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ARTICLE
Year : 2011  |  Volume : 34  |  Issue : 2  |  Page : 137-143  

Assessment of occupational and public exposures due to natural radioactivity in saline soil at Panbros salt industry limited in the Accra metropolis of Ghana


1 Graduate School of Nuclear and Allied Sciences, University of Ghana; Radiation Protection Institute, Ghana Atomic Energy Commission, Box LG 80, Legon, Ghana
2 Graduate School of Nuclear and Allied Sciences, University of Ghana, Legon, Ghana
3 Radiation Protection Institute, Ghana Atomic Energy Commission, Box LG 80, Legon, Ghana

Date of Web Publication12-Jul-2012

Correspondence Address:
C Kansaana
Graduate School of Nuclear and Allied Sciences, University of Ghana; Radiation Protection Institute, Ghana Atomic Energy Commission, Box LG 80, Legon
Ghana
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Source of Support: None, Conflict of Interest: None


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  Abstract 

This study was conducted to assess occupational and public exposures due to natural radioactivity from 238 U, 232 Th, and 40 K in saline soil at Panbros Salt Industry Limited in the Greater Accra Region of Ghana. Activity concentrations in samples were measured using gamma spectrometry. The average annual effective dose in an outdoor environment for an individual worker was estimated to be 0.06 mSv/year, and for a member of the public, it was estimated to be 0.05 mSv/year. These values are below the average effective dose of 0.07 mSv/year received per caput worldwide due to external terrestrial radiation outdoors, given in UNSCEAR 2000 report. The results indicate insignificant contamination of the salt mining environment, and hence the workers and the public are not exposed to any significant radiological health hazard.

Keywords: Natural radioactivity, public exposure, saline soil


How to cite this article:
Kansaana C, Darko E O, Schandorf C, Adukpo O K, Faanu A, Lawluvi H, Kpeglo D O. Assessment of occupational and public exposures due to natural radioactivity in saline soil at Panbros salt industry limited in the Accra metropolis of Ghana. Radiat Prot Environ 2011;34:137-43

How to cite this URL:
Kansaana C, Darko E O, Schandorf C, Adukpo O K, Faanu A, Lawluvi H, Kpeglo D O. Assessment of occupational and public exposures due to natural radioactivity in saline soil at Panbros salt industry limited in the Accra metropolis of Ghana. Radiat Prot Environ [serial online] 2011 [cited 2020 Sep 25];34:137-43. Available from: http://www.rpe.org.in/text.asp?2011/34/2/137/98403


  1. Introduction Top


There is a great interest in the study of natural environmental radioactivity in soil because the population is exposed at different levels depending on the concentration of natural radioactive minerals in each region of the world. [1] Soil is a mixture of natural bodies on the earth's surface containing living matter and supporting plants. Soil is a complex substance because of its extreme variability in physical and chemical composition. It consists of small but significant quantities of organic and inorganic compounds, which are essential for plants growth. [2] There are many types of soils depending on the physical and chemical composition. The soil is classified as saline, saline sodic, alkali, etc. In saline soil, the concentration of salt is increased to the level at which crop's growth is adversely affected. Saline soils have a high content of natural salts and their pH is generally above 7.3. [3] Living organisms of the planet are exposed to natural radiation, which is mainly due to the activity concentration of primordial radionuclides 232 Th, 238 U, and their decay products, in addition to the presence of other natural radionuclides such as 40 K in the earth's crust. [4] According to Ramli, [5] one of the major interests in the study of natural background radiation is the need to establish reference levels, especially in areas where the risk of radioactive material being released to the environment is high. There is also a worldwide interest in identifying new areas with high natural radiation.

In Ghana, a number of registered companies are operating large-, medium-, and small-scale salt mining. Activities in the salt mining sector have increased in recent times, and as a result, it has been recognized that there is a need to address the challenges of naturally occurring radioactive materials (NORMs). Several authors have studied the levels of natural background radiation by analysis of radionuclides' concentrations in saline soil. [6],[7] Until now, no studies have been carried out on radioactivity levels associated with NORMs in saline soil in the salt mines in Ghana. This situation could pose public and occupational health hazards as well as environmental contamination challenges as the country makes frantic efforts in promoting the salt industry.

The objectives of the study include identifying and determining the activity concentrations of NORMs in saline soil. The second objective was to calculate the absorbed dose rates to estimate the effective doses in order to assess the associated occupational and public exposures. Finally, we aimed to assess the current state of radiological protection of the workers, the public, and the surrounding environment from the findings.

This work, therefore, assessed occupational and public exposures associated with NORMs in saline soil at Panbros Salt Industry Limited operations as part of the national effort to establish baseline data of natural radioactivity in the salt mining industry in Ghana.


  2. Materials and Methods Top


2.1. Description of the study area

The study was conducted in Weija, in the Ga South Municipality of the Greater Accra Region of Ghana and is within the Accra-Winneba road. It lies between longitudes 0° 17' 57.16"W and 0° 55' 15"W and latitudes 5° 33' 26.27"N and 5° 35' 30"N. It is bounded on the north and west by McCarty Hills and Tetegu, respectively, on the east by Mendskrom-Dansoman, and on the south by the Gulf of Guinea [Figure 1].
Figure 1: A map of Panbros Salt Industry Limited showing sampling points

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This study area is generally flat and at an elevation of 70 m from the sea level. The total area of the mine's concession is about 1130 ha of which 784 ha have been developed into pans. The climate is characterized by two rainfall maxima. The major rainy season occurs between May and July with the peak occurring in June, while the minor one occurs between September and October with the peak occurring in October. Generally, the rainfall in the area is low with mean annual rainfall of approximately 900 mm per annum. The mean temperature is 26°C. The vegetation is mainly coastal grassland and scrub. The soil is sandy clay with salinity ranging from 6 to 21 desi semin/meter (ds/m) at 1 m depth of soil profile. [8]

2.2. Selection of sampling sites

The choice of the sampling sites was an important factor in this study. The geological map of the study area was examined in order to select sampling sites which would give possibly an accurate representation of the real situation. The sampling areas were the salt production sites, the offices, and the residential areas [Figure 1]. The sampling sites were chosen from various geological formations within the study area. It is, therefore, anticipated that the type of geological formation upon which a person works at the salt mine or lives within the vicinity will have a significant effect on the exposure, indicating the extent of the exposure range of the operating staff and the inhabitants.

2.3. Sampling and sample preparation

The sampling was carried out in the month of October 2010 where the weather conditions were fairly stable. A total number of 60 samples were collected from the salt production sites, the offices, and the residential areas. Five samples were collected at depths of about 0.05-0.10 m from each of the 12 identified sampling sites. The samples were then transferred into polyethylene bags and labeled accordingly. At the laboratory, composite samples were made to represent each sampling site and subsamples taken for the analysis. [Table 1] shows the sampling sites with their respective co-ordinates. Background measurements were taken at each site using a Rados Universal survey meter, model RDS-200, manufactured by Rados Technology of Finland, with a sensitivity range of 0.01-10 μSv/hour in the energy range up to 1.3 MeV.
Table 1: Sample location with co-ordinates for saline soil samples

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The soil samples were air dried on trays for 1 week and then oven dried at a temperature of 105°C. They were then ground into powder and sieved through a 2-mm wire mesh-size sieve to remove stones, pebbles, and other macro-impurities. The prepared samples were then measured in 1.0 l Marinelli beakers, i.e. in the same geometry as the standard. The prepared samples were stored in the laboratory for a minimum of 1 month to allow the short-lived daughters of 238 U and 232 Th decay series to achieve secular equilibrium with their long-lived parent radionuclides. [9]

2.4. Measurement of samples

The measurements were performed with a computerized gamma spectrometry system made up of a 3 inch × 3 inch NaI (Tl) detector and measuring assembly. The NaI (Tl) was located inside a cylindrical lead shield of thickness 5 cm, internal diameter of 22.5 cm, and height of 50 cm. The lead shield was lined with concentric layers of cadmium, copper, and plexiglass, each of 3 mm thickness. A high voltage supply provided the appropriate bias to the detector system. Oak Ridge multichannel analyzer (MCA) and PCA software were used for data acquisition and evaluation of the various gamma-emitting radionuclides.

Prior to the analysis of the samples, energy and efficiency calibrations were performed to enable identification and quantification of the radionuclides. The detector system was calibrated using the multinuclide reference standard solution. The calibrations were carried out for Marinelli beaker containing a mixed radionuclide solution supplied by the Dentscher Kalibrierdienst (DKD) of Germany. The standard solution contains the following radionuclides with the corresponding energies: 241 Am (59.54 keV), 137 Cs (661.6 keV), and 60 Co (1172.3 and 1332.5 keV).

2.5. Calculation of activity concentrations and estimation of doses from spectral data

A counting time of 36,000 seconds was used to acquire spectral data for each sample and the data were evaluated. The activity concentrations of 238 U, 232 Th, and 40 K were determined after correction for background and inhomogeneities. [10],[11]

The activity of 238 U was calculated from the average of 295.3 keV of 214 Pb and 1764.5 keV of 214 Bi, 232 Th from the average of 2614.0 keV peak of 208 Tl and 969.1 keV of 228 Ac, and 40 K was determined from 1460.0 keV.

The analytical expression used in the calculation of the activity concentrations in Bq/kg for soil samples is as shown in equation 1:



where Aac is the activity concentration, λ the decay constant, Nsam the total net counts for the sample in the peak range, PE the gamma-ray emission probability, Td the decay time between sampling and counting, Tc the counting time, ε(E) the total counting efficiency of the detector system, M the mass of sample (kg), and the expression exp (λT d ) is the correction factor for decay between sampling and counting. [11]

The decay of naturally occurring radionuclides in soil results in exposures to humans. External exposures outdoors arise from terrestrial radionuclides present at trace levels in all soils. The gamma absorbed dose rates in air outdoor were estimated from the concentrations of each of the nuclides of 238 U and 232 Th series and 40 K, and the individual results were added in order to obtain the total absorbed dose rate in air due to gamma radiation from these radionuclides.

The external gamma dose rate (Dγ) at 1.0 m above the ground for the soil samples was calculated from the activity concentrations using equation 2:



where ATh , AK and AU are the activity concentrations in Bq/kg of 232 Th, 40 K, and 238 U, with dose conversion factors of 0.604, 0.0417, and 0.462 in nGy/Bq, respectively. [4]

The annual effective dose Eγ from external gamma exposure in an outdoor environment was calculated from the absorbed dose rate by applying equation 3:



where Eγ is the annual effective dose in Sv and Dγ the estimated absorbed dose rate in nGy/hour, T the outdoor exposure time (0.2 × 8760 = 1752 h/year for members of the public and 2000 h/year for workers), and F is the dose conversion factor of 0.7 in Sv/Gy. [4]


  3. Results and Discussion Top


[Table 1] shows the salt production sites, the offices, and the residential areas where the samples were collected for analysis, with their respective co-ordinates.

The results of the activity concentrations in the saline soil samples are summarized in [Table 2]. The three most common primordial radionuclides investigated in the study area were 40 K, 238 U, and 232 Th.
Table 2: Activity concentrations of 238U, 232Th, and 40K and the salinity values in the saline soil samples

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The activity concentrations of 40 K in saline soil for the various locations varied from 145.47±11.12 to 165.82±15.45 Bq/kg, with an average value of 154.59±12.35 Bq/kg. The highest activity concentration of 40 K was recorded at the pump house site, whilst the lowest was obtained at the Katanga site.

The other naturally occurring radionuclides were 238 U and 232 Th. The measured activity concentrations of 238 U for the various locations varied from 21.76±1.32 to 37.67±4.32 Bq/kg, with an average value of 27.05±2.09 Bq/kg. The lowest activity concentration was recorded at the office areas, whilst the highest was obtained at the football field. For 232 Th (T1/2 = 1.4×10 10 years), the measured activity concentrations for the various locations varied from 21.67±1.32 to 41.06±3.24 Bq/kg, with an average value of 32.86±2.93 Bq/kg. The lowest activity concentration of 232 Th was recorded at the office areas, whilst the highest was obtained at the Congo site. [Figure 2] shows the comparison of the average activity concentrations of 238 U, 232 Th, and 40 K in the saline soil samples.
Figure 2: Comparison of the average activity concentrations of 238U, 232Th, and 40K in the saline soil samples

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The concentrations of 238 U, 232 Th, and 40 K vary at different locations, and in the earth's crust, the concentrations vary with the half-lives of these radionuclides. Thus, the longer the half-life, the higher is the activity concentration of the radionuclide. The ratio of 232 Th to 238 U in the earth's crust is generally greater than 1, [6] and in this study, the ratio was also found to be greater than 1 for all the study sites [Table 2]. The activity concentration of 40 K in soil is of the order of magnitude higher than those of 238 U and 232 Th. For all the saline soils under the study area, the activity concentration of 40 K was about three times higher than those of 238 U and 232 Th. This is also in accordance with the well-known fact that potassium in the earth's crust is expressed in terms of percentage, whereas uranium and thorium are expressed in parts per million. [6]

To compare the radionuclides' content in the samples with the salinity values, each of the parameters was normalized to unity and expressed as a percentage index, Pind (%). The salinity values are assumed to have some bearing on the radionuclide content. Comparing the trends in the salinity with the radionuclide content as shown in [Figure 3], however, reveals nothing conclusive probably due to the geographical locations or close proximity of the sampling sites. The sampling sites of the area also have similar geology and meteorological conditions. Variation in the parameters may also be due to statistical fluctuations since the sampling sites are very close to each other. A long-term study is necessary to investigate the dependence of the salinity on the radionuclide content and other important parameters.
Figure 3: A graph showing the relationship between the salinity and the radionuclide content in the saline soil samples. Each parameter was expressed as a percentage index, Pind (%)

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For purposes of comparison, the activity concentrations of 238 U, 232 Th, and 40 K have been collected from the literature for saline soils from other countries and are presented in [Table 3]. The area of Dhaka, Bangladesh, has a high value of 238 U compared to other countries and is slightly higher than the values given in UNSCEAR 2000 report. The activity concentrations of 238 U and 232 Th in the saline soil of the other countries are found to be within the range given in UNSCEAR 2000 report and are comparable to that of the saline soil of the area under investigation. The activity concentration of 40 K in the saline soil of Madurai, India, is also found to be lower than that of Panbros Salt Industry Limited and the range given in UNSCEAR 2000 report. The activity concentration of 40 K of the other countries is found to be in the range given in UNSCEAR 2000 report but higher than that of Panbros. The variations in the activity concentrations could be due to differences in the geological structure of the soil of the areas. The reason for the higher content of potassium present in the saline soil of the other countries than that of the saline soil of Panbros may be the use of phosphatic fertilizers in these areas. It has been estimated that phosphatic fertilizers applied to the fields in recommended amounts could raise the radioactivity level in soils. The use of fertilizers to a large extent has affected radionuclide concentration, especially potassium containing fertilizers which are the cause of high activity concentration of 40 K in the soil. [12]
Table 3: Comparison of the radioactivity concentrations in the saline soil with those of other countries

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The variations in the activity concentration levels are within the activity values measured all over the world. [4],[13] According to UNSCEAR 2000 report, the world's range of activity concentrations for 238 U, 232 Th, and 40 K are 17-60, 11-64 and 140-850 Bq/kg, respectively. The measured activity concentrations of 238 U, 232 Th, and 40 K for the saline soil samples from the study area were found to be within the world averages.

[Table 4] shows the estimated total gamma absorbed dose rate in air outdoors due to 238 U, 232 Th, and 40 K radionuclides in saline soils of the area under investigation. The estimated dose rates varied from 27.82±2.21 at the office areas to 46.77±4.85 n/Gy at the football field, with an average value of 37.46±3.90 nGy/hour. The estimated average dose rates lie within the global average dose rate range of 18-93 nGy/hour in UNSCEAR 2000 report.
Table 4: Absorbed dose rates and annual effective doses of 238U, 232Th, and 40K in the saline soil samples

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The estimated total annual effective dose to adult workers for the saline soils at the various locations varied from 0.04±0.01 to 0.07±0.02 mSv, with an average value of 0.06±0.01 mSv. For members of the public, it varied from 0.03±0.01 to 0.06±0.02 mSv, with an average value of 0.05±0.01 mSv. The total effective dose is due to external irradiation from radionuclides outside the body by gamma radiation from radionuclides in the soil. There is no drastic difference in the effective doses received by the workers and members of the public. A slight increasing trend was observed in the effective dose received by the workers at all the locations. [Figure 4] shows the trend of the estimated total annual effective dose due to 40 K, 238 U, and 232 Th radionuclides in saline soils of the area under investigation with that of UNSCEAR 2000 report.
Figure 4: Comparison of the annual effective dose due to external irradiation from radionuclides in the soil of Panbros Salt Industry Limited with that of UNSCEAR 2000 report

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According to UNSCEAR 2000 report, the worldwide annual per caput effective dose is 0.07 mSv/year received due to external terrestrial radiation outdoors. The values of the effective doses estimated for the workers and members of the public are below the UNSCEAR values. Comparing the values of the absorbed dose rates and the effective doses with the published data from UNSCEAR 2000 report, it would be seen that the values obtained from this study are less than those given in the UNSCEAR report, and this indicates that the activities of the salt mine do not impact negatively on the workers and the public from exposure to NORMS in the mining environment.


  4. Conclusions Top


Natural radioactivity in saline soil at Panbros Salt Industry Limited in the Greater Accra Region of Ghana has been measured using gamma spectrometry. The samples were collected from the salt production sites, the offices, and the residential areas for analysis.

The variations in the radionuclide activity concentrations in the samples and the absorbed dose rates from the measurements were found to be within the world average. The annual effective doses in an outdoor environment to an individual worker and member of the public were estimated to be 0.06 and 0.05 mSv/year, respectively, which are below the average annual effective dose of 0.07 mSv/year received per caput worldwide due to external terrestrial radiation outdoors. [4],[14]

The results indicate insignificant contamination of the salt mining environment, and hence the workers and the general public are not exposed to any significant radiological health hazard. The results provide data and information for dose assessment and further studies.


  5. Acknowledgments Top


The authors would like to acknowledge with thanks the assistance received from the management of Panbros Salt Industry Limited during the period of sampling. The authors are also grateful to the Radiation Protection Institute (RPI) of the Ghana Atomic Energy Commission for making their laboratories available for this research work. The authors also appreciate the contribution of Mr. Ali Ibrahim Doe, and Mrs. Rita Kpodzro of RPI for their assistance in the completion of this work.

 
  References Top

1.Radhakrishna AP, Somashekarappa HM, NarayanaY, Sidappa K. A new natural background radiation area on the southwest cost of India. Health Physics 1993; 65:390-5.  Back to cited text no. 1
    
2.Bool SW, Hole M. Soil Genesis and Classification, Iowa State University Press Publisher.1976; 3:288-314.  Back to cited text no. 2
    
3.Brady NC. The nature and properties of soils. 10 th ed. London:Macmillan Publisher;1990. p. 243-6.  Back to cited text no. 3
    
4.UNSCEAR. Sources and effects of ionizing radiation. United Nations Scientific Committee on the Effects of Atomic Radiation, United Nations, New York, 2000.  Back to cited text no. 4
    
5.Ramli AT. Environmental terrestrial gamma radiation dose and its relationship with soil type and underlying geological formations in Pontian District, Malaysia. Appl Radiat Isot 1997; 48:407-12.  Back to cited text no. 5
    
6.Miah FK, Roy S, Touhiduzzaman N, Alan B. Distribution of radionuclides in soil samples in an around Dhaka city. Appl Radiat Isot 1998; 49:133-7.  Back to cited text no. 6
    
7.Akhtar N, Tufail M, Chaudhry A, Iqbal M. Estimation of Radiation Exposure Associated with the Saline soil of Lahore, Pakistan. J Res Sci 2004; 15:59-65.  Back to cited text no. 7
    
8.Dickson KB, Benneh G. A New Geography of Ghana. London:Longmans Group Limited; 2004.  Back to cited text no. 8
    
9.Tufail M, Iqbal M, Mirza MS. Radiation doses due to natural radioactivity in Pakistan marbles. Radioprotection 2000; 34:355-9.  Back to cited text no. 9
    
10.Gilmore G, Hemingway JD. Practical Gamma Spectrometry. NY: John Wiley and Sons; 1995.  Back to cited text no. 10
    
11.Oresengun MO, Decker KM, Sanderson CG. Determination of self absorption correction by computation in routine gamma-ray spectrometry for typical environmental samples. J Radioact Radiochem1993; 4:38-45.  Back to cited text no. 11
    
12.Bhatti TM. Phosphate fertilizers a potential source for Uranium recovery as by product. A technical report No. PAEC/NIBGE-2/1994. Faisalabad, Pakistan:National institute for biotechnology and genetic engineering (NIBGE); 1994.  Back to cited text no. 12
    
13.IAEA. Measurement of radionuclides in food and environment. Technical reports series No. 295., 1989.  Back to cited text no. 13
    
14.Saravanan S, Jodha AS, Gopalani D, Bhalti SS, Kuma S. Preliminary measurements of natural radioactivity at Maduri District of Tamilnadu, India. Radiat Meas 2003; 36:397-9.  Back to cited text no. 14
    


    Figures

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

  [Table 1], [Table 2], [Table 3], [Table 4]



 

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Abstract
1. Introduction
2. Materials and...
3. Results and D...
4. Conclusions
5. Acknowledgments
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