Radioactivity and radiological hazards from a kaolin mining field in Ifonyintedo, Nigeria

Graphical abstract

Nigeria is on the Pan-African mobile belt, which separates Congo Cratons and West Africa [16]. In Nigeria, the two pronouncing geology are the Basement rocks and sedimentary Basins. From the literature, few works from both of the terrains could be found in Refs. [17][18][19][20][21][22][23]. Ifonyintedo is directly seated on the Eastern region of Dahomey Basin, which is one of the Nigerian Basins. This Basin is situated in SW region of Nigeria, which is separated from the prolific Niger Delta Basin by Okitipupa Ridge. Its depositional constituents are grouped into six classes, these are: Benin, Abeokuta, Oshosun, Akinbo, Ilaro and Ewekoro Formations. The descriptions of these classes have been presented by Adagunodo et al. [1]. The Hydrogeology of Dahomey basin comprises Ogun River and Owena basin. Fig. 1b is the diagrammatic representation of the geological domains in Ogun state revealing the study area.

Materials and methods
In situ measurements of activity concentrations of K-40, Th-232, U-238 and the gamma dose rates were taken over kaolin deposits in Ifonyintedo, Dahomey Basin, SW Nigeria. The data were randomly occupied at the upper axis of the field from eleven (11) locations using a hand-held detector known as Super-Spec (RS 125). The coordinate and elevation of each location were determined with the aid of global positioning system (GPSMAP 78). The radioactivity measurements were taken four times at each location while their averages and standard deviations were estimated in order to ensure accuracy. All the measured parameters are shown in Table 1. The radiometric survey was carried out in the month of January 2018. The detector used was manufactured by Canadian Geophysical Institute. It has high accuracy with probable measurement errors of about 5%. It offers an integrated design with a large detector, direct assay readout, data storage and high sensitivity. The assay mode of RS-125 Super SPEC provides sample concentration analysis with direct data display of potassium (K) in percentage (%), uranium (U) in part per million (ppm) and thorium (Th) in part per million (ppm). The spectrometer is calibrated on 1 Â 1 m test pads, which utilizes 5 min spectra accumulation on potassium, thorium and uranium pads and 10 min accumulation on the Background (BG) pad as calibrated by Canadian Geophysical Institute. It uses sodium iodide (NaI) crystal doped with thallium [Tl] as activator. The approximate linear energy of the detector falls between 0.80 and 1.2 MeV, this range covers the majority of significant gamma-ray emissions from terrestrial sources. The detection of gamma-ray from cosmic ray is negligible due to the detector's low response to high-energy gamma radiation. The full count of 120 s per assay was adopted for best accuracy as stated in Radiation Solutions Inc. [24]. The recorded activity concentrations of K-40, Th-232, U-238 from the detector were converted to Becquerel per kilogram (Bq kg À1 ) in accordance with the conversion factor of International Atomic Energy Agency [25,26]. The advantage of in situ radiation measurement method over ex situ is that, the measurements are faster; less costly; greater data points can be measured; more than two measurements per station can be achieved, which minimizes the uncertainty on the mean of radioactivity concentrations [27]. Calibration pad for Super-Spec (RS-125) according to Canadian Geophysical Institute is presented in Eqs. (1)-(3).

Method descriptions
Measured radionuclides and gamma dose The mean and standard deviation of the measured radionuclides ( 238 U, 232 Th and 40 K) and gamma Dose Rates (DR) per station from the upper axis of kaolin deposits in Ifonyintedo are revealed in Table 1. The highest recorded values for 238 U, 232 Th, 40 À1 and 59.56 nGy h À1 in the same order for the radionuclides and DR respectively. The global averages from four standards were compared with the overall mean as revealed in Table 1. The NEA-OECD [28] and UNSCEAR [29] standards revealed that 238 U and 40 K were below the community weighted values of 50 and 500 Bq kg À1 respectively. The overall mean values of 232 Th and DR showed that they were above the permissible limits of 50 Bq kg À1 and 55 nGy h À1 by the factor of 1.3 and 1.1 respectively. The EC [30] standard revealed that the overall mean values of 238 U and 40 K were below the community weighted values of 50 and 670 Bq kg À1 respectively. For 232 Th and DR, the overall mean values were above the community weighted values of 50 Bq kg À1 and 50 nGy h À1 by the factors of 1.3 and 1.2 respectively. The latest standard considered from Table 1 is UNSCEAR [31], which gave the community weighted values for radionuclides ( 238 U, 232 Th and 40 K) and DR as 32 Bq kg À1 , 45 Bq kg À1 , 420 Bq kg À1 and 84 nGy h À1 respectively. By comparing the community weighted values with the overall mean values in Table 1, it is revealed that 40 K and DR were below the permissible limit, while 238 U and 232 Th were above the community weighted values by the factors of 1.2 and 1.4 respectively. Since the estimated mean values presented in Table 1 are greater than their respective standard deviation values, it indicates that there is high degree of uniformity in the presented data sets [32]. The comparative analysis of the measured radionuclides and DR with some selected studies from literature is revealed in Table 2.
The isouranium, isothorium, isopotassium, and isodose maps of kaolin deposits in the upper axis of the field in Ifonyintedo are presented in Figs. 2-5 respectively. Based on the standard set by UNSCEAR [31], the enhanced activity concentrations of uranium are depicted with red colour on Fig. 2. The uranium distributions in the study area trend in NW -SE orientation, with its peak towards the western region. The activity concentrations of thorium trend in NNW -SSE orientation, with its order of increment explained from the colour scale (Fig. 3). Very low potassium activity dominates the study area, which is far lower than the global mean. Nonetheless, two distribution trends were observed from the isopotassium map, which are NE -SW and SSW -NNE orientations (Fig. 4). The gamma isodose map (Fig. 5) revealed that the enhanced activity trend from north to south, and spread towards the SW and SE of the study area respectively. The distributions of the doses are explained from the colour scale of Fig. 5.
The correlation studies between the radionuclides and the gamma dose were achieved by plotting the graphs of dose rate against 238 U (Fig. 6a), dose rate against 232 Th (Fig. 6b), and dose rate against 40 K (Fig. 6c) respectively. Correlation study is usually performed between the pairs of radionuclides or / and its gamma dose rate when someone is keen to quick check the relationships that exist between the activity and gamma dose in the area of interest [38]. A weak correlation of 0.355 existed between 238 U and DR, a fairly good correlation of 0.676 existed between 232 Th and DR, while a poor correlation of 0.072 existed between 40 K and DR as revealed in Fig. 6a-c. The correlation results showed that the area of study is enriched in thorium. Hence, the gamma dose received from the kaolin deposits in the upper axis of Ifonyintedo is insignificant as a result of potassium isotopes, but might be weakly significant due to uranium series. Despite the transfer of radionuclides from the raw material to their finished product (such as the case of kaolin to tile), it is imperative to state that the miners on this field need to be aware of the hazards from overexposure to thorium. Thorium is one of radioactive metals that exist in soil, rock, water (surface and ground), and man's environment. It does not dissolve easily in water, or evaporate to the surface and environs of the Crust. Overexposure to thorium has been linked with cancers of various kind, liver diseases, malfunctioning of the body systems and blood stream related diseases. Generally, overexposure to background radiations has been linked with severe health related problems such as disease of lung, bone, mouth, skin and failure of the body systems, which could result to death in the long term [40].

Assessment of radiological hazards from kaolin deposits
Eight radiological hazards were determined in order to evaluate the risks that are associated with the mined kaolin deposits in Ifonyintedo as well as the miners. The estimated hazards were radium equivalent, external and internal hazards, outdoor and indoor annual effective doses, gamma and alpha indices, and representative level index. All these estimated hazards are presented in Table 3.  Radium equivalent Since the measured activity of 40 K, 232 Th and 238 U are inhomogeneous, it is essential to introduce a common radiological index that evaluates the level of each of the radionuclides in the kaolin deposits. The estimated index, which is known as radium equivalent (Ra Eq ) is presented in Eq. (4) as presented by Turhan [36].    where AC U , AC Th and AC K are the activity concentrations of 238 U, 232 Th and 40 K in Bq kg À1 respectively. The implication of Eq. (4) is that the maximum limit of the Ra Eq must not be up to 370 Bq kg À1 , such that the external dose of less than 1.5 mGy y À1 will be maintained [37]. The Ra Eq activity in this study is presented in Table 3. The values ranged between 118.23 and 172.37 Bq kg À1 , with the mean of 138.51 Bq kg À1 . Both the range and the mean of Ra Eq values were below the limit of 370 Bq kg À1 as reported by UNSCEAR [31].

External and internal hazards
Exposure to radiation could be external and/or internal. Eqs. (5) and (6) were used to determined the radiation hazards emanating from the field and the mined kaolin [32].
where AC U , AC Th and AC K have been defined in Eq. (4). The reduction of the limit of 238 U to half the numeric value essential to external exposure only is known as the internal hazard index, such that the internal dose received will be <1.5 mSv y À1 [39]. The H Ex in this study varied between 0.32 and 0.47, with the mean of 0.37. As reported by Ravisankar [32], the internal exposure to radon and its progeny is managed by H In . Therefore, the H In from the kaolin deposits in this study as presented in Table 3 ranged from 0.37 to 0.61, with the overall mean of 0.48. In both cases, the external and internal hazards estimated over a kaolin mining field in Ifonyintedo were less than unity. This implies that the field poses no threat to the miners and the mined kaolin is safe for economic use.

Outdoor and indoor annual effective doses
In this present article, outdoor and indoor annual effective doses were estimated from the measured absorbed doses and other factors as reported from literature [37]. In order to estimate the Outdoor Annual Effective Dose (AED Outdoor ), the dose conversion coefficient (0.7 Sv Gy À1 ) from absorbed dose in air to the effective dose received by the body and outdoor occupancy factor of 0.2 was adopted as given by UNSCEAR [31]. For the Indoor Annual Effective Dose (AED Indoor ), the occupancy factor of 0.8 was adopted for AED Indoor [37]. This implies 8760 h are in a year. Individuals stayed longer indoor than outdoor in a day, hence the variation in the occupancy factors for outdoor and indoor respectively. Eqs. (7) and (8) were used to estimate the AED Outdoor and AED Indoor respectively [14,29,31,32,37]. AED Outdoor (mSv y À1 ) = Dose rate (nGy h À1 ) Â (365 Â 24) Â 0.2 Â 0.7 (Sv Gy À1 ) Â 10 À6 (7) AED Indoor (mSv y À1 ) = Dose rate (nGy h À1 ) Â (365 Â 24) Â 0.8 Â 0.7 (Sv Gy À1 ) Â 10 À6 The expected mean annual external effective dose from naturally occurring radionuclides is 0.70 mSv y À1 , while its internal counterpart is 0.05 mSv y À1 [37]. In this study, the AED Outddor ranged from 0.25 to 0.36 mSv y À1 , while that of AED Indoor ranged from 0.06 to 0.09 respectively. The estimated means for the AED Outddor and the AED Indoor are 0.29 and 0.07 mSv y À1 respectively. These results indicate that the study area poses no risk to the miners as well as the materials that will be produced from the kaolin (such as tile).

Gamma and alpha indices
Other key hazards that were considered in this study are gamma (I g ) and alpha (I a ) indices respectively. These indices were estimated based on the European Commission [30] standard. Gamma index (I g ) is the factor that assesses the g-radiation hazard(s) associated with the naturally occurring radionuclides in a material. The I g is determined based on Eq. (9) as given by [28,30].
Where AC U , AC Th and AC K are the same as for other estimated hazards. The permissible range of the outdoor annual effective doses' contributions to the g-radiation is 0.3 to 1 mSv y À1 . Any material or sample that poses the AED Outdoor > this range should be exempted from use as raw materials or finished products [36]. If the I g 1, it corresponds to an outdoor dose of 1 mSv y À1 . However, if the I g 0.5, it corresponds to an outdoor dose of 0.3 mSv y À1 [14]. From Table 3, the I g ranged from 0.42 to 0.61, with a mean of 0.48. These results correspond to I g 0.5, which gives the outdoor effective dose of 0.3 mSv y À1 .
The alpha index (I a ) is used to estimate the exposure to a-radiation associated with radon inhalation from a material. The I a is determined based on Eq. (10) [36].
where AC U is the activity concentration of uranium in each location. It is measured in Bq kg À1 . As suggested by the by the European Commission [30], the exhalating radon from a material can be greater than 200 Bq m À3 if and only if the activity from uranium concentration is greater than 200 Bq kg À1 . An I a that is less than or equals 1 corresponds to uranium activity ( 238 U) 200 Bq kg À1 . The I a as presented in Table 3 ranged from 0.09 to 0.32, with a geometric mean of 0.19. Both the range and the geometric mean results showed that the exposure as a result of a-radiation on the kaolin field is minimal. However, period monitoring is recommended for g-radiation emanating from the kaolin, since I g 0.5, which corresponds to 0.3 mSv y À1 of outdoor effective dose that was established on the field.

Representative level index
The Representative Level Index (RLI) activity was also determined in this study. The RLI is used to determine the g-radioactivity level associated with the concentrations of these radionuclides. Eq. (11) was used to estimate the RLI over a kaolin field in Ifonyintedo as proposed by [28] and [38].
where AC U , AC Th and AC K are the activity concentrations of 238 U, 232 Th and 40 K (Bq kg À1 ) respectively. The safety rule is that the RLI < 1 [38,39]. The estimated RLI values are presented in Table 3. The results fluctuated from 0.84 to 1.21, with the overall mean of 0.97. When compared to the limit, it has been revealed that the range is slightly above the limit, while the overall mean %1. This implies that the RLI of kaolin deposits in the upper axis of Ifonyintedo field may pose radiation hazard, which might be harmful to the miners and the users of the product(s) from the kaolin deposits if proper periodic monitoring and assessment are neglected on the field.

Conclusion
For the eleven locations covered in this study, the radiometric measurements of radioactivity concentrations of 40 K, 232 Th and 238 U as well as the gamma doses over a kaolin mining field in Ifonyintedo, Nigeria were achieved using Super-Spec (RS-125) detector. The radiological hazards associated with mining of this mineral deposits and its usability as building material (either as raw material or finished product, as in case of tile) were evaluated. The numbers of conclusions drawn from this study are: i The range of the estimated mean from four-time in situ measurements per location of 238 U, 232 Th, 40 [31], the mean activity concentrations of 238 U and 232 Th were above the limit by the factors of 1.2 and 1.4 respectively, while 40 K and DR were below the limit. ii The Ra Eq activity ranged from 118.23 to 172.37 Bq kg À1 were below the recommended limit of 370 Bq kg À1 as given by UNSCEAR [31]. iii The external and internal hazards which ranged from 0.32 to 0.47, and 0.37-0.61 respectively were below the recommended limit of unity as reported by [32] and [39]. iv The outdoor and indoor annual effective doses ranged from 0.25 to 0.36 mSv y À1 , and 0.06-0.09 mSv y À1 .
The mean of AED Outdoor and AED Indoor were below the limits of 0.70 and 0.05 mSv y À1 as presented by UNSCEAR [31] and Avwiri et al. [37]. v The I g ranged from 0.42 to 0.61, with a mean of 0.48 % 0.5. The gamma index in Ifonyintedo corresponds to I g 0.5, which gives the outdoor effective dose of 0.3 mSv y À1 . The I a ranged from 0.09 to 0.32, with a geometric mean of 0.19. The a-radiation exposures as a result of usage of the mineral deposits as raw material or finished product, or to the miners are minimal, but that of g-radiation exposure needs periodic monitoring [28,30,36].
vi The RLI ranged from 0.84 to 1.21, which revealed that some locations (UA1, UA4, UA5 and UA7) are hazardous, because their RLI were beyond the recommended value (i.e. 1.0), while the remaining locations are close to unity as presented in Ravisankar et al. [32] and Chandrasekaran et al. [38]. The overall mean of 0.97 was achieved, which could be approximated to unity.
Ogun state is the leading state producer of solid minerals in Nigeria, with kaolin being one of the major solid minerals mined from the state. It is however recommended that periodic assessment of radiological exposure to the miners and the mined kaolin deposits should be of utmost concern to the Nigerian Environmental Standards and Regulatory Enforcement Agency, since some of the estimated hazards are close or could be approximated to the permissible limit.