Radiation dose assessment of soil from Ijero Ekiti, Nigeria

Abstract The activity concentration of 238U, 232Th and 40K in the soils of mining and living areas of Ijero Ekiti were determined using hyper pure germanium gamma ray spectrometer. The mean concentrations of 238U, 232Th and 40K were found to be 128.05 Bqkg−1, 24.8 Bqkg−1 and 455.05 Bqkg−1, respectively, for mining areas, while it was found to be 42.02 Bqkg−1, 43.27 Bqkg−1 and 635.41 Bqkg−1, respectively, for the living areas. The mean absorbed dose rate, annual effective dose equivalent, gamma index and the excess lifetime cancer risk evaluated for the locations were 89.70 nGyh−1, 0.11 mSvy−1 and 1.4 for mining areas; while it was 72.22 nGyh−1, 0.089 mSvy−1 and 1.14 for living areas, respectively. The absorbed dose rate, annual effective dose equivalent and gamma index estimated for the mining site were higher than the living areas. The mean excess lifetime cancer risk estimated for the mining areas was consecutively higher than the recommended limit of 0.29 × 10−3 for background radiation. It is therefore necessary for government to barn illegal mining activities going on in the town so as to reduce natural radiation burden from their operations in the town.

M. R. Usikalu 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 and compare the measured activity to where there are no mining activities. The radioactivity measurement was done using hyper pure germanium 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
The radiation exposure from natural primordial radionuclides 238 U and 232 Th and their progenies constitute the largest source of radiation to man. 238 U and 232 Th condense in crystalline rocks such as granites and other alkaline magmatic because of their big atom and they often accompany by rare earth elements (UNSCEAR, 2000). The associated health hazards of these radionuclides are as a result of their ability to pile up in human tissues. In reaction, the radionuclides emit gamma rays as well as high-LET charged particles that are capable of damaging the tissues where they are localized and also to some extent nearby organs. Banat, Howari, and Al-Hamad (2005), Akinlua, Ajayi, and Adeleke (2006), Birkefeld, Schulin, and Newack (2006), Nyarko et al. (2006), Adagunodo, Hammed, Usikalu, Ayara, and Ravisankar (2018), Omeje et al. (2018) reported that radionuclides of both uranium and thorium decay series can be found in high concentration in the natural occurring materials in the vicinity where mining activities are going on, which can be considered as technological enhanced naturally radioactive material (TENORM). The digging of huge quantities of sand which is usually piled up to form large volume of tailings is one of the negative implications of mining activities on the environment. These activities usually affect the natural radionuclides in the soil and equally produce change in the ecosystem. UNSCEAR, (2000) and Lipsztein et al. (2001) observed that mining, milling and processing of uranium-and thorium-bearing minerals can lead to increase in radiation exposures not only to the workers but also to the inhabitants of the mining sites. Another issue is the haphazard dumping of the tailings which resulted into forming of huge slopes and this leads to increase in their movement and the risk of transportation to a large extent. 238 U and 232 Th from the sites find their ways of getting to the ground water potential through percolation, thereby contaminating the water and soil in the area (Usikalu, Anoka, & Balogun, 2011). Therefore, the tailings are a nuisance and source of pollution to the ground water and surrounding soil (Sam & Awad Al-Geed, 2000). The implication of the pollution to man is of great concern since the radionuclides will find their way to the food and inhalation in the air thereby increasing the radiation burden of the inhabitant in the surrounding.
Ijero Ekiti with coordinates 7.8120°N, 5.0677°E is situated in the northwest part of Ekiti State. The Ijero local government has a largely agrarian population producing cash crops such as cocoa, kola nuts, coffee, cashew and timber. Food crops such as yam, cocoyam, cassava, pepper, tomatoes and banana are equally grown in the town. It also has mineral resources such as tourmaline, colombat, vesper, and crystal stone in the northern part of the town called Kusa Mountain. Kusa Mountain in Ijero falls within the basement complex of southwestern Nigeria. It is distinguished by the abundance of pegmatites that contain different minerals like rare earth metals, gem-stones and metallic-ores (Oyawoye, 1972). It is intruded by the Mesozoic calc-alkaline ring complexes (Younger Granites) of the Jos Plateau and is overlain by Cretaceous and younger sediments as shown in Figure 1. The pegmatite comprises of quartz, biotite, albite, microcline, muscovite, tourmaline, hornblende and other minerals in varying quantity.
As earlier stated, high percentages of the inhabitants of the town engage in farming activities and food crops such as cocoyam, yam, cereals and vegetables are grown for consumption. Also, the inhabitants leaving around the vicinity see the tailings as source of cheap materials that can be used for building purposes (Usikalu et al., 2011). The radiological impact on the inhabitants and workers are of interest due to the unchecked mining processes going on in the area. In Nigeria, high activities concentrations of natural radionuclides had been reported in foodstuffs obtained from Bisitchi a mining site in Jos Plateau (Jibiri, Farai, & Alausa, 2007). The natural radionucludes in soils from the Kusa Mountain and its radiological implications have not been studied as was done for other mining sites in Nigeria. Hence, there is need to assess the radiological implications of the mining activities in the area. In this research, radiological parameters such as hazard index, radium equivalent, gamma index and excess lifetime cancer risk have been estimated from the activity concentration of 238 U, 232 Th and 40 K. The aim of this study is to assess the health risks associated with the mining activities and develops a baseline of natural background radiation levels for the area.

Materials and methods
Samples were collected by digging the ground to at least 3 cm so as to take samples free from debris and vegetation. Five soil samples were taken from different points at each location for better sampling. These were kept in Ziploc bags and labeled accordingly making a total of 90 soil samples from all the locations. Forty were from the mining area and fifty from non-mining sites of the town. The samples were oven dried at 110°C to ensure complete removal of moisture and were made to pass through a 2 mm sieve. 100 g of each sample were placed in plastic vessels, 9 cm in diameter with a total capacity of 300 cm 3 . The vessels were weighted and sealed for 30 days to allow secular equilibrium in the 238 U and 232 Th with their respective progeny.
The activity measurement was described fully by Usikalu et al. (2017a). P-type high-purity germanium (HPGe) detector (Canberra model GC4520) coupled to a multichannel analyzer (MCA) with 45 % efficiency was used. It has energy resolution of 2.2 KeV (FWHM) for the 1332.5 KeV gamma-ray transition of 60 Co source. The detector was positioned in a 5 cm thick lead to protect the measuring laboratory from external background radiation. The spectra for 238 U, 232 Th and 40 K were obtained using IAEA reference materials RGU-1 (4940 Bqkg −1 uranium ore); RGTh-1 (3250 Bqkg −1 thorium ore) and potassium chloride (16259 Bqkg −1 40 K) from Merck company with 99.5 % purity. Each sample was counted for 36,000 s so as to achieve minimum counting error. The analysis of gamma ray spectrometry used here has been used by other researchers so as to ascertain good quality (Iqbal & Tufail M Amd Mirza, 2000;Kant, Gupta, Kumari Gupta, & Garg, 2015;Usikalu, Maleka, Malik, Oyeyemi, & Adewoyin, 2015). The activity concentrations were calculated using the photo peaks corresponding to each radionuclide. 40 K was obtained at photopeak of 1.460 MeV; 214 Bi (1238 keV and 1378 keV), 214 Pb (295 keV and 351 keV) for 238 U and 208 Tl (860 keV), 228 Ac (338 keV, 911 keV, 969 keV) for 232 Th using palmtop MCA software for computation: where A net is the net area under energy peak (count), P ɣ is the gamma emission probability at energy E, t c is the counting time ε is the absolute efficiency of the detector, and M s is the mass of the dried sample (kg).

Results and discussion
The measured activities concentrations of 238 U, 232 Th and 40 K from Ijero Ekiti soils both in the mining area and Odo-Oye Ijero (control) are presented in Table 1. It was observed that the mean activities of 238 U measured in the mining areas (128.05 Bqkg −1 ) is much higher than the area where mining is not going on (42.02 Bqkg −1 ). Meanwhile, the mean activity of 232 Th measured in the mining area (24.80 Bqkg −1 ) is lower than the control area (42.02 Bqkg −1 ). The mean activity of 40 K measured in the mining area (455.05 Bqkg −1 ) was lower than the control area (635.41 Bqkg −1 ). Figures 2 and 3 display the ratio of the three activity concentrations in the two locations. The ratio of uranium to thorium in the control locations is nearly equal to one while in the mining area uranium is five times higher than thorium concentration. The mining activities might be responsible for the enhanced concentrations of 238 U in the mining area. This may be as a result of the mineral contents of the mine deposit or during mining and mineral extraction process due to various physical processes and extraction of the mineral being exploited by chemical processes. Also, there is limited number of in-situ leaching activities since uranium is more soluble than thorium. This may be the reason for the elevated uranium concentration in the mining area. The high-activity concentration level of 40 K in both locations as shown in Figures 2 and 3 may have perhaps been from the denudation of the phosphate rocks (Usikalu et al., 2015(Usikalu et al., , 2017b) but most probably from inorganic fertilizer used in agricultural activities in the control area. The continuous application of fertilizers in farms within this region enhances the radioactive levels which in turn increases the radionuclide concentration. Table 2 shows the mean activity measured in different similar areas. Many international organizations have studied and reported average world value for natural background radiations which are taken as recommended limit or permissible level with which radiation of any location can be compared with (UNSCEAR, 2000;IAEA, 1989;ICRP, 1994) as shown in in the last row of Table 2. The results obtained in the present study in the mining area is higher than those obtained in Egypt, Australia and in gold mine Nigeria where high radioactivity had been recorded but compared well with the measured activities in Ijero living area (Ademola et al., 2014;Berekta & Mathew, 1985;El Afifi, Hilal, Khalifa, & Aly, 2006).

Evaluation of radiological parameters
The absorbed dose rates in air at 1 m above the ground level were evaluated using Equation (2). The measured activity of 238 U, 232 Th and 40 K were converted into dose (nGyh −1 ) by applying the conversion factors 0.462, 0.604, and 0.0417 for uranium, thorium and potassium, respectively,   C U , C Th , and C K are the concentrations of uranium, thorium and potassium in the soil samples (Bqkg −1 ). The average absorbed dose rates calculated ranged from 50.28 to 126.98 nGyh −1 with a mean value of 89.70 nGyh −1 for the mining area and 72.22 nGyh −1 for the control area. The mean absorbed dose rates estimated for the two locations exceeded the world average recommended limit 60 nGyh −1 may be because the soils originated from mineral bearing igneous rocks. The annual outdoor effective dose equivalent (AEDE) received by a member was calculated using Equation (3) with a conversion factor of 0.7 SvGy −1 applied to convert the absorbed rate to annual effective dose with an outdoor occupancy of 20%. Table 3, column 3, shows the outdoor annual effective doses. It was found to vary from 0.06 mSvy −1 to 0.16 mSvy −1 with mean from 0.09 mSvy −1 for the control and 0.11 mSvy −1 for the mining areas. The estimated values for both locations exceeded the world average (0.07 mSvy −1 ) for outdoor effective. This may be associated with the soils being rich in minerals: Outdoor annual effective dose AEDEmSvy À1 Figures 4 and 5 show the correlation of absorbed doses with the radium equivalent activity within the study areas. Regression analysis technique was used in drawing a trend line between the points. The regression correlation was positive, linear and high but higher in the control area than the mining area.
The internal hazard index was evaluated using Equation (4). The aim of radiation protection is to limit the index to a value less than unity. The H in in one location in mining area is higher than unity but the mean H in for both mining area is less than unity (0.88) and control area (0.53) was less than recommended limit of unity. The gamma representative index (I ɣ ) was estimated using Equation (5). This was done in order to examine how the soil materials from these locations are suitable for construction purposes. The calculated I ɣ is greater than unity in the samples collected from the mining site except in one location with mean value of 1.4. While, it was found to be greater than unity in half of the control locations with a mean value of 1.13. The evaluated mean I ɣ obtained for both locations were higher than the recommended permissible limit of unity which suggest that the materials may not be suitable for construction purposes from radiation protection point of view: where C U , C Th , and C K are the concentrations of uranium, thorium and potassium in the soil samples (Bqkg −1 ).
Excess Life time Cancer risk (ELCR), the additional or extra risk of developing cancer due to exposure to a toxic substance incurred over the life time of an individual as a result of exposure to a toxic substance accumulated over a period of lifetime (Average 70 years) was calculated as follows: where AEDE had been obtained from Equation (3), LT is the average life span (70 years) and FR is the risk factor. The calculated ELCR was found to be higher than the permissible limit of 0.29 × 10 −3 Ra eq Activity (Bqkg -1 ) Dose Rate (nGyh -1 ) Figure 4. Correlation of radium equivalent with dose rate in mining area. (UNSCEAR, 2000) in all the samples collected from the mining site except in one location and the mean ELCR value of 0.38 × 10 −3 was obtained for the mining sites. The increase was found to be 76% above the recommended value (UNSCEAR, 2000). This suggests that dwellers and workers in the vicinity are likely to be exposed to elevated level of natural radioactivity and they may be prone to the development of cancer within their lifetime.

Conclusions
• The natural radioactivity contents of soils from Ijero mining and living areas had been measured using hyper pure germanium detector.
• It was revealed that the mining area had the highest activities concentrations of 238 U (128.05 Bqkg −1 ) which exceeded the recommended permissible limit.
• Highest mean absorbed dose rate, annual effective dose equivalent, gamma index and the excess lifetime cancer risk were consistently obtained from the mining sites.
• The high radioactivity measured in the town may be connected with the mining activities going on in the town.
• The study therefore concludes that the soils in most of the studied locations are not suitable for building purpose.
• The research also recommended that government should see to it that illegal mining be stopped in the town so as not to expose the inhabitants to high radiation burden.