|Year : 2016 | Volume
| Issue : 2 | Page : 83-90
Study of the radiological parameters associated with small-scale mining activities at Dunkwa-on-Offin in the central region of Ghana
Marfo Emmanuel1, O Darko Emmanuel2, Faanu Augustine2, Mayin Sey1
1 Department for Nuclear Safety and Security, Graduate School of Nuclear and Allied Sciences, P. O. Box AE1, Atomic, Accra, Ghana
2 Radiation Protection Institute, Atomic Energy Commission, P. O. Box LG 80, Accra, Ghana
|Date of Web Publication||13-Sep-2016|
Graduate School of Nuclear and Allied Sciences, P.O. Box AE 1, Atomic, Accra
Source of Support: None, Conflict of Interest: None
Studies have been carried out to assess the radiological exposure to the general public from small-scale mining activities in Dunkwa-on-Offin and its surrounding communities. Direct gamma spectrometry was used to determine the concentration of naturally occurring radionuclides 226 Ra (238 U),232 Th, and 40 K in the soil samples. The mean activity concentrations measured for 226 Ra (238 U),232 Th, and 40 K in the soil samples were 25.4 ± 11.1, 29.4 ± 15.6, and 225.9 ± 93.8 Bq/kg, respectively. The total annual effective dose to the public was estimated to be 0.05 mSv. The results, thus, indicate an insignificant exposure of the general public. The radiological parameters assessment as a result of 226 Ra (238 U),232 Th, and 40 K was also carried out. Data established from the study could aid in formulating guidelines, to educate and create awareness on the levels of naturally occurring radionuclides resulting from small-scale mining activities in Dunkwa-on-Offin and Ghana as a whole.
Keywords: Gamma spectrometry, mining, radium equivalent activity
|How to cite this article:|
Emmanuel M, Emmanuel O D, Augustine F, Sey M. Study of the radiological parameters associated with small-scale mining activities at Dunkwa-on-Offin in the central region of Ghana. Radiat Prot Environ 2016;39:83-90
|How to cite this URL:|
Emmanuel M, Emmanuel O D, Augustine F, Sey M. Study of the radiological parameters associated with small-scale mining activities at Dunkwa-on-Offin in the central region of Ghana. Radiat Prot Environ [serial online] 2016 [cited 2022 Jan 19];39:83-90. Available from: https://www.rpe.org.in/text.asp?2016/39/2/83/190390
| Introduction|| |
One of the potential sources of exposure to naturally occurring radioactive materials (NORM) has been found to be mining. However, mining companies are not being regulated and monitored for NORM in most countries including Ghana since there are no guidelines for their regulation and monitoring by the National Regulatory Authority. A recent study on one mining company in Ghana reported mean annual effective dose of about 0.69 mSv. However, at radiation doses below 0.1 Sv in a year, the rise in the incidence of stochastic effects is assumed by the International Commission on Radiological Protection to occur with a small likelihood and in proportion to the rise in radiation dose over the background dose and this is known as linear nonthreshold (LNT) model. The commission, therefore, considers this model to be the best practical approach to managing risk from irradiation, although there are differing opinions on this consideration, world over. The recommendation by the commission of the LNT model combined with a dose and dose rate effectiveness factor for extrapolation from higher doses remains a prudent basis for radiological protection at low dose and low dose rates.
With the recent increase in awareness and knowledge of the potential exposure situations of NORM, many developed countries are amending their legislations and putting in place measures to curtail the problems of NORM, and a little is being done in the developing countries. It is also worth noting that most of the NORM industries such as mining and mineral processing, oil and gas exploration, and extraction among others are located in developing countries such as Ghana., Small-scale and artisanal mining has been defined differently around the world. Small-scale gold mining in Ghana is defined as mining by any method not involving substantial expenditure by an individual or group of persons not exceeding nine in number or by a co-operative society made up of ten or more persons.
Mining, no matter the level of operation, has some magnitude of impact on the surroundings. The degree of damage depends largely on the mining and processing methods being used. Although legalized small-scale mining activities have some negative impacts on the environment, in most cases, they can be minimized and regulated through environmental permitting and regular monitoring by field officers.
The general aim of the studies is to assess the radiological exposure to members of the general public living in Dunkwa-on-Offin and its surrounding communities due to NORMs as a result of the rampant small-scale mining activities. This study will add to the data available to aid in formulating guidelines and to educate and create awareness of the enhanced levels of naturally occurring radionuclides resulting from small-scale mining activities.
| Materials and Methods|| |
Description of study area
The study area is Dunkwa-on-Offin and its surroundings. Dunkwa-on-Offin or simply Dunkwa is a town and the capital of the upper Denkyira east municipal district in the Central Region of Ghana. Dunkwa-on-Offin is selected as the study area because of the high incidence of practices of small-scale mining (“galemsay”) activities. The upper Denkyira east municipal district is 1 of the 13 administrative districts of the central region. It lies within latitudes 5° 30' and 6° 02' north of the equator and longitudes 1° W and 2° W of the Greenwich Meridian. Dunkwa-on-Offin has a 2013 settlement population of 33,379 people. [Figure 1] shows the location of Dunkwa-on-Offin in Ghana and the surrounding communities where sampling was carried out. The municipality falls within the semi-equatorial zone with its characteristics. The mean annual temperatures are 29°C on the hottest months and about 24°C in the coolest months. There are two rainfall regimes, but the total annual mean rainfall is between 1200 and 2000 mm. The first rainy season is from May to June with the heaviest in June, while the second rainy season is from September to Mid-November. The main dry season is from late-November to February. The rainfall data obtained from the Ghana Meteorological Agency at Dunkwa-on-Offin during the study period (2013) show total annual rainfall figures of 1129.0 mm with mean rainfall of 94.1 ± 75.1 mm. The highest rainfall 234.9 mm was recorded in October.
Sampling, sample preparation, and analysis of soil
Twenty composite samples were collected randomly within the selected areas of the small-scale site of the study area. The selection of the sampling locations was based on the accessibility to the public and closeness to the small-scale mine. Based on these criteria, twenty soil sampling locations were identified. The sampling location for soil is shown in [Table 1].
Soil samples were collected from the following locations within Dunkwa and the surrounding communities including Dunkwa Township Nyamebekyere, Brentuo, Amoafo, Akropong, Buabinso, Tekyikrom, Abesewa, and Babianiha (control). The soil samples were taken using a coring tool to a depth of 5–10 cm. One kilogram of each sample was collected for analysis. The samples were then transported to the laboratory for preparation and analysis. Geo Explorer II Global Position System with serial number 28801-00 and version 2.11 was used to record the location of the sampling sites.
At the laboratory, the soil samples were spread in trays and allowed to dry at room temperature for 2 weeks and then air dried and then oven dried at a temperature of 105°C for between 3 and 4 h until the samples were well dried with a constant weight. The samples were then ground into fine homogenous powder using a ball mill grinder and sieved through a 2 mm pore size mesh. They were then transferred into a weighed 1000 cm 3 (1 L) Marinelli beakers. The Marinelli beakers with its content were then weighed again to determine the weight of the sample. The beakers were covered and sealed with a paper tape to prevent the escape of the gaseous radionuclides in the sample. The samples were then stored for 30 days to allow for secular equilibrium between the long-lived parent radionuclide and their short-lived daughter radionuclides.
The samples were counted using gamma spectrometer consisting of an ORTEC GEM Coaxial n-type high purity germanium detector with ORTEC multichannel analyzer MAESTRO-32 evaluation software for spectrum acquisition, and processing was used to determine the concentration of naturally occurring radionuclides 226 Ra,232 Th, and 40 K in the samples at the laboratory. The detector was calibrated with respect to energy and efficiency before analysis. A standard of known concentrations of radionuclides homogenously distributed on solid water in a 1 L Marinelli beaker was used for calibration of the gamma spectrometry system. The standard was supplied by Deutscher Kalibrierdienst-3, QSA Global GmBH, Germany. The relative efficiency of the detector was 25% with energy resolution of 1.8 keV at gamma-ray energy of 1332 keV of 60 Co.
Calculation of specific activity concentrations (C
The activity concentrations of 226 Ra (238 U),232 Th, and 40 K in the soil sample were calculated using the following analytical expression shown in Equation 1.
where N is the net counts of the radionuclide in the samples, td is the delay time between sampling and counting, P is the gamma emission probability (gamma yield), η is the absolute counting efficiency of the detector system, Tc is the sample counting time, m is the mass of the sample (kg), is the decay correction factor for delay between time of sampling and counting, and λp is the decay constant of the parent radionuclide.
Estimation of annual effective dose from external gamma dose rate measurements
The outdoor external gamma dose rate was measured using RDS-200 Universal Radiation Survey Meter, which had been calibrated at the Secondary Standard Dosimetry Laboratory at the Radiation Protection Institute of the GAEC. At each location, five measurements were made at 1 m above the ground and the average value taken in nGy/h. The annual effective dose (Eγ,ext) was then estimated from the measured average outdoor external gamma dose rate from the Equation 2.
Eγ,ext (mSV/year) = Dγ,extTexpDCFext(2)
where, Dγ,ext is the average outdoor external gamma dose rate nGy/h, Texp is the exposure duration per year, 8760 h (365 days) and applying an outdoor occupancy factor of 0.2, DCFext is the absorbed dose to effective dose conversion factor of 0.7 Sv/Gy for environmental exposure to gamma rays.
Determination of radiological hazard indices
Radiological equivalent activity index (Raeq)
The hazard due to NORM in soils in the study area which may be used as building materials was assessed to compare the specific activities of building materials containing concentration of radium, thorium, and potassium. A single quantity called radium equivalent activity (Raeq), takes into account the radiation hazard associated with the building materials and is calculated using a method proposed by Beretka and Mathew as shown in Equation 3.
Raeq = CRa + 1.43CTh + 0.77Ck (Bq/kg)(3)
where CRa, CTh, and CKare the specific radioactivity activity concentrations of 226 Ra,232 Th, and 40 K, respectively.
In the definition of Raeq, it is assumed that 370 Bq/kg of 226 Ra, 259 Bq/kg of 232 Th, and 4810 Bq/kg of 40 K produce the same gamma ray dose rate. The above criterion only considers the external hazard due to gamma rays in building materials. The maximum recommended value of Raeq in raw building materials and products must be <370 Bq/kg for safe use. This means that the external gamma dose must be <1.5 mSv/year.
External gamma dose rates
The absorbed dose rates due to gamma radiation in air at 1 m above the ground for the uniform distribution of the naturally occurring radionuclides (226 Ra,232 Th, and 40 K) was calculated based on guidelines provided by UNSCEAR (2000) as in Equation 4.
D = 0.462CRa + 0.604CTh + 0.0417CK (nGy/h)(4)
where CRa, CTh, and CK are the specific radioactivity concentrations of 226 Ra,232 Th, and 40 K, respectively.
Annual effective dose
To estimate the annual effective dose, two main parameters were taken into consideration namely (i) conversion coefficient from the absorbed dose in air to effective dose and (ii) the occupancy factor. Annual estimated averaged effective dose equivalent received by a member was calculated using a conversion factor of 0.7 Sv/Gy, which is used to convert the absorbed dose rate to annual effective dose equivalent with an outdoor occupancy of 20%. The annual effective dose is determined as shown in Equation 5.
Eext (mSv/year) = (Absorbed dose)nGy/h × 0.7 Sv/Gy (Conversion coefficient) × 0.2 occupancy factor × 8760 h(5)
External hazard index (Hex)
The external hazard index (Hex) is a parameter to keep the external gamma radiation dose from building materials to <1.5 mGy/year or unity. The model below was used as dose criterion to calculate external hazard index. This is shown in Equation 6.
Hex = CRa/370 + CTh/259 + Ck/4810 ≤ 1(6)
where, CRa, CTh, and CK are the specific radioactivity activity concentrations of 226 Ra,232 Th, and 40 K, respectively. The value of the external hazard index must be less than unity for the external gamma radiation hazard to be considered negligible. The radiation exposure due to the radioactivity from construction materials is limited to 1.5 mSv/year.
Internal hazard index (Hin)
The calculation of internal hazard index was based on the fact that radon and its daughters are also hazardous to the respiratory organs. This is shown in Equation 7.
Hin = CRa/185 + CTh/259 + CK/4810 ≤ 1(7)
where, CRa, CTh, and CK are the specific radioactivity activity concentrations of 226 Ra,232 Th, and 40 K, respectively. For construction materials to be considered safe for construction of dwellings, the internal hazard index should be less than unity.
| Discussion|| |
[Table 2] shows both calculated absorbed dose rate due to the presence of 226 Ra,232 Th, and 40 K in the soil samples and measured absorbed dose rate in air at 1m above the ground. The measured absorbed dose rate varied in the range of 32.0 (SS1) to 68.0 (SS14) nGy/h with an average value of 50.1 nGy/h and that of calculated absorbed dose rate also varied in the range of 15.2–78.8 nGy/h with an average value of 38.5 nGy/h. The world average value of absorbed dose rate as estimated by the United Nation Scientific Committee on Effects of Atomic Radiations (UNSCEAR) is 60 nGy/h., From the results, it can be deduced that the average absorbed dose rate from both measured and calculated values are almost on par with the values of UNSCAER (2000), when cosmic ray component is added to calculated dose rates. According to UNSCEAR (2000), in normal background areas, the average annual effective dose from terrestrial radionuclides is 0.46 mSv/year. The measured annual effective dose rate varied in the range of 0.04–0.08 mSv/year with an average value of 0.06 ± 0.01 mSv/year and that of calculated annual effective dose rate also varies in the range of 0.02–0.10 mSv/year with an average value of 0.05 ± 0.02 mSv/year. In the determination of the calculated annual effective dose rate value, a dose conversion factor of 0.7 Sv/Gy, and outdoor occupancy factors of 0.2 was applied., The annual effective dose rates obtained are at par with the worldwide average of normal background dose received from terrestrial source.
|Table 2: Average absorbed dose rate and effective dose at 1 m above the ground|
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To determine the effect of one radionuclide on another, the Pearson Correlation Matrix Method in Microsoft Excel 2000 was adopted. This method was used to assess the correlation between 226 Ra,232 Th, and 40 K activities concentration due to soil samples and the results are shown in [Table 3]. The results show a strong positive correlation between Ra, Th, and K activities concentration. The order of increasing positive correlation is as follows K versus Th (0.79) <K versus Ra (0.86) <Th versus Ra (0.90). This implies that thorium and radium seem to coexist well and have a greater influenced on each other compared to the rest.
|Table 3: Correlation analysis (Pearson correlation matrix method) used to assess the correlation between 226Ra (238U), 232Th and 40K respectively due to soil samples|
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The specific activity concentrations of the 226 Ra,232 Th, and 40 K in all the soil samples have been determined in the study area and the results are shown in [Table 4]. The mean specific activity of 226 Ra (238 U) is 25.4 Bq/kg in a range of 11.2–45.3 Bq/kg. For 232 Th, the mean activity concentration is 29.4 Bq/kg in a range of 9.5–67.5 Bq/kg and that of 232 K is 225.9 Bq/kg in range is 96.4–409.0 Bq/kg. The mean activity concentration of 40 K is higher than that of 226 Ra and 232 Th. This can be attributed to fact that potassium is usually the most dominant nuclide in the world. The highest activity concentrations of 226 Ra,232 Th, and 40 K were obtained in samples taken from a mining site at Amoafo a community in the study area. This could also be attributed to the geology of the sampling site. The UNSCEAR average activity concentration due to U, Th, and K in soil samples are 35, 30, and 400 Bq/kg. From the results obtained, the mean activity concentrations of 226 Ra (238 U),232 Th, and 40 K are lower than the world average values.
|Table 4: Average specific activities of 226Ra, 232Th and 40K in the soil samples|
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The percentage contribution of 226 Ra,232 Th and 40 K to the total mean absorbed dose in the soil in the study area in decreasing order are 232 Th (45.6%),226 Ra (28.2%) and 40 K (24.2%). In terms of activity concentration, the percentages with respect to total natural activity are 232 Th (10%),226 Ra (9.0%) and 40 K (81%). The UNSCEAR reported worldwide average values for the percentage contribution to the total natural activity concentration are 232 Th (40%),40 K (35%) and 226 Ra (25%). [Figure 2] shows the pie chart representation of percentage natural activity of individual radionucides of the study area.
|Figure 2: Percentage contribution of mean activity of radionuclides to the total natural activity concentration from the study area|
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[Table 5] shows a comparison of specific activity concentration of 226 Ra,232 Th, and 40 K in the soil samples in the study area to similar published research works in Ghana and other countries.,, The study area results compare well with data from other published data from other countries. The results, therefore, do not show any significant difference from the published works.
|Table 5: Comparison of activity concentration of 226Ra, 232Th and 40K in soils in the study area with published data,,|
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To assess whether the soil in the study area could be used for building purposes without significant exposures to the public in the study area, the following hazard assessments were made; radium equivalent activity index (Raeq) in Bq/kg, external (Hex) and the internal hazard (Hin). The maximum recommended value of Raeq in raw building materials and products must be <370 Bq/kg for safe use. This means that the external gamma dose must be <1.5 mSv/year. The external and internal hazard indices must also be less than unity to keep the radiation hazard insignificant. The results of radium equivalent activity (Raeq) and hazard indices of the soil samples from the study area are presented in [Table 6]. The radium equivalent activity (Raeq), External hazard index (Hex) and Internal hazard index (Hin) varied in the range 32.8–173.4, 0.1–0.4, and 0.1–0.6 Bq/kg with mean values of 87.0 ± 38.9, 0.2 ± 0.1, and 0.3 ± 0.1, respectively. The values of the Raeq, Hex, and Hin are below the acceptable values. Hence, soils from the study area that could be used for building purposes might not pose any significant radiological hazard and thus are considered safe for use as building materials.
|Table 6: Results of radium equivalent activity (Raeq) and hazard indices of the soil samples from the study area|
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| Conclusions|| |
In this study, activity concentration of 226 Ra,232 Th, and 40 K in water and soil samples as well as absorbed dose rate have been determined. In addition, the annual effective dose rate and radiological indices have also been determined. The mean activity concentration of 226 Ra,232 Th, and 40 K in the soil samples are 25.4, 29.4, and 225.9 Bq/kg, respectively. The results from the study area compared well with other similar studies carried out in other countries including Ghana and are below the recommended UNSCEAR guideline value. The results of absorbed dose rate from both external irradiation due to 226 Ra,232 Th, and 40 K and the measured ambient dose rate are on par with the UNSCEAR recommended value. The radiological hazard to the general public in the study area were determined to be 87.0 Bq/kg, 0.3 and 0.2 for mean radium equivalent activity, internal and external hazard, respectively, for soil samples. From the results, it can be concluded that all the values are within the world acceptable limit. From the results of the study, it can be inferred that the worldwide average annual effective dose for normal background are similar to the measured average annual effective dose as well as the calculated in the study area. A major concern raised now is with radiological safety of small-scale miners since the present practice of the miners are unprofessional such as lack of usage of nose mask to prevent against inhalation of radon gas and the principles of radiation protection is not practiced. In conclusion, data obtained from the study will go a long way to serve as a radiological baseline data for Dunkwa-on-Offin and its host communities.
The authors are very grateful to the Radiation Protection Institute of the Ghana Atomic Energy Commission, Department of Nuclear Safety and Security of the Graduate School of Nuclear and Allied Sciences of University of Ghana with their support. The authors would like to thank Mr. Wanab Wilson Zooga (Ghana Mineral Commission, Dunkwa) for his assistance during sample collection at the small-scale mine sites and also to Mr. Manu (Agricultural Institute, Dunkwa-on-Offin). The authors also acknowledge the contribution of the following persons, namely, Oscar Adukpo, Lordford Tettey, Bernice Konadu Agyeman (Technologist), Ali Ibrahim (Technologist), Mubarak Sadauki, and Mary Ann Aikins (National Service person) and Mr. Issac Baidoo and Mr. Ekow Quagurine of the National Nuclear Research Institute.
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[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3, [Table 4], [Table 5], [Table 6]