|Year : 2019 | Volume
| Issue : 3 | Page : 90-95
Assessing naturally occurring radionuclides in soil of Egbeda Local Government for a baseline data of Oyo State, Nigeria
Latifat Ronke Owoade, Samuel Mofolorunso Oyeyemi, Fatai Abiodun Lawal, Adekunle Joseph Adeyemo, Nojeeb Oyeyemi Adamoh, Faidat Mosunmola Adebowale
Department of Environmental Monitoring, National Institute of Radiation Protection and Research, Nigerian Nuclear Regulatory Authority, University of Ibadan, Ibadan, Nigeria
|Date of Submission||09-Apr-2019|
|Date of Decision||13-Apr-2019|
|Date of Acceptance||21-Apr-2019|
|Date of Web Publication||06-Nov-2019|
Latifat Ronke Owoade
National Institute of Radiation Protection and Research, Nigerian Nuclear Regulatory Authority, University of Ibadan, Ibadan
Source of Support: None, Conflict of Interest: None
The activity concentrations of the natural radionuclides, namely226Ra,232Th, and40K were measured from soil samples collected from different locations of Egbeda Local Government Area, Oyo state, Nigeria, with the aim of establishing radioactivity baseline data for the area. High-resolution gamma spectrometry detector was used to determine the activity concentration of these radionuclides in 45 soil samples taken at a depth of about 15 cm in various communities of Egbeda Local Government Area. The concentration of226Ra was in the range 5.9–61.3 Bq/kg with an average value of 30.5 Bq/kg,232Th in the range 4.0–116.5 Bq/kg with an average value of 50.8 Bq/kg, and40K in the range 27–798 Bq/kg with an average value of 200 Bq/kg. The results obtained in this study were compared with those of different countries in the world. Radiological index parameters were used for the assessment of radiological exposure of the natural radioactivity, and the absorbed dose rate, the radium equivalent activity, the annual effective dose, the external exposure index, and internal exposure index were calculated. There is no radiological risk from soil that may threat the residents around Egbeda Local Government Area. Hence, the probability of occurrence of any of the health effects of radiation is insignificant. Therefore, the radioactivity measurements taken represent the baseline data of the study area.
Keywords: Baseline data, high-resolution gamma spectrometry, radiological exposure, soil radioactivity
|How to cite this article:|
Owoade LR, Oyeyemi SM, Lawal FA, Adeyemo AJ, Adamoh NO, Adebowale FM. Assessing naturally occurring radionuclides in soil of Egbeda Local Government for a baseline data of Oyo State, Nigeria. Radiat Prot Environ 2019;42:90-5
|How to cite this URL:|
Owoade LR, Oyeyemi SM, Lawal FA, Adeyemo AJ, Adamoh NO, Adebowale FM. Assessing naturally occurring radionuclides in soil of Egbeda Local Government for a baseline data of Oyo State, Nigeria. Radiat Prot Environ [serial online] 2019 [cited 2019 Nov 22];42:90-5. Available from: http://www.rpe.org.in/text.asp?2019/42/3/90/270439
| Introduction|| |
Natural radioactivity varies from one place to another due to variation of natural minerals and local geology of each region. The presence of naturally occurring radionuclides in the environment may result in an external and internal dose, received by a population exposed to them directly and via the ingestion and inhalation pathways. The assessment of the radiological impact on a population as a result of the radiation emitted by these radionuclides is particularly important in giving the baseline data on radiation levels which will be used by policymakers and residents, for appropriate utilization.
Natural radioactivity is a source of continuous exposure to human beings. It is present in the human environment due to the presence of cosmogenic and primordial radionuclides in the Earth's crust. Cosmogenic radionuclides are produced by the interaction of cosmic rays with atomic nuclei in the atmosphere, while primordial ones (terrestrial background radiation) were formed by the process of nucleosynthesis. The great interest expressed worldwide for the study of naturally occurring radiation and environmental radioactivity has led to interest in extensive surveys in many countries. Natural sources still contribute almost 80% of the collective radiation exposure of the world's population.,
There are many sources of radiation and radioactivity in the environment. Gamma radiation emitted from naturally occurring radionuclides, also called terrestrial background radiation, represent the main external source of irradiation of the human body once present in the environment. These radionuclides are available for uptake by plants and animals and so make their way into the food chain. Human beings are exposed to radiation from sources outside their bodies; mainly, cosmic rays and gamma ray emitters in soils, building materials, water, food, and air. Studying the levels of radionuclide distribution in the environment provides essential radiological information. The amount of radioactivity in soil varies widely; hence, it is important to monitor the terrestrial background radiation mainly due to natural radionuclides in soil.
The present work aims at estimating the activity concentration of radionuclides,226 Ra,232 Th, and40 K in soil samples collected from Egbeda Local Government Area, Oyo State, Nigeria and evaluates the radiological indexes including their impacts on the population who live in those environments. Therefore, the results were used to assess the potential radiological exposures associated with these soils. Data on the radioactivity levels of soil in this Local Government were not reported previously. The result obtained from this assessment serves as the baseline data for Egbeda Local Government Area.
| Description of the Study Area|| |
Egbeda is a Local Government Area in Oyo State, Nigeria. The Local Government is made up of about 40 local communities. It lies within latitude 7°22“ N to 7°28” N and longitude 3°58“ E to 4°08” E with its headquarters in the town of Egbeda. It covers a landmass of 185.508 square kilometers with a population density of 1722 persons per square kilometer. The 2010 estimated population figure of the Local Government is put at 319,388 people based on a growth rate of 3.2% using 2006 census figure. The local government area is subdivided into 11 wards: Erunmu, Ayede/Alugbo/Koloko, Owo Baale/Kasumu, Olodan/Ajiwogbo, Olodo/Kumapayi I, Olodo II, Olodo III, Osegere/Awaye, Egbeda, Olode/Alakia, and Olubadan Estate. The Local Government Area share boundaries with Osun State to the east, Ibadan North Local Government area to the north, Ibadan North East Local Government Area to the west, and Ona Ara Local Government to the south. It is dominated by the Yorubas and endowed with a wide expanse of land for the production of livestock and arable farming. About half of the Local Government area is rural in nature and suburb to Ibadan Metropolis. The town lies on a gently undulating plain which falls below 180 m (600ft) above sea level in most part of the Local Government area while the lower part are very close to the flood plain of river Osun on each side of its banks with the height 150 m above sea level. The entire land of Ibadan is made up of basement complex rock. More than three-fourth of the basement rock in Egbeda occur as banded gneisses, while granite and quarzites share the rest in almost equal proportion. The rock types are covered in most places by weathered materials and outcrop in a few places.
| Experimental Procedure|| |
Sample collection and sample preparation
A total of 45 soil samples were collected from different communities of the Egbeda Local Government Area. A geographical positioning system and RadEye personal radiation detector survey meter (model number 42506/71) were used to measure the sample locations and dose rate, respectively, at every sampling point [Table 1]. Samples were collected during the dry season, and the sampling was done by random grid sampling system with the aid of Egbeda Local Government map. The top layers of the soil which contained wastes that are yet to decompose were removed, and the soil samples were collected to a depth of 15 cm, using a hand trowel tool that was thoroughly cleaned and dried before each sample was collected. Ultimate care was taken in the extraction of soil sections to avoid mixing or cross contamination of soil samples. About 1 kg of each sample was collected in a polythene bag at the sampling points, which was given sample identification tag and name to prevent sample mix-up. The soil samples were moved to the laboratory and were prepared according to the IAEA recommended procedure. The samples were dried in an oven at 105°C, after cooling; the samples were pulverized and sieved with a 0.5 mm mesh screen to obtain a homogenize sample matrix. The dried and sieved samples were packed in 500 ml Marinelli beaker and properly sealed for 4 weeks to attain secular equilibrium between226 Ra (daughter of238 U) and232 Th with their daughters.
|Table 1: Names of villages with their coordinates including the dose rate of the sampling area|
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Calibration and energy lines
The activity concentrations of226 Ra (which is the most important radiologically and the most referenced among238 U series),232 Th, and40 K in the samples were determined by gamma spectrometry using high-purity germanium (HPGe) detector (Canberra) with 80% relative efficiency and 2.3 keV resolution for the 1332.5 keV60 Co gamma line. The detector is shielded with three inner concentric shells of lead, cadmium, and copper on all sides to reduce the background level of radiation. Energy Calibration was performed using point sources like;241 Am of energy line 59.5 keV,137 Cs of energy line 661.6 keV and60 Co of energy lines 1173.2 and 1332.5 keV, while efficiency calibration was done with a certified mixed multi-gamma ray standard (MGS6M315),125 Sb (176.3, 427.9 and 600.6 keV),155 Eu (60.0, 86.5 and 105.3 keV),54 Mn (834.8 keV) and40 K (1460.8 keV). The absolute efficiency and uncertainty of the HPGe detector were determined using Genie 2000 software, and this is presented in [Table 2]. The standard source had the same geometry as that of the measured samples. The detector background and the samples were counted for 18,000s.226 Ra activity concentration was determined using energy lines of 295.2 keV and 51.9 keV of214 Pb and energy lines of 609.3, 1120.3 and 1238.1 keV of214 Bi. The232 Th activity concentration was determined using 300.1 keV of212 Pb, 277.4 and 860.6 keV of208 Tl, 209.3 and 911.1 keV of228 Ac, and 723.3 and 785.3 keV of212 Bi gamma lines. The activity of40 K was determined directly from the 1460.8 keV gamma line. The net count rates under the most prominent photopeaks of all radionuclides were calculated by subtracting the respective background count rate from the gross count rate for all the radionuclides obtained for the same counting time. Then, the activities of the radionuclides were calculated using equation 1.
|Table 2: The absolute efficiency and uncertainty of high purity germanium detector|
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| Theoretical Calculations|| |
The activity concentration
The activity concentrations of the radionuclides in the measured samples were computed using the following relation:
Where A is the specific activity concentration, Nc is the net gamma counting rate (counts per se cond), ɛ is the detector efficiency of the specific γ-ray, Ms is the mass of the sample (kg), and Pϒ is the emission probability of radionuclide.
The efficiencies of other energy lines used in the calculation of specific activity concentrations were relatively determined using equation 2.
Where ɛn, ɛ1 and ɛ2 are the respective efficiencies for energy En, E1, and E2
The radium equivalent activity
For the purpose of comparing the radiological effect or activity of materials that contain226 Ra,232 Th, and40 K by a single quantity, which takes into account the radiation exposures associated with them, a common index termed as the radium equivalent activity (Raeq) is used. This index parameter provides a useful guideline in regulating the safety standards on radiation protection for the general public residing in the area under investigation. The Raeq index represents a weighted sum of activities of the above-mentioned natural radionuclides and is based on the estimation that 1 Bq/kg of226 Ra, 0.7 Bq/kg of232 Th, and 13 Bq/kg of40 K produce the same gamma radiation dose rates. The index is given as:
Where CRa, CTh, and CK are the average activity concentration in the sample in Bq/kg of226 Ra,232 Th, and40 K, respectively.
The absorbed dose rate
The absorbed dose rate (AD) in air at average gonad height of 1 m above the surface of ground due to the natural radionuclides226 Ra,232 Th, and40 K was estimated using equation 4.
Where dose conversion factors (DCFRa), DCFTh, and DCFK are the DCF for226 Ra,232 Th, and40 K in nSv/h/Bq/kg and CRa, CTh, and CK have the same meaning as in Equation 3.
DCFRa is 0.462 nGy/h/Bq/kg, DCFTh is 0.640 nGy/h/Bq/kg, and DCFK is 0.0417 nGy/h/Bq/kg.
The annual effective dose
The annual effective dose (AED) to the population can be calculated using the conversion coefficient from AD rate in air to the effective dose received by adults (0.7 Sv/Gy) and 0.2 for the outdoor occupancy factor. Therefore, the outdoors AEDs are calculated using equation 5.,
Doutdoor(mSv/y) = (AD [mGy/h] × 24 h × 365.25d × 0.2 × 0.7Sv/Gy) × 10−6 (5)
The external and internal exposure indexes
The external exposure index (EEI) and internal exposure index (IEI) due to the emitted γ-rays of the soil samples were calculated using equation 6 and 7, respectively:
The value of EEI must be lower than unity to keep the radiation exposure insignificant.
| Results and Discussion|| |
The activity concentration
The results of analysis of activity concentration of226 Ra,232 Th, and40 K in soil samples for different locations of the study area are presented in [Table 3]. The range of measured activity of226 Ra in the soil of Egbeda Local Government Area was 6.0-61.3 Bq/kg with an average of 30.5 Bq/kg. The minimum value for226 Ra was obtained in sample ELG 25 at Kusela village and a maximum for the sample was in sample ELG 13 at Ibiti village. The range of measured activity concentration of232 Th for the soil was 4.0–116.5 Bq/kg with an average value of 50.8 Bq/kg. The minimum value was obtained in sample ID ELG25 at Kusela village and a maximum value was obtained in sample ID ELG12 at Erunmu village. The range of activity concentration of40 K was 27–798 Bq/kg with an average value of 200 Bq/kg. The minimum value for40 K was obtained in sample ELG 35 at Onisade village and maximum for the sample was in sample ELG 41 at Owobaale village.
Moreover, our obtained average values fall within the range of corresponding world average values and other published results mentioned in [Table 4]. The world average activity concentration of226 Ra is 35 Bq/kg with ranges of 17–60 Bq/kg,232 Th is 30 Bq/kg with ranges of 11–64 Bq/kg, and40 K is 400 Bq/kg with ranges of 140–850 Bq/kg. The observed results in some samples show that the activity concentrations for226 Ra and232 Th for the study area are higher than the reported international radioactivity levels of226 Ra and232 Th in UNSCEAR 2000.
|Table 4: Comparison of average of natural radioactivity levels in soil and absorbed dose rate at different locations of Egbeda Local Government Area with those in other countries|
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[Table 5] shows the radiological effects such as the radium equivalent, the AD rate, AED, external, and internal exposure indexes of the soil samples collected from local government under investigation.
Using equation 3, the Raeq found in the soil samples are shown in [Table 5]. The Raeq calculated for the same soil samples vary from 20 Bq/kg to 230 Bq/kg with an average value of 119 Bq/kg. It is inferred that for all the soil samples analyzed, the Raeq value is well within the permissible limits of 370 Bq/kg.
The calculated AD rate varied from 10 to 107 nGy/h, with an average value of 55 nGy/h. This weighted mean value represents 96.5% of the world average outdoor exposure due to terrestrial gamma radiation of 57 nGy/h. Thus, the radioactive impact and the additional external radiation exposure for population due to soils were negligible. The calculated AED varied from 0.01-0.13 mSv/y with an average value of 0.07 mSv/y, and these results lie within the world wide average values (0.07 mSv/y for outdoor exposure), which is also within the dose limit of 0.48 mSv/y (indoor + outdoor) recommended by UNSCEAR 2008.
The range of EEI was 0.07–0.48, with a mean value of 0.21, while the IEI ranged between 0.10 and 0.71 with a mean value of 0.30.
The average AED in this study is within the world average value.
[Table 5] compares the reported values of natural radionuclides and AD rate in the soil samples, obtained in other countries, with those determined in the present study. On comparison, it is found that the average value of226 Ra and40 K are lower when compared with world average while232 Th is higher than world average but lower than Malaysia and India.
Gamma ray spectrometry was used to determine the activity concentrations due to naturally occurring radioisotopes of226 Ra,232 Th, and40 K with the associated radiation exposure levels in 45 soil samples from different locations of Egbeda Local Government Area, Oyo State, Nigeria. The average concentration for226 Ra,232 Th, and40 K were 30.5, 50.8, and 200 Bq/kg, respectively. These average activity concentrations were lower than the world average values. It is concluded that no harmful radiation effects were posed to the population who live in the study area.
The average dose rates (0.03 ± 0.01) μSv/h and other calculated hazard indices were lower than the average national and world recommended values, therefore, will not pose health risks to the population of the area. The total AED was lower than 0.48 mSv/y from external terrestrial radiation and 0.07 mSv/y for external outdoor for public radiation exposure control. The results in this study compared well with other studies carried out in other countries and with the worldwide average. This study is considered to be the first conducted radiological survey in Egbeda Local Government Area, Oyo State. Hence, the data will serve as a baseline data for information or further research.
Financial support and sponsorship
This research was sponsored and carried out by the National Institute of Radiation Protection and Research, Nigerian Nuclear Regulatory Authority.
Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]