A study of environmental radioactivity measurement of selected Kaolin mining fields in Kwara, Nigeria

Abstract This article reports an in-situ measurements of the background gamma radiation dose rates and the activity concentrations of 238U, 232Th and 40K, at kaolin mining-fields in Ilorin-south and Ilorin-west, Kwara, Nigeria. Readings were recorded in 90 randomly selected sample points. For Ilorin-south mining site 50 sample points were recorded, while 40 randomly selected sample points were considered for Ilorin-west mining site. A handheld RS-125 Super-Spec gamma spectrometer was utilized to perform the radioactivity measurements on both mine fields. The results of the activity concentrations showed that the locations are enhanced with 40K activity concentration compared with 238U and 232Th. The mean values of 40K, 238U, 232Th and DR for Ilorin-west were found to be 492.19, 35.63, 44.07 Bqkg−1 and 63.28 nGyh−1, respectively. While the mean values for the measured activity concentrations of 40K, 238U, 232Th and DR for Ilorin-south are 263.55, 52.24, 31.29 Bqkg−1 and 54.71 nGyh−1, respectively. Consequently, the mean values of the estimated radiological hazard parameters of Ilorin-west were higher than the estimated mean values for Ilorin-south. This shows that the Ilorin-west Kaolin mine field poses more significant source of radiation hazard. The results in this current work can be used as a significant baseline radioactivity data of the mining areas in Nigeria for future epidemiology and monitoring purposes.


Usikalu Mojisola Rachael is a Lecturer and Head
Radiation and Health Physics Research Cluster in Department of Physics, Covenant University. She has worked extensively on the measurement of radioactivity in the soil, rock and water for the estimation of the associated radiological risks in various part of the country. Through these researches, she has been able to identify soils and rocks that are not fit for construction purposes due to high radiation burden (cancer incidence, untimely death) associated with them. The research outcome provides useful information for policy makers on setting guidelines for the populace on type of soil that could be used for building and construction purposes and the safety distance to build houses from mining sites.

PUBLIC INTEREST STATEMENT
Radiation is inevitable as long as we walk on soil, drink water and carry out our day-to-day activities under the sun. This is because soil, water, rock, etc., contain different natural radioactivity concentration in varying proportion. The radiation level in a location depends mainly on the geological makeup of the rock, soil and different activities taking place in the area. In this work, we assessed the natural radioactivity concentration in locations where illegal mining are taking place. The radioactivity measurement was carried out with sodium iodide detector. This was done to verify whether mining activities has impact on the radiation dose from a location. The results from the research established that mining activities increase the radiation dose and the excess lifetime cancer risk of the study area.

Introduction
People's exposure to ionizing radiation has become a growing source of public concern because of its associated health effects such as cancer (Ajibola et al., 2021;Joel et al., 2019;Orosun et al., 2020a). The background radiation is made up of radioactive nuclei that are found in air, soil and water either naturally or as a result of human activities. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) estimates that the global average human exposure from natural radiation sources is 2.4 mSvy −1 , with natural sources of terrestrial and cosmic origin accounting for 82% of this amount (Oyeyemi et al., 2017; United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR, 2000). The terrestrial component is made up of long-lived radionuclides in the earth's crust, whereas the cosmic component is made up of cosmic rays from space. Furthermore, man-made sources such as nuclear disasters, reactor accidents, nuclear testing and the use of technical items have an impact on background radiation levels in a region. Although natural radiation is the primary source of the world's population's external dosage, the potential dangers of increased or heightened levels of radioactive chemicals in air, water and soil are usually considered a public health concern. As a result, environmental radioactivity measurements are routinely carried out by researchers all over the world in order to ascertain the nationwide background radiation levels.
The Nigerian Nuclear Regulatory Authority, which is mandated by law to guarantee that radiation protection and safety rules are followed, is in charge of nuclear and radiation generating sources in Nigeria. Several studies have been carried out around the world to analyze natural radioactivity levels in soil/sediment in specific places, as well as raw materials utilized in construction and building. Natural radionuclides and radiological risk assessment of a granite mining field in Asa, North-central Nigeria (Orosun et al., 2019), dataset on ground radiometric survey in part of the Eastern Dahomey Basin, SW Nigeria (Orosun et al., 2019), dataset on ground radiometric survey in part of the Eastern Dahomey Basin, SW Nigeria (Orosun et al., 2019), dataset on ground radiometric survey in part of the Eastern Dahomey (Oyeyemi et al., 2017), dataset on radioactivity measurement of Beryllium mining field in Ifelodun and Gold mining field in Moro, Kwara State (Orosun et al., 2020a), natural radioactivity concentration and its health implication on dwellers in selected locations of Ota, , investigation of natural environmental radioactivity concentration in soil of coastline area of Ado-Odo/Ota Nigeria and its radiological implications (Joel et al., 2019), and natural radioactivity and geological influence on subsurface layers at Kubwa and Gosa area of Abuja, Northcentral Nigeria. In recent years, there has been widespread dumping of mining tailings in the vicinity of mining sites throughout the zone. One of the main activities of the residents in the chosen location is mining. Due to the existence of naturally occurring radioactive materials (NORM) in the earth, mining by-products, and wastes resulting from mining operations, individuals are likely to be exposed to radiation. The objective of this study is to assess natural environmental radioactivity levels in the soil/sediment of Kaolin mining field in Ilorin-South and Ilorin-West, Kwara State, Nigeria. The air absorbed external gamma-radiation exposure, annual effective radiation dose, and external radiation hazard index were all determined during this investigation. The information gathered in this study will serve as a baseline for radiation exposure in an environment where mining is taking place, and it may be useful to authorities in developing radiation protection standards for the general public in the country, as well as conducting further research on the subject.

Materials and methods
Figure 1 below provides the flow chart of the research method process. Pearson's correlation technique was employed to further investigate the degree of strength and nature of relationship between the measured activities of 238 U, 232 Th, 40 K and the radiation dose rate at both mining sites.

Study area
The areas under study are Fufu village in Ilorin-south and Akerebiata area of Ilorin-west LGA in Kwara, Nigeria. It is located within latitudes 8°20 ' N and 8°50 ' N and Longitudes 4°25' E and 4°65ʹE (Figures 2 a, b). Kwara is situated in the North-central part of the country with tropical wet and harmattan times of year with normal annual precipitation of about 1,200 mm. A temperature of 26.2°C is its mean annual temperature; which tops in the month of March with about 30°C (Orosun et al., 2020a). Wet period usually take place between the months of April and October, while dry periods are experienced between the months of March and November.
The geology of the study area is of crystalline pre-Cambrian basement complex rocks. The soils are formed from the 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 (Orosun et al., 2019(Orosun et al., , 2020a(Orosun et al., , 2021b. Detail geology of the study area can be found in Orosun et al. (2019Orosun et al. ( , 2020bOrosun et al. ( , 2021a.

Field survey
Estimations of the activities of 40 K, 232 Th, 238 U and the radiation dose exposures were done insitu with help of Super SPEC RS-125 spectrometer (see, Figure 3) with enormous 2.0 × 2.0 NaI crystal (106 cm 3 ). The estimation of the activities of the primordial radionuclides and radiation doses was done at around 1 meter over the ground level (Orosun et al., 2021b(Orosun et al., , 2019(Orosun et al., , 2021a. The RS-125 spectrometer is a small handheld detector with high precision and a 5-percentagepoint error. It has a well-integrated design with a pleasant user interface. The detector was made in Canada by the Canadian Geophysical Institute. It has the ability to store large amounts of data, allowing it to track a variety of activities. The detector was calibrated in compliance with Canadian Geophysical Institute guidelines. On a 1 × 1 m testpad, 5 minutes of spectra accumulation on potassium, uranium, and thorium pads were used, followed by 10 minutes of aggregation on the ambient pad. It uses a thallium [Tl] doped Sodium Iodide [NaI] crystal as activator. The energy range of the instrument varies from 30 to 3000 keV, which is adequate to measure the greater part of the radiation emitted from the earth sources (for example, 214 Bi (609.31 and 1764.49 keV) gamma beams was used to measure 238 U, 212 Pb (238.63 keV), 208 Tl (583.19 keV) and 228 Ac (911.21 keV) gamma beams were employed to measure 232 Th and the energy peaks of 40 K which occurs in the background spectrum at 1460.83 keV. Runtime of 120 s for each test was utilized for greater accuracy and precision as expressed in the Radiation Solutions Inc (Orosun et al., 2019(Orosun et al., , 2020c(Orosun et al., , 2021aRadiation Solution Inc, 2015). The assay mode of the RS-125 gamma detector gives the activities of 238 U and 232 Th in part per million (ppm) and 40 K in percentage (%). The measured dataset was converted to Bqkg −1 that was the conventional unit using conversion rates provided by the International Atomic Energy Agency (1989,2000).
In this current study, measurements were repeated four (4) times at every geolocation at the interim of 120 seconds. Ninety (90) sampling locations were recorded altogether for the two (2) mining fields (For Ilorin-south mining site, 50 sample points were recorded together with their standard error while 40 randomly selected sample points were considered for Ilorin-west mining site. The number of sampling points was based on the area of each location). At each of these sampling points, the coordinates and elevation were determined using global positioning system (GPSMAP78). Detail information about this detector can be found in works where this device was utilized (Orosun et al., 2019(Orosun et al., , 2021b(Orosun et al., , 2021cUsikalu et al., 2018;Omeje et al., 2014;Oyeyemi et al., 2017).

Evaluation of the radiological impact parameter
The data acquired were used to calculate both the radiological impact a ssessment and risk evaluation of human and then comparing them with the universal recommended limits, by calculating the dose rates, effective doses, hazard indices and cancer risk because of the concentration of characteristic natural radionuclides in the samples evaluated.

Radium equivalent activity index (Ra eq )
The distribution of 238 U, 232 Th and 40 K in environment is not uniform, so that with respect to exposure to radiation, the radioactivity has been defined in terms of radium equivalent activity (Raeq) in Bqkg −1 (United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR, 2000;Usikalu et al., 2020;Orosun et al., 2021c).
where A U ,A Th and A K are radioactivity activity concentration in Bqkg À 1 for 238 U, 232 Th and 40 K respectively.

Hazard indices (H int and H ext )
Equations (2) and (3) were used to calculate the hazard indices, which are the external radiation hazard (H ext ) and the internal radiation hazard (H int ). A small radiation hazard for both the Hint and Hext should not be greater than or equal to unity for the soil to be declared less toxic (Orosun et al., 2019).
where A U ,A Th and A K are radioactivity activity concentration in Bqkg À 1 for 238 U, 232 Th and 40 K respectively.

Estimated absorbed dose rate (D outdoor and D indoor )
The outdoor absorbed dose rate was measured in situ using the RS-125 Gamma Spec; however, equation (4) was used to calculate the outdoor absorbed dose so as to be able to compare the calculated result with the result from the detector (Orosun et al., 2020b).

Annual Effective Dose (AED)
The annual effective dose received indoor and outdoor by a workers in the mining field and member of the public was calculated using Equations (6) and (7). Dose conversion factor of 0.7 Sv Gy −1 and occupancy factor for outdoor and indoor as 0.2 and 0.8 were adopted (Orosun et al., 2019) Eout μSv y

Results and discussion
The results of this work is dataset that covers the estimated levels of activities of 40 K, 238 U, 232 Th and the dose rate for Kaolin mining areas in Ilorin-south and Ilorin-west LGAs, Kwara, Nigeria. Tables 1 and 2 presented the estimated activities from the points measurements and their geolocations and the summary of the descriptive statistical analyses of the obtained data. Furthermore, detailed statistical analyses were done on the original dataset to grasp the statistical distribution of the measured levels of activities. The depth descriptive statistical analyses of the in-situ measurement of activity concentrations of 238 U, 232 Th, 40 K and the gamma dose rate (DR) using the Super-Spec RS125 Gamma-Spectrometer is also given in Table 1. It presents the lowest, highest, mean, standard deviation, range, coefficient of variation (CV) and Skewness. The estimated values for 238 U, 232 Th, 40 K and DR were slightly skewed as a large portion of the proportion of the asymmetry of their probability distribution about their means ranges between −1 and +1 ("Normality Testing, Skewness and Kurtosis," 2020). The computation of the coefficient of variation shows the variability in the distribution of the measured activities of the primordial radionuclides and the gamma dose rate at the mining sites. Coefficient of variation ≤ 20% shows slight variability, 20 < coefficient of variation ≤ 50% suggests moderate variability, whereas 50% < coefficient of variation ≤ 100% demonstrates high variability and coefficient of variation value greater than 100% is viewed as been exceptionally high (Isinkaye, 2018).
From Table 1, eleven sample sites were found to have a dose rate value fall below the global average of 59 ± 00 nGyh −1 . However, twenty-nine sample sites have the absorbed dose rate value higher than the global average, with the highest found at site IWS27 with a factor of 1.64 ± 7.16. This implies that the miners and habitat at this mining field are at risk of over exposure to radiation with time. Twenty-one sample locations have their 40 K value below the global average as shown in Table 1. IWS5 recorded highest value of 40 K with a factor of 1.86 ± 27.04. Twenty-five sample codes have a 238 U values higher than the global average of 32.00 the highest value was found at IWS5 with a factor of 2.16. IWS29 recorded the highest value of 232 Th, which is higher than the world average with a factor of 1.49. The mean activity concentrations of 238 U, 40 K and the gamma radiation dose rates measured at Kaolin fields in Akerebiata area of Ilorin-west LGA were found to be higher than the global average value.
From Table 2, thirty-five sample sites were found to have a dose rate value below the global average of 59 ± 00. However, fifteen sample sites have the absorbed dose rate value higher than the global average, with the highest found at site ISK40 with a factor of 1.50 ± 6.45. This implies that the miners and habitat at this mining field are at risk of over exposure to radiation with time. Forty one sample locations have their 40 K value below the global average value for 40 K as shown in Table 2. ISK325 recorded highest value of potassium with a factor of 1.27. Forty-one sample codes have a 238 U values higher than the global average of 32.00 the highest value was found at ISK36 with a factor of 4.13. ISK35 recorded the highest value of 232 Th, which is higher than the world average with a factor of 1.53. Only the mean activity concentrations of 238 U at Kaolin mining field in Fufu area of Ilorin-south LGA was found to be higher than the global average value.
The continuous dumping of waste and tailings from mine sites into the immediate environment during mining exercises has been known to cause the enhancement and bioaccumulation of radionuclides and other toxic elements in water, air (dust), soil, and the food crops. As provided by International Commission on Radiological Protection (1991,1991), United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR, 2000) reports, the threshold values recommended for public due to exposure to 238 U, 232 Th, 40 K and DR are given as 32.00, 45.00, 420.00 Bqkg −1 and 59.00 nGyh −1 , respectively.
Comparing the mean values of 40 K, 238 U, 232 Th and DR for the two studied fields with selected studies from literatures (local and international), as shown in Table 3

Correlation analyses
To further investigate the degree of strength and nature of relationship between the measured activities of 238 U, 232 Th, 40 K and the radiation dose rate at both mining sites, Pearson's correlation technique was employed. The outcomes of the Pearson's correlation analysis are presented in Tables 4 and 5 For Ilorin-west, a significant correlation was found to exist between 238 Th and dose rate (R = 0.5515) and weak correlation (R = 0.4234, and 0.3940) exist between 40 K and dose rate, and 238 U and dose rate respectively. For Ilorin-south, a significant correlation was observed between 238 Th and dose rate (R = 0.7289) and between 238 U and dose rate (R = 0.6474), respectively. However, an insignificant correlation was observed to exist between the primordial radionuclides  Usikalu et al., Cogent Engineering (2022) for all the mining locations. The correlation results confirm that the enhanced outdoor dose rates was caused principally by 232 Th followed by 238 U and then 40 K.

Evaluation of the radiological hazard indices for the locations
The radiological risk parameters were computed to appraise the radiological hazards associated with the primordial radionuclides in the locations under study. The summary of the estimated hazards indices are provided in Tables 6 and 7

Conclusion
A handheld Super Spec RS-125 gamma spectrometer was used to measure the activity concentrations of 40 K, 238 U, 232 Th and the gamma radiation dose rate of Kaolin mining fields in Ilorin-west and Ilorin-south in Nigeria. The results of the activity concentrations obtained were used to estimate the corresponding radiation impact parameters in order to assess the level of radiological hazards to the populace in the study area. The mean values of 40 K, 238 U, 232 Th and DR for Ilorin-    K, 232 Th and DR for Ilorin-west are greater than the corresponding values at Ilorin-south. In contrast, the mean value of 238 U for Ilorin-west is less than the estimated mean value for Ilorin-south. Consequently, the mean values of the estimated radiological hazard parameters of Ilorin-west were higher than the estimated mean values for Ilorin-south. This shows that the Ilorin-west Kaolin mine field poses more significant source of radiation hazard. Considering that recommended values for background radiation were exceeded at the Ilorin-west Kaolin mine field, it follows that the Kaolin from Ilorinwest should not be used for building and construction purposes and the local populace should be mindful of health effects like cancer and other radiation induced health effects. Similarly, the dataset generated in this study could be utilized by the Nigerian Nuclear Regulatory Agency (NNRA) and related authorities for enforcement of rules and laws to reduce the mining exercises in the nation. Hext AED(outdoor) ELCR Figure 4. Summary of the estimated radiological parameters at Ilorin-west.