People are exposed to ionizing radiation from radionuclides that present in different types of natural sources (Ghosh et al. 2008). Owing to the health risks associated with exposure to natural radionuclides, international and local organizations—such as the International Commission on Radiological Protection and the United States Environmental Protection Agency (USEPA)—have set standards and legislation to minimize such exposure (Hammond et al. 2007; Saleh and Shayeb 2014). Traditional gold mining in the Sudan has become increasingly attractive in the last decades owing to rising gold prices. For this reason, gold markets have prospered in many Sudanese provinces. The miners might be exposed to ionizing radiation from primordial radionuclides during gold extraction and processing. People residing close to the mines may be directly exposed to radionuclides through the ingestion of drinking water or uptake through the food chain. In addition, the general public may also be exposed to ionizing radiation because of reuse of mine wastes as construction materials (International Commission on Radiological Protection [ICRP] 1991). Indiscriminate mining, for example, removes vegetation and soil leading to an imbalance in the ecosystem and resulting devastation and destruction of the agricultural land. In addition, mining activities may sometimes produce toxic waste that can cause health problems and contamination (USEPA 2007). Radioactivity in soil is a good indicator of the distribution and accumulation of radioactivity in the ecosystem. Furthermore, the concentration of primordial radionuclides provides helpful data for monitoring environmental radioactivity (UNSCEAR 2000). Much consideration has been directed to natural radioactivity in the soil around gold-mining sites, and estimations have been made in numerous countries by many researchers (Ademola et al. 2014; Africa et al. 2000; Anjos et al. 2010; Durand 2012; Doyi et al. 2013; Vivian et al. 2011). Minerals are mined by artisanal miners using different methods such as digging pits and opening small, narrow holes up to several meters in depth. Mining activities in Sudan are not subject to radiological regulatory control. Therefore, there is no general awareness and knowledge about the radiation hazards and exposure levels to natural radioactivity in these mining areas. Thus, the current study aimed at examining the radioactivity concentrations of 226Ra, 232Th, and 40K in soil samples and assessing the radiological hazards. In addition, it lays focus on increasing the awareness and knowledge about radioactivity in mining areas.

Materials and Methods

Study Area

The area under consideration (Fig. 1) is located around 18°12′37″N and 33°57′7″E in a northern Berber town 18 km from the Al-Ibedia locality in the River Nile State, Sudan. It is bordered by the Nile to the west, the Albaoukh and Mberekhto areas to the north, the Red Sea to the east, and the Alhfab area to the south. Mining activities for the extraction of gold such as drilling, leaching, handling, storing, and transportation of raw materials—as well as the use of contaminated equipment, and the contaminated waste produced—have caused various environmental problems. Furthermore, some of the naturally occurring radioactive materials soluble in water, which have a tendency to leach into water bodies and farmlands, are an environmental and public health concern. The reasons mentioned above are the basis for the choice of the study area.

Fig. 1
figure 1

Location map of the study area (Idriss et al. 2014)

Full size image

Geology of the Study Area

The study area consists of a series of outcrops of Upper Proterozoic green schist assemblage. Mafic–ultramafic lenses exist along the contact consisting of a variety of rock types: peridotites, pyroxenites, and layered gabbros. East of the River Nile, the mafic–ultramafic lenses are intercalated with volcano-sedimentary sequences, highly sheared and transformed to talc, and rich in talc carbonate (Ali and Abdelrahman 2011). The core of the synform is dominated by highly deformed volcano-sedimentary sequences and intruded by syntectonic granitoid plutons. The mafic–ultramafic succession consists of a basal unit of serpentinite and talc-chlorite schist overlain by thick cumulate facies of peridotites, pyroxenites, and layered gabbros as well as basaltic pillow lava associated with thin layers of carbonates and metachert (Fig. 2). Hydrothermally ferruginated chert is observed at Qurun Mountain (Mohamedai 2014). The basal ultramafic rocks are in contact with the volcano-sedimentary rocks and subjected to very strong deformation and intense shearing giving rise to boudinage structures (Ali and Abdelrahman 2011). The area of mineralizations consists of metavolcano-sedimentary rocks intruded by syntectonic granitoid splutons. Both had been subjected to dynamic metamorphism and obliterated by recrystallization forming rock units of variable composition of greenschist facies (Küster and Liégeois 2001; Kuster et al. 2008). Mineral deposits in Al-Ibedia are divided into two parts. First, the upper portion of the mineralized zone has been weathered and transported for deposition forming a conglomeratic placer lying on top of the intrusive body. Later, these mineralized horizons eroded forming a thick overburden, which is mined by artisanal miners as a medium for native gold grains and flakes (Kuster et al. 2008). The lower portion is a hydrothermal solution primarily composed of auriferous quartz veins and stringers filling into tectonic fissures in the host rock. The hydrothermal system related to a magmatic solution of high temperature is associated with granitoid intrusion and contains high concentrations of 226Ra, 232Th, and 40K (Mohamedai 2014).

Fig. 2
figure 2

Geological map of the study area (Küster and Liégeois 2001; Küster et al. 2008)

Full size image

Sample Collection and Preparation

A total of 35 soil samples were taken from different locations around the open-pit of traditional gold mining sites at Al-Ibedia as shown (Fig. 3). The study area comprised gold mine waste and tailings dumps. The soil samples were crushed into a fine powder and sieved through a 100-μm sieve. The samples were then sealed in 500-ml plastic containers for approximately 4 weeks to allow radon (half-life 3.8 days) and its short-lived decay daughters, 214Bi and 214Pb, to reach secular equilibrium with the long-lived 226Ra precursor in the samples. The sample weights ranged from 240 to 564 g. A background sample was also prepared using a similar empty container.

Fig. 3
figure 3

Photographs (a through e) showing different activities of artisanal gold mining (digging, crushing, and ore processing)

Full size image

Gamma Spectrometric Measurement

The gamma-ray spectrometry set-up used in this analysis consisted of a highly shielded and well calibrated 7.6 × 7.6-cm NaI (Tl) detector enclosed in a 5 cm-thick lead shield to assist in reducing background radiation. In addition, the set-up was coupled with a computer-based multichannel analyzer, which was used for data acquisition and analysis of gamma spectra. The spectrometer was tested for its linearity and then calibrated for energy using gamma sources supplied by the International Atomic Energy Agency, Vienna. This was achieved by collection of spectra data from standard sources with energies in the range of 0.25–2.62 MeV. The channel numbers of the photopeaks corresponding to the different gamma energies were recorded after 900 s, and the energy-channel linear relationship was obtained. The efficiency calibration of the system was performed using a known source (Mixed Gamma Standard, Amersham) and then validated with three well-known reference materials obtained from the International Atomic Energy Agency for K, Ra, and Th activity measurements: RGK-1, RGU-1, and RGTH-1. The background count was determined by counting an empty container of the same dimension as those containing the samples and subtracting from the gross count. Each sample was measured during an accumulating time between 10 and 24 h. Activity concentrations of the samples were determined using the net area under the photopeaks using Eq. 1. The activity concentration (Bq/kg) of 232Th was determined from the photo-peaks of 208Tl (583 keV) and 228Ac (911 keV); that of 226Ra was obtained from the gamma-lines of 214Pb (352 keV) and 214Bi (609 keV); and that of 40K was measured directly from the photo-peaks at 1460 keV. This spectral analysis was performed with the aid of computer software Genie 2000 obtained from CANBERRA. The software uses an interactive photo-peak fit that corrects for interferences from various energies. Manual calculations were also performed to validate the results by defining the region of interest for each energy obtained for the sample and comparing it with the reference material (has the same matrix). The detection limit was required to estimate the minimum detectable activity in a sample, and it was obtained using the equation given elsewhere. Detection limits obtained were 2.7, 9.2, and 0.2 for 226Ra and 232Th, and 40K, respectively (IAEA 1989; Kolapo 2014; Godfred et al. 2015).

$$A_{c} \, \left( {\text{Bq/kg}} \right) = \, {{C_{n} } \mathord{\left/ {\vphantom {{C_{n} } {P\gamma M\varepsilon }}} \right. \kern-0pt} {P\gamma M\varepsilon }},$$
(1)

where A c  is the activity concentration of the radionuclide in the sample given in Bq/kg; C n is the net count rate under the corresponding peak; Pγ is the absolute transition probability of the specific γ-ray; M is the mass of the sample (kg); and ε is the detector efficiency at the specific γ-ray energy (Kolapo 2014).

Calculation of 222Rn Activity Concentrations in Soil

222Rn activity concentrations in soil were determined using Eq. 2 obtained from UNSCEAR (2000):

$${}^{222}{\text{Rn}}_{\text{Soil}} \, = C_{\text{Ra}} \times f \times \rho_{\text{s}} \times \varepsilon^{ - 1} \, \left( {1 - \varepsilon } \right) \, \left( {m\left[ {K_{\text{T}} - 1} \right] + 1} \right)^{ - 1} ,$$
(2)

where 222RnSoil is the 222Rn activity concentrations in soil (kBq/m3); C Ra is the concentration of Ra in soil (Bq/kg); f is the emanation factor; ρ s is the density of the soil grains (2700 kg m3); \(\varepsilon\) is the total porosity including both water and air phases; m is the fraction of the porosity that is water-filled (also called the “fraction of saturation”); and K T is the partition coefficient for radon between the water and air phases. For dry soil, m is zero, and the last term on the right side of the equation can be omitted. Warm, moist soil (25 °C, K T = 0.23, m 0.95) with typical soil parameters (f = 0.2, \(\varepsilon\) = 0.25).

Rn in air (222Rnair) was calculated using Eq. 3 (UNSCEAR 1988):

$${}^{222}{\text{Rn}}_{\text{air}} \, = {}^{222}{\text{Rn}}_{\text{s}} \, \times \left( {\tfrac{{D{}_{\text{soil}}}}{{D_{\text{air}} }}} \right)^{{\frac{1}{2}}} ,$$
(3)

where 222Rnair is the 222Rn concentration in the air (Bq/m3); 222Rns is the 222Rn concentration in soil (Bq/m3); D soil is the diffusion rate constant of 222Rn in the soil (0.5 × 10−4 m2/s); and D air is the diffusion rate constant of 222Rn in the air (5 m2/s).

Calculation of Uptake of Radium by Vegetables

Uptake of radium by vegetables from the soil around the studied area was determined using Eq. 4 (UNSCEAR 1988; IAEA 1996):

$${}^{226}{\text{Ra}}_{\text{AC}} = \, A \times {}^{226}{\text{Ra}}_{\text{soil}} ,$$
(4)

where 226RaAC is the 226Ra activity concentration in vegetables (Bq/kg); A is the transfer coefficient of 226Ra from soil to vegetables (0.04) (IAEA 1990); and 226Rasoil is the 226Ra activity concentration in soil samples (Bq/kg).

Calculation of Absorbed Dose Rate

The absorbed dose rate was calculated using Eq. 5:

$$D\left( {{\text{nGyh}}^{ - 1} } \right) = \, 0.461 \, \left( {A_{\text{Ra}} } \right) + 0.623 \, \left( {A_{\text{Th}} } \right) + \, 0.0414\left( {A_{\text{K}} } \right),$$
(5)

where A Ra, A Th, and A K are the activity concentrations of 226Ra, 232Th, and 40K, respectively (Kocher and Sjoreen 1985; Leung et al. 1990).

Calculation of Annual Effective Dose

The annual effective dose was calculated using Eq. 6:

$$H\;(\mu {\text{Sv}}/{\text{year}})\; = \;D\left( {{\text{nGh}}^{ - 1} } \right)\; \times \;24\;{\text{h}}\; \times \;365.25\;{\text{days}}\; \times \;0.2\; \times \;0.7\;{\text{SvGy}}^{ - 1} \; \times \;10^{ - 3} ,$$
(6)

where 0.7 Sv/Gy is the quotient of annual effective dose rate to absorbed dose rate in air for environmental exposure to gamma rays, and 0.2 is the outdoor annual occupancy factor (Hamed et al. 2012).

Ra-Equivalent Activity

Ra-equivalent activity (Raeq) is expressed mathematically using Eq. 7:

$${\text{Ra}}_{\text{eq}} = \, A_{\text{Ra}} + \, 1.43 \, A_{\text{Th}} \, + \, 0.077A_{\text{K}} ,$$
(7)

where A Ra, A Th, and A K are the activity concentrations of 226Ra, 232Th, and 40K in Bq/kg, respectively (UNSCEAR 1993).

Calculation of the External Hazard Index

The external hazard index (H ex) was calculated using Eq. 8:

$$H_{ex} = \frac{{A_{Ra} }}{370} + \frac{{A_{Th} }}{259} + \frac{{A_{K} }}{4810} \, \le \, 1,$$
(8)

where ARa, ATh, and AK are the activity concentrations of 226Ra, 232Th, and 40K in Bq/kg, respectively. The values of this index must be less than unity to keep the radiation hazard significant (Beretka et al. 1985).

AGDE

The organs of interest considered by UNSCEAR (1988) are the gonads, active bone marrow, and bone surface cells. Therefore, AGDEs were calculated using Eq. 9.

$$AGDE \, \left( {{{\mu {\text{Sv}}} \mathord{\left/ {\vphantom {{\mu {\text{Sv}}} {\text{year}}}} \right. \kern-0pt} {\text{year}}}} \right) = \, 3.09 \, A_{\text{Ra}} \, + \, 4.18 \, A_{\text{Th}} \, + \, 0.314 \, A_{\text{K}} .$$
(9)

Calculation of the Annual Effective Dose Due to 222Rn Inhalation and Vegetable Consumption (Xinwei et al. 2006)

The overall annual effective dose due to 222Rn inhalation and vegetable consumption was determined using Eq. 10 (IAEA 1996).

$$H_{\text{p}} = C_{\text{p}} \times I_{\text{p}} \cdot {\text{DCF,}}$$
(10)

where H p is the dose rate due to 222Rn inhalation or vegetables consumption in (Sv/year); C P is the 226Ra concentration in vegetables (Bq/kg) or the concentration of 222Rn in the air (Bq/m3); I p is the amount of consumption of vegetables per year (90 kg/year) and for air outside the home (600 m3/year) (IAEA 1990); and DCF is the dose-conversion factor for 226Ra (2.8 × 10−7 sv/Bq) and 222Rn (1.3 × 10−9 sv/Bq) (UNSCEAR 1988).

Results and Discussion

Table 1 lists the activity concentrations of 226Ra, 232Th, and 40K in soil samples from Al-Ibedia’s traditional mining area, which ranged from 2.66 to 18.47, 9.20 to 51.87, and 0.17 to 419.77 Bq/kg with an average value of 7.54 ± 4.91, 20.74 ± 11.29, and 111.87 ± 136.84 Bq/kg, respectively. 226Ra, 232Th, and 40K activity concentrations in soil samples varied within the study area due to differences in geological structure. The literature has repeatedly indicated that 226Ra, 232Th, and 40K concentrations in soil vary according to geological formation, physico-geological characteristics of soil types, topographical differences, geomorphology, and meteorological conditions of the region. Geologically these locations are characterized by sedimentary rocks known as Nubian sandstone, basement rocks, and modern sedimentary rocks with 2 m of soft sand and mudstone on top. The basement rocks are metamorphic consisting of a variety of schist and gneiss (Küster and Liégeois 2001).

Table 1 Mean concentrations with associated SDs of 226Ra, 232Th, and 40K activity concentrations in soil samples around Al-Ibedia traditional mining area
Full size table

The highest activity concentrations of 226Ra (18.47 Bq/kg) and40K (419.77 Bq/kg) were measured in location no. 4 and those of 232Th (51.87 Bq/kg) in location no. 33. These areas are characterised mineralogically by a hydrothermal solution composed mainly of auriferous quartz veins and stringers filling into the tectonic fissures in the host rock. The hydrothermal system relates to a magmatic solution of high temperature associated with granitoid intrusion containing high values of elements such as 226Ra, 232Th, and 40K. The average values of 226Ra 232Th, and 40K of the present study are less than the world average of 35, 45, and 420 Bq/kg, respectively (UNSCEAR 2000) and were thus compared with those from similar investigations in other countries as shown (Table 2). Regression analysis showed that no correlation emerged.

Table 2 Comparison of 226Ra, 232Th, and 40K concentrations in soil samples around Al-Ibedia traditional mining areas and with those in some other countries as given in UNSCEAR (2000)
Full size table

222Rn gas, which constitutes 40 % of the annual accumulated dose (UNSCEAR 1993; Isam et al. 2014), is considered to be the primary source of human exposure to natural radioactivity. Moreover, exposure to radon and its progeny is believed to be associated with an increased risk of developing several types of cancer (Clamp and Pritchard 1998; Idriss et al. 2011, 2015). Therefore, mathematical models obtained from UNSCEAR (1988, 2000) were used to determine 222Rn concentrations in soil and air samples. 222Rn concentrations in soil and air were computed to characterize the building materials as an indoor radon source; knowledge of the radon-exhalation rate from these materials is very important. The activity concentrations of 222Rn in soil and air were determined in various locations throughout the study area shown (Table 3). 222Rn activity concentrations in soil ranged from 4.374 to 29.97 kBqcm3 with an average value of 12 ± 224 kBq/m3. On comparing the results with global data, it was discovered that the obtained values of 222Rn in the soil were considerably lower then the reported range for Slovenia (0.9–32.9 kBq/m3) (Vaupoti et al. 2010). However, the range of 222Rn concentrations in the soil observed in this study is significantly high relative to similar studies reported from Syria [76–3143 Bq/m3 (Shweikani and Hushari 2005)], India (0.4–25.78 kBq/m3, Prasada et al. 2008), Libya (31.17–469 Bq/m3, Saad et al. 2013), Portugal (102–2.982 Bq/m3, Pereira et al. 2011), and Sudan (20–1.359 Bq/m3, Idriss et al. 2014). 222Rn concentrations in air were in the range of 13.832–86.065 Bq/m3 with an average of 35.683 ± 26.535 Bq/m3.The concentrations of radon in air were far below the action level of 200–600 Bq/m3 recommended by the International Commission on Radiological Protection [ICRP] 1991); the 200 Bq/m3 set by the National Radiological Protection Board (1990), UK; the reference level of 100 Bq/m3 set by the World Health Organization (WHO 2009); and the recommended activity concentration of 148 Bq/m3 set by the USEPA (2004).

Table 3 Activity concentrations of 222Rn in soil and air, 226Ra in vegetables, and radiation dose of 222Rn due to inhalation and 226Ra due to vegetable consumption
Full size table

The average concentration of 222Rn in the air was compared with data from different countries (Table 4). Radium transfer to vegetables was in the range of 0.11–0.74 with an average value of 0.301 ± 0.198 Bq/kg. The results of 226Ra transferred to vegetables appear to be negligible compared with the allowable limit (UNSCEAR 1988). Table 5 lists the values of corresponding absorbed dose rates, H ex index, Raeq, AGDE, annual effective dose rates, and percentage contribution to the total dose rate from 226Ra, 232Th, and 40K in soil samples around the Al-Ibedia traditional mining area. The absorbed dose rates were computed from the measured activity concentrations of 40K, 232Th, and 226Ra in the soil using the UNSCEAR (1993) dose-rate conversion factors of 0.0414, 0.623, and 0.461 nGy/h/Bq/kg, respectively. Factoring in the calculation of dose-rate conversion factors was built on the premise that all daughters of 238U and 232Th series are in radioactive equilibrium with the parent, that the soil density is 1.4 g/cm3, and that activity distribution is homogeneous up to 1 m in depth.

Table 4 Ra concentrations in air and action levels in different countries as given in UNSCEAR (2000)
Full size table
Table 5 Corresponding, H ex, Raeq, and AGDE and percentage contributions to the total dose rate from 238U, 232Th, and 40K in soil samples around Al-Ibedia traditional mining area
Full size table

The estimated absorbed dose rate ranged from 9.46 to 51.91 with an average value of 21.03 ± 12.80 nGy/h (Table 5). The observed absorbed dose in this study was lower than the global average of 60 nGy/h (UNSCEAR 2000). Calculation of the relative contribution of 226Ra, 232Th, and 40K to the total absorbed dose in air showed that the majority of the contribution comes from 40K (79.72 %).

The computed annual effective dose varied from 11.61 to 63.71 with an average value of 25.81 ± 15.71 µSv/year shown (Table 5). The estimated effective dose was below the allowable limit of 20 mSv/year for occupational exposure control as recommended by the ICRP (1991). To assess gamma radiation hazards to humans associated with the use of the soil surrounding the mining sites for the construction of houses (filling and local brick-making); this provides a single index that describes the gamma output from a different mixture of 226Ra, 232Th, and 40K in the samples.

Raeq activity was in the range of 21.51–112.95 with average values of 45.81 ± 27.06 Bq/kg (Table 5). Evidently the results were lower than the maximum allowable limit for materials to be used in the construction of dwellings of 370 Bq/kg (UNSCEAR 1982). The estimated Hex index ranged from 0.06 to 0.31 with an average of 0.12 ± 0.07. These values were below the criterion of unity. AGDE ranged from 63.49 to 358.78 with an average of 145.12 ± 0.26 µSv/year (Table 5). The average value of AGDE is lower than the global average of 300 µSv/year (UNSCEAR 2000). However, locations nos. 32 (328.11 µSv/year), 33 (358.78 µSv/year), and 34 (346.79 µSv/year) exhibit values >300 µSv/year. The overall annual effective dose due to radon inhalation and vegetable consumption range between 0.01 and 0.07 and 0.003 and 0.019 mSv/year with an average of 0.03 ± 0.02 and 0.008 ± 0.005 mSv/year, respectively (Table 3). The overall annual effective dose for radon inhalation and vegetable consumption were lower than the recommended reference dose level of 1.0 mSv/year set by UNSCEAR (2000), the 1.0 mSv/year set by the FAO (1977), and the 1.0 mSv/year set by the IAEA (1996).

Conclusion

Radionuclide measurements of 226Ra, 232Th, and 40K in soil samples from traditional mining areas are necessary from the perspective of environmental radiation protection. Activity concentrations measured for 226Ra, 232Th, and 40K in soil samples within the Al-Ibedia traditional mining area are relatively normal. Potential radiological effects to miners and the public due to 226Ra, 232Th,40K, and 222Rn are insignificant. 226Ra transferred to vegetables appears to be negligible compared with the allowable limit of 1.0 mSv/year set by UNSCEAR. Raeq activity, absorbed dose rate, H ex index, and annual effective dose equivalent were all lower than the allowable limits proposed by ICRP (1991) and UNSCEAR (2000). In contrast, the average value of AGDE is lower than the global average of 300 µSv/year (UNSCEAR 2000). However, locations 32 (328.11 µSv/year), 33 (358.78 µSv/year), and 34 (346.79 µSv/year) exhibit values >300 µSv/year. There is a need to initiate a comprehensive nation-wide radiation survey of mining to bring this industry under regulatory control. Legislation resulting in mandatory radiation monitoring in national mining is critical for the protection of workers and the public from the dangers posed by natural radionuclides.