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 Table of Contents 
ORIGINAL ARTICLE
Year : 2017  |  Volume : 40  |  Issue : 2  |  Page : 60-68  

Environmental impact assessment due to natural radioactivity in the mountain rocks of the Red Sea coast, Egypt


1 Department of Radiation Physics, National Center of Radiation Research and Technology, Atomic Energy Authority, Cairo, Egypt; Department of Physics, Faculty of Science, Al Baha University, Al Bahah, Saudi Arabia
2 Department of Physics, Faculty of Science, Al Baha University, Al Bahah, Saudi Arabia
3 Department of Physics, Faculty of Science, King Khalid University, Abha, Saudi Arabia

Date of Submission14-Feb-2017
Date of Decision09-Mar-2017
Date of Acceptance01-Jun-2017
Date of Web Publication13-Jul-2017

Correspondence Address:
Soad Saad Fares
Department of Radiation Physics, National Center of Radiation Research and Technology, Atomic Energy Authority, Cairo

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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_6_17

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  Abstract 

The mountains' composition, especially the granitic mountains, contains a certain amount of natural radioactivity due to the decay of uranium, thorium, and potassium isotopes. As a part of the impact assessment study, thirty rock samples were collected from natural mountains in the Red Sea coast, Egypt, considered as the most popular ones, and were measured for their natural radioactivity to assess the radiological impact when they are used as building materials. Rock samples were examined by high-resolution γ-spectrometry. The average activity concentration of 238U, 226Ra, 232Th, and 40K were 192 ± 24, 178 ± 27, 66 ± 7 and 193 ± 23, and 287 ± 31 Bq/kg, respectively. The annual effective dose rate (mSv/y), the mean of the absorbed dose rates (D), radium equivalent (Raeq), the external hazard index (Hex) and the internal hazard index (Hin) and the representative level index (Iγr, Iαr) were; 0.25 mSv/y, 205.64 nGy/h, 286.9 Bq/kg, 0.81, 1.33, 1.97 and 0.89, respectively. The specific activity ratios of 226Ra/238U and 232Th/238U were evaluated to analyze the behavior of these radionuclides.

Keywords: Mountains Red Sea, natural radionuclides, radiation hazard index, γ-spectrometry


How to cite this article:
Fares SS, Korna AH, Elsaeedy HI, Alshahrani BE, Yakout H. Environmental impact assessment due to natural radioactivity in the mountain rocks of the Red Sea coast, Egypt. Radiat Prot Environ 2017;40:60-8

How to cite this URL:
Fares SS, Korna AH, Elsaeedy HI, Alshahrani BE, Yakout H. Environmental impact assessment due to natural radioactivity in the mountain rocks of the Red Sea coast, Egypt. Radiat Prot Environ [serial online] 2017 [cited 2017 Aug 19];40:60-8. Available from: http://www.rpe.org.in/text.asp?2017/40/2/60/210579


  Introduction Top


Radionuclides exist everywhere on the earth's surface and can generally be grouped into four classes according to their origin: primordial radionuclides, cosmogenic radionuclides, natural decay series daughters, and anthropogenic radionuclides.[1] Primordial radionuclides have existed on the earth since its creation during the formation of the earth and are distinguished by their extreme long half-lives compared to the life of the earth, such as 40K (T1/2 = 1.248 × 109years),232Th (T1/2 = 1.405 × 1010years), and 238U (T1/2 = 4.468 × 109years). Cosmogenic radionuclides are produced by the interaction of cosmic radiation with the earth's atmosphere and surface. Examples of commonly used cosmogenic radionuclides in chronology are 14C and 10Be.[2] Natural decay series radionuclides are generated from the continuous decay of primordial radioactive isotopes (e.g. 232Th,235U, and 238U). The decay processes comprise nuclear transformation associated with emission of different types of subatomic particles.[3] The decay of these daughters' nuclides induces more than 80% of the total effective radiation dose to the environment and is a major source of radiation. Some of the short-lived radionuclides, such as 131I and 137Cs, are introduced to the environment through human activities including nuclear weapon testing, accidental releases from nuclear power plants, nuclear fuel reprocessing, and many other industrial and medical uses; these radionuclides are called anthropogenic radionuclides whereas the other three origins of radionuclides are natural occurring.

The main sources of radiation exposure to human beings are natural radionuclides. The inescapable feature of life on the earth includes radiation exposure due to terrestrial from naturally occurring radioactive material (NORM), cosmic, and internal source.[4] The distribution of radioactive isotopes is not uniform in nature, and the knowledge of their dispersion in the rocks and soil generally enables to assess any radiation exposure potential to occupants that use these materials to build houses. It is very important to estimate the levels of natural radioactivity in the rocks and to evaluate the gamma dose rate emerging from the earth's crust for the outdoor occupation. Since 98.5% of the radiological effects of the uranium series are produced by radium and its daughter products, the contribution from 238U and other 226Ra precursors is normally ignored.[4] Nationwide surveys have been done to determine the radium equivalent (Raeq) activity of rock and rock samples in many countries.[5],[6],[7],[8]


  Experimental Technique Top


Site description

Thirty rock samples were collected along the Red Sea coast from the city of Suez to Marsa Alam city by long 660 km away. Two samples were collected from each of the 15 locations along the Red Sea coast [Figure 1]. Samples from S1 to S7 were collected from the interior coastal region of the Suez Gulf and the Aqaba Gulf on the Red Sea coast, while the samples from S8 to S16 were collected from the mountains of the Red Sea chains along the entire area of the study, which extended from Suez city (latitude 28°30'3900” N) to Marsa Alam city south (Latitude 25°40' 2900” N).
Figure 1: The mountain Red Sea location and location map of the collected samples

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Sample collection and preparation

Samples of 30 different types of rocks were collected directly from 15 Egyptian sites [Figure 1]. The samples were dried in an oven at 110°C till constant dry weight was obtained, crushed, and homogenized. Rock samples were powdered by grinding using a ball mill and sieved at different grain size (360, 250, 200, 100, and 63 μm); ≤63 μm powders were used. The homogenized samples were packed in a 250 ml plastic container to its full volume with uniform mass. These containers were tightly sealed to ensure that gaseous daughter products of uranium and thorium (radon gas) do not escape from the bottle. These samples were then stored for 30–40 days before counting to ensure 226Ra and short-lived offspring have radioactive equilibrium.[9],[10]

Radioactivity measurements

The activity concentration of the natural radioactivity from 238U,226Ra,232Th, and 40K in the samples was determined using a high-resolution high-purity germanium γ-spectrometry system with 40% relative efficiency. The resolution of this spectrometer was 1.89 keV at 1332 keV γ-rays of 60Co. The efficiency calibration of the gamma-ray spectrometer was performed with the radionuclide-specific efficiency method to avoid any uncertainty in gamma-ray intensities as well as the influence of coincidence summation and self-absorption effects of the emitting gamma photons. A set of certified reference materials (IAEA, RG-set) was used, with densities similar to the rock materials measured after pulverization. This was performed by taking 250 cm 3counting bottles filled up to a height of 7 cm, which corresponds to 170 cm 3, with reference. The measurement duration was up to 90,000 s and the process was carried out in the laboratory, chemical warfare, radioactive materials department, the Egyptian Ministry of Defense.

The obtained spectra were analyzed using Canberra Genie 2000 software version 3.0. Based on the following gamma-ray transitions (in keV), the activity concentrations for the radionuclide were calculated. The 226Ra activities (or 238U activities for samples assumed to be in radioactive equilibrium) were estimated from 234Th (92.38 keV, 5.6%), while γ-energies of 214Pb (351.9 keV, 35.8%) and 214Bi (609.3 keV, 45%), (1764.5 keV, 17%), and 226Ra (185.99 KeV, 3.5%) were used to estimate the concentration of 226Ra. The gamma-ray energies of 212Pb (238.6 keV, 45%), and 228Ac (338.4 keV, 12.3%), (911.07 keV, 29%), (968.90 keV, 17%) were used to estimate the concentration of 232Th. The natural abundance of 235U is only 0.72% of the total uranium content and hence was not considered in the present study. The activity concentrations of 40K were measured directly by its own gamma rays (1460.8 keV, 10.7%).

The following relation was used to obtain the lowest limits of detection (LLD):[11]



where Sb is the estimated standard the error of the net background count rate in the spectrum of the radionuclide, ε represents the counting efficiency, and Iγ is the abundance of gamma emissions per radioactive decay The LLD value for 238U was obtained to be 3.21 Bq/kg while that of 232Th and 40K were 2.44 and 10.81 Bq/kg, respectively.


  Results and Discussion Top


Activity concentrations and ratios

Activity concentrations were determined by measuring their respective decay daughters of 238U and 232Th [12] after subtraction of the background. Empty polystyrene containers were counted in the same way as samples to determine the distribution of the background due to naturally occurring radionuclides in the environment surrounding the detector. Activity concentrations were computed from the intensity of every line taking into consideration, mass of the sample (M), branching percentages of the γ-decay (I), time of counting, and efficiencies of the detector.[13] The activity concentrations of the samples were determined from Equation 2:



where CPS is count per se cond.

The results of analysis of activity concentration of 238U,226Ra,232Th, and 40K radionuclides in all samples for different locations of the study area are presented in [Table 1]. The average activity concentrations of 238U,226Ra,232Th, and 40K in the rocks samples were found to be 192 ± 24, 178 ± 27, 66 ± 7, and 193 ± 23 Bq/kg, respectively. The range of measured activity of 238U in the rock samples was 98–289 Bq/kg. The range of measured activity concentration of 232Th for the rock samples was 13–99 Bq/kg. The activity concentration range of 40K was 111–287 Bq/kg. These differences also attributable to the rock type differences in the region under investigation. The mean activity concentrations of rock materials are 5.4, 5.1, 2.2, and 0.48 times of the worldwide average concentrations for these radionuclides in soil as: 35 Bq/Kg for 238U and 226Ra and 30 Bq/kg for 232Th and 400 Bq/kg for 40K. The wide variations of the activity concentration values are due to their presence in the mountain environment and their physical, chemical, and geochemical properties.
Table 1: Activity concentration of 238U, 226Ra, 232Th, and 40K and radium equivalent doses, ratio between 226Ra, 232Th, and 40K in activity concentration (Bq/kg) and (ppm)

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For a detailed study,238U/226Ra,238U/40K, and 232Th/40K ratios are calculated and are shown in [Table 1]. It is generally expected that 238U and 226Ra being in the same series are in equilibrium and the same was found true, when the associated statistical uncertainties are accounted (10% for 238U and 15%–30% for 226Ra). The activity concentrations ratios of 238U/40K were found to have a wide range from 0.46 to 2.34, with an average value of 1.08, which is almost equal to unity, the global ratio (UNSCEAR, 2000).[4] The ratio of 232Th/40K ranged from 0.08 to 0.69 with an average value of 0.33. This ratio can be used as an indicator of the relative occurrence of these radionuclides.

The elemental ratio of eth/eu (in ppm) was varied from 0.26 (sample no. 29) to 2.23 (sample no. 1) with an average of 1.13. eth/eu ratio is an indicative for the relative depletion or enrichment of radioisotopes. eth/eu ratio for continental crust varies from 3.84 to 4.2.[14],[15] The concentration ratios are higher than unity for most of the sampling sites, which shows low geochemical mobility of thorium. The arithmetic mean of all studied rock samples (1.13) of eth/eu ratio is much lower than the Clark's value (3.5), which indicates U-enrichment in the rock samples in the studied area. The high concentrations results of 238U in these areas of study are due to the presence of phosphate and granite rocks with highly enriched with this radioactive nuclide and the weathering effects.[16]

Radium equivalent and exposure rate

Raeq index is a radiation hazard index used on a large scale. It is an appropriate indicator to compare the specific activity of samples containing different concentrations of 226Ra,232Th (228Ra), and 40K. It is defined on the assumption that 10 Bq/kg of 226Ra, 7 Bq/kg of 232Th, and 130 Bq/kg of 40K produce the same gamma dose rate. It was calculated as follows:[17]



where CRa, CTh, and Ck are the activity concentrations of 226Ra,232Th, and 40K in Bq/kg, respectively. In this study, the Raeq ranged between 116.8 and 449.4 Bq/kg, with a mean of 286.13 Bq/kg. It is inferred that for all the rock samples analyzed, the Raeq activity value is well within and less the permissible limits of 370 Bq/kg, except for samples (3, 6, 8, 9, and 10). No regular trend in the variation of the terrestrial radioactivity has been observed from the study area.

The exposure to ionizing radiations from natural sources occurs due to radioactive elements in the soil and rocks. Their effects in the air can be expressed in terms of exposure rate or absorbed dose rate using the conversion factors from radioelement concentrations in the samples to exposure rate or absorbed dose rate. The ground level exposure rate can be calculated from the apparent concentrations of K (%), AU (ppm), and ATh (ppm) using the expression:[18],[19]





The estimated exposure rate (μR/h) for the studied samples is listed in [Table 2]. In this study, E (μR/h)of the samples was calculated using Equation 5. The values of E (μR/h) ranged from 5.68 to 22.52 (μR/h), with the mean value of 16.15 (μR/h).
Table 2: Exposure rate E, representative level index Iγ, Iα, external hazard index, internal hazard index and activity concentration of 238U, 226Ra, 232Th (ppm), and 40K

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External and internal hazard indexes for finite thickness of walls

External hazard index

The external hazard index (Hex) represents the external radiation exposure associated with gamma irradiation from radionuclides of concern. Hex value must not exceed the maximum acceptable value than one in order to maintain the considerable danger. The Hex definition is as follows:[20]



External hazard index for finite thickness of walls

To estimate the gamma-radiation dose expected to be delivered externally from building materials, a model was suggested by various researchers to limit the radiation dose from building materials to 1.5 mSv/year. In this model, the Hex has been defined (Hex) by some researchers.[13],[17] This model was corrected after considering a finite thickness of walls and the existence of windows and doors.[21] Taking these considerations into account, the equation used for the calculation of Hex is as follows:

Hex= (CRa/740 + CTh/520 + CK/9620) ≤1 (7)

Internal hazard index

The internal hazard index (Hin) is used to control the internal exposure to 222Rn and its radioactive progeny.[15] The internal exposure to radon and its daughter products is quantified by the Hin,[17] which is given by the following equation:

Hin = CRa/185 + CTh/259 + CK/4810 ≤ 1(8)

Where CRa, CTh, and CK are the activity concentrations of 226Ra,232Th, and 40K in Bq/kg, respectively.

The calculated external hazard values of Hex are between 0.43 and 1.14 [Table 2]. The mean value of the Hex (0.81) is lower than the recommended limit and is insignificant. Five locations such as L2, L4, L5, L10, and L11 exceed the recommended limit. This exceedance in these sites is due to the higher concentration of radionuclides. Here also, average relative contribution to the gamma-index due to the 238U is higher followed by the contributions due to 232Th and 40K. The values for Hin in this study ranged between 0.70 and 1.92 with a mean value of 1.33. The Hin exceeds the permissible value in the most rock samples; this means that 222Rn and its progeny contributes significantly to Hin due to radiation from the samples under consideration. The values derived from the second model of Hex for finite thickness of walls ranged between 0.16 and 0.61, with a mean value of 0. 39, since these values are lower than unity.

Representative level index for gamma

An additional hazard index so-called representative level index for gamma (Iγr) is calculated using the formula as follows:[22]

Iγr= (CRa/150 + CTh/100 + CK/1500)(9)

Where CRa, CTh, and CK are the specific activities (Bq/kg) of 226Ra,232Th, and 40K, respectively. The value of this index must be less than unity to keep the radiation exposure insignificant. The index Iγr is correlated with the annual dose due to the excess external gamma radiation caused by superficial material. Values of index of Iγr ≤ 1 correspond to 0.3 mSv/year whereas Iγr ≤ 3 correspond to 1 mSv/year. Thus, the activity concentration index should be used only as a screening tool for identifying materials which might be of concern to be used. According to this dose criterion, materials with Iγr ≥ 3 should be avoided, since these values correspond to dose rates higher than 1 mSv/year.[23]

The calculated Iγr values for all the samples are presented in [Table 2]. The values range from 0.80 to 3.08 with an average of 1.97. The calculated values for most samples are ≤3 which corresponds to an annual effective dose <1.0 mSv/year.

Representative level index for alpha

Several alpha indices have been proposed to assess the exposure level due to radon inhalation originating from rock materials.[24] The representative level alpha index (Iαr) was determined using the following formula:

Iαr = CRa/200 (Bq/kg)(10)

where CRa (Bq/kg) is the activity concentration of 226Ra assumed in equilibrium with 238U. The recommended exemption and upper level of 226Ra activity concentrations in building materials are 100 and 200 Bq/kg, respectively, as suggested by European Commission.[23] These considerations are reflected in the alpha index. The recommended upper limit concentration of 226Ra is 200 Bq/kg, for which Iαr = 1. The mean computed Iαr values for the studied samples are given in [Table 2] for the different rock types and the locations where they were collected. The values of Iαr ranged from 0.45 to 1.52, with the mean value of 0.89. The alpha indexes were lower than unity as shown in [Table 2]. Thus, radon inhalation from investigated rock samples was below the upper level and the study area is safe from the view of environmental radiation hazard.

Correlation studies

To find the existence of these radioactive nuclides together at a particular place, correlation studies were performed between the combinations of radionuclides such as 226Ra,238U,232Th, and 40K. A search was carried out to detect the presence of a statistically significant correlation between the measured radionuclides in the present rock samples.[25]

In this context, [Figure 2] shows linear regression of the activity concentrations of 238U versus 226Ra for all samples. As shown in [Figure 2], concentrations of 238U and 226Ra showed a statistically significant correlation as P < 0.05 at 95.0% confidence level. The R2statistic indicates 67.5% of the variability in activity concentration of 226Ra. The correlation coefficient is equal to 0.82, indicating a moderately strong relationship between the variables. Furthermore, the good correlation coefficient of the 238U/226Ra activity ratio indicates a common source of the parent materials.[26]
Figure 2: Linear regression of the activity concentration of 238U versus 226Ra for all rock samples under study

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Other correlations among measured radionuclide were also investigated which may provide information on the relative depletion or enrichment of the natural radioelement's. [Figure 3] shows a correlation coefficient equals to 0.28, indicating a relatively weak relationship between the variables (232Th and 238U), which disagreed with a previous study in rock samples obtained at the Red Sea coast area.[27],[28] In general, most sites have the ratio 232Th/238U higher than one and this agrees with the reported mean activity concentration ratios of 232Th/238U in sandstone and shale areas, which are 1.7 and 2.5, respectively.[26] This coincides with the same ratio derived from [Figure 3], which is equal to 1.13.
Figure 3: Linear regression of the activity concentration of 238U (ppm) versus 232Th (ppm) for all rock samples under study

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On the other hand, weak correlations were observed between (238U,40K) and (232Th,40K) in the collected samples, with n = 30 and correlation coefficients equals 0.14 and 0.42, respectively, indicating a relatively weak relationship between the variables. Weak correlation may be due to rock processes that affect differently the mobility of the two radionuclides.

The levels of detected radionuclide in all samples indicated wide variations and this may be attributed to the diversity of formations and textures of the rock in the studied area. However, the variability among levels of 238U and levels 232Th are frequently associated with the type of geological minerals. Therefore, detailed mineralogical investigations are needed for more interpretations.

Radon emanation coefficient and radon mass exhalation

The radon emanation coefficient and radon mass exhalation coefficient of samples was calculated based on two γ measurements. The first measurement was carried out directly after sealing of samples, while the second measurement was carried out after attainment of secular equilibrium between radon and its short lived decay daughters (after 30 days). Based on these measurements, the radon emanation coefficient was calculated according to the following expression:[29]



where RnEC is the radon emanation coefficient, No is the net count rate of 222Rn at the time of sealing the sample container, N is the net count rate of 222Rn emanated at the radioactive equilibrium with 226Ra and its progeny.

The mass exhalation rate of radon is the product of the emanation coefficient of radon (RnEC),226Ra content (CRa), and decay constant of radon (λRn = 2.1 × 10−6/s).[30] The mass exhalation rate (ERn in Bq/kg.s) can be expressed as follows:

ERn = CRa × RnEC× λRn(12)

The radon emanation coefficient RnEC and 222Rn mass exhalation rate of rock samples under the current study are shown in [Table 3]. It is clear that the values of the emanation coefficient and the 222Rn and exhalation rate for all samples under investigation was ranged from 0.39 to 0.48 and 83.52 to 265.42 (μBq/kg.s), respectively. This variation in radon concentration confirms an earlier position that the uranium content in the earth crust is different at different locations. [Figure 4] shows a weak correlation between the specific activity of 226Ra and 222Rn radon emanation coefficient with (R2= 0. 0102137, N = 30) for rock samples, which means that 222Rn and 226Ra not accompanied each other and that the individual result for any one of the radionuclide concentration is not a good predictor of the concentration of the other.
Table 3: The specific activity of 226Ra, activity of 238U before and after sealing time, the emanation coefficient, and the radon mass exhalation from the rock samples used in the study area (round off last four columns to one decimal)

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Figure 4: Correlations between the specific activity of 226Ra and 222Rn radon emanation coefficient

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  Conclusions Top


Natural radioactivity levels in rock samples from the Red Sea coast of Egypt have been evaluated and the corresponding radiological index parameters were assessed. The activity concentration of 238U,226Ra,232Th, and 40K in the rocks samples was found to be 289 ± 39, 303 ± 46, 99 ± 13, and 287 ± 31 Bq/kg, respectively, which exceed the world average levels except for 40K.238U/226Ra ratios for most of the 30 rock samples are close to unity, reflecting a state of radioactive equilibrium between U and its daughter,226Ra.

A good correlation coefficient for 238U/226Ra activity indicates a common source of the parent materials. The levels of detected radionuclides in all samples indicated wide variations, and this may be attributed to the diversity of formations and textures of the rock in the studied area.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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    Figures

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

  [Table 1], [Table 2], [Table 3]



 

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