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 Table of Contents 
ORIGINAL ARTICLE
Year : 2020  |  Volume : 43  |  Issue : 1  |  Page : 6-12  

Measurement of natural radioactivity in soil dust samples along roadways in high commercial areas of the Ketu South district of the Volta Region, Ghana


1 National Nuclear Research Institute, Ghana Atomic Energy Commission, Accra, Ghana
2 Novell Community Development Solutions, Canada
3 Nuclear Security Department, Nuclear Regulatory Authority, Ghana

Date of Submission15-Nov-2019
Date of Decision29-Nov-2019
Date of Acceptance18-Jan-2020
Date of Web Publication12-May-2020

Correspondence Address:
Bernard Osei
P. O. Box LG 80, Legon, Accra
Ghana
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_34_19

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  Abstract 


There are increasing awareness and concern about the radiological impact of dust from soils along the roadways on petty traders and commuters in major commercial areas of Ghana. Soils in such commercial areas can be contaminated by road dust that could alter their radiological characteristics. This is because of the general exposure of road dust to environmental elements such as industrial and urban activities and also exhaust fumes from the vehicles. In the present study, 52 surface soil samples were collected along six roadways in the Ketu South District of the Volta Region, Ghana. The study area is one of the high commercial activities and consequently high traffic density. The measures of the specific activity concentration of238U,232Th, and40K in the samples were taken using high-purity germanium (HPGe) gamma-ray spectrometry. The results were evaluated to assess the radiological impact due to roadside soil in the area. The activity concentration in the samples ranged from 74.62 to 156.3 Bq/kg for238U, with an average of 112.4 Bq/kg. The activity concentrations of232Th ranged from 6.5 to 29.0 Bq/kg, with an average of 14.6 Bq/kg and that of40K ranged from 83.76 to 224.27 Bq/kg with an average of 141.02 Bq/kg. The results were used to estimate the radiological parameters of the study soils. The levels of radium equivalent activity (Raeq), absorbed dose rate (D), and annual effective dose (E) in them were lower than the recommended or safe limits proposed by international bodies such as UNSCEAR (2000) or ICRP (1991). These findings indicated that soils in the studied area had the normal levels of radiation and were therefore radiologically safe.

Keywords: Absorbed dose rate, annual effective dose, radium equivalent activity, specific activity


How to cite this article:
Addo MA, Lomotey JS, Osei B, Appiah K. Measurement of natural radioactivity in soil dust samples along roadways in high commercial areas of the Ketu South district of the Volta Region, Ghana. Radiat Prot Environ 2020;43:6-12

How to cite this URL:
Addo MA, Lomotey JS, Osei B, Appiah K. Measurement of natural radioactivity in soil dust samples along roadways in high commercial areas of the Ketu South district of the Volta Region, Ghana. Radiat Prot Environ [serial online] 2020 [cited 2020 Oct 31];43:6-12. Available from: https://www.rpe.org.in/text.asp?2020/43/1/6/284228




  Introduction Top


The presence of primordial radionuclides in human habitats has always been a source of prolong exposure. Terrestrial radiation due to naturally occurring radioactive materials contributes significantly to natural radiation exposure.[1] Understanding the different levels of radiation and radionuclide distributions in any environment is vital for evaluating the effects of radiation exposure to humans as a result of cosmogenic, human, and terrestrial activities.[2] Natural radioactivity in the soil is one of the main sources of exposure to humans, and the associated external radiation exposure due to the gamma radiation depends primarily on the geological and geographical conditions of the region where the soil is formed.[3],[4] Natural radioactivity in soil, rock, and mineral comes from the decay of radionuclides whose half-lives are comparable to the age of the earth, and the most important ones include238 U,232 Th, and40 K. A significant part of the total dose contribution to humans by way of natural radiation comes from these radionuclides. There are increasing awareness and concern about the radiological impact of dust from soils along the roadways on petty traders and commuters in major commercial areas of Ghana. Pedestrians and petty traders and hawkers who ply their trade along major roads are exposed to roadside soil and dust. It is also not uncommon for roadside soil to be mined and used as building materials. High levels of radiation in the environment which are often influenced by human activity are a natural health hazard. This makes the monitoring of soil radioactivity a topic of interest to many scientists.[5]

Due to the high exposure of road dust to such environmental elements as exhaust fumes from the vehicles, soils in highly commercial areas can become contaminated, resulting in changes in their radiological characteristics. In recent times, studies of air pollution, especially in the urban environment, have focused largely on the road-deposited dust.[6],[7],[8],[9] The particles of dust from the atmosphere that accumulate along roadside form the soils generally referred to as road dust or road deposit dust.[10] Studies have shown that road dust has a very complex chemical composition due to interaction with solid, liquid, and gaseous materials from different sources and activities.[11],[12] Singh et al.[13] described road dust as a complex mixture of particulates and contaminants derived from an extensive range of urban and industrial sources and processes.

Several studies about the level of radioactivity in soils and their radiological safety as building materials have been completed in many countries.[14],[15],[16],[17],[18] However, there is little research into the activities of238 U,232 Th, and40 K in roadside soils in Ghana.

The purpose of the present study was to assess the radiological safety of roadside dust in the Ketu South District (KSD) of Volta Region of Ghana. Furthermore, the results will provide baseline data for the area toward future monitoring.


  Materials and Methods Top


The study area

The KSD is located in the Volta Region in the South-eastern corner of Ghana [Figure 1]. The district has a total land area of about 400 km and an altitude of about 41 m above the sea level. KSD has dry equatorial climate with average monthly temperatures between 24°C and 30°C. The rainfall is double maxima type occurring from April to July and September to October. The mean annual rainfall for the district is 850 mm at the coast increasing to 1000 mm inland. The dry season, which is dominated by the harmattan winds, extends from December to February. The original vegetation of the district is coastal savanna woodland. It is made up of short grassland with small clumps of bush and trees in the northern part of the district and coastal shrub, mangrove forest, and marshland in the south. The investigated area is underlaid by the three main geological formations mainly the Dahomayan formation of the Precambrian age. The crust is made up of tropical gray and black earth, regosolic groundwater literates, recent deposits of the littoral consisting of marine sands, and a tertiary formation of savannah ochrosols.
Figure 1: The Ketu South district is located in the Volta Region at the South-Eastern corner of Ghana

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The KSD is an area of high commercial activity and traffic density. It shares a boundary with the Republic of Togo. Aflao, one of its main towns, is a border town between Ghanaand Togo. The West African truck highway from Cote I'voire through Ghana, Togo and Benin to Nigeria passes through Aflao.

Sample collection and preparation

Fifty-two sampling sites were selected in the KSD of the Volta Region for road dust sample collection. The samples were collected from six popular roads that experiences intense traffic conditions within the district. Those roadways include Diamond Cement (DC) road, Denu-Aflao (DA) road, Market-District Hospital (MH) road, Adafienu-Denu (AD) road, Agbozome-Heglakope (AH) road, and Heglakope-Ho (HH) road. At every sampling location, a dust composite sample was collected by sweeping using polyethylene brush and tray from four to six points of road edges during the dry season in March 2015. On the field, each site samples were separately stored in labeled 1 L plastic bottles. Along the roadways, sites with obvious pollution sources such as a factory, filling stations, car parking points, fitting shops, and soiled or oil-stained samples were avoided. The samples were also collected within 2 consecutive days such as to minimize temporal changes. At the sampling sites, a Geographical Positioning System was employed to record the geographical position. The individual samples were stored in a large ice-chest container and then transported into the laboratory of the Nuclear Reactors Research Center, Ghana Atomic Energy Commission (GAEC).

The samples were air dried in the laboratory for 10 days and then screened through a 250-μm mesh nylon sieve by the aid of a sieve shaker for 2 min to remove small stones and oversized residue which were not needed for the analysis. The sieve and container were cleaned thoroughly to prevent the contamination of the subsequent sample to be sieved; the process underwent repetitions until all the samples were thoroughly sieved.

All the soil samples were oven dried at 110°C for 72 h. The homogenized samples were weighted and hermetically sealed in 450 ml Marinelli plastic containers. Containers with the same size and geometry were used for the reference materials for the efficiency calibration of the detector system for the radioactivity measurement. The samples were filled to an indicated mark on the Marinelli container and the mass determined by the simple calculation of the difference between the weight of the container and the weight of the container with each sample. The samples were closed tightly to limit as far as the possible escape of radon. Each Marinelli container was analyzed after 4 weeks after232 Th and226 Ra had reached secular equilibrium with their decay products using HPGe detector setup and 8192 channel multichannel analyzer (MCA). The detector was calibrated for absolute efficiency using mixed radionuclides gamma-ray standard QCY4 solution (obtained from Physikalisch Technische Bundesanstalt (PTB), Braunschweig, Germany) supplied by the IAEA. The standard solution contains the following radionuclides with corresponding energies241 Am (60 keV),109 Cd (88 keV),57 Co (122 keV),139 Ce (165 keV),203 Hg (279 keV),113 Sn (391.69 keV),85 Sr (514 keV),137 Cs (662 keV),88 Y (898 keV and 1836 keV), and60 Co (1173 keV and 1333 keV).

Radioactivity determination

The measurement of activity concentration of radionuclides was taken using HPGe gamma-ray spectrometry system. The gamma-ray spectrometry system was equipped with an HPGe detector model GR 2518-7500L (Canberra Industries Inc.), high-resolution gamma-ray spectrometry coupled to a computer-based PCA-MR 8192 MCA mounted in a cylindrical 90-mm thick lead shield, and an internal volume of approximately 99.53 L. The detector is cooled by liquid nitrogen from a vertical dipstick cryostat dipped in 35 L liquid nitrogen dewar. The detector has a relative efficiency of 25% to NaI detector, 1.8 keV energy resolution at the energy peak of 1333 keV of60 Co isotope, and a peak-to-Compton ratio of 55:1. The radionuclides were identified using the gamma-ray spectrum analysis software, ORTEC MAESTRO-32.

The radioactivity measurement of the samples was made by placing them on the detector inside the lead shielding, and the spectrum was collected for accumulation. The same geometry was used to determine the peak area of samples and reference materials. Each sample was measured during an accumulating time for 36,000 s. The activity concentrations were calculated based on the weighted mean value of their respective decay products in equilibrium. The gamma-ray lines of 295.2 (18.2) and 351.9 (35.1) keV from214 Pb and the 609.3 (44.6) and 1764.5 (15.1) keV from214 Bi were used to determine the activity concentration of226 Ra. The gamma lines of 338.4 and 911.2 (26.6) keV from228 Ac, the723 keV from212 Bi, and 583.2 (30.6) keV from208 Tl were used to determine the activity concentration of232 Th. The activity concentration of40 K was measured directly by its own gamma-ray line at 1460.8 (10.7) keV. The values in parentheses following gamma-ray energy indicate the absolute emission probability of the gamma decay.

The gamma-ray background inside the detector shielding was determined using an empty container under identical measurement conditions. This background was subtracted from the measured gamma-ray spectra of each sample before calculating the activity concentrations. The specific activity concentration, AEi of a radionuclide i and for a photopeak at energy E, is given by the following analytical expression (1):[15]



Where NEi is the net count for a sample at energy E, ÔEi is the detector efficiency at energy E, Tc the counting live time, γd the gamma emission probability, and Ms the mass (dry weight) in kilogram of the sample.

Estimation of the radiological health risk parameters

To estimate the radiological health risk associated with the road soil dust, commuters, roadside traders, and people using the soil material in building were considered. Six radiological parameters were used for the assessment of the suitability of the soil material and include radium equivalent activity (Raeq), representative gamma index (Iγr), absorb dose rates (Dr), and annual effective dose rates (E).

Radium equivalent activity

To compare the radioactivity concentration of any building material containing226 Ra,232 Th, and40 Ka, common index is required to obtain the sum of the radioactivities. The Raeq has been used for the purpose.[19] Raeq is calculated according to the following Eq. (1).[19],[20]

Raeq (Bq/kg) = ARa + 1.43 ATh + 0.077 AK (1)

where ARa, ATh, and AK are the specific activity concentrations of226 Ra,232 Th, and40 K in Bq/kg. Eq. (1) was created on the premise that 1 Bq/kg of226 Ra, 0.7 Bq/kg of232 Th, or 13 Bq/kg of40 K produce the same gamma dose rate and that Raeq should not exceed a maximum of 370 Bq/kg[21] for the annual effective to be less than 1.5 mSv. It may be noted that238 U is replaced by the decay product226 Ra, although there may be disequilibrium between238 U and226 Ra. This is given as 1.03 by UNSCEAR.[19]

Representative gamma index

The representative gamma index (Iγ), is one of the radiological indices used to assess human safety when exposed to γ-radiation.[19] The Iγ is estimated based on the following Eq. (2):[22]

Iγr = ARa/150 + ATh/100 + AK/1500 (2)

For the safe use of building material in the construction of buildings, Iγr should be less than unity. There are other indices that are equally used to evaluate the radiological safety in building materials. For example, the European Commission's Radiation Protection Report 112[23] reveals an activity concentration index (I) for the representation of activity concentration due to238 U,232 Th, and40 K in building materials corresponding to dose criteria. For instance, I = 1 corresponds to an annual effective dose of 1 mSv.[24] Meanwhile, Rao[24] suggested that since all the index parameters are interrelated and correspond to dose criteria, it is not necessary to use the decade-old Hex and Hin terms of hazard exposure index. Rao[24] then recommended the replacement of Hex with external exposure index and Hin with internal exposure index instead of hazard index, if at all needed.

Absorbed dose rates

The outdoor air-absorbed dose rates due to terrestrial gamma rays at 1 m above the ground can be calculated from238 U,232 Th, and40 K concentration values in the soil if other radionuclides, such as137 Cs,90 Sr, and235 U decay series can be neglected as they contribute little to the total dose from the environmental background.[13] The corresponding factor used to calculate the absorbed dose rate is given by UNSCEAR[19] as in Eq. (3):

D (nGy/h) =0.462 AU + 0.604 ATh + 0.0417AK (3)

where D is the absorbed dose rates at 1 m above the ground due to238 U (AU),232 Th (ATh), and40 K (AK) in the soil samples. Eq. (3) was derived by using the conversion factor of 0.0417 nGy/h/Bq/kg for40 K, 0.604 nGy/h/Bq/kg for232 Th, and 0.462 nGy/h/Bq/kg for238 U.

Annual effective dose rates

To estimate the annual effective dose, accounts were taken into conversion coefficient from the absorbed dose in the air to effective dose and the indoor or outdoor occupancy factor. In order to estimate the annual effective dose rate (HE), the conversion coefficient from the absorbed dose (D) in the air to effective dose of 0.7 Sv/Gy and outdoor occupancy factor (0.2) and indoor occupancy factor (0.8) proposed by UNSCEAR[19] were used. The HE was calculated from the following formula:

HE (mSv/y) = D × 8760 (h/y) × Q (occupancy factor) × 0.7 SvGy/h (conversion factor) × 10-6 (4)

where, Q is the occupancy factor which is determined by the indoor or outdoor situation where HE is being estimated.


  Results and Discussion Top


The activity concentrations of238 U,232 Th, and40 K from the 52 road soil samples collected from the six roadways in the KSD are presented in [Table 1]. All values were obtained in Bq/kg dry weight. The mean activity concentration of238 U,232 Th, and40 K in all the roadways soil samples was 112.36, 14.55, and 141.02 Bq/kg, respectively, with ranges of 74.62–156.13, 6.54–29.01, and 83.76–224 Bq/kg. The results indicated that the distribution of radionuclides within the investigated soils follow the order40 K > 238 U > 232 Th. The pattern of the radionuclide distribution among the individual roadways fits perfectly into the overall pattern, suggesting that the roadway soils may be derived from common geology. Similar studies by Saleh and Abu-Shaye[25] and Sahoo et al.[1] conducted along highways in Jordan and India, respectively, observed similar distribution patterns.
Table 1: Statistics of activity concentration of238U,232Th, and40K in the road soil samples of the selected roadways of study area

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As shown in [Table 1], the activity concentration of radionuclides in the investigated soils varied over a wide range in each roadway. However, comparisons between the roadways showed some similar results. For instance, 116 and 116 Bq/kg of238 U were recorded for AD and MH roadways, respectively, and 19.7 and 19.6 Bq/kg of232 Th were recorded for the same roadways. Furthermore, the values of 110 Bq/kq of40 K recorded for both DC and AH roadways, respectively, were similar. These similarities of the radionuclide concentration results cannot be attributed only to same regional geology; there is a possibility of vehicular influence accounting for changes in the soil parameters.

The implication of the results from [Table 1] is that the soils in the investigated area are over enriched with uranium by the global standard of 35 Bq/kg (UNSCEAR[19]) by a factor of 3.21. Overexposure to background radiation has been linked to health challenges such as lung diseases, cancer, cataract, and teeth fracture among others.[26] The natural radioactivity values for232 Th and40 K remain lower than the world average values of 30 Bq/kg and 400 Bq/kg, respectively.[27] Pearson's correlation coefficient was computed between the permutation pairs of the three radionuclides for the activity concentration of all the investigated samples. All the results between each permutation of the radionuclides were positive. The correlation coefficient of232 Th and238 U was observed to be high with a value of 0.69, whereas the correlation between232 Th and40 K and238 U and40 K was low with 0.2 and 0.38, respectively. According to Usikalu et al.,[28] the results of the relationship between the pair of the radionuclides were not surprising, as the high correlation between232 Th and238 U is expected as they come from the natural decay series while40 K, though a primordial radionuclide, does not undergo any decay.

Evaluation of radiation indices

In order to estimate the radiological health risks associated with the roadside soils and how they could impact commuters, six different radiological parameters were estimated using different models provided by international bodies including UNSCEAR and OCEA. The results are presented in [Table 2].
Table 2: Average values of radium equivalent activity, absorbed dose rate, and annual effective dose equivalent rate in soil samples

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As shown in [Table 2], the individual Raeq values for the soil samples ranged from 133.75–154.46 Bq/kg. The overall average was 144.84 Bq/kg, which was about 39% lower than the UNSCEAR[19] Raeq limit of 370 Bq/kg or an external dose equivalent of 1.5 mSv/y.[22] This implies that these roadside soils present no significant radiological health risk to users. Roadside users within the study area are, therefore, free from any radiological health risk. Although the results showed that Raeq activities in AD roadway soils were slightly higher than other roadways, the results are still very comparable, suggesting a common bedrock for all the roadway soils investigated.

Representative level index (Iγr), an estimation of gamma radiation associated with natural radionuclides (238 U,232 Th, and40 K), was used for radiological health risk assessment in the soil samples. The Iγr values for the road soil samples ranged from 0.88 to 1.05, with an average of 0.98. The values for soil samples from DA, HA, and HH roadways were lower than the criterion limit of unity indicating that they were radiologically safe. However, the values for the soil samples from DC, MH, and AD roadways were found to be slightly higher than unity.

The absorbed dose rate (D) in air at 1 m above the ground level owing to the studied radionuclides in the soil samples in each roadway was estimated using the Eq. (3), as presented in section 2.4. The average absorbed dose rate for the current study ranged from 62.22 nGy/h in the HH roadway to 71.06 nGy/h in the AD roadway [Table 2]. The averages for all the roadways in studied were higher than the world average value of 57 nGy/h.[19]

The annual effective dose rate equivalent was calculated using a conversion factor of 0.7 Sv/Gy to convert the absorbed dose rate to the effective dose equivalent and 0.2 and 0.8 for the outdoor and indoor occupancy factors, respectively. The outdoor occupancy factor focused more on pedestrians and petty traders' safety along the roadways, whereas the indoor occupancy factor was used to determine the natural radiation safety for dwellers in building constructions when the soil is used as building materials. The average annual outdoor effective dose rate was 0.09 mSv/y, which was slightly higher than the worldwide average (0.07 mSv/y) indicating that commuters and petty traders along the investigated roadways are safe from radiation risk. The corresponding average annual indoor effective dose rate was 0.33 mSv/y, which was almost the same as the worldwide average of 0.34 mSv/y indicating that the roadside soils investigated were radiologically safe for building construction.


  Conclusions Top


The levels and distribution of terrestrial natural radionuclides (238 U,232 Th, and40 K) in roadside soil samples collected from six roadways in the KSD in the Volta Region of Ghana were measured to evaluate any health radiological risks posed by the soil to commuters or for use as building material. Four radiation index parameters comprising radium equivalent activity (Raeq), representative gamma index (Iγr), absorbed dose rates (D), and annual effective dose rates (E) were utilized. The findings indicated that of the three radionuclides,40 K had the highest activity concentration in all except one roadway (i.e., MH). The investigated area was found to be over enriched with238 U compared to the global standard of 35 Bq/kg by a factor of 3.21. All the radiation health parameters with the exception of the absorbed dose rate were below the permissible limits set by professional international bodies, including UNSCEAR, ICRP, and OECD. Specifically, the average absorbed dose rate of 66.92 nGy/h obtained in this study was higher than the world average of 57 nGy/h; the annual effective dose rates of 0.33 and 0.09 mSv for indoor and outdoor occupancy factors, respectively, were <1 mSv;[29] the Raeq value was lower than the international limit of 370 Bq/kg; and representative gamma index (Iγr) was lower than the international limit of unity. These results indicated that the roadway soils investigated posed no radiation hazard to commuters from direct exposure or to occupants of buildings constructed with those soils.

Acknowledgment

The author would like to acknowledge the tremendous support received from the field assistants from GAEC. We are also indebted to the technical staff of gamma spectrometry laboratory, Radiation Protection Institute, and GAEC for their kind support in the radioactivity measurement.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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