|Year : 2012 | Volume
| Issue : 1 | Page : 22-28
Radiological impact of soil as a source of building material
Francis Otoo1, Emmanuel Ofori Darko1, Geoffrey Emi-Reynolds1, Aba Bentil Andam2, Oscar Kwaku Adukpo1
1 Radiation Protection Institute, Ghana Atomic Energy Commission; Graduate School of Nuclear and Allied Sciences, University of Ghana, Legon, Accra, Ghana
2 Graduate School of Nuclear and Allied Sciences, University of Ghana, Legon, Accra, Ghana
|Date of Web Publication||6-May-2013|
Radiation Protection Institute, Ghana Atomic Energy Commission, P.O. Box LG 80, Legon-Accra
Source of Support: Radiation Protection Institute, Ghana Atomic Energy Commission., Conflict of Interest: None
The radiological hazards associated with naturally occurring radioactive materials in soil samples from different geological locations have been studied using gamma spectrometry. The average activity concentration of 226 Ra, 232 Th, and 40 K were (16.4-74.6 Bq/kg), (12.0-44.7 Bq/kg), and (215.4-498.6 Bq/kg). The highest values of 226 Ra and 232 Th occurred in Dodowa and Oyibi respectively. The 40 K recorded the highest activity concentration of 498.6 Bq/kg, measured in soil from McCarthy Hills. The radium equivalent activity (Ra eq ; 46.9-135.7 Bq/kg), the internal hazards index (H in ; 0.29-0.52) the external hazard index (H ex ; 0.22-0.37), the absorbed dose rate in air (23.3-75.8 nGy/h), and the annual effective dose (E T ) (44.4-79.4 uSv/y) were evaluated to assess the radiation hazard to the populace living in dwellings made of these soil as a building material. The results obtained were found to be within the acceptable limits for public exposure control recommended by the European Commission, International Commission on Radiological Protection, and Organization for Economic Cooperation and Development-Nuclear Energy Agency.
Keywords: Activity concentration, hazards indices, naturally occurring radioactive materials, soil
|How to cite this article:|
Otoo F, Darko EO, Emi-Reynolds G, Andam AB, Adukpo OK. Radiological impact of soil as a source of building material. Radiat Prot Environ 2012;35:22-8
|How to cite this URL:|
Otoo F, Darko EO, Emi-Reynolds G, Andam AB, Adukpo OK. Radiological impact of soil as a source of building material. Radiat Prot Environ [serial online] 2012 [cited 2021 Apr 13];35:22-8. Available from: https://www.rpe.org.in/text.asp?2012/35/1/22/111406
| Introduction|| |
Radioactive contamination of the environment occurs as a result of natural and man-made sources. The man-made sources are classified as those sources used in the industrial and domestic activities. ,, Soil is regarded as being the main source of radiation exposure to human it acts as a medium of migration for transfer of radionuclides to the biological systems and serves as the basic indicator of radiological contamination in the environment. ,
Natural radionuclides and their associated external exposure depend primarily on the geological and geographical conditions, and vary considerably in the soils around the globe. ,,, The dose rates (DRs) also differ from one place to another depending upon the concentration of natural radionuclides of 238 U, 226 Ra, 232 Th, and 40 K present in the soil. These radionuclides pose radiation exposure risks externally due to their gamma-ray emissions and internally due to radon and its progeny that emit alpha particles. ,, The specific levels of terrestrial radiation are related to the composition of each lithologically separated area and to the content of the rock from which the soils originate. ,
Measurement of natural background radiation and activity concentration in soil used as building materials have been carried out in many countries for the purposes of estimating radiation exposure, and to establish the baseline data for radiation impact assessment and radiation protection. ,,,,,,, This study complements a few other studies, which were carried out and limited to specific locations in Ghana. ,,,,, In view of this, it is important to assess different type of indices to aggregate and assess the contamination level in soil as result of human and industrial activities. The study determines activity concentrations of natural radionuclides in the soil around industrial areas, commercial areas, and residential areas to estimates the annual effective doses associated with these radionuclides within the areas and assess the radiological health impact on the population and the environment.
| Materials and Methods|| |
Background of the study area
[Figure 1] shows the geological map and the sampling points of the study area. The main rock types are the Dahomeyan and the Togo series.
The soil of the study area is classified into four main groups: Drift materials resulting from deposits by windblown erosion; alluvial and marine motted clays of comparatively recent origin derived from underlying shales; residual clays and gravels derived from weathered quartzites, gneiss and schist rocks, and lateritic sandy clay soils derived from weathered Accraian sandstone bedrock formations.
In many low lying poorly drained areas, pockets of alluvial 'black cotton' soils are found. These soils have a heavy organic content, expand, and contract readily causing major problems with foundations and footings.
In some areas, lateritic soils are strongly acidic and when saturated are prone to attack concrete foundations causing honeycombing. Near the foothills are the large areas of alluvial laterite gravels and sands. Many of these deposits are being exploited in an uncontrolled manner for constructional purposes.  There are two rainfall patterns within the region for all the climatic zones, but their intensity differ for the climatic zones. The major rainy season occurs between May and July with the peak occurring in June, whereas the minor rainy season occurs between the months of September to October, with the maximum recorded in October.  The mean annual rainfall generally varies between 740 and 890 mm. The mean monthly temperature ranges from 26 o C to 30 o C.  In general, the study area is characterized by a rugged topography due to the presence of several mountains of different elevations above the sea level. Climatically, some of the areas are arid and the vegetation is scarce, except some grasses and trees, which grow after rainy seasons. Many of these deposits are being exploited in an uncontrolled manner for constructional purposes.
Sample collection and preparation
A total of 154 samples were collected from 11 different geological locations within greater Accra and some part of eastern region of Ghana. The samples were collected from construction industries and open pits which are mostly used for building and farming, respectively. The investigated soil samples are mostly used in construction of houses, bridges, and roads in the country. The samples were then sent to the Environmental Laboratory of Radiation Waste and Safety Centre, Radiation Protection Institute, for analysis. The samples were ground, homogenized, air-dried, and sieved to a uniform mixture with a particle size of about 5 μm, sealed in 1.0 L Marinelli beaker, and stored at room temperature for a period of 3-4 weeks to allow 238 U and 232 Th decay series to reach radioactive equilibrium with the short-lived progenies. ,,
Gamma spectrometry system and activity concentration measurements
The gamma spectrometer system consisting of a high-purity germanium detector coupled to a multichannel analyzer (MCA) with software for data acquisition was used for this work. The detector crystal has a diameter of about 36 mm and thickness of about 10 mm. The crystal is housed in an aluminum canister with a 0.5-mm-thick beryllium entrance window. A lead shield, built with 5-cm-thick lead bricks, surrounds the detector to reduce background from building materials and cosmic rays. The detector is coupled to a Canberra 1510 signal processing unit which contains the power supply, amplifier, and analog-to-digital converter. Digitized counts are collected in a Canberra S100 MCA. Spectrum acquisition and analysis are performed with APTEC software. The detector was cooled down to about 77 K by liquid N 2 .
The energy and efficiency calibration of the system were carried out using solid water in 1.0 L Marinelli geometry from Deutscher Kalibrierdienst of Germany. The standard solution contains the following radionuclides with the corresponding energies: 241 Am (60 keV), 109 Cd (88 keV), 57 Co (122 keV), 139 Ce (1656 keV), 203 Hg (279 keV), 113 Sn (391.69 keV), 85 Sr (514 keV), 137 Cs (662 keV), 88 Yt (898 keV and 1836 keV), and 60 Co (1173 keV and 1333 keV). The samples were counted for 36,000 s. Background measurements were also made for the same period and subtracted from the sample spectra. The Canberra S100 MCA and APTEC software program were used for spectrum acquisition and analysis. The weight of each sample, sampling date, and time, and counting date and time were recorded. Corrections for density and inhomogeneity were also made. , The activity concentration, Ac (Bq/kg), of each radionuclide in any given sample was calculated from the spectrum using the following analytical expression: 
where, M sam (kg) is the mass of sample, N sam is the peak area for the sample in peak range, P(E) is the gamma emission probability, T c is the counting time in seconds, and η(E) is the photopeak efficiency which had been obtained from the standard solution.
Where the parent nuclides were in secular equilibrium with their daughter nuclides, the activity concentrations of the parent nuclides were estimated from their respective daughter concentrations. The transition lines (609.31, 1764.61 keV) of 214 Bi and (583.32, 2614.57 keV) of 208 TI were used to determine the activity concentrations of 226 Ra and 232 Th, respectively. 40 K was determined directly with its only 1460.75 keV peak transition line. 
| Results and Discussions|| |
Activity concentration of the studied samples
The results of activity concentration of the studied samples from different locations in the greater Accra and some part of the eastern regions of Ghana are summarized in [Table 1] and [Figure 2] respectively.
|Table 1: The results of activity concentration of the studied soil samples|
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The table shows the mean activity concentrations and the standard deviation of the radionuclides in the soil while the figure also shows the calculated average activity concentrations of the radionuclides in the soil samples. The distributions of 226 Ra, 232 Th, and 40 K do show strong evidence of normality. However, radioactivity levels vary greatly from one location to another location depending on soil type, depth, sampling periods, and radionuclides present.  It can be seen from [Table 1] and [Figure 2], that the activity concentrations of 226 Ra, 232 Th, and 40 K vary widely in the investigated soils according to the variation of in terrestrial environment, their physical; chemical; geochemical properties and the pertinent to the environment.  From [Table 1] and [Figure 2], the activity of 226 Ra for the samples ranged from 16.4 to 74.6 Bq/kg. The activity concentration of 232 Th ranged from 12.0 to 44.7 Bq/kg. The activity of 40 K has also been observed to vary in the range 215.37-498.57 Bq/kg. The highest values for the activity concentration of 226 Ra and 232 Th were measured from the soil samples collected from Dodowa and Oyibi, respectively. The highest activity concentration of the 226 Ra, 232 Th, and 40 K present in the studied samples were higher than the world average reported in United Nations Scientific Committee on effects of Atomic Radiations,  and 5 times smaller than the reported 226 Ra 232 Th, and 40 K recorded values less than studies from Jordan,  and with the other countries recorded activity concentration greater than the current studies which is given in [Table 2] below. It is also important to mention that these values were not for the whole country as stated, but for the areas from which the samples were collected.
|Table 2: Comparison of activity concentration values (Bq/kg) of soil samples with other studies|
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Estimation of radiation hazards
Radium equivalent activity
The main objective of calculating the Ra eq is to make an estimate of radiation dose likely to be externally due to gamma radiation. This Ra eq provides a useful guideline in regulating the safety standards on radiation protection for the general public. The Ra eq was calculated using the following equation. 
Where, A C Ra, A C Th, and A C K are the mean activities of 226 Ra, 232 Th, and 40 K (Bq/kg), in the soil respectively. The Ra eq is the summation of above mentioned radionuclides and based on the assumption that, 1 Bq/kg of 226 Ra, 0 .7 Bq/kg of 232 Th, and 13 Bq/kg of 40 K produce the same gamma radiation D R . ,
The maximum value of Ra eq must be 370 Bq/kg to keep the external dose to 1.5 mSv/y.  The building materials especially soil whose Ra eq exceeds 370 Bq/kg is discarded to reduce radiaton hazards associated with these materials. The Ra eq values ranged from 46.8 to 135.7 Bq/kg with the highest and lowest values recorded in Ofankor and Oyibi respectively. The highest Ra eq value of 135.7 Bq/kg from Ofankor is lower than the acceptable average of 370 Bq/kg. 
Internal and external hazards indices
The internal and external hazard indices were also evaluated to determine the radiation hazards associated with this investigated soil samples. The external and internal hazard indices are then calculated for the samples with the aim of limiting the radiation dose to permissible limit of 1 mSv/y. 
The H ex was calculated using the following equation,
Where, A C (Ra), A C (Th), and A C (K) are the activity concentrations of 238 U, 232 Th, and 40 K, respectively, expressed in Bq/kg. The H ex value must be less than unity to keep the external gamma dose of soil to a radiation exposure limit of 1.5 mSv per year or radiation hazards to be negligible. ,
The internal hazards index (H in ) gives the internal exposure to carcinogenic radon and its short-lived progeny to the respiratory organs and also the bone marrow and the bone surface cells are considered as the organs of interest.  H in was calculated by a proposed model. 
The values of H in must also be less than unity to have negligible hazardous effects of radon and its short-lived progeny to the respiratory organs. 
The average values of H ex and H in are presented in [Table 3]. From the result, it was found out that, in general they do not exceed the recommended limits, indicating that the hazardous effects of these radiations associated with the soil samples are negligible. The overall calculated mean values of both H ex and H in from investigated soil samples were found to be 0.22-0.37 and 0.29-0.52, respectively. However, in other studies, it was reported that the average values of H ex were found to be 2.03 for Eastern Desert of Egypt,  0.99 for Southeast part of Eskisehir (Turkey)  and 0.84 for Xiazhuang Granite Area (China)  while that of H in were also calculated to be 0.45 for Highlands of Northern Jordan  and 0.51 for Punjab Province-Pakistan.  All the H ex values compared are higher than the current studies, but the highest value of H in occurring in Dodowa was higher than that of other countries.
|Table 3: Radium equivalent activity, internal, external hazard index, absorbed dose rate, and annual effective dose equivalent for the investigated samples|
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Absorbed dose rate
The gamma-radiation absorbed D R (nGy/h) in air at 1 m above the ground was calculated using the conversion factor of 0.462 nGy/h per Bq/kg for 226 Ra, 0.604 nGy/h per Bq/kg for 232 Th and 0.0417 nGy/h per Bq/kg for 40 K.  It is given by the equation below.
where Ac (Ra), Ac (Th) and Ac (K), are the activity concentrations expressed in (Bq/kg) of radium, thorium and potassium in the soil samples. The absorbed D R associate was calculate to range from 23.3 to 93.0 nGy/h occurring at the depth of 10-15 cm to 5-10 cm to 0-5 cm, with the highest value recorded in McCarthy hills. The calculated mean value also varied from 37.1 to 64.7 nGy/h with both values occurring at atomic hills and Dodowa as shown in [Table 3], respectively. The calculated absorbed D R and its mean values were lesser than the worldwide average value of soil (55 nGy/h)  with the exception of few locations which recorded values which are greater than the worldwide average value and within UNSCEAR reported average values (18-93 nGy/h) of terrestrial gamma-ray in normal circumstances. ,
Annual effective dose
To estimate annual effective E T in μSv/y to the public from the measured radioactivity associated with the soil samples the E T was calculated according to the following formula. ,
where, D R is in nGy/h, 0.7 Sv/Gy is the effective to absorbed dose conversion factor, and 0.2 is the outdoor occupancy factor.  The mean values for annual effective dose equivalent calculated for the study area were found to be (44.4-79.4) uSv/y, respectively. According to UNSCEAR (2008) report, the worldwide average annual effective dose for terrestrial gamma-rays in normal environment is approximately 70 uSv/y. The result from the present study summarized in [Table 3], have good agreement with the average worldwide levels. However, for few instances where the annual effective dose do exceed the normal levels at specific locations of the study areas due to the presence of high concentrations of 226 Ra and 232 Th. The values were also found to be less than the limit for public exposure control set by the International Commission on Radiological Protection (ICRP) and Organization for Economic Cooperation and Development-Nuclear Energy Agency (NEA-OECD). ,
| Conclusion|| |
The activity concentration of natural terrestrial radionuclides of 226 Ra, 232 Th, and 40 K and the associated hazards were measured using with gamma-ray spectrometry system for soil samples collected from the most populated areas and industrial areas within greater Accra and some part of Eastern region of Ghana. The activity concentrations of the samples are comparable to the background levels of radiations obtained from other countries and within the world-wide average. A comparison of mean activities of 226 Ra, 232 Th, and 40 K were also made. The studies, indicates that 40 K is the dominant gamma-emitting source in the soil. The radium equivalent activity, external hazards index, internal hazards index, absorbed dose in air, and annual effective dose equivalent calculated were generally found to be within the values found in other countries, with the exception of three samples which showed greater values more than the recommended level for annual effective dose recommended by UNSCEAR (2000). The values were also found to be within the limit for public exposure control set by the European Commission (1999), ICRP 60, and NEA-OECD. , Therefore, the soils from studied areas do not pose any radiation or heavy metals hazard to health when used in building or construction of houses and other facilities.
| Acknowledgment|| |
The authors are grateful to the Environmental Laboratory of Environmental Protection and Waste Management Centre of the Radiation Protection Institute of the Ghana Atomic Energy Commission for the use of their facilities for this study.
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[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3]
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