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ORIGINAL ARTICLE
Year : 2013  |  Volume : 36  |  Issue : 3  |  Page : 122-127  

Natural radioactivity levels and the radiological health implications of tailing enriched soil and sediment samples around two mining sites in Southwest Nigeria


Department of Physics, Ekiti State University, P. M. B. 5363, Ado Ekiti, Nigeria

Date of Web Publication28-Jul-2014

Correspondence Address:
Matthew Omoniyi Isinkaye
Department of Physics, Ekiti State University, P. M. B. 5363, Ado Ekiti
Nigeria
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.137477

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  Abstract 

The activity concentrations of natural radionuclides in tailing enriched soil and sediment samples collected from two mining sites in southwest Nigeria are reported. The samples were analyzed by gamma spectrometry with low background NaI (Tl) detector. The activity concentrations of 40 K, 226 Ra and 232 Th in all the measured samples varied from 249.66 to 1459.25 Bq/kg, 7.62 to 50.31 Bq/kg and 12.68 to 234.18 Bq/kg, respectively in soil samples. For sediment samples, the values varied from 241.86 to 1590.40 Bq/kg, 9.86 to 74.8 Bq/kg and 15.47 to 145.46 Bq/kg for 40 K, 226 Ra and 232 Th, respectively. The results show that the mean activity concentrations of the radionuclides in soil and sediment of the study area are higher than their world-wide average crustal values. The mean concentration of 232 Th is >6 times higher than that of 226 Ra in soil samples from Ijero mining site. This shows that 232 Th is slightly enhanced in the soil from this location than 226 Ra. In order to evaluate the radiological hazards of the natural radioactivity, the radium equivalent activity, external hazard index, absorbed gamma dose rates and the annual effective dose rates were determined. All these hazard indices have mean values, which are within their acceptable limits but higher than their world average values.

Keywords: Mining sites, Nigeria, natural radioactivity, sediment, tailing enriched soil


How to cite this article:
Isinkaye MO. Natural radioactivity levels and the radiological health implications of tailing enriched soil and sediment samples around two mining sites in Southwest Nigeria. Radiat Prot Environ 2013;36:122-7

How to cite this URL:
Isinkaye MO. Natural radioactivity levels and the radiological health implications of tailing enriched soil and sediment samples around two mining sites in Southwest Nigeria. Radiat Prot Environ [serial online] 2013 [cited 2020 Sep 24];36:122-7. Available from: http://www.rpe.org.in/text.asp?2013/36/3/122/137477


  Introduction Top


Mining and processing of natural resources have being in practice since the earliest centuries for the benefit of mankind. Even though, mining and processing of natural resources is beneficial to man, it has also cause various types of environmental damages such as air pollution, contamination of soil and water bodies and other environmental degradations. Mining activities all over the world have resulted in the accumulation of large amount of tailings which contain high concentrations of natural radionuclides in the vicinity of mining sites leading to health concerns among the populace. The health risk to human due to the radionuclides in tailing could increase significantly when these materials accumulate in farmlands. [1] At the mining sites, exposure to radiation could either be by direct inhalation of suspended dust particles in air or indirect ingestion through the consumption of food crops grown on tailing enriched soil. Population exposure could also occur when soil, sediment or rock from the mining sites are used as components of building materials.

Natural radioactivity originates from extra-terrestrial sources as well as from radioactive elements found in varying concentrations in the earth's crust. [2] Only those radionuclides whose half-lives are comparable to the age of the earth and their decay products, exist in sufficient and measurable quantity to contribute significantly to population exposure. [3] Human exposure to natural radiation can either be external or internal. The major contribution to external exposure comes from gamma-emitting radionuclides present in trace amounts in soil and sediment, mainly from 40 K and the 328 U and 232 Th decay series. Internal exposures arise from the intake of terrestrial radionuclides by inhalation and ingestion. The dominant components of exposure to inhalation are the short-lived decay products of radon, while the doses from ingestion are due to 238 U and 232 Th decay series and 40 K present in food and water. [3]

Human activities such as mining and processing of minerals can lead to enhanced levels of naturally occurring radioactive materials (NORM). Enhanced levels of natural background radiation have been reported in surface and underground mines around the world. [3],[4],[5],[6],[7],[8],[9] In South Africa alone, the average annual doses received by underground gold miners were estimated at 6.3 mSv in 1997, 4.9 mSv in 1998, 5.4 mSv in 1999 and 7.0 mSv in 2000. [3],[9],[10] Also, an evaluation of the occupational radiation exposure to NORM in surface and underground mining operations in Ghana shows an annual effective dose of 0.26 ± 0.11 mSv for surface mining and 1.83 ± 0.56 mSv for underground mines. [8] The level of exposures in mines depends on a number of factors, which include; the type of mine, geology of the local environment and the working conditions such as the degree of ventilation. In the study area, no specific safety standards are followed in the mining operations as most of the mining activities are carried out by illegal miners who employ the service of quarks. This could significantly increase the radiation exposure both to the public and the workers at the mining sites due to uncontrolled disposal of waste and tailing from the extracted ore. No significant attempt had been made before now to quantify the radioactivity levels of tailing enriched soil and sediment from these mining sites.

The study of the concentrations and distributions of natural radionuclides in soil and sediment of mining environment is important for environmental protection. It allows the understanding of the radiological consequences of natural radionuclides due to the gamma ray exposure of the body and irradiation of lung tissue by 222 Rn and its deacy products. [11],[12],[13] The main objective of this study therefore, is to determine the radioactivity levels of 226 Ra, 232 Th and 40 K in soil and sediment around an abandoned gold mining site located at Itagunmodi village in Ilesa and tin, tantalite and kaolin mining sites in Ijero-Ekiti both in Southwest Nigeria, and to assess the radiological health hazards associated with the mining operations. The radium equivalent activity, external hazard index (Hex), the air absorbed dose rates and the annual effective dose rates were evaluated and compared with values reported in literature as well as with internationally approved limits.


  Materials and methods Top


Geological setting of the study area

The study area lies within the Nigerian basement complex, which is situated approximately between latitudes 7°N and 10°N and longitude 3°E and 6°E in the equatorial rain forest region of Africa. The lithologies of the basement complex include; the poly-metamorphic migmatite gneiss of various compositions, the low-grade sediment dominated schist and the sync-tectonic to late tectonic granitic rock of different compositions. [1] The rock types includes; amphibolites, the hornblended gneiss and the granite gneiss. The amphibolites rock types occur widely in the study area with outcrops of massive melanocratic amphibolites exposed in streams and river channels. [14] The overburden soil in these areas is strikingly red due to the presence of hematite and magmatite liberated during the weathering of the amphibolites to form the overburden soil. [14] Itagumodi gold field is located few kilometers south of Ilesa town in Osun State, Nigeria. The area lies within the tropical rainforest region of southwest Nigeria [Figure 1]. The mine is essentially abandoned leading to the gold field been covered by thick and dense forest. Prominent in the mine is a top layer with abundance of tall and thick bamboo trees with evergreen canopy and very dense undergrowth made up of creeping and climbing vines. The Ijero tin and tantalite deposits fall within the politic schist belt, which is extremely exposed due to the tropical conditions and rainforest vegetation. [1] Open cast mines exist in Ijero mining site and the waste disposal method is essentially indiscriminate dumping as tailings within the locality of the mine.
Figure 1: Map of south west Nigeria showing the sampling sites (Source: Google earth map)

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Sample collection and preparation

In all, 60 samples were analyzed in this study, consisting of 15 soil and 15 sediment samples each from Ijero-Ekiti (Ekiti State) and Ilesha (Osun State) mining sites. The soil samples include mines tailing and soil samples collected around the mines. The samples were collected from different representative mine locations. Sediment samples were collected from streams within the locality of the mines. The samples were initially sun dried to remove moisture and aerated at laboratory temperature until samples weights became constant. The dried samples were then crushed to smaller particle sizes and homogenized. They were then weighed and packed into plastic containers of dimension 7.5 × 6.5 cm. The weight of the samples ranged from 107.0 g to 421.7 g. The containers were hermetically sealed to avoid the escape of gaseous radioactive products and kept for 30 d prior to analysis to bring radon ( 222 Rn) and its short-lived daughters in equilibrium with radium ( 226 Ra). [15],[16]

Activity concentration measurements

The activity concentrations of 226 Ra (in equilibrium with 238 U), 232 Th and 40 K in the soil and sediment samples were determined using gamma spectrometric technique. The spectrometry system consisted of low-background 7.6 × 7.6 cm NaI (Tl) detector manufactured by Bircom with pre-amplifier (model 2001), multi-channel analyzer card with in-built spectroscopy amplifier (model 2020), analog-to-digital converter (model 8075) and high voltage power supply (model 3105) installed in a Windows operated PC loaded with spectrometry software. The background spectrum was collected by measuring an empty container with the same dimension with those of samples for 10 h. The efficiencies of the spectra peaks for the energies of the radionuclides were determined by measuring a reference material (International Atomic Energy Agency [IAEA] soil-375) obtained from IAEA, Vienna, Austria. The spectrum acquisition for the reference material and those of samples were carried out for 10 h and each and the spectra were stored in the memory of the PC. The activity concentration of each sample was determined by subtracting the background from the spectrum of the sample. The activities of 226 Ra and 232 Th were obtained from the peaks of their respective daughter products, 1.760 Mev peak from 214 Bi and 2.615 Mev from 208 Tl were use for 226 Ra and 232 Th, respectively while that of the 40 K was based on its single peak of 1.465 MeV. The activity concentrations (A s ) of 226 Ra, 232 Th and 40 K in each sample in Bqkg -1 (dry mass) were calculated using the following equation:



where, A s is the specific activity concentration of radionuclide in the analysed sample, A st is the specific activity concentration of radionuclide in the reference sample, N s is the net count rate due to the analysed sample, N std is the net count rate due to the reference sample, M s is the mass of the soil samples in g and M std is the mass of the reference soil samples in g.


  Results and discussion Top


The results of the activity concentration measurements for 40 K, 226 Ra and 232 Th in tailing enriched soil and sediment samples collected from the two mining sites are presented in [Table 1]. The range and mean activity concentrations ± standard deviation (SD) are presented. The SDs depict the spatial distributions of the activity concentrations of the natural radionuclides within the locations. The activity concentration of 40 K in soil ranged from 249.66 to 1459.25 Bq/kg in all the measured samples with the highest mean concentration of 901.67 ± 305.37 Bq/kg obtained in Ijero mining site. For 226 Ra, the concentrations ranged from 7.62 to 79.73 Bq/kg with the highest mean concentration of 37.51 ± 20.30 Bq/kg obtained in Ilesa mining site. 232 Th activity concentrations ranged from 12.68 to 234.18 Bq/kg with a highest mean concentration of 160.40 ± 48.67 Bq/kg obtained in Ijero mining site. In sediment samples, 40 K ranged from 241.86 to 1427.28 Bq/kg while 226 Ra and 232 Th ranged from 9.85 to 74.80 and 15.47 to 145.46 Bq/kg, respectively. The maximum measured concentration of 232 Th in soil samples from Ijero mining site is about 6 times higher than the minimum concentration of 232 Th measured in the same location. This was the case with the concentrations of other radionuclides in soil and sediment measured in the two sites. Exception to these were observed in the range of 40 K concentrations in soil samples from Ilesa, as well as 40 K and 232 Th concentrations in Ijero sediment, where the range exhibits a narrower region. The activity concentrations of 40 K and 232 Th are higher in soil and sediment of Ijero mining site than those of Ilesa mining site. The distributions of 40 K, 226 Ra and 232 Th in tailing enriched soil and sediment samples collected from the two mining sites are presented in [Figure 2]. The mean concentrations of 40 K and 232 Th obtained in this study for both soil and sediment samples are higher than the latest worldwide average concentrations of 412 Bq/kg for 40 K and 45 Bq/kg for 232 Th reported in United Nation Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) (2008), [3] except for soil of Ilesa mining site, which is slightly lower. The mean activity concentration of 226 Ra obtained in Ilesa is higher than the worldwide average of 32 Bq/kg, while those obtained in Ijero mining site are lower for both soil and sediment samples.
Figure 2: Mean Activity concentrations of 40K, 226Ra and 232Th in tailings enriched soil and sediment samples of Ijero and Ilesa mining sites

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Table 1: Mean values and ranges with the activity concentrations ratios of natural radionuclides measured in soil and sediment samples of Ijero and Ilesa mining sites


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The ratios of the activity concentrations of different radionuclides were calculated for all the soil and sediment samples. The range and mean values obtained are presented in [Table 1]. The 226 Ra/ 40 K and 232 Th/ 40 K ratios are all <1, indicating that the activity concentrations of 40 K is higher than those of 226 Ra and 232 Th and in some cases with several order of magnitudes higher. 232 Th/ 238 U ratio is higher than 1 in all the soil and sediment samples from Ijero mining site. For the soil samples, the mean 232 Th concentrations is about 6 times higher than the concentrations of 226 Ra, this is far greater than the 1.2 factor reported in Eisenbud and Gesell. [2] This result indicates that 232 Th is slightly enhanced in the soil of Ijero mining site when compared with 226 Ra.

The distribution of 40 K, 226 Ra and 232 Th in soil and sediment is not uniform, but uniformity can be achieved in terms of exposure to radiation by a single quantity which takes into account the radiation hazards associated with each radionuclide. This quantity is termed radium equivalent activity (Ra eq ). It is a widely used radiation hazard index that compares the specific activity of materials containing varying concentrations of 40 K, 226 Ra and 232 Th. [17],[18],[19] It is given mathematical as:



Where, A Ra , A Th and A K are the mean activity concentrations of 226 Ra, 232 Th and 40 K in Bq/kg, respectively. In this relation, it is assumed that 1 Bq/kg of 226 Ra, 0.7 Bq/kg of 232 Th or 13 Bq/kg of 40 K yields the same gamma dose. [20],[21] The calculated radium equivalent activity for all the samples ranged from 68.64 to 397.12 Bq/kg with the highest mean equivalent activity of 240.28 Bq/kg obtained from Ijero soil samples [Table 2]. The highest calculated radium equivalent activity obtained in this study is higher than the upper limit of 370 Bq/kg but the mean values obtained for both soil and sediment samples from the two mining sites are lower than this maximum permissible level for soil and sediment. [22],[23] This indicates that, if the soil and sediment of the mining sites are used as components of building materials, they will pose no significant radiation health hazard to the occupants of such building.

Another radiation hazard index that reflects the external exposure to radiation dose from 226 Ra, 232 Th and 40 K is called external hazard index Hex. This index is given as:
Table 2: Radiological indices of soil and sediment of Ijero and Ilesa mining sites


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where A Ra , A Th and A K are the mean activity concentrations of 226 Ra, 232 Th and 40 K in Bq/kg, respectively. The value for this index which is lower than unity, indicates insignificant radiation hazard. [24] The values obtained in this study ranged from 0.19 to 1.07. The calculated mean values for soil and sediment are all lower than unity. It can then be inferred that the radiation hazards in the mining sites are low.

The external gamma absorbed dose rate in air at 1 m above the ground surface was calculated from the measured activity concentrations of 40 K, 226 Ra and 232 Th in soil and sediment since other radionuclides, such as 137 Cs, 90 Sr and the 235 U series can be neglected as they contribute very little to the total dose from environmental background. [24],[25] In order to calculate the absorbed dose rate (Ḋ) in the samples, the following equation was used: [21],[26]



where A x (Bq/kg) is the mean activity concentration of 226 Ra, 232 Th and 40 K, while C x (nGy/h/Bq/kg) is the corresponding dose conversion factor. The dose conversion factors used for the calculations were 0.462 for 226 Ra, 0.604 for 232 Th and 0.0417 for 40 K. [3] The absorbed dose rate in air obtained for all the analyzed samples ranged from 33.39 to 173.70 nGy/h. The mean absorbed dose rates for soil were 110.77 and 61.18 nGy/h for Ijero and Ilesa mining sites, respectively, while those obtained for sediment were 108.55 and 79.39 nGy/h, respectively for Ijero and Ilesa mines. The mean absorbed dose rates obtained for soil and sediment of the two mining sites are all higher than the world crustal average value. The value of the average absorbed dose rate in soil samples from Ijero mining site is 1.91 times higher than the 58 nGy/h given as the average value in the latest report of UNSCEAR. [3] From the results of this study, it is observed that the soil of Ijero mining site contained a slightly enhanced level of radioactivity. This may be as a result of tin deposit in the area. In earlier studies, enhanced levels of radioactivity have been reported in tin mining sites of Jos-Plateau, Nigeria. [27],[28],[29],[30]

The annual effective dose to the public due to the absorbed dose rate in air was calculated, taking into account the conversion coefficient from absorbed dose rate in air to effective dose (0.7) and the outdoor occupancy factor (0.2). The calculations were performed according to the following equation: [31],[32],[33],[34]



where Ḋ is the dose rate given in equation 3 and 8760 is the total number of hours/year. The annual effective dose obtained for all the samples ranged from 0.041 to 0.213 mSv. The mean values obtained are 0.136 ± 0.040 and 0.075 ± 0.015 mSv for soil of Ijero and Ilesa mining sites, respectively, while those obtained for sediment are 0.133 ± 0.021 and 0.097 ± 0.032 mSv, respectively for Ijero and Ilesa mines. All the mean annual effective doses evaluated for tailing enriched soil and sediment are higher than the 0.07 mSv average annual effective dose received worldwide outdoors by the member of public as reported in UNSCEAR, 2000 [31] but lower than the 0.26 ± 0.11 mSv obtained for surface mines in Ghana as reported in Darko et al. [9]


  Conclusion Top


The activity concentrations of natural radionuclides 40 K, 226 Ra and 232 Th of terrestrial origin have been measured in the tailing enriched soil and sediment samples collected around two mining sites in Southwest Nigeria. The results show that the mean activity concentrations of the radionuclides in soil and sediment in the study area are above the worldwide average crustal values. Radiological health hazard indices were inferred from the measured activity concentrations. The values of radium equivalent activity, external hazard index, absorbed gamma dose rate and annual effective dose are all higher than their world average values but are still within their acceptable limits. Ijero mining site seems to have slightly above normal background radioactivity level due to enhanced level of 232 Th measured in soil and sediment samples.

 
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    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1], [Table 2]


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