H.A.S. Aly* and F.M. Ragab
Physics Department, University College for Women, for Arts, Science and Education, Ain Shams University, Cairo, Egypt
Received February 19, 2013; Accepted April 29, 2013; Published May 02, 2013
Citation: Aly HAS, Ragab FM (2013) Effect of Ground Water Chemistry and Surrounding Rocks on Radionuclides Distributions and their Environmental Hazard in Southwestern Sinai, Egypt. J Environ Anal Toxicol 3: 172. doi: 10.4172/2161-0525.1000172
Copyright: © 2013 Aly HAS, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Ground water samples from four drilled wells with different depths were collected in May 2011 from Southwestern Sinai to study the distribution of radionuclides and their hazards effect. The depths are ranging between 30 and 150 m. The collected water has very low salinity as the total dissolved salt (TDS) is ranging between 1226 and 1836 ppm. The chemistry is mainly chloride as the anions distribution are in the order Cl- > SO42-> HCO3-> CO32- and the cations are in the order Na+ > Ca+2> K+> Mg+2. Results shown that the correlation coefficient between Na+ and Cl- was strongly positive (0.99), while it was 0.82 between Ca2+ and SO4 2-. The activity concentration of 238U is ranging between 8.2 and 14.0 Bq/L and it is slightly correlated with Mg+. 232Th activity concentration is very low and ranges between 0.316 and 0.683 Bq/L, while 40K ranges between 0.868 and 2.3 Bq/L. The activity concentrations of 222Rn are ranging between 540.5 and 1163.3 Bq/L and its progenies (214Pb, 214Bi) did not reflect its concentration. Radon exhalation rate was calculated using a-track detector. The annual effective dose was calculated for the different personal ages. The babies (< one year) are the most annually dosed.
Radioisotopes; Ground water; Radium-Radon; Exhalation rate; Hazards parameters
Ground water naturally contains several chemical components which can lead to different kinds of health problems. According to Reiman and Banks , a groundwater source can potentially contain several naturally occurring chemical elements with their toxicity. Uranium and its daughter product radon are two naturally occurring elements that can lead to health problems if present in high concentrations in groundwater. Uranium is more harmful due to its toxic nature rather than its radioactivity. Ground water can either be extracted from bed rock (drilled well) or from soil aquifer (dug well). Radon is principally a problem in well drilled in bed rock that contains average or high concentrations of uranium. Regularly limits for 238U and 222Rn vary in different countries over the years , the standards and guide lines that are commonly adopted are shown in Table 1, together with the results of Ost Ostergren et al. .
|Source||Radon Bq/L||Uranium (μg/L)|
|Ostergren et al||100#
It is worth to mention that person consuming water with a uranium content of 100 μg/L of a daily for basis, will receive a dose of 0.1 mSv/y . Also a daily consumption of water with an activity concentration of 0.5 Bq/L of 226Ra leads to a dose of 0.1mSv/y. The reference of 0.5 Bq/L is used as regularity limit for 226Ra in Europe, an even lower value of 0.185 Bq/L (5pCi/L) is adopted for total radium (226Ra and 228Ra) in the US.
In groundwater, uranium (VI) as UO2+ ions, can form complexes with commonly existing ions in ground water such as OH-, CO32-, F-, PO43- and SO42- , and may also be strongly complexed by dissolved humic substances . U(VI) is also strongly bounded by Fe oxides at pH>5 .
The present study has been carried out to calculate the level of natural radioactivity in wells water Southwestern Sinai, Egypt. These wells are mainly the drinking water source for the people populated this area. Measurement of the activity due to 238U, 226Ra, 232Th and 40K in samples was performed by means of gamma spectrometry, using a hyper-pure germanium detector, and to measure radon and thoron concentrations by CR39- detector. Finally calculation of the hazards at different personal ages has been take place.
Three main rock units are exposed in the area connecting the studied four water wells their depths in meters are 150, 90, 75, and 30 named 1, 2, 3 and 4 as shown in Figure 1. The Cambrian unit (450 my) is consisted of three formations known as Sarabit El Khadim, Abu Hamata and Adediya . The Adediya Formation (131 m) is composed mainly of weakly cemented sandstone and represents the main aquifer of the ground water. The Adediya Formation overlies the Abu Hamata Formation which is consisted from siltstone and shale. Sarabit El Khadim Formation is underlying Abu Hamata Formation and is consisted of sandstone and conglomerate. This unit is nonconformably overlying the granite. The second rock unit is the Um Bogma Formation (0.0 – 40 m) which is unconformably overlying the Adediya Formation; the unconformity surface is poly metallic with anomalous uranium and thorium contents and shows gypsum (CaSO4 – 2H2O). The third rock unit is consisted of three formations named El Hashash Formation (60 m) mainly of sandstone, Magharet El Miah Formation (33m) mainly of carbonaceous shale and Abu Zarab Formation (100m) mainly of sandstones.
Samples collection and preparation
A total of four samples were collected from four drilled wells of different depths. Water samples are transformed to Marinelli beakers having (250 ml) volume, and sealed for a period of about four weeks before counting by gamma spectrometry then sealed and stored for four to eight weeks to prevent the escape of the radiogenic gases (222Rn and 220Rn), and to allow the attainment of radioactive equilibrium in the decay chain in order to reach secular equilibrium and to ensure that radiogenic gases are confined within the volume. Gamma spectrum has been collected for 72 hours for each sample.
The total dissolved salts (TDS) are determined by evaporation of a certain volume of water sample till dryness. The difference in weight between empty container and container with precipitate is equal to the amount of the dissolved salts. SO42- anions are determined by evaporation after precipitation with BaCl2. Na+ and K+ are determined by flame photometric technique, while the other cations and anions are determined by titration.
HP-Ge detector and γ-spectrometry
The samples have been analyzed non-destructively, using gammaray spectrometry with high purity germanium (HP- Ge) detector. This detector has a relative efficiency of about 50% of the 3″ × 3″ NaI(Tl) crystal efficiency with resolution of 1.90 keV and peak/Compton ratio of 69.9:1 at the 1.33 MeV gamma transition of 60Co. It is coupled to conventional electronics and connected to a multi- channel analyzer card (MCA) installed in a PC computer. The detector is shielded from the background radiation, using a 10 cm thick lead, internally lined with a 2mm copper foil. The software program (MAESTRO-32) was used to accumulate and analyze the data. The system is calibrated for energy to display gamma photo-peaks between 63 and 3000 keV.
The efficiency calibration was performed by using three wellknown reference materials obtained from the International Atomic Energy Agency for U, Th and K activity measurements: RGU-1, RGTh- 1andRGK-1 .
Uranium-238 activity was determined indirectly from the gamma rays emitted by its daughter products (234mPa) whose activities are determined from the 1001 keV photo-peaks, respectively . Activity of 235U was determined by its gamma ray photo-peaks; 143.8, 163.4, 185.7, and 205.3 keV . The specific activity of 40K was measured by its own gamma- ray at 1460.8 keV, while activities of 226Ra and 232Th were calculated based on the weighed mean value of the irrespective decay products in secular equilibrium. The specific activity of 226Ra was measured using the 186.1 keV from its own gamma-ray (after the subtraction of the185.7 keV of 235U). The specific activity of 214Pb was measured using the 295.2 keV and 351.9 keV while the specific activity of 214Bi was measured using the 609.3 keV. The specific activity of 232Th was measured using the 338.4 keV and 911.2 keV for 228Ac and 583 keV and 2614.4 keV for 208Tl.
Samples are measured by γ-spectrometry using hyper pure germanium detector to determine the activity concentrations in Bq/kg for 238U, 235U, 226Ra, 214Pb, 214Bi, 232Th and 40K.
For radiometric analysis, each sample was transferred to 250 ml capacity polyethylene Marinelli beakers after the attainment of secular equilibrium between 238U, 232Th and their progenies, the samples were subjected to gamma- ray spectrometric analysis. After each sample counting, an empty cylindrical plastic container (polyethylene Marinelli beaker) was placed in the detection system, for a counting period of 48 h, in order to collect the background count rates.
The net area count after background corrections in each photopeak was used in the computation of the activity concentration (C) in Bq kg-1 for each of the radionuclides in the samples using the following
expression after Jibiri et al. : (1)
Where Cn is the count rate under each photo- peak due to each radionuclide, ε is the detector efficiency for the specific γ-ray, Pλ is the absolute transition probability of the specific γ-ray and Ms is the mass of the sample (Kg). The lowest limits of detection (LLD) were obtained from the relation [12,13]:
LLD = (2)
Where Sb is the estimated standard error of the net background count rate in the spectrum of the radionuclide and Iγ is the abundance of gamma emissions per radioactive decay. The LLD values obtained were 9.347, 1.307, 1.344 and 0.025 Bq kg-1 for 40K, 238U, 232Th and 137Cs, respectively .
Alpha particles detections
To determine the concentrations of 220Rn (Thoron) and 222Rn (Radon), CR39 SSNTD (Solid State Nuclear Track Detector) is used to record the produced α-particles activity. The irradiation was performed in air tied closed chamber designed such that the produced track only represents 220Rn and 222Rn. Figure 2 shows used chamber, A represents the CR39 detector which is used to record α-track of particles produced by 222Rn gas that travel a distance more than the alpha particles range and 220Rn diffusion length, B represents another CR39 detector provided for the whole 220Rn + 222Rn activities rather than other α-emitter isotopes activity present in the sample, F is a filter paper that allows 220Rn and 222Rn to pass
Chemical characterization of ground water
The results of chemical analyses of the studied ground water are shown in Table 2. From these results, it can be concluded that there is good correlation between Na+ and Cl- as shown in Figure 3. This relation defines these waters as chloride type. Ca2+ is more related to SO42- than CO32- Figure 4 and 5 it is noticed that HCO3- is more dominating than CO32-. The domination of anions is in the order Cl- >SO42-> HCO3-> CO32-, while cations are in the order Na+ >Ca2+> K+.> Mg2+.
Table 2: Results of chemical analyses (ppm) of the studied ground water.
The activity concentration values in Bq/L of 232Th, 238U and 40K for the measured samples are listed in Table 3.
|GW1||0.656 ± 0.12||10.103 ± 0.73||2.28 ± 0.17|
|GW2||0.525 ± 0.08||14.004 ± 0.52||1.156 ± 0.33|
|GW3||0.683 ± 0.11||13.424± 1.02||0.868 ± 0.41|
|GW4||0.316 ± 0.02||8.214 ± 1.14||1.050 ± 0.06|
Table 3: Activity concentrations of 232Th, 238U and 40K (Bq/L) in the studied samples.
The results show that the lowest 238U and 232Th activity concentrations are noticed in the shallowest well GW4 located in Wadi El Dibbabat in which the surrounding surface rocks are low in radioactivity. The highest 238U activities are noticed in the two wells GW2 and GW3 which are located at the downstream of wadies Moried, Naseib and Seih in which the uraniferous Um Bogma Formation is exposed. The highest 40K content is noticed in well GW1 which is the deepest one (150m). In this depth, thin clay beds are known to occur in the section below the reservoir sandstone of Adediya Formation which may increase the K content of the groundwater in this drilled well. The relation between uranium activity concentrations vs HCO3-, CO32-, Cland SO42- ; are clear in Figure 6. It is noticed that 238U is more related to SO42-, in the form of urinal sulphate [UO2 (SO4)2]2- complexes, than the other anions. Radium is easily removed from solution by adsorption on clays and silicates or by co-precipitation with insoluble sulphate. 226Ra was not detected in the studied water as it does not dissolve in water, while high concentrations of Ca2+, Mg2+ and Cl- are present in the studied water; this could be because these ions compete for adsorption site. Adsorption of 226Ra on clays and silicates could have occurred. 232Th and 40K activities are very low as listed in Table 3, which may be produced due to mechanical washing during flashfloods as they are of low mobility. Activity concentrations of 214Pb and 214Bi have been gamma-measured and listed Table 4.
|222Rn (Bq/L)||214Pb (Bq/L)||214Bi (Bq/L)||Radon Exhalation rate (Bq/h)|
Table 4: Activity concentrations of Radon (Bq/L) and its progenies (Bq/L) and exhalation rate (Bq/h).
Alpha track measurements
After 4 weeks of irradiation for the exposed and unexposed CR39 detectors, they were collected and immediately etched chemically in NaOH with optimum conditions which are 6.25 N of for 6 h at constant temperature 70°C with an accuracy of + 0.1°C . During the etching process the solution was constantly stirred. The detectors were then washed under running tap water for about 5 min and dried using tissue paper.
An optical microscope (400 x magnifications) was used to count the number of tracks per cm2 on each detector; Figure 7 shows the counting system. The track density was converted into radon concentration Bq/L using the calibration factor of CR39 where integrating radon's concentration is known .
The concentrations were determined according to the equation given by Tanner : (3)
Where, CRn is the mean Rn-222 concentration (in Bqm-3), N is the track density (Track.cm-2), B is the background track density (Track. cm-2), CF is the calibration factor in terms of α- tracks. (cm-2 d-1per Bqm-3) and t is the exposure time (hours). Radon exhalation rate E, has been calculated using the following equation.4 :
where A, V, λ and T are the area of the chamber in m2, effective volume of the chamber in m3, decay constant for radon in h-1 (0.00756 h-1), and the exposure time in hours, respectively. The results of alpha track showed that thoron concentration was within the background level in the laboratory. Results have been listed in Table 4, radon concentrations range between 540 and 1163 Bq/L. Although the 214Pb and 214Bi activities measured using gamma spectroscopy show no relation with radon concentrations as shown in Figure 8, yet this may be explained as a result of radon being transmitted to water by bed rock interaction of radium migrated from other areas as previously suggested. This explains that the studied water samples did not contain radium content.
Calculation of annual effective dose
Equation.5 is used to calculate the annual effective dose due to the intake of natural radionuclides from drinking water: D = CIE
Where D is the annual effective dose (Sv/ y) to an individual due to the ingestion of radionuclides from drinking water, C is the activity concentration of radionuclides in the ingested drinking water (Bq/ l), I the annual intake of drinking water (l /y) and E the ingested dose conversion factor for radionuclides (Sv/Bq) [17,18]. E values used in the calculations are listed in Table 5.
|Radioisotope||Dose conversion factors (mSv Bq-1)|
|<1 y||2–7 y||≥ 17 y|
|214Bi||1.4 × 10-9||3.6 ×10-10||1.1 × 10-10 (Ajayi and Owolabi 2007)|
|226Ra||4.7× 10-6||6.2 ×10-7||2.8 × 10-7 (Ajayi and Owolabi 2007)|
|228Ac||7.4 × 10-9||1.4 × 10-9||4.3 × 10-10 (Ajayi and Owolabi 2007)|
|212Pb||1.5 ×10-7||3.3 × 10-8||6.0 × 10-9 (Ajayi and Owolabi 2007)|
|235U||3.5 × 10-7||8.5 ×10-8||4.7 ×10-8 (Ajayi and Owolabi 2007)|
|40K||6.2 × 10-8||2.1 × 10-8||6.2 × 10-9 (Ajayi and Owolabi 2007)|
|238U||3.4× 10-7||8 ×10-8||4.5 ×10-8 IRPA 2010|
The annual effective dose has been calculated for different age groups as listed in Table 6 for babies (age below 1y), children (age from 2 to 7 y) and adults (age from 17 y and above), with annual water consumption per year, as daily drinking volume of water equal two litters. Contribution of each radionuclide depends on its ingested dose conversion factor E and its present activity. Babies are more annually dosed from these wells water intake, and the adults are the less affected by drinking water as shown in Figure 8. Similar results have been found in some areas in Yemen , and some aquifer systems in Brazil .
|Sample code||Annual effective dose|
Table 6: Net annual effective (mSv/y) dose has been calculated for different age groups.
In the present study the levels of radionuclides are determined through four selected water from well water using Gamma-spectrometry with high purity germanium detectors which is a very efficient analysis technique for measuring radionuclides in environment. Also solid state nuclear track detectors are used, for radon and thoron measurements.
The mean activity concentration of the radionuclides in different samples is presented and indicates that 238U concentration was significant in all measured samples. The results obtained show undetectable radium while its parent was present 238U from the source rocks to water as a result of bed rock interaction. The general conclusions of this document may be listed in the following points:
1. There is a good relation between uranium in the studied samples and the uraniferous rock exposures around the wells where 238U activity concentration present in the studied sample reflect the higher uranium activity of the bed rock around the wells.
2. Results of radon and its progenies measurements prove undetectable radium content; this could explain the origin of collected water as seasonally affected by young transmitted uranium.
3. Ratio of radionuclides in the studied water is completely different from normal ratio this reflect the effect of chemical properties of the water wells and their lithologies on the distribution of radionuclides.
4. Presence of significant activity 238U without 226Ra shows that the decay path is affected by water processing on bed rock of the studied wells.
5. Babies are more exposed to hazards due to studied water intake, so this document recommends another drinking water source for babies' stage.
Authors would like to thank Prof. Ibrahim E. El Aassy, Professor of Uranium Geology, at the Nuclear Materials Authority, (NMA), Egypt, for his continuous help throughout the present work.