Natural radionuclides and radiological risk assessment of granite mining field in Asa, North-central Nigeria

Graphical abstract


Method details
Natural radionuclides are broadly dispersed in the Earth crust. They are found in significant concentrations in many mineral rocks. Granites, just like other mineral rocks, may possibly hold deposits of natural radionuclides like 238 U, 232 Th, their progenies and the non-series 40 K [1,2]. The activity concentrations of these radionuclides may differ even within a particular block of granite. If present, these radionuclides will decay to give off radon and some amounts of gamma and beta radiations. Human exposure to ionizing radiation resulting from these radionuclides and their progenies can cause cancer and other radiation health effects, damaging critical organs of the body which could even lead to death [1,[3][4][5]. For granites used for building and construction of houses, these dangerous radiations will be released over the lifetime of using such buildings. So the knowledge of the concentrations of these radionuclides in building materials is fundamental for estimating the level of public exposure to radiations, since most residents spend approximately 80% of their time indoors. In order to reduce these radiation risks, the United State Environmental Protection Agency recommended that all houses should be tested for these radionuclides, whether they contains granite countertops or not [1]. Such an action is not economically feasible for a third world country like Nigeria. So researchers resolve to monitoring and assessments of the mine fields where the building materials (mineral rocks or soils) are mined originally and their finished products.
The levels of 238 U, 232 Th, their respective progenies and the non-series 40 K have been studied in different building materials (both raw and finished products) from different parts of the country [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22], but none has been carried out in Kwara State despite the increasing level of granite mining and usage in this part of the country. Also, data from University of Ilorin Teaching Hospital (UITH) shows that 74 different cancers of 2246 (891 male and 1355 female) cancer patients within the age of 1-105 were recorded at the University of Ilorin Teaching Hospital (UITH) cancer registry between the period of 2007 and 2016 [23]. Therefore, a pioneer study which is based on internationally verified methodology regarding assessment of radiological health implications on the general populace due to granite mining in this part of the country is apposite.

Study area
Asa is a Local Government Area in Kwara State, Nigeria. It has an area of 1286 km 2 and a population of 126,435 according to 2006 census. It is located at the southwestern part of Kwara State and it is surrounded by Moro local government to the north, Oyun and Offa local government to the South and Ilorin west local government to the East. The study area lies between latitudes 4 0 12'N and 4 0 29'N and longitudes 8 0 7'E and 8 0 42'E ( Fig. 1a and b). The study area is underlain by basement complex rock. The soils are formed from basement complex rocks (metamorphic and igneous rocks) which is about 95%. The metamorphic rocks consist of biotite gnesiss, banded gnesiss, quartzite augitegnesiss and granitic gnesiss. The intrusive rock comprises of pegmatite and vein quartz [24][25][26]. Detail geology of the study area can be found in [24][25][26][27][28].

Field survey
For the in situ measurements of activity concentrations of 40 K, 232 Th, 238 U and the radiation dose exposures, Super SPEC RS-125 spectrometer with large 2.0 Â 2.0 NaI crystal was used. The measurement of the activity concentration of the radionuclides was carried out at about 1 m above the topsoil [15,29]. The RS-125 is a transportable handheld radiation detector with high accuracy and likely error of about 5%. It presents superior integrated design with big detector, good sensitivity and easy to use. The RS-125 is manufactured by Canadian Geophysical Institute and it comes with a large data storage which allows one to take multiple readings with ease. The RS-125 spectrometer was calibrated in accordance with Canadian Geophysical Institute i.e., the instrument was calibrated on 1 Â1 m test pads, which employs 5 min spectra accumulation on potassium, uranium and thorium pads and 10 min accumulation on the Background pad. It makes use of sodiumiodide (NaI) crystal doped with thallium [Tl] as activator. The energy range of the instrument, is from 30 to 3000 keV, which is enough to detect most of the radiation giving off from the terrestrial sources (i.e. 214 Bi (609.31 and 1764.49 keV) gamma rays to determine 238U, 212 Pb (238.63 keV), 208 Tl (583.19 keV) and 228 Ac (911.21 keV) gamma rays to determine 232 Th and the photopeaks of 40 K which occours in the background spectrum at 1460.83 keV). The detection of gamma-ray from cosmic ray is small and negligible due to the detector's low response to high-energy gamma radiation. The total count of 120 s per assay was employed for best accuracy as stated in Radiation Solutions Inc [15]. The assay mode of the instrument gives the activity concentration of 40 K in percentage (%), 238 U and 232 Th in part per million (ppm). The data was converted to the conventional unit Bqkg À1 using conversion factors given by [15,30].
In this work, four (4) readings were recorded at each data point at the interval of 120 s. 50 sample points were recorded to cover the area of the mining field. The field was divided into grids of approximately equal size (i.e. 50 semi-rectangular boxes) with each box representing a data collection point. At each of these samples location (point), the coordinate and elevation were determined using a global positioning system (GPSMAP78). More details about the instrument can be found in [15,17,19,29].
Estimation of the radiological impact parameters (RIP)

Radium equivalent activity index (Ra eq )
The distributions of the measured radionuclides are not uniform in the environment. So exposure to radiation has been defined in terms of radium equivalent activity (Ra eq ) in Bqkg À1 to compare the specific activity of materials containing different amounts of 238 U, 232 Th and 40 K. This is based on the assumption that 1 Bqkg À1 of 238 U, 0.7 Bqkg À1 of 232 Th and 13 Bqkg À1 of 40 K produce the same radiation dose rates. This allows a single number to be used to represent the gamma output due to different combination of 238 U, 232 Th and 40 K in the granite material. The Ra eq was calculated using Eq. (1) [31,32]: where C u , C Th and C K are the radioactivity concentration in Bqkg À1 for 238 U, 232 Th and 40 K respectively. The average value of 370 Bqkg -1 is recommended normal background radiation value [31].

Radiation hazard indices
Eq. (2) and (3) were used to calculate the external radiation hazard (H ext ) and the internal radiation hazard (H int ).
where C u , C Th and C K are as defined in Eq. (1) above. For the radiation hazard to be small, both H int and H ext ought to be less than 1. Natural radioactive elements in soil generates external field to which the general populace are exposed. H ext equal to unity translates to the upper limit of radium equivalent dose (370 Bqkg À1 ) [19,31,32].

Absorbed dose rate
At 1 m height above the ground level, it is assumed that the naturally occurring radionuclides will have a uniform distribution. The outdoor absorbed dose rate at 1 m above the ground is calculated using Eq. (4) [2,15,31].
But fortunately, this outdoor dose rate was measured in situ using the RS-125 Gamma Spec.
The granite from Asa LGA as highlighted earlier, is primarily used for building purposes. As a result, the indoor radiation dose rate in a typical building of 4 Â 5 Â 2.8 m room size, with wall thickness of about 20 cm was calculated using Eq. (5) [13]: where C u , C Th and C K are as defined earlier.

Annual effective dose (AED)
The annual effective dose received indoor and outdoor by a member of the public was calculated from dose rates given in Eqs. (6) and (7). Dose conversion factor of 0.7 Sv Gy À1 and occupancy factor for outdoor and indoor as 0.2 and 0.8 were adopted [13,31].

Excess Lifetime Cancer Risk (ELCR)
The Excess Lifetime Cancer Risk (ELCR) was calculated using Eq. (8): where, AED indoor is the indoor Annual Equivalent Dose, DL is the average duration of life (estimated to 70 years) and RF is the Risk Factor ( Sv À1 ), i.e. fatal cancer risk per Sievert. ICRP uses RF as 0.05 for stochastic effects for the general public [19,31,32].
Annual gonadal equivalent dose (AGED) An increase in AGED has been known to result in leukemia which is very fatal. This hazard parameter for the residents using the granite for building was evaluated using Eq. (9) [19,31,32]: where C U , C Th , and C K maintain their usual definitions.
Representative level index (RLI) RLI value of 1 corresponds to an AED of less than or equal to 1 mSv. Thus, RLI is a radiological impact parameter for screening materials used for building construction and the RLI was estimated using Eq. 10 [31,32].
where C U , C Th , and C K maintain their usual definition.

Method descriptions
The record of the measured activity concentrations of 40 K, 238 U and 232 Th, the gamma dose rate, the elevations and the estimated radium equivalent activity index for the 50 sample locations is presented in Table 1. The mean activity concentration of 40 K was observed to dominate the 238 U and 232 Th mean . The estimated mean value for 40 K was relatively higher than the global average of 420.00 Bqkg À1 for normal background radiation levels recommended by [31] as shown in Fig. 2. It was observed that the measured activity concentration of 40 K were lower than the global limit in just 8 (16%) locations out of the 50. Surprisingly, all the measured and the mean activity concentrations of 238 U are lower than the global average of 32.00 Bqkg À1 [31]. However, the mean activity concentration of 232 Th was found to higher than the given global average of 30.00 Bqkg À1 . As a matter of fact, the measured values of the activity concentrations are higher than the recommended limit in about 80% (40 out 50) of the sample points. This is a reason for concern because considerable enrichment or increase in the concentration of 232 Th will enhance the level of the background radiation and maybe render the mineral rock unfit for use in building and construction purposes. The maximum, minimum and the average value for the measured outdoor dose rate are 85.30 AE 2.0, 40.10 AE 0.1 and 60.11 nGyhr À1 respectively. This mean value for the outdoor dose is higher than the recommended permissible value of 59 nGyh À1 recommended [31]. Fig. 2 revealed that the granite mine field is enriched with potassium and thorium which causes the gamma dose rate to be high. This high background ionizing radiation has been reported to cause various kinds of cancers and cruel health related harms which may possibly lead to death [5,13,15,19].
We conducted a correlation analysis to study the relationship between these measured radionuclides and the gamma dose rate. The result of the correlation analysis which is presented in Table 2, were classified according to the correlation coefficient R [33]. A significant correlation was found to exist between DR and 40 K (R = 0.7259), DR and 232 Th (R = 0.6768) and 232 Th and 238 U (0.5450). While weak correlation was observed between 40 K and 232 Th (R = 0.3768) and insignificant correlation was observed to exist between others. The correlation results confirm that the granite mine field is endowed with potassium and thorium, and they contributed significantly to the gamma dose received from the field than 238 U. However, the significant correlation observed between 232 Th and 238 U could mean that they share common origin during the rock formation.
The results of the estimated radiological parameters Ra eq , H int , H ext D in , D out , AED indoor , AED outdoor , ELCR, AGED and RLI respectively are presented in Table 3. The estimated values for the radium equivalent (Ra eq ) ranges between 175.45 and 83.82 Bqkg À1 with an average value of 123.40 Bqkg À1 . The average value of Ra eq is lower than the limit of 370 Bqkg À1 recommended by UNSCEAR [31] for   respectively. The indoor gamma dose (D in ) received by the general populace due to the radionuclides concentration in the granite ranges between 155.77 and 73.33 nGyh À1 with mean value of 109.52 nGyh À1 . The estimated mean value of EAD indoor was found to be 0.54 mSvy À1 . These mean values of D in and EAD indoor are well above the limits of 84.00 nGyh À1 and 0.41 mSvy À1 respectively [2,13,15,19,31]. This reveals that there is danger of indoor gamma radiation exposure is much and the general public is not safe from overexposure to indoor ionizing radiation if the granite is used for building purposes. The mean value for the Excess Lifetime Cancer Risk (ELCR) was estimated and found to be below the recommended limits of 3.75 Â 10 À3 . The maximum, minimum and mean values of the AGED for the residents using the granite for building are 0.59, 0.27 and 0.41 mSvy -1 respectively. The mean value of the AGED is higher than the recommended limit of 0.32 mSvy -1 . This high value of AGED further augmented our worry over the use of the granite from the mine field in Asa LGA for building purposes. The estimated RLI ranges between 1.33 and 0.61 with a mean value of 0.93. The estimated mean value is close to unity, so care should be taken in the use of the granite from this mine field for building. The contributions of 40 K, 238 U and 232 Th to the Ra eq , D out , D in , H in , H ext , RLI and AGED were investigated and presented in Figs. 3 and 4. It reveals that 40 K and 234 Th were the chief contributors to the radiological hazards.

Conclusion
A well calibrated Super-Spec (RS-125) gamma spec was used to measure the activity concentrations of 40 K, 238 U, 232 Th and gamma doses rate over a granite mining field in Asa, Kwara State, North-central Nigeria. The results of the activity concentrations were used to estimate the corresponding radiation hazard parameters to assess the suitability of the granite for building and construction purpose. The results of the activity concentrations showed that the mine field is loaded with thorium and potassium which as a result enhances the outdoor gamma radiation dose rate. The  estimated mean values of D in , EAD indoor and AGED are above the recommended limits which follows that the danger of indoor gamma radiation exposure is high and the residents may not be safe from indoor ionizing radiation overexposure if the granite is used for building. Other hazard parameters are close to the recommended limits. The study therefore concludes that Nigerian Environmental Protection Agency (NEPA) and other regulatory bodies should implement specific statutory requirements and laws to regulate the high rate of mining activities in the State and the country at large. And in accordance with international recommendations quoted in the Basic Safety Series No.115 from the IAEA, the use of building materials containing enhanced concentrations of NORM should be controlled and restricted under the application of the radiation safety standards.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationship that could have appeared to influence the work reported in this paper.