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 Table of Contents 
ORIGINAL ARTICLE
Year : 2019  |  Volume : 42  |  Issue : 1  |  Page : 40-46  

Assessment of radiological safety of abandoned tantalite mining sites in Oke-Ogun, Oyo State, Nigeria


1 Department of Physics, Federal University of Agriculture, Abeokuta, Nigeria
2 Department of Environmental Management and Toxicology, Federal University of Agriculture, Abeokuta, Nigeria
3 Department of Applied Sciences, College of Technological Studies, Public Authority for Applied Education and Training, Shuwaikh, Kuwait

Date of Submission24-Jul-2018
Date of Decision05-Aug-2018
Date of Acceptance14-Mar-2019
Date of Web Publication3-Jun-2019

Correspondence Address:
A E Ajetunmobi
Federal University of Agriculture, Abeokuta
Nigeria
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_56_18

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  Abstract 


Introduction: Tantalite and other solid mineral deposits in Oke-Ogun occupy a very large landscape, and the possibilities of further exploration in new landscape and abandoning unproductive sites cannot be ruled out. These landscapes when abandoned become hideout for criminals and ritualistic activities which pose a great threat to the wellness of the people in the area and Nigerians at large. Objectives: This study attempts to investigate the radiological safety of these abandoned mines so as to convert them to economical valuable sites that can serve as a means of employment for population of the unemployed youth in the area and Nigeria at large. Materials and Methods: A total of seventeen soil samples were randomly selected. These samples were processed and analyzed for activity concentrations of 226Ra, 232Th, and 40K using Hyper Pure Germanium Gamma Spectrometer. RESidual RADioactivity software version (6.5), a computer program developed by the Environmental Assessment Division of Argonne National Laboratory, was used for dose prediction for 70 years using the activity concentrations of soil samples from all the selected sites as its input parameter for the software. Predicted doses were estimated with respect to the three exposure pathways, namely external gamma irradiation from radionuclides in the soil samples, inhalation of dust particles, and inadvertent ingestion of soil. Results: The measured activity concentrations of radionuclides: 226Ra for all the sites is in the range of (17–177) Bq/kg, 232Th in the range of (2–107) Bq/kg, and 40K in the range of (650–1667) Bq/kg. The predicted dose for all the sites is within the range of 0.05–0.35 mSv/y with Eluku mining site having values greater than the permissible limit of 0.25 mSv/y. The health implication of these values is that three of the sites (Gbedu, Sepenteri, and Komu) are safe for other usages. Conclusion: Any of these three mining sites may be used for re-creational purposes, fishing and building of estates to mention but a few.

Keywords: Abandoned tantaliet mines, assessment, radiological safety


How to cite this article:
Ajetunmobi A E, Mustapha A O, Okeyode I C, Gbadebo A M, Al-Azmi D. Assessment of radiological safety of abandoned tantalite mining sites in Oke-Ogun, Oyo State, Nigeria. Radiat Prot Environ 2019;42:40-6

How to cite this URL:
Ajetunmobi A E, Mustapha A O, Okeyode I C, Gbadebo A M, Al-Azmi D. Assessment of radiological safety of abandoned tantalite mining sites in Oke-Ogun, Oyo State, Nigeria. Radiat Prot Environ [serial online] 2019 [cited 2019 Aug 23];42:40-6. Available from: http://www.rpe.org.in/text.asp?2019/42/1/40/259670




  Background Top


Environmental contamination and degradation at mining sites always result in waste rocks and waste such as tailing piles and can accumulate over the years of mining activities. Mining influenced water, including contaminated surface water, groundwater, and seepage from former mine openings, waste in the form of slurry that find their way into abandoned tantalite mines, waste sludge that was discharged into unlined lagoons, and aerial deposition of heavy metals and other contaminants from ore processing activities are of risks for the environment and the villagers occupying this land. Over the years, the possibility of sites not being productive cannot be overruled; hence, there may be a need for miners to relocate to other greener pasture leaving the sites with all that have been excavated over the years. Abandoned mining sites may be of profitable economic values in the nearest future by converting them to farm settlement, fish ponds and industries. Appreciable work has been done on abandoned mining sites in developed counties with little done in developing countries like Nigeria. Ndalulilwa et al.[1] in their study identified 157 abandoned mines with only a handful monitored to determine the contamination and safety level of the sites. Francie[2] reported 520 open abandoned mines contrary to the public report of 1300 abandoned mining sites. In the study, it was reported that private companies mined over 400,000 tones of uranium ore during cold war in Najavo nation, and after the war, the companies are said to abandon the mines as they were. It was also reported that environmental protection agency (EPA) contractors reported radiation level at the mines to be higher than EPA Geiger counter could read. The mines exposed Navajo nation residing close to the mines to airborne dust, contaminated drinking water and many residents' homes were built using mud and rocks near the mines that are reported to be radioactive. John[3] in his work to educate the land manager agencies saddled with the responsibility of handling radioactive sites in their lands reported that national park service identified 44 abandoned uranium sites. His work was largely on educating the land manager agencies on the fundamental concept of radioactivity, environmental characterization methods, data interpretation, risk characterization and assessment and appropriate risk communication. Premium Times[4] reported 1200 abandoned mining sites in Nigeria and these sites if radiologically safe, may be used for building companies that will employ the youth in the nation; abandoned sites may be used for farming, housing, and fish pods. The question that begs for an answer is how safe (radiologically) are these abandoned sites for the public as their virgin characteristics have been modified due to human activities. This suggests the impetus for the study that is aimed at estimating the dose level for 70 years. Vast land of abandoned sites that may be reused for other beneficial purposes that can be of economic values abounds in the selected mining locations. The result of this study will serve as a baseline for future study in the selected area and to the best of the knowledge of the researcher; there has not been any study in the area on dose prediction for safety and economic values for the selected mining sites. The study is aimed at prediction of dose levels at the sites for a period of 70 years using RESidual RADioactivity (RESRAD) software.


  Materials and Methods Top


Description of the study location

The study locations are Komu (KO), Sepenteri (SP), Gbedu (GB), and Eluku (EL) villages in Itesiwaju, Saki East, Iwajowa, and Saki Local Government areas, respectively, all in Oke-Ogun, Oyo State, Nigeria. Oke-Ogun (Latitude 8° 00'00''N-8° 39'00'' N and Longitude 2° 56'00''E-3° 46'00''E) is a populated place in Oyo State with a population of 1.4 million according to 2006 census. It is located at an elevation of 188 m above sea level (Oke-Ogun map).[5] The residents in the selected mining areas are largely artisan miners and farmers. The location map and the drainage maps are shown in [Figure 1] and [Figure 2], respectively. The drainage map shows that the water channels are enormous, thereby making the radionuclides from the effluents from the sites to easily access the food chain. [Figure 3] shows the Geological map for the study area.
Figure 1: The location map for Oke-Ogun, Oyo State

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Figure 2: The drainage map of the study area

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Figure 3: Geological map of Oke-Ogun, Oyo State

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Sample collection and preparation

Seventeen (17) soil samples were collected from the four selected mining sites which are in Oke-Ogun, Oyo State. [Figure 1] shows the various locations where the samples were collected. On site, the samples were collected in big paper envelopes and labeled with identification names indicating the mining locations. The samples were air-dried for 5 days, pulverized, and homogenized. The pulverized samples were later transferred into cylindrical plastic containers (500 ml), sealed, and kept for about a month to achieve secular radioactive equilibrium between226Ra and progenies.

Specific activity determination

The soil were analyzed using a high purity germanium detector (HPGe) at the National Institute of Radiation Protection and Research of University of Ibadan.

Instrumentation for high pure germanium spectrometer

Hyper purity germanium (HPGe) detector was used for counting and detection of radionuclide contents of all the samples. The detector has a depleted, sensitive thickness of several centimeters and therefore can be used as a total absorption detector for gamma rays up to few MeV. When the detector is cooled to liquid nitrogen temperatures (77 Kelvin), it produces spectroscopic data and pulses proportional to the captured photon energies. The HPGe detector used is a P-type and has a diameter of 78 mm, length of 69.8 mm, 16 K channels with relative efficiency of 80%, and a resolution of 2.3 keV at the 60Co line of 1.33 MeV. The spectrometer was equipped with the necessary electronics. Counting time was 10,800 s for the acquisition of the spectrum from samples. Multigamma ray standard source (mgsAm241) was used for the equipment calibration. Standard point sources' gamma emitters used for the energy calibration to determine the radionuclide present in the samples within a wide range of photopeaks and efficiency calibration was done using volume source of the same geometry as the samples in determining the activity concentration of the radionuclide present in the samples.

Energies – 295.2 keV and 351.9 keV of 214Pb and 609.3 keV, 1120.3 keV, and 1238.1 keV of 214 Bi keV gamma ray lines were used to determine the 226Ra activity concentration. 232Th activity concentration was determined using 300.1 keV of 212Pb, 277.4 keV and 860.6 keV of 208 Tl, 209.3 keV and 911.1 keV of 228 Ac, and 723.3 and 785.3 keV of 212 Bi gamma lines.226Ra has energy of 186.2 keV. The activity concentrations of 40 K were determined directly from 1460.8 keV gamma lines. The experimental setup is shown in [Figure 4].
Figure 4: Experimental setup for hyper pure germanium spectrometer. (a) Lead shield. (b) Liquid nitrogen. (c) Power supply, amplifier, and multichannel analyzer. (d) Monitor showing acquired spectrum

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RESidual RADioactivity Software – a tool for dose prediction at mining sites

RESRAD software version (6.5) – a computer program developed by the Environmental Assessment Division of Argonne National Laboratory, first released in 1989 – was used for dose prediction for 70 years using the activity concentrations of soil samples from all the selected sites as stated by Bruce.[6] For the work scenarios in the study, potential radiation exposure pathways include direct exposure to external radiation from the soil, internal radiation from inhalation of dust, and internal radiation from inadvertent ingestion of contaminated soil. RESRAD performs calculations for external irradiation, ingestion, and inhalation of radionuclides using dose factors published by EPA[7] in Federal Guidance report No. 12 from 1993 and dose conversion factors by International Commission of Radiological Protection, 72[8] as summarized in its publication 72 entitled Age-dependent doses to the members of the public from intakes radionuclides: Part 5 compilations of ingestion and inhalation coefficients respectively [Table 1].
Table 1: International Commission of Radiological Protection 72[8] dose coefficient for the public

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Predicted doses are estimated with respect to the three exposure pathways, and the dose summation of all the pathways was also estimated by the software.

Validity of RESidual RADioactivity Software

The validity of the RESRAD software was checked using equations (1) and (2) below with occupancy factor of 0.3 which describes the period of stay of the artisan miners in mining sites. The results of the two approaches for estimating the radiological parameters is given in [Table 2] (Validity of software). The purpose for using equation (1) is only to show the validity of the software (RESRAD).
Table 2: Comparison of values of radiological parameters estimated using RESidual RADioactivity software and conventional equations

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Annual effective dose

The annual effective dose He can be calculated from equation (1), where He is the annual effective dose rate in mSvy−1, D is the value of absorbed dose rate calculated, T is the occupancy time (T = f × 24 × 365.25 h year−1), f is the occupancy factor with value of 0.3 because the miners spend 8 h out of 24 h at the mining site for outdoor measurement, and Fo is the conversion factor (0.7 SvGy−1) UNSCEAR.[9]

He= DTFo (1)


  Results and Discussion Top


The section presents the results of the study.

Result of soil analysis using RESidual RADioactivity Software for dose prediction mining sites for safety and economic value of the sites

The section presents the predicted dose for 70 years. The dose prediction for the future is to ascertain the safety of the sites for any other future uses if the sites were to be used for other purposes.

For GB site, the predicted dose for all the pathways is 0.08 mSv/y and is lower than permissible limit of 0.25 mSv/y set by National Research Council.[10] [Table 3] shows the average activity concentrations of the radionuclides in the soil samples from the selected sites. [Table 4], [Table 5], [Table 6], [Table 7] shows the predicted doses for the sites. The highest dose is the pathway of exposure to external gamma radiation from radionuclide in the soil (0.06 mSv/y) and lowest exposure pathway of inhalation of dust (0.0 mSv/y). The radioactivity of 40 K is lower than that of 226Ra and 232Th; hence, the contribution of 40 K to the different exposure pathways is lower despite its high activity concentration (676.31 Bq/kg).
Table 3: Summary of average activity concentration (Bq/kg) of radionuclide in soil at the selected mining sites

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Table 4: Dose prediction for 70 years in Gbedu mining site

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Table 5: Dose prediction for 70 years in Eluku mining site

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Table 6: Dose prediction for 70 years in Komu mining site

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Table 7: Dose prediction for 70 years in Sepenteri mining site

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The sum of the activity concentrations of all the radionuclides is 803.7 Bq/kg and this resulted in the predicted of 0.08 mSv/y.

For EL mining site, the predicted dose for all the pathways is 0.30 mSv/y and is higher than 0.25 mSv/y. The highest dose is the pathway of exposure to external gamma radiation from radionuclide in the soil (0.30 mSv/y) and the lowest exposures are from inhalation of dust particles and inadvertent ingestion of soil with value of 0.0mSv/y.

For KO mining site, the predicted doses for all the pathways are 0.07 mSv/y and are higher than the permissible limit of 0.25 mSv/y. The highest dose is the pathway of exposure to external gamma radiation from radionuclide in the soil (0.07 mSv/y) and lowest is the exposure pathway of inhalation of dust (0.0 mSv/y).

Finally, for SP site, the sum of predicted dose for all the pathways is 0.15 mSv/y and again the highest for external gamma radiation (0.15 mSv/y). In general, this may be due to the large quantity of naturally occurring radionuclides in soil that the miners interact with during the work activities at the site. In summary, for the next 70 years, the mining site may not be of negative health effect to the miners as a result of radioactivity from the soil since the predicted dose is less than the permissible limit of 0.25 mSv/y.


  Conclusions Top


In general, external radiation of the miners to radionuclides in soil samples from all the mining sites seems to contribute the highest dose while inhalation of dust has the lowest values. The activity concentrations of Ra-226, Th-234, and K-40 is highest in EL, GB, and KO sites with values – 176.90 Bq/kg, 106.49 Bq/kg, and 1666.16 Bq/kg, respectively. Sites of GB, KO, and SP are safe for other uses if mining activities stop in the far future (70 years). However, site EL seems to be safe radiologically for other uses in the future since the predicted dose with a value of 0.30 mSv/y is marginally greater than the permissible limit of 0.25 mSv/y and is also within the errors of measurements.

Financial support and sponsorship

Appreciation goes to TETFUND Nigeria for the sponsorship during the research work.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Ndalulilwa K, Haidula A, Leonard R. Risk Assessment of Abandoned Mine Sites in Namibia. IGCP/SIDA Project 594, Inaugural Workshop, ITWE. Zambia: Czech Geological Survey; 2011.  Back to cited text no. 1
    
2.
Francie D. Abandoned Uranium Mines: An “Overwhelming Problem” in the Navajo Nation. Available from: https://www.scientificamerican.com/article/abandoned-uranium-mines-a/. [Last accessed on 2019 Mar 10].  Back to cited text no. 2
    
3.
John EB. Effective Management of Radiological Hazards at Abandoned Radioactive Mine and Mill Sites. Available from: https://www.nps.gov/subjects/abandonedminerallands/upload/radpaper.pdf. [Last accessed on 2019 Mar 10].  Back to cited text no. 3
    
4.
Premium Times. Nigeria Identifies 1,200 Abandoned Mining Sites; 2019. Available from: http://www.abandoned-mining-sites.html. [Last accessed on 2019 Mar 10].  Back to cited text no. 4
    
5.
Oke-Ogun; 2015. Available from: http://www.getamap.net/maps/nigeria/oyo/_okeogun/. [Last accessed on 2015 Apr 26].  Back to cited text no. 5
    
6.
Bruce S. Residual Radioactivity in Your Neighborhood: A Community Guide to Estimating Radiation Doses Resulting from Radioactive Contamination. Institute for Energy and Environmental Research; 2009.  Back to cited text no. 6
    
7.
Enviromental Protection Agency. Federal Guidance Report No. 12: External Exposure to Radionuclides in Air, Water, and Soil. 1993.  Back to cited text no. 7
    
8.
Age-dependent doses to members of the public from intake of radionuclides: Part 5. Compilation of ingestion and inhalation dose coefficients. Ann ICRP 1996;26:1-91.  Back to cited text no. 8
    
9.
UNESCEAR. Report to the General Assembly, with Scientific Annexes. New York: United Nations Scientific Committee on the Effects of Atomic Radiation; 1993.  Back to cited text no. 9
    
10.
NRC, NUREG-1727. NMSS Decommissioning Standard Review Plan; September, 2000. Available from: http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1727/. [Last accessed on 2019 Mar 10].  Back to cited text no. 10
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7]



 

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