|Year : 2016 | Volume
| Issue : 3 | Page : 155-164
Impact assessment of naturally occurring radioactive materials on the public from gold mining and processing at Newmont Golden Ridge Limited, Akyem, Eastern Region of Ghana
Augustine Faanu, Oscar Kwaku Adukpo, Charles Kansaana, Lordford Tettey-Larbi, Henry Lawluvi, David Okoh Kpeglo, Emmanuel Ofori Darko, Geoffrey Emi-Reynolds, Razak Abdul Awudu, Peter Atta Amoah, Alex Opoku Efa, Ali Doe Ibrahim, Benice Agyeman, Rita Kpodzro, Lilian Agyeman
Environmental Protection and Waste Management Centre, Radiation Protection Institute, Ghana Atomic Energy Commission, Legon, Accra, Ghana
|Date of Web Publication||30-Nov-2016|
Environmental Protection and Waste Management Centre, Radiation Protection Institute, Ghana Atomic Energy Commission, P. O. Box LG 80, Legon, Accra
Source of Support: None, Conflict of Interest: None
Baseline radioactivity levels of naturally occurring radioactive materials within the operational area and surrounding communities of Akyem Gold Mine of Newmont Golden Ridge Limited of Ghana were determined based on direct gamma-ray spectrometry to quantify the radionuclides of interest, namely,238 U,232 Th, and 40 K in soil samples. The average activity concentrations of 238 U,232 Th, and 40 K in the soil samples were 11.90, 11.39, and 139.71 Bq/kg, respectively. For the water samples, the concentration values of gross-alpha and gross-beta for all the water samples were below the Ghana Standards Board (now Ghana Standards Authority) and World Health Organization recommended guideline values for drinking water quality. The annual average effective dose to the public due to gamma ray exposures from the soil samples was estimated to be 0.03 mSv which is below the UNSCEAR 2000 average reference level of 0.07 mSv for public exposure control. The results obtained in this study also show that radiation levels are within the natural background radiation levels found in literature and compare well with the results of similar studies in Ghana.
Keywords: Annual effective dose, gamma spectrometry, gold mine, gross-alpha/beta, radioactivity
|How to cite this article:|
Faanu A, Adukpo OK, Kansaana C, Tettey-Larbi L, Lawluvi H, Kpeglo DO, Darko EO, Emi-Reynolds G, Awudu RA, Amoah PA, Efa AO, Ibrahim AD, Agyeman B, Kpodzro R, Agyeman L. Impact assessment of naturally occurring radioactive materials on the public from gold mining and processing at Newmont Golden Ridge Limited, Akyem, Eastern Region of Ghana. Radiat Prot Environ 2016;39:155-64
|How to cite this URL:|
Faanu A, Adukpo OK, Kansaana C, Tettey-Larbi L, Lawluvi H, Kpeglo DO, Darko EO, Emi-Reynolds G, Awudu RA, Amoah PA, Efa AO, Ibrahim AD, Agyeman B, Kpodzro R, Agyeman L. Impact assessment of naturally occurring radioactive materials on the public from gold mining and processing at Newmont Golden Ridge Limited, Akyem, Eastern Region of Ghana. Radiat Prot Environ [serial online] 2016 [cited 2020 Oct 21];39:155-64. Available from: https://www.rpe.org.in/text.asp?2016/39/3/155/194962
| Introduction|| |
Mining has been identified as one of the potential sources of exposure to naturally occurring radioactive materials (NORM). NORM constitute the largest source of population dose by external or internal irradiation. Higher concentrations may arise as a result of mining and mineral processing., The specific levels are related to the type of rock from which the soil originates. Higher radioactivity levels are associated with igneous rocks such as granite and lower levels with sedimentary rocks. There are exceptions, however, as some shales and phosphate rocks have relatively high content of radionuclides. Mining companies are not being regulated for NORM in most countries including Ghana. In Ghana, there are over 200 registered mining companies operating small-, medium-, and large-scale mining. Studies to establish radionuclide concentrations resulting from mining and mineral processing industries are ongoing in Ghana. There is a national program to establish baseline radioactivity measurements for mines that are yet to start operations and existing ones for the purpose of gathering reference data. This is a program vigorously being pursued by the Radiation Protection Institute of Ghana. Some studies already done in this field are published as follows.,,,,, Data from one of the studies on two mines in Ghana have reported average values for 238 U,232 Th, and 40 K in the soil for the two mines as follows: 28.7, 25.4, 581.8 and 34.5, 20.7, 682.4 Bq/kg, respectively. The reported average annual effective dose for the two mines was 0.3 ± 0.06 mSv. Similar studies carried out on different types of mines in other countries have been reported. The World Health Organization (WHO) screening levels for drinking water below which no further action is required are 0.5 Bq/l for gross-alpha and 1.0 Bq/l for gross-beta, and therefore, the study also seeks to determine the gross-alpha and gross-beta activity concentrations of natural radionuclides in drinking water of the mine and the surrounding communities to ascertain the radiological risk associated with the use of water for domestic purposes. The primary objective of this study was to measure and assess the baseline radioactivity levels of the Akyem Newmont Mine and the immediate surroundings and communities so that reference data could be established. The study focused on the determination of the activity concentration and distribution of the naturally occurring radionuclides of the U/Th decay series and 40 K in soil by gamma spectrometry and gross-alpha and gross-beta activities in water samples. Airborne-absorbed gamma dose rates were also measured at 1 m above the ground at all sampling points with a gamma radiation survey meter. The availability of data from these studies is very vital to all stakeholders involved in environmental protection measures.
| Description of Study Area|| |
Location of the study area
The Akyem Gold Mine of Newmont Golden Ridge Limited (the “Company”) is a subsidiary of Newmont Mining Corporation (Newmont). The mine is located in the Birim North District of the eastern region of Ghana. The mine is located approximately 3 km west of the district capital New Abirem, 133 km west of Koforidua the regional capital, and 180 km northwest of Accra. The study area extends from Ntronang in the west to the Nkawkaw to New Abirem highway in the east and is bounded by the Ajenjua Bepo and Mamang River Forest Reserves on the north and south, respectively. The study area includes the settlements of New Abirem, Afosu, Ntronang, Mamanso, Old Abirem, Yayaaso, Adausena, Hweakwae, and Obohema and several other hamlets and farmsteads.
Local geology of the area
The Akyem deposit is localized in the hanging wall (upper side or plate) of a regional fault that trends to the northeast (N70°E) parallel to regional structures and dips to the southeast (S60°E) parallel to the foliation developed in a Birimian host rock. The planar fault structure (thrust fault), is intensely sheared and exhibits a mylonitic to cataclastic fragmental texture consisting of lens of metavolcanic fragments in a matrix of sheared and plastically deformed graphitic material. The presence of graphitic rubble zones suggests reactivation of the fault zone over time. The fault occurs in fine-grained, gray-green massive to locally sheared Birimian mafic metavolcanic rocks exhibiting chlorite and carbonate alteration that locally contain euhedral magnetite. Immediately overlying the metavolcanics is a distinctive gray-green-pink unit containing blue quartz phenocrysts in a mylonitic matrix (quartz epiclastic rock). The upper and lower contacts of this unit are typically sheared and brecciated, and the upper contact of this unit is locally in sharp contact with overlying light gray-tan chert. This unit is considered sedimentary and represents a distinctive marker between the metavolcanic and metasedimentary units of the Birimian. The overlying metasedimentary rocks are for the most part turbidite sequences consisting of graywacke, argillites, black carbonaceous siltstone, and fine-grained arkosic sandstones. This unit grades upward into a saprolitically weathered zone (deeply weathered bedrock largely altered to clay) that ranges from 10 to 50 m thick. The saprolite consists of lateritic clay and quartz fragments with as much as 25% weathered rock remaining within the saprolite. Near the surface, in the upper 1–5 m, red lateritic clay is developed as subsoil.
| Materials and Methods|| |
Sampling and sample preparation for gamma spectrometry analysis
A total of 46 soil samples and 53 water samples were collected within selected areas of the mine concession and the surrounding communities. [Figure 1] shows soil sampling and surface water locations in and around the mine concession. It should be noted that not all locations on the map were covered and this was based on assessment of direction of impart to humans. Similarly, [Figure 2] also shows groundwater sampling locations within the mine concession. In the laboratory, each of the soil samples was air dried on trays for 7 days and then oven dried at a temperature of 105°C between 3 and 4 h until all moisture was completely lost. The samples were grinded into fine powder using a ball mill and sieved through a 2 mm pore size mesh into 1 L Marinelli beakers. The Marinelli beakers with the samples were weighed, completely sealed, and stored for 1 month, to allow the short-lived daughters of 238 U and 232 Th decay series to attain equilibrium with their long-lived parent radionuclides., Each sample was counted using a high purity germanium detector (HPGe) based gamma spectrometer.
|Figure 1: Map showing soil sampling and surface water locations in and around the mine concession|
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|Figure 2: Map showing groundwater sampling locations within the mine concession|
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Instrumentation and calibration
Direct gamma spectrometry analysis without pretreatment (nondestructive) was used for the measurement of gamma rays for the soil and water samples using an HPGE. The gamma spectrometry system consists of an n-type HPGE detector coupled to a computer-based multichannel analyzer. The relative efficiency of the detector is 40% with energy resolution of 2.0 keV at gamma ray energy of 1332 keV of 60 Co. The identification of individual radionuclides was performed using their characteristic gamma-ray energies, and the quantitative analysis of radionuclides was performed using the Genie 2000 gamma acquisition and analysis software. The detector is housed in a 100 mm passive shielding of ancient lead lined with copper, cadmium, and plexiglass (3 mm each) to reduce the background radiation. The detector is cooled in liquid nitrogen at a temperature of −196°C (77 k). Energy and efficiency calibrations were carried out by counting standard radionuclides of known activities, with well-defined energies in the energy range of 60 keV–~2000 keV. For the analysis of soil/rock and water samples, the efficiency calibration was carried out using standard radionuclides uniformly distributed in solid water with volume and density of 1000 cm 3 and 0.98 g/cm 3, respectively (source number: 9031-OL-146/14 and manufactured by Czech Metrology Institute). To determine the background distribution in the environment around the detector (quality control), ten empty Marinelli beakers were thoroughly cleaned and counted for 36,000 s in the same geometry as the samples. The background spectra were used to correct the net peak area of gamma rays of measured isotopes. The background spectra were also used to determine the minimum detectable activities of 238 U (0.12 Bq/kg),232 Th (0.11 Bq/kg), and 40 K (0.15 Bq/kg) of the detector.
Calculation of activity concentration and estimation of doses
The activity concentration of 238 U in the samples was determined using the energy peak of 609.31 of 214 Bi. Similarly, the activity concentration of 232 Th was determined using 911.21 keV peak of 228 Ac. The activity concentration of 40 K was determined from the energy of 1460.83 keV. The analytical expression is used in the calculation of the activity concentrations in Bq/kg for soil and Bq/l for water samples as shown in Equation 1.
where ND is the net counts of the radionuclide in the samples, P is the gamma ray emission probability (gamma ray yield), η (E) is the absolute counting efficiency of the detector system, Tc is the sample counting time, and m is the mass of the sample (kg).
The external gamma dose rate (Dγ) at 1.0 m above ground for the soil samples was calculated from the activity concentrations using Equation 2.
where DCFK, DCFU, DCFTh are the absorbed dose rate conversion factors for 40 K,238 U, and 232 Th in nGy/h/Bq/kg and AK, Au, and ATh are the activity concentrations for 40 K,238 U, and 232 Th, respectively.
DCFK = 0.0417 nGy/h/Bq/kg; DCFU = 0.462 nGy/h/Bq/kg; DCFTh = 0.604 nGy/h/Bq/kg.
The average annual effective dose was calculated from the absorbed dose rate by applying the dose conversion factor of 0.7 Sv/Gy and an outdoor occupancy factor of 0.2 represented by Equation 3.
where Eγ is the average annual effective dose and Dγ is the absorbed dose rate in soil.
At each sampling location, five measurements of the ambient gamma dose rates were made at 1 m above the ground using Rados Universal Survey Meter (Model No.: Rds-200) and the average value taken in μGy/h. The annual average external effective dose (Eγ, ext) was then estimated from the measured average outdoor external gamma dose rate as in Equation 3.
Determination of natural radioactivity in water samples using gross-alpha and gross-beta counter
Twenty-five water samples taken from boreholes (underground) and 24 taken from surface water bodies including streams and mine pits were analyzed for gross-alpha and gross-beta radioactivity. Five hundred milliliters of each water sample was acidified with 1 ml of concentrated HNO3 and evaporated to near dryness on a hot plate in a fume hood. The residue in the beaker was rinsed with 1M HNO3 and evaporated again to near dryness. The residue was dissolved in minimum amount of 1M HNO3 and transferred into a weighed 25 mm stainless steel planchet. The planchet with its content was heated until all moisture had evaporated. It was then stored in a desiccator and allowed to cool and to prevent the sample from absorbing moisture. The prepared samples were then counted for 200 min to determine alpha and beta activity concentrations using the low background Gasless Automatic Alpha/Beta Counting System (Canberra iMatic™) calibrated with alpha source (241 Am) and beta source (90 Sr) standards. The system uses a solid state passivated implanted planar silicon detector for alpha and beta detection. The alpha and beta efficiencies were determined to be 36.4 ± 2.1% and 36.6 ± 2.2%, respectively. The background readings of the detector for alpha and beta activity concentrations were 0.04 ± 0.01 and 0.22 ± 0.03 cpm with minimum detection limit of 0.01 cpm.
| Results and Discussion|| |
[Table 1] shows the physical parameters of the water samples taken from surface water bodies including streams and mine pits and boreholes (underground). [Table 2] shows the absorbed dose rate measured in air at 1 m above the ground using Rados Universal Survey Meter (Model No.: Rds-200) at the soil and water sampling points in the mine site and in the surrounding communities. It also shows the range and average values of the absorbed dose as well as the calculated annual effective doses. As can be observed, measured absorbed dose rates varied in a range of 0.03–0.11 μGy/h (30–110 nGy/h), with an average value of 0.10 ± 0.02 μGy/h (100.0 ± 20.0 nGy/h). The corresponding average annual effective dose was calculated to be 0.09 ± 0.05 mSv in a range of 0.01–0.20 mSv. By comparison, the results of the absorbed dose rates in this study compare well with the range of dose rates values reported for other countries  as well as results from similar studies carried out in other mines in Ghana,,, although the average absorbed dose rate measured in air from the study area is above the worldwide average. According to the UNSCEAR 2000 report, the worldwide average absorbed dose rate measured in air outdoor from terrestrial gamma radiation is 59 nGy/h (≈0.06 μGy/h). The possible reasons for the higher values of the doses for external gamma could be due to difference in geological formations as well as contribution from cosmogenic radionuclides in addition to terrestrial radionuclides.
|Table 1: Physical parameters of the water samples from surface and underground sources|
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|Table 2: Average absorbed dose rate in air at 1 m above the ground at the sampling points in the study areas and calculated annual effective dose|
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[Table 3] shows the activity concentrations of 238 U,232 Th, and 40 K in the soil samples as well as the calculated absorbed dose rates and the estimated annual effective doses. The average value of the activity concentrations of 238 U is 11.90 ± 0.21 Bq/kg in a range of 5.34–20.55 Bq/kg. For 232 Th, the average activity concentration is 11.39 ± 2.28 Bq/kg in a range of 2.51–24.09 Bq/kg and that of 40 K is 139.71 ± 275.11 Bq/kg in a range of 24.27–425.67 Bq/kg. The reported worldwide average activity concentration of 238 U,232 Th, and 40 K in soil samples from similar studies carried out around the world is 35, 30, and 400 Bq/kg, respectively. By comparison, it shows that the average values of the activity concentrations of 238 U,232 Th, and 40 K in this study are about three times lower than the world average values in normal continental soils. Since the average values in this study are less than the worldwide average values, hence the activity concentrations are even far below the exemption values of 1000 Bq/kg for 238 U and 232 Th and 100,000 Bq/g for 40 K in materials that will warrant regulatory control. The average gamma dose rate and annual effective dose from terrestrial gamma rays calculated from soil activity concentrations are also shown in [Table 3]. The average absorbed dose rate was calculated to be 18.08 ± 2.50 nGy/h in a range of 5.88–30.30 nGy/h, which is by a factor of six times lower than the dose rate measured in air at 1 m above the ground. The absorbed dose rate due to the soil concentrations is also about three times lower than the worldwide average value of 60 nGy/h., This difference could be attributed to site-specific geology and geochemical state of the sampling sites. Furthermore, there is the possibility of the survey meter picking up a wider range of gamma emitting radionuclides including cosmic ray component in the soil sampling area as against the specific gamma radionuclides of 238 U,232 Th, and 40 K determined in the soil. The corresponding average annual effective dose estimated from the soil concentrations is 0.03 ± 0.01 mSv in the range of 0.01–0.09 mSv.
|Table 3: Average activity concentrations, absorbed dose rate, and annual effective doses of 238U, 232Th, and 40K in soil samples in the study area|
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The activity concentrations of gross-alpha and gross-beta in the water samples are shown in [Table 4]. Radionuclide concentrations in groundwater depend on the dissolution of minerals from rock aquifers. The activity concentrations of gross-alpha in the water samples varied in a range of 1.25 mBq/l in surface water to 4.95 mBq/l in tailing water from underground water. For the gross-beta, the activity concentrations varied in a range of 1.20 mBq/l in surface water to 42.54 mBq/l for tailing water taken from underground. Comparing the results from the water with the WHO guideline values, it can be observed that all the values of the gross-alpha and gross-beta are lower than the guideline values. The WHO screening levels for drinking water below which no further action is required are 500 mBq/l for gross-alpha and 1000 mBq/l for gross-beta. The guideline values is based on “Individual Dose Criterion” which ensures an exposure lower than 0.1 mSv/year assuming a water consumption rate of 2 L/day. This indicates that the water sources in the study area which are designated for drinking and domestic purposes do not have significant natural radioactivity. However, it is important to note that some of the surface water bodies which are located at restricted access areas of the mines are not accessible for use by the public for domestic purposes. Further, even though the mine has provided enough boreholes to serve as drinking water sources for the communities, some members of the communities continue to resort to the use of the surface water bodies in their vicinity for domestic purposes. The main source of water supply of the mines is underground for both domestic uses and in the processing plant.
|Table 4: Gross-alpha and gross-beta activity concentrations (mBq/l) in water samples from the study area|
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| Conclusions and Recommendations|| |
The study to assess the radiological impact of the mining and mineral processing of Newmont Golden Ridge Limited, Akyem, on the public and the environment was carried out. Two exposure pathways, namely, direct external gamma ray exposure from natural radioactivity concentrations in soil from 238 U,232 Th, and 40 K and internal exposure from natural radioactivity in water samples from the pits, surface water, and underground water (boreholes), were considered. The average activity concentrations of 238 U,232 Th, and 40 K in the soil samples were calculated to be 11.90, 11.39, and 139.71 Bq/kg, respectively. The average annual effective dose estimated from direct external gamma ray exposure from natural radioactivity concentrations in soil was 0.066. The results of the calculated absorbed dose rate in the soil samples varied in a range of 5.88–30.30 nGy/h, with an average value of 18.08 nGy/h. The average absorbed dose rate in this study is about three times lower than the worldwide average value of 60 nGy/h estimated from soil concentrations. The annual effective dose is lower than the 0.07 mSv per year of world average by the UNSCEAR 2000 for public radiation exposure control. However, in view of the fact that some of the samples recorded higher values, it is recommended that the mining company establishes a periodic (every 3 years) monitoring program for environmental radioactivity as the operations may alter the geochemical and radiological state of the mine. The water samples had gross-alpha and gross-beta values below the recommended levels of the WHO and Ghana Standards Board. In general, the results of this study are comparable to similar studies carried out in other mines in Ghana , and other countries. It also indicates insignificant levels of the natural radionuclides in the study area, and these data could serve as a reference material in future studies. It also implies that previous mining activities had not imparted negatively in terms of radiological hazard to the communities in the study area.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3], [Table 4]