Introduction

Radionuclides naturally occurring in soils make an important contribution to human irradiation. According to UNSCEAR, most of the total radiation is caused by natural sources (UNSCEAR 2000). Therefore, to assess the risks of irradiation, a detailed study of natural sources of radiation in soils is important and evident. Main part of terrestrial gamma radiation is caused by natural radionuclides, such as 40K, 238U, 232Th, and their progenies distributed in the soil.

The radioactivity of the soil depends on the content and distribution of natural radioisotopes in it, while the latter depends on soil parent material and some other factors of soil formation. As a rule, relatively high level of natural radioactivity is related with magmatic rocks (e.g., granite) and the low level with sedimentary rocks. Differences are also observed in the igneous rocks themselves, for example, felsic igneous rocks are characterized by a relatively higher content of natural radioisotopes than ultramafic and mafic igneous rocks (Dhawal et al. 2013).

In Georgia, rocks with granite composition are mainly represented in the axial part of the Great Caucasus Mountains and in the crystalline missives — Dzirula, Khrami, and Loki, which represent the salients of old (Variscan) terrain of the so-called Georgian massif (Adamia et al. 2011). Our previous research is devoted to the Khrami crystalline massif (Kapanadze et al. 2019). Some studies are conducted for other regions of Georgia by other authors (Kekelidze et al. 2017; Kekelidze et al. 2018). In present research, the part of Dzirula massif is selected, which is the highest and most fragmented area of the Imereti (Upper Imereti) region of western Georgia and is characterized by humid subtropical climate and well-developed soil-forming processes. During the selection of the study area, in addition to petrology, other factors are taken into account as well, for instance, existence of settlements in the region, agriculture, and mining activities.

Materials and methods

Characteristics of the study area

The study area occupies about 100 km2 and includes parts of the river Dzirula and its tributaries basin in the Upper Imereti region. Yellow brown forest soils of different granulometry are predominant in the study area (Urushadze et al. 2015).

Based on the existing geological data (Geguchadze et al. 1972), the rocks in the study area are mainly represented by granitoids (Middle Jurassic), quartz-diorites, microclinized granitoids and migmatites (Late-Middle Paleozoic), microcline (rose) granites (Middle Paleozoic), and gabbroic rocks (Paleozoic). Mesozoic-Cenozoic sedimentary rocks (marls, limestones, sandstones, etc.) adjacent to the Dzirula massif are also sporadically presented in the study area (Fig. 1).

Fig. 1
figure 1

Geological map of the study area (based on (Geguchadze et al. 1972))

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Sampling methodology

To ensure comparability of research results, well-proven methodology, used also in previous research (Kapanadze et al. 2019), is used in the present study. Considering geological peculiarities of the study area, a specific sampling scheme has been developed. The samples were taken at a reasonable distance from buildings and other artificial structures to exclude any appearance of nonlocal amount of soil in the samples. To present the nature of the local terrestrial radioactivity uniformly, systematic grid method (Mehdi et al. 2016) of soil sampling has been used. According to the sampling methodology, five samples (45–50 m apart) were collected in each sampling site and averaged. A total of 19 mixed samples were taken, which amounted to 95 samples. Sampling sites themselves were, on average, 1000 m apart. Each sample was taken at a depth of 5–20 cm from the soil surface, as most of impact from terrestrial radiation comes from the top layers of the soil (Dhawal et al. 2013). Initial processing of samples has been performed on site and, consequently, up to 500 g of soil fraction was obtained and sealed.

Laboratory measurements

Gamma spectroscopy, as a well-known technique, is used to define the activity concentrations of radionuclides in the soil samples. For the gamma spectroscopy analysis, samples have been prepared in accordance with approved and tested methodologies. Primarily, samples have been air dried and sifted to get them almost in powder form (weighing up to 150 g). Then samples have been sealed hermetically and stored for about 2 months, to attain equilibrium of 214Bi and 214Pb with 222Rn (Kapanadze et al. 2019). Taking into account the storage period, it can be assumed that 228U and 226Ra reach the equilibrium (Keser et al. 2013; Rahman et al. 2012).

Measurements of the activity concentrations of radionuclides in soil samples have been conducted at the Institute of Physics of Tbilisi State University. CANBERRA high-purity germanium detector (HPGe) coupled with multichannel analyzer (MCA) has been used for laboratory works. ISOCS and LaBOCS software have been used for detector calibration. The detector has been calibrated using standard sources with known energies such as 137Cs (662 keV), 60Co (1173 keV and 1332 keV), and 241Am (59 keV). The full width half maximum (FWHM) resolution of the detector at 1332 keV of 60Co gamma ray was 1.8 keV. To reduce background effects, the detector is covered with a lead shield (10-cm thick). Genie-2000 software has been used for gamma spectrum analysis. Correction in each measurement has been made by subtraction of background levels. The counting time (to collect the spectrum for each sample) was 25000 s. To determine activity concentrations of 137Cs and 40K, single peaks of 662 keV and 1461 keV have been used respectively. Activity concentration of 238U has been determined by measuring the 609 keV peak from 214Bi and 352 keV peak from 214Pb. Activity concentration of 232Th has been determined by measuring 583 keV peak from 208Tl and 911 keV peak from 228Ac (Kapanadze et al. 2019).

Results of activity concentrations are calculated by using Eq. (1) (Kapanadze et al. 2019):

$$ {A}_{Ei}=\frac{C_{Ei}}{D_{eff}\bullet \gamma \bullet t\bullet m} $$
(1)

where AEi is the activity concentration of the radionuclide i for a peak at energy E, CEi is the total count of a photopeak, Deff is efficiency of detection, γ is the percentage of gamma emission probability, t is the counting time, and m is the mass of the sample (Kapanadze et al. 2019).

To determine contents of radionuclides (in ppm), following Eq. (2) is used (Knoll 1989):

$$ m\ (ppm)=\left(A\ M/{N}_{Av}\ \mathrm{ln}2\right)\ {t}_{1/ 2} $$
(2)

where A is activity concentration in (Bq/g) of the matter or daughter in radioactive equilibrium, M is molecular weight (g/mol), NAv is Avogadro’s number (6.02×1023), and t1/2 is half-life in seconds.

Results and discussion

Radionuclide concentrations

The results of activity concentrations (in Bq/kg) and calculated contents (in ppm) of natural radionuclides 40K, 238U, and 232Th as well as technogenic radionuclide 137Cs are provided in Table 1. Apart from radionuclide concentrations, Table 1 also presents local characteristics of the sampling sites such as petrology, soil type, and altitude.

Table 1 Radionuclide concentrations and characteristics of sampling sites
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According to the results presented in Table 1, the average values ​of activity concentrations are 994, 44, and 58 Bq/kg for 40K, 238U, and 232Th respectively. World median values of 40K, 238U, and 232Th activity concentrations are 400, 35, and 30 Bq/kg respectively (UNSCEAR 2000). Comparison of obtained results with world median values shows that for 40K, our results exceed world median value by 594 Bq/kg, for 238U — by 9 Bq/kg, and for 232Th — by 28 Bq/kg.

Activity concentrations of 137Cs vary in range from 12 to 46 Bq/kg, with the average value of 27 Bq/kg. The maximum value of activity concentrations of 137Cs (46 Bq/kg) is recorded at the 11th site (as well as for 238U and 232Th). Table 2 compares obtained results for 137Cs with some studies conducted in Georgia and in various countries. According to the investigations carried out in Georgia, it is considered that average background values of the 137Cs concentrations in the soils vary in the range from 1 to 10 Bq/kg for the whole territory of Georgia (Kekelidze et al. 2017). We assume that the sufficiently appreciable quantities for 137Cs, in our results, indicate the trace, left after Chernobyl 1986 accident, and atmospheric weapon tests in the 1950s and 1960s. However, it is clear that some places of the study area and especially highland regions were relatively more polluted by technogenic radionuclides than eastern and lowland regions of the country. Generally, fallout and sedimentation of technogenic radionuclides after the Chernobyl accident depended on peculiarities of atmospheric circulations. Apparently, due to the kind of high-altitude barrier for the atmospheric currents and relatively high intensity of precipitation, contamination in the highlands is higher than in the lowlands.

Table 2 Comparison of the results obtained for 137Cs with some data from other regions of the globe
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Outdoor absorbed dose rate in air (D)

As is known, the contribution of the natural radionuclide to the absorbed dose rate in air depends on the concentration of the radionuclide in the soil. Consequently, based on the determined results of activity concentrations of radionuclides in soil, the absorbed dose rate in air can be calculated. For this, Eq. (3) is used (UNSCEAR 2000):

$$ D\ \left( nGy/h\right)=0.0417{A}_K+0.462{A}_U+0.604{A}_{Th} $$
(3)

where D is the dose rate in the air at 1-m high from the ground; AK, AU, and ATh are activity concentrations of 40K, 238U, and 232Th respectively, determined from Eq. (1); and 0.0417, 0.462, and 0.604 are dose conversion factors for 40K, 238U, and 232Th respectively;

From Table 3, presenting the calculation results for all radiological parameters, it is seen that results of outdoor absorbed dose rate vary from 66 to 122 nGy/h, with an average of 97 nGy/h. This average value is relatively higher than corresponding mean worldwide value of 57 nGy/h (UNSCEAR 2000). From Fig. 2, graphically presenting the components of absorbed dose rate, it is seen that 40K makes the significant contribution to the formation of the outdoor absorbed dose rate in air.

Table 3 Determined radiological parameters
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Fig. 2
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Contribution of 40K, 238U, and 232Th towards the dose rate

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Annual effective dose rate (AEDR)

To estimate the annual effective doses, it is necessary to take into account the conversion coefficient of 0.7 Sv/Gy from the absorbed dose in air to the effective dose received by adults, as well as the occupancy factors, which are 0.2 for outdoor and 0.8 indoor spaces respectively (UNSCEAR 2000). The effective dose rate is determined by using Eq. (4) (UNSCEAR 2000):

$$ AEDR\ \left( mSv/y\right)=D\times Q\times T\times OF\times {10}^{-6} $$
(4)

where D denotes the absorbed dose rate in the air, Q is the conversion coefficient, T is the time for 1 year, i.e., 8760 h, and OF is an occupancy factor.

From Table 3, it is observed that values of outdoor annual effective dose rate are in range from 0.08 to 0.15 mSv/y, with mean value of 0.12 mSv/y, which is higher than world global value of 0.07 mSv/y for outdoor annual effective dose rate (UNSCEAR 2000).

Radium equivalent activity (Ra eq)

Radium equivalent activity is determined taking into account the risks associated with the use of building materials that contain 40K, 238U, and 232Th in acceptable quantities. Assuming that 130 Bq/kg of 40K, 10 Bq/kg of 238U, and 7 Bq/kg of 232Th initiate almost the same dose of gamma radiation (Hussain and Hussain 2011), the sum of activity concentrations of 40K, 238U, and 232Th is to be calculated. Radium equivalent activity is defined by the following formula (5) (Dhawal et al. 2013):

$$ {Ra}_{eq}\ \left( Bq/ kg\right)=0.077{A}_K+{A}_U+1.43{A}_{Th} $$
(5)

where AK, AU, and ATh are activity concentrations of 40K, 238U, and 232Th respectively. To reduce risks of irradiation, the material that has a radium equivalent activity greater than 370 Bq/kg, equivalent to a gamma dose of 1.5 mSv/y (UNSCEAR 2000), should not be used (Alaamer 2008; Dhawal et al. 2013). From Table 3, it is seen that calculated values for radium equivalent activity vary in range of 139–258 Bq/kg, with a mean value of 203 Bq/kg. All obtained values of radium equivalent activity are less than the limit value of 370 Bq/kg (UNSCEAR 2000).

External and internal hazard indexes (H ex and H in)

To assess the external effects of gamma radiation, a hazard index is commonly used, which is known as the external hazard index and is defined as follows (6.1) (Hussain and Hussain 2011):

$$ {H}_{ex}=\frac{A_K}{4810}+\frac{A_U}{370}+\frac{A_{Th}}{259} $$
(6.1)

Another parameter, used in such calculations, is known as the internal hazard index, which is determined by the following way (6.2) (Hussain and Hussain 2011):

$$ {H}_{in}=\frac{A_K}{4810}+\frac{A_U}{185}+\frac{A_{Th}}{259} $$
(6.2)

In Eqs. 6.1 and 6.2, AK, AU, and ATh denote activity concentrations. According to limitations, hazard indexes should be less than one, which equals to radium equivalent activity of 370 Bq/kg (Kessaratikoon and Awaekechi 2008). In our case (Table 3), the average values of hazard indexes are 0.55 and 0.67 for Hex and Hin respectively, which are less than the limit value.

Cancer risk

To calculate and estimate the fatal cancer risk (outdoor) to an individual living on the territory, the following Eq. (7) was used (Ramli et al. 2009):

$$ \hat{R_i}=a{E}_{out} $$
(7)

where a is the risk factor that is 0.05 of mortality per sievert, and Eout is outdoor effective annual dose rate. In our case, value of fatal cancer risk due to external gamma radiation is about 6.0×10−7 per year to each individual living in this region. This value is small and cannot serve the grounds for any concern.

Table 4 compares the results (average values) obtained for 40K, 238U, and 232Th activity concentrations and air absorbed dose rate with similar results from different parts of the world.

Table 4 Comparison of average values of 40K, 238U, and 232Th activity concentrations and absorbed dose rate with similar data from other regions of the world
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Relationships

Based on the existing geological data and using ArcGIS tools, a geological map of the study area is created (Fig. 1). Using Kriging interpolation method, a spatial distribution scheme of the values of outdoor air absorbed dose rates is developed (Fig. 3).

Fig. 3
figure 3

Spatial distribution of the outdoor absorbed dose rates in air

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The analysis of Figs. 1 and 3 shows that the results of absorbed dose rates in the air (which in its turn is directly related to the concentrations of natural radionuclides in the soil) depend on the geology and topography of the territory. In particular, soils formed as a result of weathering of the Late Variscan granitoids of the Dzirula crystalline massif obviously reveal a higher level of natural radioactivity in contrast to the soils originated from sedimentary rocks. Gabbroids, as igneous mafic rocks, show a relatively low level of radioactivity. Consequently, it is apparent that the spatial distribution of natural radionuclides in the study area noticeably depends on the type of soil parent material.

Figure 4 demonstrates the relationship between the annual effective dose rate and petrology of the area (parent rocks) in accordance with sampling sites. According to the results, relatively increased values of annual effective dose can be observed. For instance, increased values are observed at the sites 3, 5, 10, and 11, which are associated with granite rocks. Relatively decreased values of annual effective dose rate are recorded for the sites 15, 16, 18, and 19 presenting gabbroidal and sedimentary rocks.

Fig. 4
figure 4

Relationship between annual effective dose and petrology of the area

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Conclusion

An analysis of the results shows that soils originated from sialic igneous rocks (granitoids in our case) revealed relatively high level of natural radioactivity than soils originated from mafic (gabbroids) or sedimentary rocks (marlstones). Results obtained for radionuclide concentrations, outdoor absorbed dose, and annual effective dose rate are slightly higher than the corresponding global averages. Consequently, terrestrial gamma radiation of the study area, of course, is not life threatening, but deserves certain attention and maybe further investigation.

According to results obtained, average value of radium equivalent activity equals to 203 Bq/kg, which is less than internationally established limit of 370 Bq/kg (UNSCEAR 2000). Considering that in the area under study there is no deep regional tectonic fault, it can be concluded that territory is free from the hazards due to radium and its progeny radon. All values obtained for external and internal hazard indexes are less than unity. This indicates that the population living in the study area is not exposed to radiation exceeding permissible limits.

Results of natural radionuclide concentrations provided in Table 1 do not significantly differ from the results previously obtained for the Khrami crystalline massif, where average values of activity concentrations are 880, 39, and 53 Bq/kg for 40K, 238U, and 232Th respectively (Kapanadze et al. 2019). This apparently shows some similarities between these two crystalline missifs in terms of natural radioactivity.

The results of the study can serve to develop unified analytical information base on the levels of natural radioactivity and radiation pollution of the environment and can be used during the planning of different social-economic projects.