Natural radioactivity in groundwater arises from 238U, 232Th, 40K and their decay products which are produced as a result of the interactions between rocks and water ( Todorović et al., 2012 ; Kraemer & Genereux, 1998 ). Consequently these radionuclides transported in ground water can enter the food chain through irrigation waters and the water source through ground water wells ( Malanca et al., 1998 ). Thus, the ingestion of radionuclides in drinking water causes human internal exposure ( Degerlier & Karahan, 2010 ). The estimated exposure of natural radioactivity sources contributes dose was 2.4 mSv y−1 (cosmic ray 0.4, terrestrial gamma ray 0.5, radon 1.2, and food and drinking water 0.3) ( UNSCEAR, 2000 ). So, determinations of natural radioactivity concentrations in bedrock wells were important to estimate the dose in the water samples ( Salih et al., 2002 ). In the south-west of Saudi Arabia (Al-Baha region), there are no rivers, lakes, and springs. Therefore, people depend on the underground water especially those coming from wells. These wells must be safe and of good quality. Concentration levels of 226Ra, 232Th and 40K in ground water used in this region and the radiological impacts of the consumed drinking water have not been reported in literature previously. So, the aim of this work is to determine the concentration values of natural radionuclides present in the well water samples collected from different location in Al-Baha region and estimate the corresponding radiation doses for people consuming these waters. This work is necessary due to the demands of having the database available on the waters and is highly required to keep human drinking water standards. The obtained data in this study may afford baseline levels of natural radioactivity in such water and provide background information for future research on drinking water for radiological protection of the human.
2. Materials and Method
2.1. Study Area
In order to determine the natural radioactivity in ground water, twenty water samples were collected from most frequently used wells spread in different locations of Al-Baha region. It is located in the south-east of Saudi Arabia (20˚0'0"N, 41˚30'0"E), as shown in Figure 1. The sampling locations were chosen mostly based on population density and accessibility. Al-Baha region is an agricultural area, the main agro products, are wheat, vegetables and fruits. This region was chosen based on the active usage of ground water for drinking and for irrigation purposes made by locals of the area which can also be a source of radionuclides in foods.
2.2. Samples Collection, Preparation and Measuring
The collected water samples were acidified with nitric acid to avoid the collection of organic materials, then each sample was filled into 500 ml capacity polyethylene Marinelli beakers ( IAEA, 1989 ). Before use the containers were washed
Figure 1. The study area (Al-Baha region—Saudi Arabia).
with dilute HCl and rinsed with distilled water. The Marinelli beakers were sealed and stored for more than 4 weeks before counting to reach the secular equilibrium between 226Ra nuclides and 232Th nuclides and their daughters for gamma ray measurements ( Abbady, 2004 ). Detection and measurements of the sample concentrations were carried out using a coaxial high-purity germanium (HPGe) detector with relative efficiency of 25% and FWHM 2.0 keV at 1332 keV, of 60Co. The detector was housed inside a thick lead shield to reduce the background of the system. Genie 2000 basic spectroscopic software was installed in the computer for data acquisition and analysis. The system was calibrated for energy and efficiency on a regular basis in ( IAEA, 1989 ). Each sample after equilibrium was kept on the top of the HPGe detector and counted for 36,000 s. The background was measured every week under the same conditions of sample measurement.
2.3. Calculations of the Activity Concentration
The activity of 226Ra was determined based on the gamma ray energies of 351.9 keV from 214Pb and 609.3, 1120.3 and 1764.5 keV (from 214Bi). The activity concentration of 232Th was determined based on gamma ray energies of 338.8, 911.2 and 968.97 keV from 228Ac and 583.0 keV from 208Tl. 40K was determined directly by its emission energy of 1461.8 keV gamma-ray line. The activity concentrations Aw of the natural radionuclides in the measured samples were calculated using the equation ( UNSCEAR, 2000 ):
where: Ca is the net gamma counting rate (counts per second), ε the detector efficiency of the specific γ-ray, Pγ the absolute transition probability of Gamma-decay and m the mass of the sample (kg).
3. Results and Discussion
3.1. Activity Concentrations of 226Ra, 232Th and 40K
The concentrations in Bq∙L−1 of radionuclides 226Ra, 232Th, and 40K measured in the water samples were listed in Table 1. As shown, in this table, 226Ra activities It ranged from 0.24 Bq∙L−1 (w17) to 1.54 Bq∙L−1 (w11). It was not detected in four samples (w3, w9, w18 and w20) and the measured samples (w5, w6, w7, w10 and w13) were higher than the recommended limits of 1.0 Bq∙L−1 ( WHO, 2006 ). 232Th series were detected in eight samples only, the lowest value was 0.15 Bq∙L−1 (sample 15) and the highest value was 0.81 Bq∙L−1 (w16). The results show that the concentration values of 226Ra were higher than those of 232Th. This may because of the radium being more soluble in water ( Kitto & Kim, 2005 ). 40K activities ranged from 0.66 Bq∙L−1 (w7) to 5.64 Bq∙L−1 (w4), all samples concentration were below the recommended limit 10.0 Bq∙L−1 of 40K reported by WHO 2006 ( WHO, 2006 ). The obtained average concentrations of the three nuclides (226Ra, 232Th, and 40K) were 0.85, 0.43 and 2.84 Bq∙L−1, respectively. The average values
Table 1. Activity concentrations of 226Ra, 232Th, and 40K of the ground water samples.
ND: not detected.
for 226Ra and 40K do not exceed the WHOO 2006 recommended limit in drinking water, whereas, slightly higher for 232Th than the limit. It should be noted that, in the present results, 40K is the most abundant concentration, about 76% of the total (226Ra + 232Th + 40K). Potassium-40 is an isotope of an essential element which is under homeostatic control ( Jabir et al., 2007 ). The activity concentration of 226Ra, 232Th and 40K in the studied ground water samples illustrated in Figure 2.
3.2. Radiation Dose Estimation
where Dw is the annual effective dose equivalent from consumption of drinking water (mSv/year), Aw is the activity concentration of radionuclides in the ingested water (Bq/L), Cw is the consumption rate of water (L/year). According to WHO (2003) , the dose was estimated by considering a consumption rate is 730 L/year for adults. The dose conversion factors Fw (Sv/Bq) for adults were (2.8 × 10−7, 2.3 × 10−7 and 6.2 × 10−9 Sv Bq−1) for 226Ra, 232Th and 40K, respectively, ( WHO, 2006 ).
The calculated total annual effective doses for adults ingested radionuclides 226Ra, 232Th and 40K from the ground water samples were tabulated in Table 1. The highest value (0.237 mSv/y) of effective dose was calculated in sample w6 due to the radium high concentration. The range of effective doses due to intake of 226Ra, 232Th, and 40K were from 0.004 to 0.237 mSv/y, with an average value of 0.058 mSv/y which is below the average limit (0.1 mSv/y) reported by WHO ( WHO, 2006 ). Consequently, we recommended that, the investigated waters are acceptable as drinking water for life-long human without any treatment to reduce the concentrations of radioactive contaminants. Figure 3, shows the effective dose for the ingestion of 226Ra, 232Th and 40K by adults.
3.3. Lifetime Risk Assessment (R)
Lifetime risk assessment was calculated using the Eq ( EPA, 1999 ):
Figure 2. Activity concentration of 226Ra, 232Th and 40K in ground water samples collected from Al-Baha region, Saudi Arabia.
Figure 3. A comparison of total annual effective dose (mSv/y) for adults ingested radionuclides 226Ra, 232Th and 40K from the ground water samples.
where Dw is annual effective dose equivalent (Sv/y), DL is duration of life (70 years) and RF is risk factor (Sv−1). For risk assessment, the nominal probability coefficient of 7.3 × 10−2 Sv−1 was adopted ( ICRP, 1996 ).
The risk levels from the direct ingestion of the natural radionuclides in ground water were estimated as shown in Table 1. The cancer risk R for the adults, i.e. corresponding to the total ingestion dose of 0.058 mSv/y is estimated as 0.299 × 10−3. This risk is much lower than the limit 8.4 × 10−3, if the total natural radiation dose of 2.4 mSv/y as given by UNSCEAR 2000 UNSCEAR (2000) .
3.4. Comparison of Results with Similar in Other Countries
Table 2 shows a comparison between the measured activity concentration values of the groundwater samples in present work and other samples obtained from different countries. Where, the average concentration of 226Ra (0.56 Bq∙L−1) is acceptable with all countries except the maximum values for Yemen and Nigeria. 232Th average value (0.16 Bq∙L−1) is almost matched with Saudi Arabia ( Alseroury et al., 2018 ), higher than Jordan ( Al-Shboul et al., 2017 ) and lower than other countries. The present results show that the average of 40K concentration is nearly matching the value of Nigeria ( Ajayi & Owolabi, 2007 ), higher than Yemen ( Abdurabu et al., 2016 ) and Jordan ( Al-Shboul et al., 2017 ), while it is lower than the other observed values. Therefore, we can indicate that the obtained results are in agreement with the data obtained from various locations in the world, taking into account the data of the countries of different geographical locations differ regarding specific mineral.
The activities of 226Ra, 232Th and 40K in the well water samples collected from Al-Baha region, south-east of Saudi Arabia were investigated in the present study. The obtained average concentrations of the three nuclides (226Ra, 232Th, and 40K) were 0.85, 0.43 and 2.84 Bq∙L−1, respectively. The results indicate that
Table 2. Activity concentration of natural radionuclides in ground water from different countries.
the natural radioactivity concentrations of these nuclides were below the WHO guidance levels and were within the values reported by the other researchers. The risk assessment data show that the investigated radionuclides in water were below limit values and pose no detrimental health effect.
A special thanks to Nada Alshehri for helping in proofreading and organizing the paper.
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