Introduction

Assessment of natural radionuclides and related radiation hazards play an important role in radioactivity research regarding human health and have been received much attention from worldwide researchers. In which, the distribution of natural radionuclides in different types of soil and rock and assessment of their radiological hazards have been also widely investigated. For urban soils in Armenia, Belyaeva et al. [1] indicated that in the Kapan area, although the natural radioactivity (238U, 232Th, 40K) levels in soil were enhanced by mining activities, they did not significantly affect human health. By contrast, these authors showed that the level of radioactivity in the soil at some locations was higher than the background values, but it did not come from the mining activities. In Egypt, for some rocks such as marble, granite, serpentine, Shohda et al. [2] presented that the measured activity concentration of 226Ra, 232Th, and 40K in these rock samples was within the safety limits and had also no significant radiation hazards to human health. According to the research results of Birami et al. [3], the high radioactivity levels and radiation hazards were observed in Arud granitic rock and Ramsar’s soil in Northern Iran. These authors indicated that the main contributors to the high level of radiation in the study area are 40K-bearing feldspars and 226Ra-rich carbonates. In general, as reported in the literature, the level of natural radioactivity in soil and rock significantly depends on the feature of geological formation. Thus, the distribution of natural radionuclides in soil, rock which comes from different geological formations in different areas needs to be further investigated with aiming to evaluate the natural radiation hazards to human health as well as providing special information for different economic policies of the local authority.

The Khammouan province, a high population density which is one of the key economic areas of Laos. This province borders Bolikhamxay province to the north, Xavanakhet province to the South, Ha Tinh, Quang Binh provinces (Vietnam) to the east, and Thailand to the west (Fig. 1). In Laos, the mining activities in Khammouan province are not significant. However, the copper and gold mining activities in Bolikhamxay and Xavanakhet provinces are the most significant mining interest [4, 5]. Thus, investigation of the natural radionuclide concentration in soil samples in the Khammouan area will provide baseline data for further research on TENORM. Recently, the assessment of natural radionuclide in soil samples and building materials distributed in different areas of Laos also attracted many researchers [4,5,6,7,8].

Fig. 1
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Geological map and sampling points

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From the geological point of view, the geological formations of Khammoun province include Mz1 (Lower Mesozoic), Mz2 (Middle Mesozoic), N2Q (Upper Neogene-Quaternary), PR (Proterozoic), Pz2 (Middle Paleozoic), Pz3 (Upper Paleozoic), cPz2 (Middle Paleozoic), cPz3 (Upper Paleozoic), gPz2 (Middle Paleozoic), gPz3 (Upper Paleozoic), vPz3 (Upper Paleozoic) and can be divided into two main groups of geological formations in the direction from the North East to the South West strike: sedimentary and magmatic—volcanic formations. In the spatial distribution, in the Northeastern, there are intrusive magmatic rocks, basalt rocks, and volcanoes (magmatic—volcanic) whose composition is granitoid gneiss, granite, gneiss micas, marble, and mafic volcanic rocks. While the Southwestern part is the sedimentary formation which contains red arenite, gravel, sandstone, clay, limestone, coal, and conglomerate. In this study, the activity of natural radionuclides in soil samples of different geological formations in the centre province of Laos will be measured and analyzed. The 67 points for soil and rock sampling were chosen in densely populated areas, agricultural zones, tourist resorts, and schools. The samples were collected in an undisturbed area, away from trees, buildings, roads, and other structures, and followed the trending from the North East to the South West at different geological formations to observe the alteration—related formations (Fig. 1). The results of radioactivity measurement were also used to calculate the radiological hazards.

Sample preparation and methods

Sample preparation

In this study, the samples were taken at Khammouan province of Laos in the dry season in 2020 (Fig. 1). The soil samples after taking were removed from gravel, rock fragments, and tree roots. These soil samples were dried at 105 °C temperature for 10 to 24 h and then milled less than 0.2 mm in size. Each fine soil sample was weighed, then packed into a cylindrical plastic box. Each soil sample box was sealed for 30 days to reach a secular equilibrium between the radium and its daughter radionuclides.

Methods

The method for measurement of radionuclide concentration in this study follows the method which was used in previous studies [9,10,11,12,13,14]. Accordingly, a high-resolution HPGe detector with a low background of Ortec™ was used to measure the radionuclide activities and Gamma Vision software was employed to analyze the spectrum. The detector energy resolution is 1.9 keV at the 1.33 MeV 60Co gamma-ray peak. A 10-cm thick old-lead cylinder with a 1 mm cadmium and 1 mm copper inner lining detector was used to reduce the radiation inside the lead shield. The time for the measurement process was about two days to minimize the statistical counting error. The standard reference materials (reference materials RG produced by IAEA and IAEA-375) were employed for activity calculation and calibration.

The activity concentration of each soil sample was calculated based on its respective gamma lines with the gamma lines of 609.3 keV, 1120.3 keV, and 1764.5 keV for 226Ra, the gamma line of 1460 keV for 40K. The lines of 911.2 keV, 969.0 keV, 2614.5 keV, 583.0 keV were used for 232Th (228Ra). The 232Th was mentioned and measured with the assumption of equilibrium between 232Th and 228Ra (228Th) in soil samples [11, 14,15,16].

The calculation of activity concentrations of natural radionuclides, radiation hazard indices including the radium equivalent activity concentration (Raeq), the absorbed gamma dose rate (D), the annual effective dose equivalent (AEDE), and the excess lifetime cancer risk (ELCR) were conducted following the methods shown in Duong et al. (2021) [14]. In this study, the life expectancy of Laos people is 68.9 years [17].

Results and discussions

Activity concentration

The results of measured activity concentration of natural radionuclides are presented in Table 1. As shown, the activity concentrations of 226Ra, 232Th (228Ra), and 40K in the study area range from (6.0 ± 0.61) Bq/kg to (68.5 ± 8.0) Bq/kg, from (8.7 ± 1.0) Bq/kg to (78.9 ± 8.3) Bq/kg, from (32.1 ± 3.1) Bq/kg to (812 ± 63) Bq/kg with the mean values of (32.2 ± 1.7) Bq/kg, (41.6 ± 2.2) Bq/kg, (279 ± 24) Bq/kg respectively. The highest concentration of 226Ra is found in sampling point P3 which belongs to Pz3 formation while the lowest one is found in P67 (Mz1). For 232Th (228Ra) concentration, the highest value is observed in P53 (Mz2) while the lowest one is in P64 (Pz3). For 40K concentration, the highest value is found in P25 (vPz3) whereas the lowest one is observed in P53 (Mz2). The asymmetric distribution curve of these natural radionuclides is plotted in Fig. 2. As shown in this figure, since the mean and median values of 226Ra, 232Th (228Ra) are almost similar and the Skewness values are 0.49 and 0.21 respectively, the concentrations of 226Ra, 232Th (228Ra) are nearly normal distribution. By contrast, the concentration of 40K is the right-skewed (positive skew) distribution with the Skewness value of 0.72. It can be seen that the highest variation of measured activity concentration is also observed for 40K with the standard deviation, SD of 200 Bq/kg. This phenomenon is due to the high mobility of potassium and it is easily transported by water [18, 19]. The large variation of 40K concentration was also found in soil sample outside the ore body in Muong Hum, Viet Nam [14]; in surface soil sample in Bolikhamxay, Laos [5]; in soil sample in Savannakhet, Laos [4].

Table 1 Activity concentration of 226Ra, 232Th, and 40K
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Fig. 2
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Distribution curve of 226Ra, 232Th(228Ra), and 40K concentration in the study area

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In general, there are about 38.8% (26 points), 41.8% (28 points), and 22.4% (15 points) of total sampling points that has concentrations of 226Ra, 232Th (228Ra), and 40K are higher than the global average values respectively. However, the average concentration of 226Ra is almost close to the global average value (32 Bq/kg) whereas the average concentrations of 232Th (228Ra) and 40K in the study area are lower than the global average values which are 45 and 420 Bq/kg respectively [16]. In comparison to some surrounding areas of the study area, the variation of average concentrations of 226Ra, 232Th (228Ra), and 40K in these areas are shown in Fig. 3. The values of these radionuclides in these areas are taken from Leuangtakoun et al. 5]. As shown, in Laos, the average concentrations of studied radionuclides in Khammouan province (this study) are slightly higher than those in Savannakhet province [4] but lower than those in Bolikhamxay province [5]. Additionally, the average concentrations of natural radionuclides in the soil in this study are almost similar to those in other areas which are shown in Fig. 3.

Fig. 3
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Variation of natural radionuclides in soil in some surrounding areas

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The variation of natural radionuclides in terms of geological formations is shown in Figs. 4 and 5. As presented in Fig. 4, the average concentrations of 226Ra and 232Th (228Ra) slightly range from 21.8 Bq/kg (Mz1) to 41.0 Bq/kg (cPz2) Bq/kg and from 33.3 Bq/kg (Mz1) to 60.2 Bq/kg (cPz2) Bq/kg while the average concentration of 40K significantly varies from 112 Bq/kg (Mz1) to 571 Bq/kg (cPz2). This indicates that the radionuclide concentration in soil significantly depends on the geological formations. Some previous studies, such as Birami et al. [3] revealed that the types of rock are the main factor enhancing the radionuclide concentration in soil samples. In this study, it is found that the highest average concentrations of 226Ra, 232Th, and 40K are found in cPz2 formation whereas the lowest average concentrations of these radionuclides are detected in Mz1 formation. The cPz2 formation is mainly composed of muddy limestone, claystone, and shale. Among geological formations, the muddy limestone in the cPz2 formation has the highest dissolution ability and claystone, and shale are easily weathered. These phenomena may lead to a high concentration of natural radionuclides in this geological formation. By contrast, the Mz1 formation is the youngest formation in the study area and is mainly composed of coal, red arenite, and conglomerate which have a low ability of weathering, so it may lead to a low concentration of studied radionuclides in this formation. Additionally, as shown in Fig. 5, for all geological formations, the highest activity concentration is observed for 40K while the lowest one is seen for 226Ra. This phenomenon can be explained based on the mobility of radionuclides. The potassium has the highest mobility while the Ra has the lowest mobility among studied radionuclides [18, 19]

Fig. 4
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Variation of natural radionuclide concentration

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Fig. 5
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Natural radionuclide concentration at different geological formations

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Radiological hazards

The calculated results of radiological hazard indices for the soil in Khammouan province are presented in Table 2. The world average values are also listed in this table.

Table 2 Results of radiological hazard indices
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As shown in Table 2, the radium equivalent activity (Raeq) in the soil samples in the study area varies from 31.1 to 213 Bq/kg with a mean value of 113 Bq/kg. As reported in UNSCEAR (2000), the maximum value of Raeq should be lower than the limit value of 370 Bq/kg to ensure the external dose less than 1.5 mGy/h. It can be seen that the average value of Raeq in the study area is significantly lower than the recommended value of UNSCEAR (2000) [16]. Additionally, the highest value of Raeq in the study area observed in sampling point P25 is still lower than the limit value of 370 Bq/kg.

The correlation between Raeq and studied radionuclide concentrations is plotted in Fig. 6. As shown, the correlation coefficients (R2) of the Raeq and 226Ra, 232Th (228Ra), 40K concentrations are 0.56, 0.88, 0.56 respectively. It can be seen that among studied radionuclides, the Raeq has the strongest correlation with 232Th concentration. This indicates that the concentration of 232Th has a major contribution to the Raeq. This research result well agrees with the research result reported in Bolikhamxay province, Laos [5].

Fig. 6
figure 6

Correlation between 226Ra, 232Th, 40K and Raeq

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The variation of absorbed dose rate (D), annual effective dose equivalent (AEDE), and excess lifetime cancer risk (ELCR) values are shown in Figs. 7, 8 and 9. For details as shown in Table 3, the calculated results of (D) range from 14.4 to 100 nGy/h with the mean value of 52.3 nGy/h; the results of the AEDE values vary from 17.6 to 123 μSv/y with the mean value of 64.1 μSv/y, and the ELCR values range from 0.07 × 10–3 to 0.48 × 10–3 and the mean value of ELCR is 0.25 × 10–3. In general, the values of D, AEDE, and ELCR in some sampling points are higher than the world average values while those in some other points are lower than the world average one. However, the average values of D, AEDE, and ELCR in the study area are still lower than the world average values (Table 2).

Table 3 Calculated results of hazard indices in different sampling points in the study area
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Fig. 7
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Variation of absorbed dose rate (D)

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Fig. 8
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Variation of annual effective dose equivalent (AEDE)

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Fig. 9
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Variation of excess lifetime cancer risk (ELCR)

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Conclusions

The concentrations of natural radionuclides (226Ra, 232Th (228Ra), 40K) in 67 soil samples in Khammouan province, Laos have been measured and used to evaluate the radiological hazards in this area. Some main conclusions are drawn as follows:

The study radionuclide concentration in soil significantly depends on the geological formations. The highest average concentration of these radionuclides is found in cPz2 formation while the lowest one is found in Mz1 formation in general. The cPz2 formation is mainly composed of muddy limestone, claystone, and shale while the Mz1 formation is composed of coal, red arenite, and conglomerate. In all studied geological formations, the highest average concentration was found for 40K whereas the lowest one was observed for 226Ra. The average concentration of 226Ra is almost similar to its limit value as reported in UNSCEAR (2000) whereas the average concentrations of 232Th, 40K in the study area are lower than their limit values. Additionally, the average values of studied radionuclide concentration in the study area are almost similar to those in some surrounding areas.

Regarding the radiation hazards, the Raeq has the strongest correlation with 232Th concentration. In general, the radiological hazard indices, including absorbed dose rate (D), annual effective dose equivalent (AEDE), and excess lifetime cancer risk (ELCR) are lower than the world limit values and there is no effect to human health.