Radiological characterization of beach sediments along the Alexandria–Rosetta coasts of Egypt Academic research paper on "Earth and related environmental sciences"

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Radiological characterization of beach sediments along the Alexandria-Rosetta coasts of Egypt

A.A. Abdel-Halima'*, I.H. Salehb

a Department of Basic and Applied Sciences, College of Engineering and Technology, The Arab Academy for Sciences and Technology and

Maritime Transport, P.O. Box 1029, Alexandria, Egypt b Department of Environmental Studies, Institute of Graduate Studies and Research, Alexandria University, P.O. Box 832, EL-Shatby, Alexandria,

Abstract

In the present study, 52 sediment samples were collected from 14 sites along the area extending from west of Alexandria (El-MAX) to the eastern side of the Rosetta promontory (the terminal of the Nile River with the Mediterranean Sea). The collected samples were analyzed for radioactive contents. 226Ra, 228Ra, 40K and 137Cs were detected. The distribution of radionuclide activity and mass concentrations of Th and U displayed a specific pattern that reflects the mineralogical formations and beach stability. Radiological hazards were investigated by calculating the following radiological parameters: the radium equivalent, radiation hazard index and annual effective dose. It was observed that the levels of radiological parameter are higher in eastern locations than in western ones. In addition, the western side displayed radiological parameters higher than the recommended world-wide values. ©2015 The Authors. Production and hosting by Elsevier B.V. on behalf of Taibah University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Effective radiation dose; Beach sands; Radiation hazard index; Radioactivity; Th and U

radiation [1]. Littoral areas of the environment receive radioactive pollution, either natural or anthropogenic, through rivers and rainfall [2]. Marine sediments are essential reservoirs for natural and artificial radionuclides due to their diverse composition. The uptake of radionuclides by marine sediments depends on their physical and chemical properties [3]. The radionuclides distribution in marine sediments provides essential information concerning sediments movement and accumulation that provide a strong signal indicating sediment origin [3]. The concentration and distribution of 226Ra, 232Th and 40K in sands are not uniform throughout the world [4], and their distribution in soil is based on the nature of its geological formation [5,6].

Mahdavi [7] studied the concentrations of thorium, uranium and potassium in beach sands of the Atlantic and Gulf coasts. He concluded that thorium and uranium in

http://dx.doi.org/10.1016/j.jtusci.2015.02.016

1658-3655 © 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of Taibah University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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1. Introduction

Human environment is naturally radioactive, and human beings are exposed to radiation arising from natural sources, including cosmic and terrestrial origin, in addition to artificial radioactivity from fallout in nuclear testing and medical applications. Natural sources contribute approximately 80% of the environmental

* Corresponding author. Tel.: +20 1065601168; fax: +20 34285792. E-mail addresses: amoneim@aast.edu (A.A. Abdel-Halim), ibshsh@yahoo.com (I.H. Saleh). Peer review under responsibility of Taibah University.

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beach sand are contained mainly in resistant heavy minerals such as monazite, zircon and xenotime. Moreover, the highest concentrations found in beach sands were 1-2mg/kg for thorium and 0.3-0.6 mg/kg for uranium. He also found that beach sands have a thorium/uranium ratio of approximately 2.5-3.0. The locations and areas of black sands that contain monazite minerals are of interest for researchers. This is because monazite sand is considered an important geological material [8] because it may contain 0.1-0.3% uranium and 5-7% thorium, which are the main elements used in nuclear power plants [9]. Around the world, several authors have been studying radionuclide concentrations in sand beaches in the Kerala and Tamil Nadu coastal regions of India [10], in Bangladesh [9] and in southwestern Australia [11]. Also in India, Kannan et al. [12] analyzed the distribution of natural and anthropogenic radionuclides in beach sand and soil from the Kalpakkam area by using gamma ray spectrometry.

In certain beaches of Egypt, Brazil and along the west coast of India, there are areas that are well known for their high background radiation. The Rosetta and Damietta beaches on the Mediterranean coast of Egypt present high radiation due to the presence of black sands that contain zircon and monazite minerals.

2. Study area

The study area extends along the Mediterranean coast of Alexandria-Rosetta in Egypt and covers 14 sampling sites for beach sands from west of Alexandria (El-Max site) to the east side of the Rosetta Nile promontory (the terminal of the Nile River with the Mediterranean Sea). The ground positions of the El-Max site (1) to the east side of the Rosetta site (14) are (31°09' N, 29°50' E) to (31°27' N, 30°22' E), respectively, as illustrated on the map in Fig. 1.

The ground positions (latitudes and longitudes) and the local names of the studied sites are given in Table 1.

The beach sediments in the study area are characterized by their variability in geological formations. The bottom sediments of the inshore area are covered predominantly by sand, which vary from fine in the west to coarse shelly in the east. This sandy zone merges seawards into a silty-sand belt. In the northwest area, the silty-sand gradually changes into sand-silt-clay. The sand-silt is followed outward by a clayey-silt in the northeast. Minerals of light fraction are represented by calcium carbonate, which is introduced in the form of shells or shell fragments. The sediments at the El-Max area were poor in the amount of heavy minerals compared with those at the Abu Qir

Table 1

The ground positions and local names of the study sites.

ID Site name Latitude Longitude

1 El-Max 31°09'17.17" 29°50'32.11'

2 Anfushi 31°12'19.69" 29°52'38.32'

3 Manshia 31°12'03.82" 29°53'40.06'

4 Chatby 31°13'00.33" 29°55'13.78'

5 Sporting 31°13'58.56" 29°56'40.45'

6 Gleem 31°14'51.44" 29°57'33.26'

7 Asafra 31°16'48.87" 30°00'27.54'

8 Abu-Qir (W) 31°19'36.07" 30°03'56.07'

9 Abu-Qir (E) 31°18'06.48" 30°04'41.43'

10 El-Tarh 31°16'41.01" 30°06'58.07'

11 El-Maadyia 31°16'54.91" 30°12'37.60'

12 Edku 31°23'33.08" 30°20'07.10'

13 Rosetta (W) 31°27'29.04" 30°21'40.00'

14 Rosetta (E) 31°28'19.02" 30°22'10.50'

site. This is characterized by high frequencies of pyroxenes, amphiboles and epidotes and by low percentages of tourmaline and zircon of 1.1% and 2.4%, respectively. The main source of heavy minerals is mostly the Nile sediments, which decrease westward [13]. Mineralog-ically, the shelf of Alexandria is subdivided into three main provinces: (1) aragonite/calcite in the western part, (2) calcite/aragonite/quartz in the middle part and (3) quartz/calcite/aragonite in the eastern part.

In the west, the major minerals are carbonates; on the other hand, on the eastern side, heavy minerals are dominant. The distribution of heavy mineral assemblages has been recognized as two mineral groups [14,15]. The first group includes heavy minerals of low density and coarse size (augite, hornblende and epidote). Heavy minerals in this group increase from west to east along the area, as it is easy to entrain and transport the coastal sediments toward the east by wave currents. In contrast, the second group includes heavy minerals of high-density (opaque, garnet, zircon, rutile, tourmaline and monazite). These minerals are difficult to entrain and transport by wave-current actions. Hence, minerals in this group form a lag deposit within the delta and sand beaches. Rosetta is highly affected by the black sand deposits that are transported to Rosetta beaches by the Nile River water current during the time period before building the High dam in Upper Egypt.

Black sands are characterized by heavy mineral contents, such as monazite, which contains, overall, two orders of magnitude more of 232Th and 238U and an order of magnitude less of 40K compared to light minerals. Radiometric analysis of various fractions of heavy mineral sands showed that the monazite and zircon sands have highly radioactive contents (U and Th) compared

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Fig. 1. The map of the study sites.

to other minerals in the heavy mineral suite [9,16]. An abundance of thorium, typically approximately 10 wt%, and of U, approximately 0.5 wt%, are found in monazite crystals. Zircon typically contains 5-4000 ppm of U and 2-2000 ppm of Th [9,17]. On the other hand, chlorite, biotite, tourmaline, apatite, magnetite and quartz may contain uranium.

3. Materials and methods

Fifty-two surface sand samples were collected from 14 locations along the investigated area extending from west of Alexandria (El-MAX) to the eastern side of the Rosetta promontory (the terminal of the Nile River with the Mediterranean Sea), as shown in Fig. 1. Each sample was collected by employing a template method of 1m x 1 m up to a depth of 8 cm, pooling the whole sample tightly; an aliquot of an approximately 1.5 kg sample was collected after mixing thoroughly. After collection, each sample was dried in an oven at 105-110 °C for approximately 24 h and sieved through a 2-mm mesh-sized sieve to remove stones, pebbles and other macro-impurities. The homogenized sample was placed in a PVC beaker. It was sealed hermetically and externally by using cellophane tape and kept aside for approximately 28 days to ensure equilibrium between 226Ra and its daughters before being used for gamma spectroscopic analysis.

The prepared samples were measured by a gamma ray spectrometer system in the radiation laboratory at the Department of Environmental Studies, Institute of Graduate Studies and Research (IGSR), Alexandria University. The measuring system consists of a p-type coaxial HPGe with an efficiency of 24.5% and a

resolution of 1.7 keV at 1.33 MeV. The gamma spectrum was recorded by using a PC-based 8192 channel analyser and processed by using the Genie-2000 software. The spectrometer was calibrated for energy by using a set of certified gamma radiation standard sources (137Cs, 60Co, 57Co and 241Am). The absolute detection efficiencies were calibrated by using a certified standard source (152Eu) and soil reference materials prepared in geometrical shape and composition to simulate the investigated samples matrix [18]. The detector was shielded by using a cylindrical lead castle of 0.1-m thickness with an internal wall made of copper. For internal quality control requirements, reference soil samples (MAPEP-13/14, Soil) were analyzed during the measurements to confirm the calibrations. Externally, the laboratory participates periodically in the proficiency testing (PT) program (MAPEP) for radiation measurements.

The activity concentration of 40K was determined by using the 1460.8 keV gamma line. The lines 186.2, 295.2, 351.9, 609.3 and 1120.3keV were used for 226Ra (238U decay series) activity determination. The lines 338.4, 911.0 and 583.1 keV were used to determine the activity of 228Ra (232Th decay series). The activity concentration of 173Cs was determined at 661.7keV [19].

The minimum detectable activity (MDA) was calculated for each radionuclide according to Eq. (1) [20]. The levels of MDA were calculated based on the counting conditions used for measuring the studied samples and listed in Table 2.

T x Eff(E) x PY (E) x M

where T, Eff(E) and PY (E) are the counting time, full-energy peak efficiency at photon energy E and emission

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Table 2

The minimum detectable activity (MDA) for the detected radionuclides at 10 h counting time.

Radionuclide Gamma energy (keV) MDA (Bq/kg)

Ac-228 911.0 0.02

Cs-137 661.7 0.02

K-40 1460.8 0.20

Pb-214 351.9 0.06

Bi-214 609.3 0.06

Ra-226 186.2 0.20

probability, respectively. Finally, LD is the detection limit, calculated by using the following equation,

Ld = Lc + Kctd

where LC is the critical level, below which no signal can be detected, aD is the standard deviation and K is the error probability.

4. Results and discussion

The distributions of measured levels in beach sands along the study sites are discussed. Thorium and uranium mass concentrations are calculated, and their geological origin in the area are discussed.

4.1. Distribution of radionuclides level in beach sands

showed that the highest level is 600-fold of the lowest value. The very wide difference between the El-Max site in the west and the Rosetta promontory sites in the east could be explained by the presence of black sands deposits, which contain heavy minerals such as monazite. These sands contain, overall, two orders of magnitude more of 232Th and 238U decay series radionuclides [9,16]. The levels of 226Ra and 228Ra at site number 9 (eastern side of the Abu-Qir beach) are 77.09 and 40.82 Bq/kg, respectively. These values are higher than those detected at the other studied sites, except for those at Rosetta promontory sites 13 and 14. The reason for this may be due to the nature of the Abu-Qir Gulf, which receives black sands that are carried by beach littoral water currents coming from the Rosetta promontory and that circulate in the Abu-Qir Gulf. The variability in U and Th observed at the western side of Rosetta and along the Alexandria study sites can be mainly explained by the heavy and light mineral contents. Such variability is a consequence of the redistribution of the sand due to coastal processes, such as erosion and accretion, that take place in the study area or at any place in the world with the same situation.

For more explanation, 232Th and 238U concentrations in ppm were calculated for all sites according to the following formulae:

Au ATh

Cu = T2U5andCTh = 4~Ö8

where Cu and CTh are the concentrations in ppm of 238U

The values of radioactivity were calculated for 226Ra, and 232Th, respectively. AU and ATh are the radioactivity

concentrations in Bq/kg of 226Ra and 228Ra, respectively, and 12.45 and 4.08 are the specific activities of 238U and 232Th in Bq per mg of pure 238U and 232Th, respectively.

The obtained ppm concentrations for 232Th and 238U are shown in Fig. 5 for all investigated sites. It is clear that the 232Th concentration exceeds the 238U concentrations by 2-fold at site 13 and site 14, located west and east, respectively, of the Rosetta promontory. On the other hand, it exceeds only by 1-fold at site 8 (Asafra) and site 9 (Abu-Qir Gulf). According to Deer et al. [17], the abundance of thorium is typically approximately 10 wt% and that of U is approximately 0.5 wt% in monazite crystals. On the other hand, zircon typically contains 5-4000 ppm of U and 2-2000 ppm of Th. Therefore, the major contributors for enhancing the levels of radiation are monazite and, to a lesser extent, zircon, compared to other minerals [16].

228Ra, 40K and 137Cs in all measured samples in units of Bq/kg (dry weight); then, the average radioactivity of each radionuclide was calculated for each site. Additionally, the distribution of radioactivity levels for 226Ra, 228Ra, 40K and 137Cs are given. The mass concentrations of Th and U were calculated from 238U (assuming secular equilibrium between 238U and 226Ra, as well as their progenies) and from 232Th (assuming secular equilibrium between 232Th and 228Ra).

The log-linear distributions of radioactivity levels over the investigated sites are shown in Figs. 2-4 for

226Ra, 228Ra and 40K, respectively. 226Ra results indicated a range of (12.6-499.184) Bq/kg. The lowest level was observed at station number 1 (El-Max), and the highest was found at station number 13 (west of the Rosetta promontory). It is clear that the highest level of 226Ra is 40-fold of the lowest level.

Similarly, 228Ra radioactivity levels showed a range of (0.65-386.2) Bq/kg. The lowest value was recorded at site 1 (El-Max) and the highest levels were at site 13 (west of the Rosetta promontory). The range of 228Ra

The 137Cs levels distribution along the studied stations is shown in Fig. 6. The levels ranged from <0.1 Bq/kg to 3.22 Bq/kg, with an average of 0.96 Bq/kg. The lowest were detected at station 2 (Anfushi) and

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_ 1000-,

1 2 3 4 5 6 7 8 9 10 11 12 13 14 Stations

Fig. 2. The distribution of 226Ra levels along the study sites.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 Stations

Fig. 3. The distribution of 228Ra levels along the study sites.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 Stations

Fig. 4. The distribution of 40K levels along the study sites.

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Stations

Fig. 5. The distribution of 238U and 232Th concentrations, in ppm levels, along the study sites.

6 A.A. Abdel-Halim, I.H. Saleh / Journal ofTaibah University for Science xxx (2015) xxx-xxx

1 2 3 4 5 6 7

Stations

9 10 11 12 13 14

Fig. 6. The distribution of 137Cs levels along the study sites.

station 8 (west of Abu-Qir). The highest was found at station 10 (El-Tarh). The reason for this is that the El-Tarh site is closest to the agricultural drainage in that area. It is well known that such drainage water contains residues of biogenic organic matters, which have the ability to retain 137Cs nuclides. The frequent strong retention of 137Cs at the surface of different soils is due to the presence of clay minerals and organic matter [21-24]. Organic matter content was suggested to affect the retention and migration of the fallout radionuclides in the environment by modifying the adsorption properties of clay minerals in the soil [25,26].

4.2. Radiological assessment of 238U, 232Th and

Radiological assessment was performed for the detected radionuclides using the radium equivalent (Raeq), gamma radiation hazard index (I7), external absorbed dose rate at 1 m above the ground level (D) and the corresponding annual effective radiation dose (ADE).

4.2.1. Raeq

Because the diversity levels of 226Ra, 228Ra and 40K in the studied sites are not uniform, it will therefore be suitable to use Raeq (Bq/kg) as a parameter for comparing the activity of sand samples containing different amounts of theseradionuclides. The (Raeq) values were calculated according to Eq. (4) [27,28]:

eq = CRa + 1.43Crh + 0.077 Ck

where CRa, CTh and CK are the activity concentrations of 226Ra, 228Ra and 40K in Bq/kg, respectively. It has been assumed that 370 Bq/kg of 226Ra (238U), 259 Bq/kg of

228Ra (232Th) or 4810 Bq/kg of 40K produces the same gamma dose rate.

It was observed that the Raeq activity ranged from 19.28 Bq/kg to 1029.65 Bq/kg, with an average of 183.86 Bq/kg. Fig. 7 shows the distribution of levels for the sites. The lowest and highest Raeq levels were found at station 1 (El-Max) and station number 13 (west of the Rosetta promontory), respectively. It is clear that the levels at station 13 and station 14 exceed 370 Bq/kg, which is the acceptable limit for safe use [1] (UNSCEAR, 1988). In addition, station 9, at the eastern side of the Abu-Qir Gulf, recorded 144.97 Bq/kg, which indicates that Abu-Qir(E) is highly affected by the black sand that is transported by the littoral beach current coming from the Rosetta promontory area.

4.2.2. Radiation hazard index

The value of the radiation hazard index I7 must be less than unity to keep the radiation hazard insignificant. The maximum value of I7, i.e., equal to unity, corresponds to the upper limit of radium equivalent activity (370 Bq/kg). The levels of I7 were calculated according to Eq. (5) [1].

T CRa , CTh , CK

I7 =--1---1--

370 259 4810

where CRa, CTh and CK have the same meaning as in Eq. (4).

The obtained results showed that the levels ranged from 0.13 to 7.04, with an average of 1.27, over the studied area. The levels of I7 at sites 13 and 14, located west and east of the Rosetta promontory, were 7.04 and 6.5, respectively, as shown in Fig. 8. It is clear that the I7 levels recorded at Rosetta sites 13 and 14 are approximately 7-fold and 6-fold of the guidance level, respectively.

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со 600

1 2 3 4 5 6 7 Stations

I - ■ ■

9 10 11 12 13 14

Fig. 7. The Raeq levels along the study sites.

Moreover, the level at site 9 (Abu-Qir bay) matched the I7 guidance value.

4.2.3. Radiation doses from 238 U, 232 Th and 40K

The external gamma absorbed dose rate in air at 1 m above ground level was calculated according to Eq. (6) [1].

D = 0.462 CRa + 0.604 CTh + 0.042 Ck (6)

where D is the dose rate in nGy/h, and CRa, CTh and CK have the same meaning as in Eq. (4).

For assessing the health effects of absorbed doses of gamma radiation, the annual effective dose should be obtained. For the conversion from the absorbed dose to the effective dose, the conversion coefficient (0.7 Sv/Gy) was used and the outdoor occupancy factor (0.2) was taken [1] (UNSCEAR, 1988). The annual effective doses

Guidance

оНЯнннвв! 1 в В И

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Stations

Fig. 8. The levels of I7 along the study area.

(AED) in mSv/y were calculated by using Eq. (7). AED (mSVÏ = D (n|y ) x 24 X 365 (h

x 0.2 x 0.7 ( — ) x 10-6 (7)

The mean of the absorbed gamma dose rate due to the activity of 238U, 232Th and 40K in beach sands was found to be 82.49 nGy/h (ranging from 8.36nGy/h to 459.1 nGy/h). It is clear that the mean value in the present study is higher than the world average value of 55 nGy/h [29]. Additionally, the level recorded at site 9 (Abu-Qir east) was 65.25 nGy/h, and the two stations 13 and 14, located at the Rosetta promontory, recorded 459.1 nGy/h and 423.19nGy/h, respectively. On the other hand, the levels at the remaining stations were below the world average. The estimated values

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of the annual effective dose ranged from 0.01 mSv to 0.56 mSv (average 0.1 mSv), which is lower than the world average of 0.480 mSv [29].

4.3. 137Cs absorbed and effective doses

The absorbed dose rates were calculated for detected radioactivity levels of 137Cs by using the dose rate per activity concentration in sands of 0.03 nGyh-1 Bq-1 kg-1 [30]. The absorbed dose rates for all sites showed an average of 0.028 nGy/h, with the levels ranging from <0.003 nGy/h to 0.1 nGy/h.

4.4. Conclusions

After studying the distributions of radioactivity levels and ratios of thorium to uranium and evaluating the radiation doses of radionuclides that exist in beach sands in the area extending from west of Alexandria to the eastern side of the Nile Rosetta promontory, the following conclusions can be drawn:

(1) The highest radioactivity levels of 226Ra and 228Ra appeared in sands at the sites west of the Rosetta Nile promontory and were 499.184 Bq/kg and 386.2 Bq/kg, respectively.

(a) High levels of 226Ra were found, in descending order, at the Rosetta (W), Rosetta (E), Abu-Qir(E), Manshia and Anfushi sites with levels in Bq/kg of 499.18, 456.93, 77.09, 48.8 and 47.8, respectively.

(b) High levels of 228Ra were found, in descending order, at the Rosetta (W), Rosetta (E), Abu-Qir(E), El-Maadyia and Asafra sites with levels in Bq/kg of 368.2, 325.505,40.82, 9.05 and 7.3, respectively.

(c) The higher mass concentration of thorium than that of uranium in the eastern part of the study area is due to the major contributors to enhancing the levels of radiation: monazite and, to a lesser extent, zircon, compared to other minerals.

(2) The radium equivalent levels found at both Rosetta stations exceeded 370 Bq/kg, which is the acceptable limit for safe use.

(3) The external radiation hazard index exceeded the guidance value by 7-fold and 6-fold, respectively, at the east and west sites of the Rosetta promontory of the Nile River.

(4) The absorbed dose rate from each of the U-series, Th-series, 40K and 137Cs radionuclides has an

average of 82.49 nGy/hr, which exceeds the world average (55 nGy/h).

(5) U-series, Th-series, 40K and 137Cs radionuclides contributed 51.06%, 43.43%, 5.57% and 0.034%, respectively, to the annual average of external dose.

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