Introduction

Two main sources of the exposure of general public to natural radiation are cosmic ray and radioactive nuclides existing in the earth’s crust [1, 2]. The average dose rate of cosmic ray at sea level is about 30 nGy h\(^{-1}\), while the world average dose rate of terrestrial natural occurring radionuclides is about 59 nGy h\(^{-1}\) [1, 3]. Natural radioactivity in soils comes mainly from radionuclides in the decay series of uranium (\(^{238}\)U), thorium (\(^{232}\hbox {Th}\)) and potassium (\(^{40}\hbox {K}\)). The levels of natural radioactivity are also dependent on the geological and geographical structure of soils [1, 2]. \(^{226}\hbox {Ra}\) subseries contributes about 98.5% of the total external gamma dose induced by the whole \(^{238}\)U decay series, and therefore, it is usually referred to as \(^{226}\hbox {Ra}\) series instead of \(^{238}\)U series [4]. As reported in UNSCEAR report [1], the world average radionuclide concentrations in soils are 35 Bq kg\(^{-1}\) for \(^{226}\hbox {Ra}\), 30 Bq kg\(^{-1}\) for \(^{232}\hbox {Th}\) and 400 Bq kg\(^{-1}\) for \(^{40}\hbox {K}\), respectively [1]. Although the world average values of natural radionuclides in soils are low, the variation between different locations could be up to 1000 Bq kg\(^{-1}\) for \(^{238}\)U, 360 Bq kg\(^{-1}\) for \(^{232}\hbox {Th}\) and 3200 Bq kg\(^{-1}\) for \(^{40}\hbox {K}\) [2]. Several worldwide regions with higher background radiation were notified in China, Iran, India, Italy, France, Switzerland, Australia and Brazil [1]. Therefore, the data of radioactivity concentrations in a specific area should be connected with its population distribution to evaluate the health effect to human livings.

Majority of the external gamma dose rate above typical soils (95%) arises from primordial radionuclides incorporated in soils [1]. The soil layer upper 30 cm contributes predominantly to the natural terrestrial radiation exposure [5]. Soils are also the sources of spreading radionuclides to water, air, sediments and biological systems. Thus, soils are important matrices for evaluating the radiological exposure of the humans and biota, and examining the environmental radiological contamination. It means that measurement of natural radioactivity in soils is necessary to determine the change of the natural background activity with time in case of radioactive release, which is essential for environmental protection [6]. There has been increasing interest in mapping the natural radioactivity concentrations and radium equivalent activity in soils and establishing baseline data in many countries [4, 6,7,8,9,10,11,12,13,14,15]. Consequently, radiological hazard parameters can also be evaluated based on the radioactivity concentrations of natural occurring radioactive materials [1,2,3]. Therefore, these efforts are considerably important for assessing the public dose rates and the performance of epidemiological studies.

Assessment of the natural radionuclide concentrations in soil samples in Laos for establishing a baseline data is of high important. Several efforts have been made to survey and evaluate the terrestrial natural occurring radioactivity in soils and building materials in Laos. Leuangtakoun et al. [16] assessed the natural radioactivity in surface soils in Bolikhamxay province, Laos. The natural radioactivity and radiological hazards in building materials in Laos were investigated in Ref. [17, 18].

The present work aims at evaluating the natural radionuclide concentrations of \(^{232}\hbox {Th}\), \(^{226}\hbox {Ra}\) and \(^{40}\hbox {K}\) in soil samples collected widely in Savannakhet province, Laos for establishing a baseline data in this region. The results were then used to analyze radiological hazard indices such as absorbed gamma dose rate in air (D), annual effective dose equivalent (AEDE), radium equivalent activity \((\hbox {Ra}_\mathrm{eq})\), external hazard index \((H_\mathrm{ex})\) and internal hazard index \((H_\mathrm{in})\). The radioactivity concentrations of several radionuclides and radiological hazard indices in Savannakhet province have also been evaluated in comparison with neighboring and worldwide regions.

Materials and methods

Sampling area

Laos is a landlocked Southeast Asian country lying between latitude from 14.117\(^{\circ }\) to 23.684\(^{\circ }\) N and longitude from 100.413\(^{\circ }\) to 108.832\(^{\circ }\) E. It shares the borders with Myanmar and China to the northwest, Vietnam to the east, Cambodia to the southwest, and Thailand to the west and southwest. Laos has an abundance of natural resources and environmental riches with forest covering half of the country. The climate is tropical and affected by the monsoon pattern. Savannakhet is the largest province located in the southern part of Laos with the area of 21,774 km\(^2\) and the population of 970,000. The province lies on the latitude of 16° 33′ 54.18″ N and the longitude of 104° 45′ 9.83″ E as shown in Fig. 1. It shares the borders with Khammuane province to the north, Quang Tri and Thua Thien-Hue provinces of Vietnam to the east, Salavan province to the south, and Nakhon Phanom and Mukdahan provinces of Thailand to the west. The capital of Savannakhet province, also known as Kaysone Phomvihane or Muang Khanthabouly, is one of the two notable cities of Laos. Savannakhet province is administratively divided into 15 districts as displayed in Fig. 1. Sepone district is the largest mining location of copper and gold, and the most significant mining interest in Laos. Other mining locations include Vilabuly and Champhone districts. The province is also one of the main tobacco producing areas, and is an important trading post between Thailand and Vietnam.

Fig. 1
figure 1

Map of Savannakhet Province, Laos and the locations of soil samples

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Sample collection and preparation

Soil samples at 80 locations distributed widely in Savannakhet province, Laos were collected during November and December, 2018. This time period was also the dry season in Laos with the outdoor temperature of about 30–40 \(^{\circ }\)C. The sample locations are close to populated agriculture fields and tourist areas. The sampling locations are denoted as S1–S80 as depicted in Fig. 1. At the sampling sites, soil samples were collected from the surface layer with the depth of about 5–30 cm [10, 11, 19]. Five topsoil samples were collected at the four corners and the center of a square with the side of 60 cm. The soils were then mixed up, and the amount of 1–2 kg was taken using a quartile method. After removing organic materials and pieces of stone, the samples were air dried at room temperature for about 24–48 h at laboratory. Then, the samples were dried in an electric oven at the temperature of 110 \(^{\circ }\)C for about 6 h. In other related works, samples were dried for about 10–12 h at the temperature of 100–110 \(^{\circ }\)C to obtain constant masses [11, 20]. In the present work with relatively dried soil samples, the drying duration of about 6 h is considerably adequate. The samples were crushed and served with a mesh having holes with the diameter of 0.2 mm. The homogenized samples were weighted and placed in a cylindrical polyethylene box having the diameter of 7.5 cm and the height of 3.0 cm. The samples were stored in a period of four weeks for attaining secular equilibrium between \(^{226}\hbox {Ra}\) with \(^{214}\)Bi and \(^{214}\)Pb.

Analysis method

The soil samples were measured using a low background gamma spectroscopy of ORTEC P-type coaxial high purity Germanium (HPGe). The gamma spectroscopy was calibrated using the IAEA RGU–1, RGTh–1 and RGK–1 reference materials to construct the detector efficiency curve as a function of gamma energy [21]. The detector was then used to measure the IAEA–375 soil reference material. The activity concentrations of \(^{226}\hbox {Ra}\), \(^{232}\hbox {Th}\) and \(^{40}\hbox {K}\) were obtained with the deviation less than 3% compared to the reported values. Each soil sample was measured during a period of 60,000–86,400 s to ensure that the \(1\sigma\) statistic errors of important photopeaks are less than 5%. Similar measurement duration of 60,000 was also applied in Ref. [6]. In particular, to evaluate the activity concentration of \(^{226}\hbox {Ra}\), it is determined based on the photopeaks of 295.57 keV and 351.9 keV emitted from \(^{214}\)Pb and the photopeaks of 609.3 keV and 1120.3 keV emitted from \(^{214}\)Bi. The activity concentration of \(^{232}\hbox {Th}\) was determined based on the photopeaks of 338.6 keV and 911.1 keV of \(^{228}\)Ac and the peak of 583.19 keV of \(^{208}\)Tl. Whereas, the activity concentration of \(^{40}\hbox {K}\) was determined directly from its gamma line of 1460 keV. The activity concentration of a certain radionuclide is calculated as follows [13]:

$$\begin{aligned} A (\hbox {Bq}\,\hbox {kg}^{-1}) =\dfrac{n}{\epsilon \times I_\mathrm{eff} \times m_\mathrm{s}}, \end{aligned}$$
(1)

where A is the activity concentration of the radionuclide in Bq kg\(^{-1}\); n is the net gamma counting rate (cps) for a peak at a given energy; \(\epsilon\) is the detector efficiency of a specific gamma-ray, \(I_\mathrm{eff}\) is the emission probability of the photon, and \(m_\mathrm{s}\) is the mass of a soil sample. The \(2\sigma\) standard deviations of the activity concentrations of \(^{226}\hbox {Ra}\), \(^{232}\hbox {Th}\) and \(^{40}\hbox {K}\) are calculated from the errors of the net gamma counting rates, the detector efficiency, the branching ratios and the mass of soil samples.

In order to assess the radiological hazards associated with natural occurring radioactivity materials, radium equivalent activity \(\hbox {Ra}_\mathrm{eq}\), absorbed gamma dose rate D, annual effective dose equivalent AEDE, external hazard index \(H_\mathrm{ex}\) and internal hazard index \(H_\mathrm{in}\) have been evaluated from the activity concentrations of \(^{226}\hbox {Ra}\), \(^{232}\hbox {Th}\) and \(^{40}\hbox {K}\). Since the radioactivity levels of \(^{226}\hbox {Ra}\) and \(^{232}\hbox {Th}\) decay series and \(^{40}\hbox {K}\) in soils are non-uniform, the \(\hbox {Ra}_\mathrm{eq}\) is commonly used to determine the total radioactivity of a sample. The \(\hbox {Ra}_\mathrm{eq}\) is evaluated based on an estimation that 10 Bq kg\(^{-1}\) of \(^{226}\hbox {Ra}\), 7 Bq kg\(^{-1}\) of \(^{232}\hbox {Th}\) and 130 Bq kg\(^{-1}\) of \(^{40}\hbox {K}\) produce the same gamma ray dose rate. Thus, the \(\hbox {Ra}_\mathrm{eq}\) is calculated as follows [22, 23]:

$$\begin{aligned} Ra_\mathrm{eq} = A_\mathrm{Ra} + 1.43A_\mathrm{Th} + 0.077A_\mathrm{K} \end{aligned}$$
(2)

where \(A_\mathrm{Ra}\), \(A_\mathrm{Th}\) and \(A_\mathrm{K}\) are the activity concentrations of \(^{226}\hbox {Ra}\), \(^{232}\hbox {Th}\) and \(^{40}\hbox {K}\), respectively.

The calculated absorbed gamma dose rate, denoted as D, at about 1 m above the ground surface has been evaluated using the conversion factors of 0.46 nGy h\(^{-1}\) for \(^{226}\hbox {Ra}\), 0.62 nGy h\(^{-1}\) for \(^{232}\hbox {Th}\) and 0.042 nGy h\(^{-1}\) for \(^{40}\hbox {K}\). Hence, the absorbed gamma dose rate, D, can be expressed in as follows [24]:

$$\begin{aligned} D (\hbox {nGy}\,\hbox {h}^{-1}) = 0.46A_\mathrm{Ra} + 0.62A_\mathrm{Th} + 0.042A_\mathrm{K}. \end{aligned}$$
(3)

The outdoor annual effective dose equivalent, AEDE, was calculated using following equation [1]:

$$\begin{aligned} AEDE (\hbox {mSv}\,\hbox {y}^{-1}) = D \times DCF \times OF \times T \end{aligned}$$
(4)

where D is the absorbed gamma dose rate obtained in Eq. (3); DCF is a dose conversion factor; OF is an outdoor occupancy factor and T is the time factor (8760 h). The values of DCF and OF are 0.7 Sv Gy\(^{-1}\) and 0.2, respectively, as taken from UNSCEAR 2000 [1]. The outdoor occupancy factor \(OF=0.2\) is originally from considering people spending about 20% of their time outdoor [25].

The external hazard index, \(H_\mathrm{ex}\), representing the hazard of natural gamma radiation is calculated as [3, 23, 24]:

$$\begin{aligned} H_\mathrm{ex} = \dfrac{A_\mathrm{Ra}}{370}+ \dfrac{A_\mathrm{Th}}{259}+ \dfrac{A_\mathrm{K}}{4810} \end{aligned}$$
(5)

The internal hazard index, \(H_\mathrm{in}\), is calculated as [23]:

$$\begin{aligned} H_\mathrm{in} = \dfrac{A_\mathrm{Ra}}{185}+ \dfrac{A_\mathrm{Th}}{259}+ \dfrac{A_\mathrm{K}}{4810}. \end{aligned}$$
(6)

The radiation hazard is insignificant when the \(H_\mathrm{ex}\) and \(H_\mathrm{in}\) are less than unity. The value of \(H_\mathrm{ex}=1\) corresponds to the upper \(\hbox {Ra}_\mathrm{eq}\) limit of 370 Bq kg\(^{-1}\).

Results and discussion

Activity concentrations

Fig. 2
figure 2

Average activity concentrations of \(^{226}\hbox {Ra}\) in soil samples in 15 districts of Savannakhet province, Laos

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Fig. 2 shows the average \(^{226}\hbox {Ra}\) activity concentrations in soil samples in 15 districts of Savannakhet province. The activity concentrations of \(^{226}\hbox {Ra}\) in soils are vary in the range from \(6.6\pm 1.5\) to \(73.6 \pm 7.7\) Bq kg\(^{-1}\). The smallest activity concentration of \(6.7\pm 1.5\) Bq kg\(^{-1}\) is obtained with sample S7 in Outhoompone district, whereas the highest value of \(73.6 \pm 7.7\) Bq kg\(^{-1}\) is obtained with sample S80 at Nong district. The average radioactivity level of \(^{226}\hbox {Ra}\) in Savannakhet province is about \(22.4 \pm 2.1\) Bq kg\(^{-1}\). Comparing to the world average value of 35 Bq kg\(^{-1}\), the average radioactivity concentration of \(^{226}\hbox {Ra}\) is smaller by a factor of 0.64, but the highest value in this region is about two times greater than the world average value [1]. It can also be seen from Fig. 2 that three regions in Savannakhet province having greater radioactivity concentrations of \(^{226}\hbox {Ra}\) than others are Nong, Vilabuly and Sepone districts. The highest average \(^{226}\hbox {Ra}\) activity concentration of 46.3 Bq kg\(^{-1}\) is obtained at Nong district.

Fig. 3
figure 3

Average activity concentrations of \(^{232}\hbox {Th}\) in soil samples in 15 districts of Savannakhet province

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Fig. 4
figure 4

Average activity concentrations of \(^{40}\hbox {K}\) in soil samples in 15 districts of Savannakhet province

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Figures 3 and 4 display the average \(^{232}\hbox {Th}\) and \(^{40}\hbox {K}\) activity concentrations in soil samples in 15 districts of Savannakhet province. The values of \(^{232}\hbox {Th}\) activity concentrations in the soil samples vary in the range from \(3.8 \pm 1.5\) to \(113.8 \pm 3.2\) Bq kg\(^{-1}\). The smallest value of \(3.8 \pm 1.5\) Bq kg\(^{-1}\) corresponds to sample S24 collected in Xayphoothong district, while the highest value of \(113.8 \pm 3.2\) Bq kg\(^{-1}\) is obtained with sample S57 in Vilabuly district. The highest average value of 61.7 Bq kg\(^{-1}\) is obtained at Nong district as shown in Fig. 3. It is noticed that the average radioactivity concentration of \(^{232}\hbox {Th}\) in Savannakhet province is about \(30.8 \pm 2.9\) Bq kg\(^{-1}\) which is approximate the world average value of 30 Bq kg\(^{-1}\) [1]. However, the highest concentration of \(^{232}\hbox {Th}\) in Savannakhet province is greater than the world average value by a factor of 3.8.

The activity concentration of \(^{40}\hbox {K}\) in Savannakhet province is in the range from \(13.6 \pm 3.3\) to \(906.4\pm 31.4\) Bq kg\(^{-1}\). The lowest value of \(13.6 \pm 3.3\) Bq kg\(^{-1}\) is obtained with sample S13 in Khanthabuly district, and the highest value of \(906.4\pm 31.4\) Bq kg\(^{-1}\) is obtained with sample S80 in Nong district. Nong district also corresponds to the highest average value of 488.4 Bq kg\(^{-1}\) as shown in Fig. 4. The average \(^{40}\hbox {K}\) activity concentration is \(211.6\pm 16.5\) Bq kg\(^{-1}\), which is about half of the world average value (400 Bq kg\(^{-1}\)) [1]. However, the highest value in Nong district is greater than the world average value by a factor of 2.3.

Table 1 Activity concentrations of radionuclides \(^{226}\hbox {Ra}\), \(^{232}\hbox {Th}\) and \(^{40}\hbox {K}\) in soil samples in Savannakhet province, Laos
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Table 1 presents the average activity concentrations of radionuclides \(^{226}\hbox {Ra}\), \(^{232}\hbox {Th}\) and \(^{40}\hbox {K}\) in soil samples in 15 districts of Savannakhet province in comparison with the world average values. It is noticed that the activity concentrations of the radionuclides are higher at Sepone, Vilabuly and Nong districts compared to other regions in Savannakhet province. The three districts are also known as locations of mining interest. From Figs. 2, 3 and 4, one can also see the higher activity concentrations of natural occurring radioactive materials in the three districts than that in the others. The activity concentrations of \(^{226}\hbox {Ra}\) and \(^{232}\hbox {Th}\) in the three districts are greater than the average value by a factor of 1.5–2.1. Whereas, the activity concentration of \(^{40}\hbox {K}\) obtained in the three districts is greater than the average value by a factor of 1.7–2.4.

Table 2 Comparison of activity concentrations of radionuclides in Savannakhet province, Laos and other worldwide regions
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Table 2 shows the comparison of the average activity concentrations of natural radionuclides obtained in soil samples in Savannakhet province, Laos with other worldwide regions. Comparing with neighboring regions such as Hue and Da Nang provinces of Vietnam, the average activity concentrations of \(^{226}\hbox {Ra}\), \(^{232}\hbox {Th}\) and \(^{40}\hbox {K}\) in Hue province are 57.0, 47.8 and 309.0 Bq kg\(^{-1}\), while the values in Da Nang province are 51.0, 58.2 and 366.2 Bq kg\(^{-1}\), respectively, which are greater than that obtained in Savannakhet province [11]. Comparing with other worldwide regions as listed in Table 2, one can see that in general, Savannakhet province is among the regions with relatively lower activity concentrations of natural radionuclides in soils than others.

Radium equivalent activity

The maximum value of \(\hbox {Ra}_\mathrm{eq}\) in soils should be less than the limit of 370 Bq kg\(^{-1}\) to ensure the external dose less than 1.5 mGy h\(^{-1}\) as recommended by UNSCEAR reports [1, 3]. Figure 5 shows the calculated average \(\hbox {Ra}_\mathrm{eq}\) in the soil samples in 15 districts of Savannakhet province. The \(\hbox {Ra}_\mathrm{eq}\) values vary in a wide range from 17.5 to 299.9 Bq kg\(^{-1}\) with the average value of 82.3 Bq kg\(^{-1}\). One can see that similar to the radioactivity concentrations, three districts having greater average values of \(\hbox {Ra}_\mathrm{eq}\) than others are Nong, Vilabuly and Sepone. The highest \(\hbox {Ra}_\mathrm{eq}\) value of 299.9 Bq kg\(^{-1}\) obtained with sample S80 at Nong district is still smaller than the limit of 370 Bq kg\(^{-1}\), while the average \(\hbox {Ra}_\mathrm{eq}\) is much smaller than the limit value by a factor of 0.22 [1].

Fig. 5
figure 5

Average radium equivalent activity \(\hbox {Ra}_\mathrm{eq}\) in 15 districts of Savannakhet province

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Absorbed gamma dose rate

Fig. 6
figure 6

Calculated average absorbed gamma dose rate D (nGy h\(^{-1}\)) in 15 districts of Savannakhet province

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Fig. 6 shows the average D values in 15 districts of Savannakhet province. The calculated values of D due to the terrestrial gamma radiation are obtained in the range of 8.1–137.5 nGy h\(^{-1}\) with the highest value of 137.5 nGy h\(^{-1}\) occurring at Nong district. The average value of D in Savannakhet province is 37.4 nGy h\(^{-1}\), which is smaller than the world average value of 59 nGy h\(^{-1}\), but the highest value of D is greater than the world average value by a factor of 2.3.

Annual effective dose equivalent

Fig. 7
figure 7

Calculated average annual external effective dose rate AEDE (mSv y\(^{-1}\)) in 15 districts of Savannakhet province

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The AEDE values are obtained in the range from 0.01 to 0.17 mSv y\(^{-1}\). The highest AEDE value corresponds to sample S80 at Nong district, where the highest activity concentrations of radionuclides are obtained. Figure 7 show the average values of AEDE in 15 districts of Savannakhet province, which vary in the range of 0.026–0.093. The average value of AEDE in total is 0.05 mSv y\(^{-1}\) which is lower than the world average value of 0.07 mSv y\(^{-1}\) [1]. These values are less than the AEDE limit of 1 mSv y\(^{-1}\) for an individual and 20 mSv y\(^{-1}\) for radiation workers as recommended by International Commission on Radiation Protection [26].

External and internal radiological hazard indices

Fig. 8
figure 8

Calculated average external hazard index \(H_\mathrm{ex}\) in 15 districts of Savannakhet province

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Fig. 9
figure 9

Calculated average internal hazard index \(H_\mathrm{in}\) in 15 districts of Savannakhet province

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Figures 8 and 9 show the calculated average \(H_\mathrm{ex}\) and \(H_\mathrm{in}\) obtained from the activity concentrations in soil samples in 15 districts of Savannakhet province. The \(H_\mathrm{ex}\) values in the soil samples are within the range from 0.05 to 0.81. This means that the highest value of \(H_\mathrm{ex}\) is less than unity. The average values of \(H_\mathrm{ex}\) in 15 districts (0.121–0.444) as displayed in Fig. 8 and the average \(H_\mathrm{ex}\) in total of \(0.22\pm 0.03\) are much less than unity. As shown in Fig. 9, the average \(H_\mathrm{in}\) values in 15 districts and the average \(H_\mathrm{in}\) of \(0.28\pm 0.03\) in Savannakhet province are much less than unity in most of the area. There are two samples with higher values of \(H_\mathrm{in}\) (0.94 and 1.01) found in Vilabuly and Nong districts, but only one sample (S80 at Nong district) has the \(H_\mathrm{in}\) greater than unity. The results of \(H_\mathrm{ex}\) and \(H_\mathrm{in}\) imply that there is no significant radiological hazard to human health in this region.

Table 3 Radiological hazard indices in soils in Savannaket province, Laos and other worldwide regions
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Table 3 shows comparison of the average radiological hazard indices obtained in Savannakhet province with that reported for worldwide regions. In general, the radiological hazard indices obtained in Savannakhet province are relatively lower than that of other regions in Southeast Asian countries as well as world average values. The average Ra\(_\mathrm{eq}\) values in neighboring regions such as Hue and Da Nang provinces, Vietnam (149 and 162 Bq kg\(^{-1}\), respectively) are comparable with the highest values obtained in Savannakhet province (131 and 160 Bq kg\(^{-1}\) in Sepone and Nong districts, respectively) [11].

Conclusions

Measurement of the radioactivity concentrations of 80 soil samples collected widely in Savannakhet province, Laos is conducted using a HPGe gamma spectrometer for evaluating a baseline data of radioactivity concentrations and radiological hazards in the area. The activity concentrations in soil samples are in the range from 6.6 to 73.6 Bq kg\(^{-1}\) for of \(^{226}\hbox {Ra}\), from 3.8 to 113.8 Bq kg\(^{-1}\) for \(^{232}\hbox {Th}\) and from 13.6 to 906.4 Bq kg\(^{-1}\) for \(^{40}\hbox {K}\), respectively. The average activity concentrations of \(^{226}\hbox {Ra}\), \(^{232}\hbox {Th}\) and \(^{40}\hbox {K}\) are \(22.4 \pm 2.1\), \(30.8 \pm 2.9\) and \(211.6\pm 16.5\) Bq kg\(^{-1}\), respectively. The values of activity concentrations are relatively low compared to the world average values and that of other worldwide regions. The average activity concentration of \(^{232}\hbox {Th}\) is approximate the world average value (30 Bq kg\(^{-1}\)), but the value of \(^{40}\hbox {K}\) is about half of the world average value (400 Bq kg\(^{-1}\)). The average \(\hbox {Ra}_\mathrm{eq}\) is \(82.8\pm 9.7\)  Bq kg\(^{-1}\), which is much less than the safety limit of 370 Bq kg\(^{-1}\). The highest concentrations of all three nuclides were found at Nong district but the highest value of \(\hbox {Ra}_\mathrm{eq}\) (300 Bq kg\(^{-1}\)) is still less than the safety limit value. The results of radiological hazard indices such as absorbed gamma dose, annual effective dose equivalent, external and internal radiation hazard indices indicate no significant effect to human health.