Assessment of Natural Radioactivity Distribution in Surface Sediments at Erosion and Accretion Sites of Nile Delta Coastal Profiles, Egypt 
Ayman A. El-Gamal 
Department of Marine Geology, Coastal Research Institute (CoRI), National Water Research Center, (NWRC), 15 St., Elpharanaa, Elshalalat 21514, Alexandria, Egypt
Author Correspondence author
International Journal of Marine Science, 2014, Vol. 4, No. 35 doi: 10.5376/ijms.2014.04.0035
Received: 16 May, 2014 Accepted: 30 May, 2014 Published: 17 Jun., 2014
Author Correspondence author
International Journal of Marine Science, 2014, Vol. 4, No. 35 doi: 10.5376/ijms.2014.04.0035
Received: 16 May, 2014 Accepted: 30 May, 2014 Published: 17 Jun., 2014
© 2014 BioPublisher Publishing Platform
This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Preferred citation for this article:
El-Gamal A.A., 2014, Assessment of Natural Radioactivity Distribution in Surface Sediments at Erosion and Accretion Sites of Nile Delta Coastal Profiles, Egypt, International Journal of Marine Science, Vol.4, No.35 (doi: 10.5376/ijms.2014.04.0035)
Abstract
The Nile Delta coast is highly active dynamical coastal processing area. The investigation of gamma emitters of the surface sediments collected from Nile Delta coastal profiles revealed that the distribution of the detected natural radioactive materials is dependent mainly on the coastal processes. The average concentrations of 226Ra, 232Th and 40K in the Nile Delta coastal profiles were 23.78±8.54, 23.95±11.79 and 280.93±68.19 Bq/kg, respectively. These average values are lower than the worldwide averages but still within its ranges. The concentrations of the heavy radionuclides 226Ra and 232Th were found higher in erosion sites compared with the accretion ones. On the other hand, the light radionuclide 40K was behave as reverse as the heavy radionuclides that its concentration values detected at the erosion sites were lower than at the accretion ones. Radium equivalent index calculation indicated that all the sediments under investigation can use safely as building materials. The average of the outdoor gamma dose rate was 37.25±9.58 nGy/h and the annual external effective dose was 0.05±0.01 mSv/y and these values were in agreement with the corresponding values at different parts of the world. The distribution of the detected natural radionuclides and grain size in the coastal sediments at surf, breaker and offshore zones has been discussed.
Keywords
Natural radioactivity; Coastal erosion; Nile Delta; Radium equivalent index; Gamma dose; Grain size
Coastal processes are the set of mechanisms that operate along a coastline, bringing about various combinations of erosion and deposition. Sediment transport is one of the important coastal processes (Dean and Dalrymple, 2002). Erosion of the beach face has resulted in high concentration of dense heavy minerals, while the lower-density minerals were transported either to the offshore or alongshore (Frihy, et al., 1995). Heavy mineral-rich beach sands, concentrated by wave and wind action, have been found to contain significant gamma radioactivity, due primarily to trace amounts of uranium and thorium found in monazite and zircon minerals (Donoghue and Greenfield, 1991; El-Gamal and Saleh, 2012).
In marine ecosystems, radionuclides will disperse with currents, accumulate in biota, and be adsorbed by particles and sediments depending on local conditions and radionuclide properties (Jørgensen and Fath, 2011). Sediments can act as a sink for less mobile discharged radionuclides (Skipperud et al., 2000). Therefore, the use of radionuclides for studying the coastal environments can answer important scientific questions, which may be useful for coastal management (Cochran et al., 2006). The distribution of natural radionuclides at coasts can use as indicator for beach and nearshore sand transport phenomena (Greenfield et al., 1989) which indicate coastal processing such as erosion and accretion.
The assessment of gamma radiation doses from natural sources is of particular importance because natural radiation is the largest contributor of external dose to the world population (UNSCEAR, 2000; Narayana et al., 2007). The primordial and radiogenic radionuclides (e.g. 238U and 232Th series and 40K) are the most dominant radioactivity sources at the Nile Delta coast (El-Gamal et al., 2004). The higher U and Th series member's concentrations have been recorded in the black sand at Rosetta (Rashid) as one of the high background areas compared with the other stations along the Egyptian Mediterranean coast (Saleh et al., 2004). The relationship between grain size and heavy mineral distributions regarding its contents of the natural radioactive elements of the Nile Delta coastal sediments has been investigated by El-Gamal and Saleh (2012).
The present work aims to assess radioactivity distribution in erosion and accretion sites in Nile Delta Profiles and to indicate this variation in surf, breaker and off shore zones in Nile Delta coastal area and its surrounding environment. The data obtained will be used as reference information to assess any change in radioactivity background levels due to coastal processes such as erosion and accretion or any influences on the radiation environment. These results can guide us to a better understanding of oceanographic and coastal processes and phenomena in order to protect and for management of the marine environment.
1 Results and Discussions
The present study revealed that the sediment samples collected from Nile Delta coastal profiles contain primordial terrestrial radioactive elements at remarkably natural levels. Based on HPGe Detector Gamma spectrometric measurements, it could be inferred that elements such as Uranium (238U), Thorium (232Th) and Potassium (40K) are the major causative factors for the radioactivity thus confirming to the previous observations (Ibrahim et al., 1993; El-Gamal et al., 2004; Saleh et al., 2004; Hussein, 2011; El-Gamal and Saleh, 2012). The study indicated that the distribution of the sediment grain size and the natural radioactive nuclides under investigation are dependent on the geological and chemical characters of the coastal sediments and controlled by coastal processing (erosion or accretion) and the protection structure established on the coast.
It is well known that, at the erosion sites, parts of the sediments are removed and transported with the main sea current to deposit at the next sites parallel to the shoreline or transported seaward to the next position at the same profile. The depths of the profiles under investigation were compared with the previous depth records of the same profiles to indicate the actual situation of them are either erosion or accretion. Figure 1 A shows that West of Rosetta profile was recognized as erosion profile at its sampling sites. Figures 1 B and F represent East of Rosetta and East of Ras El-Bar profiles beginning as erosion at distances from 100-300 m and continue as accretion sites at the 400-500 m distances. On the other hand, Figure 1 D shows East of El-Burullus profile was recognized as accretion profile. Figures 1 C and E show that West of El-Burullus and West of Ras El-Bar profiles were started as accretion sites at distances from 100-300 m and continue as erosion sites at the 400-500 m distances.
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Results of the activity concentrations of primordial radionuclides 226Ra, 232Th and 40K (values reported as Bq/kg) and the ratio 226Ra/232Th as well as the calculated radiation hazards indices of the sediment samples are given in Table 1. Out of the 275 km stretch of the coastal line featured in our present study in Nile Delta and coastal profiles extending to 500 m distance, the average activity of 226Ra was 23.79±8.54 Bq/kg and it found to be the highest value (40.3±6.77Bq/kg) at 400 m distance west of Ras El-Bar, while the lowest activity (10.42±7.92Bq/kg) was recorded at 200 m distance east of Rosetta. Likewise, the average activity of 232Th was 23.95±11.79 Bq/kg and its highest activity (53.1±10.26 Bq/kg) was recorded at 500 m distance from west of Ras El-Bar, the lowest activity of 232Th (9.21±6.18Bq kg−1) was recorded at 500 m distance of east El-Burullus. The average 40K activity was 280.93±68.19 Bq/kg and its highest activity (399.04±41.85Bq/kg) was recorded at 400 m distance of east El-Burullus, while the lowest activity (160.55±37.48Bq/kg) was recorded at 200 m distance of west Rosetta.
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The distributions of the detected radionuclides activity concentrations are presented in Figures 2a, 2b and 2c for 226Ra, 232Th and 40K, respectively. Figure 2 a shows that generally the eastern profiles have relatively lower 226Ra (originally from the heavy nuclide 238U) activities than the western profiles. Moreover, at 400 and 500 m distances of the western side profiles at Rosetta and Ras El-Bar observed as higher concentrations than the nearest distances (100-300 m) to shore line. The behavior of the heavy radionuclide 232Th is like 226Ra as shown in Figure 2 b. Reverse behavior has been observed for the light radionuclide 40K with distribution shown in Figure 2 c. This is may be due to that the western profile sediments are subjected more to causes of erosion such as wave and currents than the eastern side profile ones. The sediments resulted from erosion at the western side transferred eastward to deposit at the eastern profiles with the dominant currents carrying the light radionuclides 40K and left the heavy radionuclides 226Ra and 232Th at the erosion sites. The worldwide average concentrations of 238U, 232Th reported by UNSCEAR (2000) are 35, 30 Bq/kg, respectively. Comparably, the detected activities of 226Ra ranged between 10.42 Bq/kg and 40.3 Bq/kg, while activities for 232Th have been found between 9.21 Bq/kg to 53.1 Bq/kg. However, the worldwide averages were close to the resulted ranges.
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Table 2 illustrates the variations between the average concentrations of radionuclides under investigation detected at erosion sites compared with the other accretion ones at Nile Delta coastal profiles. The erosion profile (W_R) shows relatively higher 226Ra and 232Th concentration values with profile averages 31.17 and 30.44 Bq/kg, respectively, compared with the accretion profile (E_B) with averages 18.13 and 18.8 Bq/kg, respectively. On the other hand, 40K detected as relatively higher in the accretion profile (E_B) with average as 298.34 Bq/kg than the erosion one (W_R) with average as 240.48 Bq/kg. Regarding the profiles have the two situations (erosion and accretion), comparison has been set between the first two sites (100-200 m) and the last two sites (400-500 m) to be away from the intersection point around 300 m. It is recognized that the heavy radionuclide 232Th was detected in erosion sites as the greater concentrations than at the accretion sites as shown in Table 2. Also, 226Ra acting as heavy radionuclide has the same behavior like 232Th. It was found in relatively higher concentrations at erosion sites than the accretion ones at the same profile except at E_R. On the other hand, the light radionuclide 40K has reverse behavior than the heavy radionuclides (226Ra and 232Th). Generally, it detected at erosion sites with lower concentrations than at accretion sites except at E_R and W_B.
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Figure 3 shows the discrimination between the erosion and accretion sites according to their contents of 232Th and 40K. The dashed line in Figure 3 is differentiating them into two separate clusters. The erosion sites are characterized by relatively high 232Th and relatively low 40K concentrations and the reverse distribution occurred at accretion sites.
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The average Raeq activity in Nile Delta coastal area was found to be 79.6±22.17 Bq/kg. The highest value was calculated as 124.46 Bq/kg at 500 m distance from west of Ras El-Bar profile, while the lowest value was calculated as 45.99 Bq/kg at 200 m distance at west of Ras El-Bar profile. The recommended maximum levels of radium equivalents for building materials to be used for homes are, 370 Bq/kg and for industries is 370–740 Bq/kg (UNSCEAR, 1982). All the materials examined are acceptable for use as building materials as defined by the Organization for Economic Cooperation and Development (OECD) criterion.
The calculated outdoor gamma doses were listed in Table 1 with average value 37.25±9.58 nGy/h. The highest value was 55.64 nGy/h measured at 500 m distance from west of Ras El-Bar profile sediment, while the lowest dose value was 21.93 nGy/h measured at 200 m distance from west of Ras El-Bar profile one. The calculated gamma dose average value is lower than other sites in Turkey but is close to the present work ranges especially the upper limits. The detected range was comparable with gamma dose measured at Balacali (54.1 nGy/h), Yeniyayla (55.8 nGy/h) and Karatas (49.5 nGy/h) (Degerlier, 2012). UNSCEAR (1993 and 2000) mentioned that the sources of radiation could vary from place to place but the dose rate generally falls between 80 and 150 nGy/hr world over which is higher than the average value of the present work.
An estimation of the annual external effective dose (AEED) rates depicts a measure of the effective dose equivalent to be received by the public due to sediments radioactivity, as computed from the activity concentrations of 232Th, 238U and 40K in bulk sand samples from Nile Delta, as listed in Table 1. AEED in Nile Delta coastal area is varied from 0.03 to 0.07 mSv/y with an average of 0.05±0.01 mSv/y. It is obvious that the average AEED values at Nile Delta are considerably lower than the world average (0.07 mSv/y) (UNSCEAR, 1993) but still close to the higher limit of its range. The values of measured AEED is in agreement with the values from Northern Jordan, for instance, which was reported as 0.06 mSv/y (Ibrahim and Mohammad, 2009) while it is lower than Preta Beach of Brazil that reported as 0.15 m Sv/y (Freitas and Alencar, 2004) and from Orissa (2.0 mSv/y) (Mohanty et al., 2004).
According to depths the sampling locations within the profiles are classified and divided into three zones; surf zone (0-2 m), breaker zone (2-4 m) and offshore zone (>4 m). Figures 4-6 are represent the distribution of the grain size analysis of the sediment samples and the detected natural radionuclides (226Ra, 232Th and 40K) in the three zones. Correlation analysis was performed between mean grain size (mm) with the radionuclides concentrations of 226Ra, 232Th and 40K (Bq/kg) at surf, breaker and offshore zones. The correlation coefficients are presented in Figure 4. High correlation was found between the mean grain size with 226Ra (r = 0.999) and 232Th (r = 0.955) concentration values at surf zone. This is my be due to the surf zone is the most dynamic part of the beach and it is characterized by Surf zone currents which can transport sediment onshore, longshore and offshore and build the (sand) bars and troughs that occupy the surf zone (Short, 1999). This sediment transport mechanism play important rule for sorting the sediment grains and also the sediments holding the radioactive materials. No correlation was found between mean grain size and the light natural radioactive nuclide 40K (r = 0.082). In breaker and offshore zone, negative weak correlations between mean grain size and the concentration of 226Ra (r = -2.61 and -0.323) and 232Th (r = -0.39 and -0.33), respectively were found.
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1.1 Surf Zone
Due to erosion status of Rosetta and Damietta promontories, the depths in front of Rosetta (east and west) and east of Ras El-Bar sea walls are more than 0-2 m and there is no surf zone at their coasts as shown in Figure 4. The other three stations (W_B, E_B and W_R) are representing accretion status with different rates at surf zone. As shown in Figure 1, the higher accretion rate has been observed at Eastern El-Burullus surf zone than west of El-Burullus during the period from 2004 to 2008. As shown in Figure 5, the eastern side of El-Burullus characterized by relatively lower amount of fine sand and appearance of coarse sand, medium sand and very fine sand. Also, relatively small amount of 226Ra, 232Th and 40K have been detected at surf zone of east of El-Burullus. The western side of Ras El-Bar surf zone has characterized by relatively high amount of fine sand, low amount of medium sand, relatively higher amount coarse sand and very coarse sand as shown in Figure 5. Relatively high amount of 226Ra, 232Th and 40K have been detected at surf zone of west of Ras El-Bar. Therefore, the accreted sands contain much more coarser grains than the eroded ones. In the natural separation the heavy minerals increase in the beach surf face due to the action of waves. On the other hand, the lighter minerals increase seaward due to their settling in a gradual decrease of energy levels (El-Askary and Badr, 1996).
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1.2 Breaker Zone
In general, it may be said that the coarse material tends to accumulate in the breaker zone and starts getting progressively finer seaward as the wave energy reduces in that direction. The breaker zone sediments revealed a higher content of less heavies and lower content of more heavies than in the beach surf face sediments (Badr et al., 1993). Due to severe erosion history of Rosetta, the depths in front of Rosetta (east and west) sea wall are also more than 2-4 m and there is no breaker zone at their coasts as shown in Figure 1. At breaker zone, east of El-Burullus is still the higher accretion rate characterized than its western side by relatively lower fine sand and relatively higher amounts of the heavy elements and the natural radionuclides under investigation is detected in relatively higher amount at 2-4 m depths as shown in Figure 6. At west of Ras El-Bar the amount of fine sand is decreased than at surf zone and the coarse sand and very coarse sand have been observed at this zone. On the other hand, erosion site at this zone is the eastern part of Ras El-Bar, which characterized by low amount of medium, coarse sand and silt and relatively higher amount of 226Ra and 232Th but low 40K.
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1.3 Offshore Zone
As shown in Figure 1, west Rosetta and east El-Burullus profiles are indicated erosion and accretion along their profiles, respectively. The other profiles under investigation were characterized by reverse processes from erosion to accretion such as east Rosetta and east Ras El-Bar, and from accretion to erosion such as west of El-Burullus and west of Ras El-Bar. Close relations of heavy elements distribution have been observed at Figure 7 between east Rosetta and east Ras El-Bar and with west El-Burullus and west of Ras El-Bar. On the other hand, no coarse sand or very coarse sand have been detected at the erosion sites as west of El-Burullus and West of Ras El-Bar have relatively higher amount of 226Ra and 232Th.
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2 Conclusion
It is concluded that the radioactive materials detected in the Nile Delta coastal profiles are naturally occurring and its distribution is dependent mainly on the coastal processes. The conclusion can be summarized as the following points:
(1) The Nile Delta coastal profiles were classified according to their coastal processing status into erosion profile (e.g. west of Rosetta), accretion profile (e.g. east of El-Burullus), erosion-accretion profiles (e.g. east of Rosetta and east of Ras El-Bar) and accretion-erosion profiles (e.g. west of El-Burullus and west of Ras El-Bar).
(2) The average concentrations of 226Ra, 232Th and 40K in the Nile Delta coastal profiles were 23.78±8.54, 23.95±11.79 and 280.93±68.19 Bq/kg, respectively. These average values are lower than the worldwide averages but still within its ranges.
(3) The concentration values of 226Ra and 232Th were found higher in erosion sites between and within the profiles compared with the accretion sites. On the other hand, the light radionuclide 40K concentration values were behave as reverse as the heavy radionuclides that it detected at the erosion sites with lower concentrations than the accretion sites.
(4) The investigation of Radium equivalent index indicated that all the sediments under investigation are acceptable to use as building materials. The average of the outdoor gamma dose rate was 37.25±9.58 nGy/h and the annual external effective dose 0.05±0.01 mSv/y and these values were in agreement with the corresponding values at different parts of the world.
(5) The distribution of the detected natural radio- nuclides and grain size in sediments at surf, breaker and offshore zones has been discussed.
3 Materials and Methods
3.1 Study area
The area under investigation is the Nile Delta coastal region which represents a part of the Mediterranean coast of Egypt (Figure 8). It covers about 280 km2 of coastal sediments profiles extended distances seaward till 500 m bounded by latitudes 26º35/- 26º45/N and longitudes 33º42/ - 33º52/E. Generally, the coast of Nile Delta is highly active dynamical coastal processing area and it is highly affected by River Nile discharge. Coastal erosion has been observed and induced on the Rosetta and Damietta Promontories after construction of the Aswan High Dam in 1964 due to elimination of alluvial sediments, although climatic factors may have been important (Frihy and Khafagy, 1991). Many artificial coastal protection structures have been established at the Nile Delta coast to control the erosion process such as sea walls, detached breakwaters and groins. The original erosion/accretion patterns along the Nile Delta promontories have been reshaped due to the massive protective structures built after 1990. The behavior of coastline pre and post construction indicates that coastal erosion fronting protective structures has declined in the case of the seawalls at the tips of the Rosetta and Damietta promontories, or has been partially replaced by sand accumulation in the case of detached breakwaters at Baltim (east of Burullus headland) and at Ras El-Bar (west of the Damietta promontory) (Frihy et al., 2003).
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3.2 Sampling and sample preparation
In order to study the behavior of the different natural radionuclides and in accordance with the hydrochemical zoning of the Nile Delta coast, different sampling points at six profiles lie east and west of Rosetta promontory, El-Burullus outlet and Damietta promontory have been selected (Figure 8). Surface sediments were collected from the coastal profiles at different distances 100, 200, 300, 400 and 500 m from fixed benchmarks at beach using grab sampler from rubber boat (Zodiac) at the end of May 2008. Each distance is corresponding to specific depth according to the sampling site. The samples were washed and dry for mechanical analysis. For radioactivity measurements, about 2.25 g of sediment sample materials were grind and homo- genized with a small marble mortar. The samples were stored in cylindrical plastic containers with a 1.0 cm diameter that fits into a germanium well detector for gamma spectrometry. Each sediment sample was covered by >1 cm layer of epoxy to isolate it from air to ensure that the radon gas is confined within the volume. Each sample was carefully sealed for four weeks to ensure secular equilibrium between 226Ra and 222Rn.
3.3 Gamma spectrometric analysis
Activity measurements were performed by using a well type high-resolution gamma-ray spectrometer with hyper-pure germanium (HPGe) detector. All the radioactivity measurements and its calculations were carried out at the main laboratory of the Environmental Radioactivity Measurement Facility, Department of Earth, Ocean and Atmospheric Sciences, Florida State University, USA. In order to reduce gamma-ray background, the detector is shielded by a cylindrical 10 cm lead shield. The accumulation counting time of activity or background ranged from 25 to 71 hours. The 226Ra activities (or 238U activities for samples assumed to be in radioactive equilibrium) were estimated from the gamma transition lines of 214Pb (295 and 351.9 keV) and 214Bi (609.3 keV). The gamma ray energies of 228Ac (338.4 and 911.2 keV) and 208Tl (583.1 keV) were used to estimate the concentration of 232Th. The activity concentrations of 40K were measured directly by its own gamma ray (1460.8 keV). The activity concentrations were calculated from the intensity of each line taking into account the mass of the sample, the branching ratios of the gamma-decay, the time of counting and the efficiencies of the detector. The background spectra were used to correct the net peak area of gamma rays of measured isotopes. Efficiency, background in CPM and minimum detectable activity (MDA) in dpm/g are listed in Table 3 for all energies used in this investigation.
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3.4Theoretical calculations
The gamma-ray radiation hazards due to the specified radionuclides were assessed by different indices. Radium equivalent activity (Raeq), a widely used hazard index, is an index that has been introduced to represent the specific activities of 226Ra, 232Th and 40K in sediments by a single quantity. Its calculations based on the assumption that 10 Bq226Ra/kg, 7 Bq232Th/kg and 130 Bq40K/kg produce the same gamma dose rate, which takes into account the radiation hazards associated with them. It calculated using the following equation (UNSCEAR, 2000; El Mamoney and Khater, 2004; Derin et al., 2012):
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Where CRa, CTh and CK are the activity concentrations (Bq/kg dry weight) of 226Ra, 232Th and 40K, respectively.
The external hazard index is obtained from Raeq expression through the supposition that its maximum allowed value (equal to unity) corresponds to the upper limit of Raeq (370 Bq/kg). This index value must be less than unity in order to keep the radiation hazard insignificant. The external hazard index can be defined as following equation (Beretka and Mathew, 1985; Singh, et al., 2005; El-Taher, 2010):
.png){kind=small}
Where ARa, ATh and AK are the specific activities of 226Ra, 232Th and 40K in Bq/kg, respectively.
The absorbed dose rates (D) due to gamma radiation from natural radionuclides at 1 meter above the ground surface, assuming uniform distribution of the naturally occurring radionuclides (226Ra, 232Th and 40K), were calculated by Monte Carlo method based on guidelines provided by UNSCEAR (2000). This calculation was performed after assuming that the contributions from other naturally occurring radionuclides are insignificant. The value of D was calculated by (UNSCEAR, 1988 and 2000; Derin et al., 2012):
D (nGy/h) = 0.462 CRa + 0.604 CTh + 0.042 CK
Where CRa, CTh and CK are the specific activities (Bq/kg dry weight) of 226Ra, 232Th and 40K, respectively.
To estimate the annual external effective dose rates, the conversion coefficient from absorbed dose in air to effective dose (0.7 Sv/Gy) and an outdoor occupancy factor (0.2) proposed by UNSCEAR (2000), were used. Accordingly, the annual effective dose rate (mSv/yr) (AEED) was calculated by:
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Acknowledgment
The authors would like to express deep gratitude to Prof. William Burnett, Department of Earth, Ocean and Atmospheric Sciences, Florida State University, Tallahassee, USA, for his contribution that the radioactivity measurements and calculations were carried out in his laboratory. The authors also thank the maritime staff of Coastal Research Institute for the help in samples collection.
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. Coastal erosion
. Nile Delta
. Radium equivalent index
. Gamma dose
. Grain size
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. Ayman A. El-Gamal
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. Natural radioactivity
. Coastal erosion
. Nile Delta
. Radium equivalent index
. Gamma dose
. Grain size
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