Assessment of Naturally Occurring Radioactive Materials (NORM) in Mining Residues and Environmental Material from Sefwi, Awaso Bauxite Mine

radionuclides Th and and the radioactivity in bauxite red and from The radioactivity in and 238 232 Th and 40 18.01±1.96 Bqkg -1 , 19.07±2.12 1 and 103.21±1.74 Bqkg -1 ; 39.42±4.18 Bqkg -1 , 97.32±10.63 Bqkg -1 14.68±1.82 Bqkg -1 ; 64.23±6.58 -1 The activity levels for both 232 U and 232 Th above world-wide average while Potassium-40 levels lower. The mean activity concentration values of 238 U, 232 Th and 40 K in were 1.49±0.45 Bql -1 , 3.68±0.69 Bql -1 and 15.69±0.28 Bql -1 within the world average activity concentrations for ore and red mud. The committed effective dose was 0.74 mSv and annual effective dose estimated to be 0.136 mSv which is below recommended dose limit of 1 mSvyear -1 for public exposure.

The radioactive materials that occur naturally in the environment are potassium, thorium, and uranium decay chains [4]. The emanated radiation from soil and rock is due to both the decay of the parent radionuclides and their daughter radionuclides and also depend upon their mineral composition. Many industries, particularly the mining industries have operated for a long time without knowledge that their operations could give rise to Naturally Occurring Radioactive Materials in mining residues and environmental materials [5,6] If the institutes fail to control the disposal site where the NORM are being disposed, this may lead to the leaching of NORM or their release into airborne dust and into the groundwater bodies. Again, leaching of the natural radionuclides from the local sources may leads to contamination of drinking water and foodstuffs and thereby expose members of the public and workers. Institutional control may need to be maintained for a longer time since the NORM is associated with long half-lives [7].
The main objective of this study was to assess the Naturally Occurring Radioactive Materials in water and soil from the environs of Sefwi, Awaso Bauxite Mine and to estimate the health risk to both mine workers and members of the public living in the vicinity of the mining area. Specific objectives were to identify and estimate the levels of the various radionuclides associated with NORM in bauxite mining and also to estimate the NORM exposure of workers and the public due to consumption of water from the area.

Study Area Description
Sefwi, Awaso is a town situated in the Municipality called Bibiani-Anhwiaso-Bekwai of the Western North Region of Ghana with the geographical coordinates of 6°14'0"N and 2°16'0"W. It is at an elevation of 166 meters above sea level with a population of 29,748 according to 2010 census. The habitats are predominantly peasant farmers with a few engage in artisanal mining.

Collection of Soil, Bauxite Ore and Red Mud Samples
Soil, mining residue and the bauxite ore were collected within the perimeter of Awaso bauxite mine and 5 km away into the surrounding communities. Simple random sampling technique was adopted where the samples were collected using manual sampling tool i.e shovel into polythene bag. This technique used to ensure that results obtained from the sample which is an aggregation from three different locations into one representative sample with single coordinate and plotted as shown in figure 1. A total of 8 samples comprising of soil, ore and red mud samples were collected at 5 cm depth using hand shovel. Labeled polythene bags were used in collection of soil, ore and red mud samples.

Collection of Water Samples
Eight (8) water samples were collected from wells which are the main source of water to the community. These wells are situated at random locations in the community a few kilometers away from the site of mining.
The wells were purged for a while before samples were collected into cleaned 2.5 L plastic bottles. Two drops of concentrated nitric acid were added after filling the water bottles in order to ensure that radionuclides do not attach themselves to the container walls.

Preparation of Soil and Water sample for Laboratory Analysis
The soil samples were air dried for 7 days and then grinded using steel ball mill into fine particles and sieved with 500 μm mesh size pore in order to have a uniform matrix size. The sieved soils were then placed in an oven to dry for 3 to 4 hours at 105 °C temperature to completely remove excess moisture from the sample.
The samples were then transferred to 500 ml labelled Marinelli beakers and completely sealed and stored for 30 days in order to allow short-lived daughters of 238 U and 232 Th decay series to attain secular equilibrium with their long-lived parent radionuclides. This step was also important to ensure that radon was removed from the sample volume before counting. The water samples were transferred to 1 Liter Marinelli beakers and sealed without any pre-treatment. The samples were weighed and recorded. They were counted for 10 hours long enough for low level radioactive materials to form peaks.

Instrumentation and Calibration
In order to obtain both qualitative and quantitative results from the samples, both energy and efficiency calibration were performed for the analytical technique used, high purity germanium gamma detector, prior to the analysis. The calibration was performed using known test sources and mixed radionuclides standard respectively for energy and efficiency in order to get accurate results. For efficiency calibration, the mixed radionuclide standard was counted for 10 hours long and a spectrum was obtained and analysed for the peak areas formed. Genie-2000 Gamma Ray Acquisition and Analysis Software was used to identify the radionuclides present in the sample with respect to their well-known gamma energies. The actual samples were counted using the same geometry as the standard used at same counting time.

Calculation of Activity Concentration
The following expression was used to calculate the activity concentrations for natural radionuclides in soil, water, ore and red mud samples, the: (1) Where: is the activity concentration, is the net count area of the radionuclides in the sample, is gamma ray emission probability (gamma yield), ( ) is the absolute counting efficiency of the detector system, is the sample counting time and is the mass of the sample

Calculation of Absorbed Dose Rate
The equation below was used to determine absorbed dose rate from soil samples [2].
Where: is Absorbed dose rate, DCF is Dose conversion factors; 0.462, 0.0417 and 0.604 (nGyh -1/Bqkg -1 ) respectively for 238 U, 40 K and 232 Th, and A is Activity concentration of the radionuclide

Calculation of Annual Effective Dose
To estimate the annual external effective dose, three factors were taken into consideration i.e. the conversion coefficient from absorbed dose in air to effective dose, the outdoor occupancy factor of 0.2 which assumes an average duration of time someone spent outdoors (8760 hours year -1 ) and the annual estimated average effective dose received by a member is calculated using a conversion factor of 0.7 SvGy -1 [1].
Where: is the Annual effective dose and is the absorbed dose rate in soil.

Committed Effective Dose
The committed effective dose is determined by the activity concentration of the radionuclides of 238 U, 232 Th and 40 K in a sample, the amount of water a person take in liters per year. According to [8] guideline for drinking water, an individual is recommended to take 730 Lyear -1 and the ingestion dose coefficient for 238 U, 232 Th, 40 K is 45, 230 and 6.4 nSvBq -1 respectively [1].
The equation below is used to calculate committed effective dose.
where: S ing ( ) is the committed effective dose, A sp is the activity concentration of the radionuclide in the sample in Bql -1 , I w is the intake of water in Lyear -1 and DCF ing is the ingestion dose coefficient in SvBq -1 . Table 1 indicates the activity concentrations of 238 U, 232 Th and 40 K radionuclides present in the soil samples, bauxite ore and red mud collected at Awaso bauxite mine and surrounding areas. The activity concentrations were used to calculate the mean absorbed dose rate as 36.21 nGyh -1 and the mean annual effective dose of 0.136 mSv in the study area. The value obtained indicates that the radiation dose or exposure is within the recommended dose limit of 1 mSv for members of the public and 20 mSv for mine workers [9].  The activity concentrations in bauxite ore from this study as indicated in figure 3 is much higher as compared to radionuclide concentrations of natural origin found in bauxite and red mud as reported in some publications. Table 2      The figures for both 238 U and 232 Th are above world-wide acceptable limit of 35 and 30 Bqkg -1 in while 40 K figure is reported to be low as compared to 400 Bqkg -1 in [1]. The higher concentration of this radionuclide in the red mud suggests an increased concentration following the removal of alumina from the bauxite ore. The data indicates that the significant accumulation of 40 K occurs in alumina by the production process, resulting in very low level of the radionuclide in the red mud waste product. As depicted in the Figure 5, the absorbed dose rate for the bauxite ore is higher than the one for the red mud and the soils. This is because of the higher activity concentration of radionuclides present in the sample and their dose conversion factors. Generally, the arithmetic means for absorbed dose for the samples (soil, bauxite and red mud) is 36.21 nGyh -1 and that is the energy deposited into tissue as a result of exposure to ionizing radiation.

Fig. 5
Comparison of the absorbed dose rates of the entire sample types (soils, bauxite ore and the red mud) Figure 6 above illustrates the annual effective dose calculated for the samples and it is observed that the dose for bauxite ore is higher than the one for the red mud and the soils. The calculated annual effective dose is 0.136 mSv. This value is below the recommended dose limit of 20 mSv for workers and 1 mSv for members of the public as recommended by [9] in radiation protection principles for public radiation exposure control.

Conclusion
The results obtained from the study indicates that the radioactivity levels in soil for 238 U, 232 Th, and 40 K for all locations where the soil samples were collected within the mine and surrounding communities were within the world activity concentration of the soil. It was also observed that the activity concentration in red mud and the bauxite ore were higher than the expected concentration of soil. The radioactivity levels in water were within the global average values for 238 U, 232 Th, and 40 K and the committed effective dose were determined.

Funding
This work was funded by International Atomic Energy Agency through a postgraduate education program in Nuclear Science and Application.

Conflicts of interest/Competing interests
The authors declare that they have no known competing interest, financial or personal relationships that could have appeared to induced the work reported in this article.

Availability of data and material
All data pertaining this article has been presented Code availability (Not Applicable)

Acknowledgement
The research work is supported by International Atomic Energy Agency (IAEA), Ghana Atomic Energy Commission (GAEC) and School of Nuclear and Allied Sciences, University of Ghana. The authors would like to thank the technicians in the Food and Environmental laboratory of Radiation Protection Institute, GAEC for their laboratory assistance in sample preparation and analysis.