability of a detector to separate or distinguish the presence of two gamma rays closely spaced in energy. The energy resolution of the 1.33 MeV energy peak for 60Co was found to be 1.69 keV at full-width half-maximum with a relative efficiency of 19.6%.

Energy and efficiency calibration

It is essential that energy and efficiency calibration of the detector be performed with great care because the accuracy of the measured data largely depends on these factors. The energy calibration of the detector was completed using standard point sources such as 22Na, 57Co, 60Co, 133Ba, 137Cs, etc. The efficiency of a detector is a measure of the number of radiation quanta (particles or photons) detected by it from the total number of emitted radiation quanta by the source. In order to determine the detector efficiency, a standard source was made by mixing 152Eu of known activity (Liquid form, 100 Bq activity) with Al2O3 matrix, prepared in identical containers to the samples. The efficiency data was also checked using the IAEA reference samples RGU-1, RGTh-1 and RGK-1 IAEA, [18]. The well-known Eq. (1) was used to determine the efficiency of the detector [19]:

$$Efficiency = CPS/(DPS\times {I}_{\gamma })$$
(1)

where, CPS = Counts per second for the radionuclides of interest present in the standard sample, DPS = Disintegration per second, and Iγ = \(\gamma\)-ray intensity of the source. The counting efficiency curve of the HPGe detector was drawn and shown in Fig. 3.

Fig. 3
figure 3

Efficiency curve of the HPGe detector for solid matrix

Full size image

Measurement of radioactivity

The activity concentration of 226Ra was calculated through characteristic gamma lines of 241.98 keV, 295.21 keV and 351.92 keV of 214Pb and 609 keV, 1120.3 keV and 1764.5 keV of 214Bi. 232Th activity was calculated through 583.14 keV of 208Tl, 911.07 keV and 969.11 keV of 228Ac, respectively [15]. For an accurate evaluation of 226Ra and 232Th radioactivity, a weighted mean approach was adopted [19, 20]. The radioactivity of 40K was calculated through singly occurring characteristic 1460.75 keV gamma line. The radioactivity concentration of each radionuclide was calculated using the following equation [17, 21]:

$${A}_{i}=\frac{cps}{\varepsilon \times {I}_{\gamma }\times w}$$
(2)

where Ai is the activity concentration of each radionuclide in the sample; cps is the net count rate per second, which is obtained by subtracting the background count from the samples radionuclide activity counts; \(\varepsilon\) is the efficiency of the detector at specific gamma-ray; Iγ is the transition probability of the specific gamma-ray, and w is the mass of the sample (kg).

The uncertainty of the measured radioactivity was obtained by using the uncertainty propagation law of the relevant quantities expressed in Eq. (2). The mathematical formulation is expressed in Eq. (3) [16]:

$$\mathrm{ Combined \, Standard \, Uncertainty } \, = \, {A}_{i}\times \sqrt{({\frac{u(N)}{N})}^{2}+({\frac{u(T)}{T})}^{2}+({\frac{u({I}_{\gamma })}{{I}_{\gamma }})}^{2}+{(\frac{u(w)}{w})}^{2}+({\frac{u(\varepsilon )}{\varepsilon })}^{2}}$$
(3)

where, N, T, Iγ, w and \(\upvarepsilon\) are the sample counts, counting time, gamma-ray emission probability, sample weight, and counting efficiency, respectively. The term u(N), u(T), u(Iγ), u(w), and u(\(\upvarepsilon\)) are respective uncertainties due to counting statistics (1–12%), measurement time (1%), gamma-ray emission probability (< 1%), sample weight (1%), and efficiency of the detector (5%). The overall uncertainties show a range of 5–13%, presented in Table 1 along with the measured activity of the radionuclides of interest.

Determination of MDA

The minimum detectable activity (MDA) of the used system was determined using the Eq. (4) as reported in [22]:

$$\mathrm{MDA}=\frac{{\mathrm{K}}_{\mathrm{\alpha }}\times \sqrt{\mathrm{B}}}{\varepsilon \times {I}_{\upgamma }\times \mathrm{T}\times \mathrm{w}}$$
(4)

where, Kα is the statistical coverage factor having a value of 1.64 (at the 95% confidence level), B is the number of background counts for the corresponding radionuclide, ε, Iγ, T, and w (in kg) have their usual meaning similar to Eq. (3). The MDAs were found to be 0.35 Bqkg–1 for 226Ra, 0.64 Bqkg–1 for 232Th, and 2.2 Bqkg–1 for 40K.

Calculation of absorbed dose rate

Effects of gamma radiation are normally expressed in terms of the absorbed dose rate in air, which originate from radioactive sources in the soil. The absorbed dose rate in the air at 1 m above the ground surface due to the radionuclides 226Ra, 232Th and 40K in soil was estimated using the formula given in UNSCEAR [6, 23]:

$${D}_{out} ({nGyh}^{-1})=0.462{A}_{Ra}+ 0.604{A}_{Th}+ 0.042 {A }_{K}$$
(5)

where, ARa, ATh and AK are the specific activities of 226Ra, 232Th and 40K in Bqkg−1 respectively.

Calculation of outdoor annual effective dose

The outdoor absorbed dose rate was converted into outdoor annual effective dose by using a conversion factor of 0.7 SvGy−1 recommended by the UNSCEAR [6] and 0.2 for the outdoor occupancy factors by considering that the people on an average, spent 20% of their time in outdoors [17, 24]:

$${E}_{out} ({mSvy}^{-1}) = D\times 24\times 365.25\times 0.2\times 0.7\times {10}^{-6}$$
(6)

where D is the outdoor absorbed dose rate in the air (nGyh−1), 8760 is the time in hours for one year, 0.2 is the outdoor occupancy factor, and 0.7 SvGy−1 is the quotient of effective dose equivalent rate to absorbed dose rate in air, and 10–6 is the factor converting nano-to-milli.

Excess lifetime cancer risk

Consequent upon the evaluation of AEDE (Outdoor Annual Effective Dose Equivalent), the excess lifetime cancer risk (ELCR) has been estimated using the equation [25]:

$$ELCR=AEDE\times DL\times RF$$
(7)

where AEDE, DL and RF are the outdoor annual effective dose equivalent, duration of life (70 years) and risk factor (0.05 Sv−1), respectively. The risk factor is fatal cancer risk per sievert, which is assigned to a value of 0.05 for the public for stochastic effects [25].

Results and discussion

The activity concentrations in the sandy soil samples collected from 15 different places are reported in Table 1, and Fig. 4 shows the fluctuations of radioactivity level of the radionuclides.

Table 1 Activity concentrations of 226Ra, 232Th and 40K in all soil samples
Full size table
Fig. 4
figure 4

Graphical representation of activity concentration of 226Ra, 232Th and 40K of sandy soil samples

Full size image

The mean values of 232Th and 40K are found to be lower than the population-weighted world average values of 32 and 420 Bqkg−1 respectively, while the mean value of 226Ra exceeded the world average of 45 Bqkg−1 [6]. Conveyances of common radionuclides in the soil rely upon their physical, substance and land properties. The relatively high radiation level found in this area may be due to the presence of a few heavy minerals as placer deposits. Several samples show relatively higher values of 40K, and this may be attributed to agricultural runoff the chemical fertilizer as well as the considerable abundance of elemental potassium in seawater. The regular tidal waves may mediate in the transportation and deposition of radioactive-, trace-, and toxic metals in the peripheral coastal strips. However, there is no attendance of 137Cs which indicates the absence of any unwanted nuclear relevant activities nearby the study areas.

Fig. 5
figure 5

Graphical representation of outdoor annual effective dose (mSv) due to soil samples

Full size image

Table 2 shows the comparison of the average activity concentrations of 226Ra, 232Th, and 40K among different places of the world [26,27,28,29,30,31,32,33,34,35,36,37,38].

Table 2 Comparison of radionuclides concentrations in soil samples with other countries
Full size table

A comparison of the average activity concentrations of 226Ra, 232Th, and 40K for the analyzed samples with the available literature is presented in Table 2. It shows that measured concentration of 226Ra in our study area is above the data reported from Iraq, Cameroon (Volcanic Area), India (Tamil Nadu), Egypt, Chile, Pakistan, Palestine and India (Maharastra), and less than the values of Jordan, India (Kerala), Malaysia (Cameroon Highlands), Malaysia and Nigeria. The activity concentration of 40K shows higher than the most of the countries data reported in Table 2 except in Pakistan, Nigeria, Malaysia (Cameron Highland) and Jordan. Similarly, activity concentration of 232Th lies above some of the countries except the data reported from India (Maharastra), Pakistan, Nigeria, and India (Kerala) and Malaysia. Such a heterogeneous behavior is usually observed following the differences in local geology of the soil in different regions in the world. The outdoor absorbed dose rate, outdoor annual effective dose, and ELCR values are reported in Table 3.

Table 3 Several radiological hazard parameters calculated from the measured radioactivity in the studied sandy soil samples
Full size table

The population-weighted average outdoor absorbed dose rate in air in the world is 59 nGyh−1 [6]. The average absorbed dose rate in air is quite similar to the world average absorbed dose rate except the sample 12. The excess lifetime cancer risk (ELCR) value is higher in sample 12 (Table 3). ELCR values of almost all the samples (except samples 6, 7, 10 & 12) are lower or very close to the recommended value of < 0.29 × 10–3 [6].

According to UNSCAER (2000) [6], the worldwide average outdoor annual effective dose due to external terrestrial radiation is 0.07 mSvy−1. As shown in Fig. 5, most of the values of annual effective dose are less than 0.07 mSvy−1 which indicates an insignificant hazards for human being.

Figure 6 shows the comparison of the average activity concentrations of 226Ra, 232Th, and 40K among different places in Bangladesh including the soil samples from a few sea beaches [9, 10, 39,40,41,42]. It shows that the activity concentration of 226Ra of the present study area is very similar to the other study areas except for the activity concentration of 226Ra in Palaeo Beach, Teknaf which is very high [39]. The activity concentration of 232Th also shows the same behavior. The activity concentration of 40K seems quite different here. The activity concentration of 40K in Savar, Dhaka and Habiganj areas is higher than the present study area, where the value is lower in Palaeo Brach, Teknaf than our study area. This happens because the soil characteristics and mineral components are differing from place to place. Although there are differences in the activities of these naturally occurring radionuclides, their values are generally below the safety limits provided by UNSCEAR [6] which reflects a general background radiation trend.

Fig. 6
figure 6

Comparison of radioactivity levels in soil samples with other studies in Bangladesh

Full size image

Conclusion

Assessment of the concentration of terrestrial radionuclides 226Ra, 232Th, and 40K and their radiological implications associated with sandy soil samples collected from the Guliakhali sea beach, Sitakunda, Chattogram, Bangladesh was carried out in this study. It determines an overall mean value of 412 ± 19, 56 ± 5 and 31 ± 3 Bqkg−1 for 40K, 226Ra, and 232Th respectively, and these values don’t exceed (except 226Ra) the corresponding world average values of 420, 32, and 45 Bqkg−1 for soil [6]. The distribution of activity concentration of these radionuclides was not uniform with respect to the study location because of the geological character of the studied area. Although one sandy soil sample # 12 (22°36′50″ NL, 91°38′57″ EL) showed comparatively higher outdoor effective dose than the recommended safety limit, most of the values of annual effective dose are less than 0.07 mSvy−1, which is the worldwide average outdoor annual effective dose [6]. Therefore, the radiation level of the sandy soil samples does not pose any health risk currently. The outcomes from this investigation may help to enrich the baseline radioactivity database in Bangladesh, and may contribute the radiological safety assessment for individuals living in the beachfront zones.