2.1 Samples collection and preparation

Thirty-six beach sand and sediments samples were collected from different locations along the beach near the desalination plant with a length interval of 6500 m (Fig. 1). Twenty-four sand samples were collected at 2 m from the shoreline. These samples were taken from surface and subsurface (10–30 cm). From the same locations, 12 sediment samples were collected from seawater at 2 m from the shoreline. Samples were bulked as a single sample and dried in an oven at 70 °C for 24 h. After that, samples were prepared for radiation counting by sieving through 2 mm mesh. Each sample was packed into 152 ml standard size beakers and tightly sealed and stored for 28 days to acquire secular equilibrium between ²²⁶Ra and its progenies. Two reference materials were packed into the same standard size beakers for efficiency calibration.

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Figure 1

Location map of the collected samples along the beach near to desalination plant.

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2.2 Experimental setup

A hyper-pure Germanium detector (HPGe), coaxial type, p-type with relative efficiency of 20% was used. The detector is shielded with a low-level background lead shield. The HPGe was calibrated for efficiency using the reference material RGU-1 from IAEA. The certified activity of uranium is 400 ppm which is equivalent to 4960 Bq kg⁻¹. The energy transitions of the ²²⁶Ra daughters (²¹⁴Pb and ²¹⁴Bi) were used to develop the efficiency calibration curve. A fourth degree polynomial fitting was performed to reach the best R² value (≈0.976).

2.3 Gamma-spectrometric analysis

After subtracting the background, the radionuclides were measured at the gamma lines (Table 1). ²²⁶Ra was measured using its progenies ²¹⁴Pb with energies 295.2 keV (19.3%) and 351.93 keV (37.6%), and ²¹⁴Bi with energies 609.31 keV (46.1%), 1120.29 keV (15.1%) and 1764.49 keV (15.4%). Radium was determined based on the above mentioned energy transitions after achieving secular equilibrium for 28 days after sample packing. For ²³²Th, the specific activity concentration was determined using the gamma lines 338.40 keV (12.4%) and 911.07 keV (25.8%) for ²²⁸Ac and 583.14 keV (33.1%) for ²⁰⁸Tl. The average values were calculated. In the case of ⁴⁰K and ¹³⁷Cs, the specific activity concentrations were estimated directly by their gamma lines of 1460.75 keV (10.7%) and 661.7 keV (85.12%), respectively.

The software used for analysis and reduction of the gamma-ray spectra was Quantum Gold, Version 4.04.00.

The minimum detectable activity (MDA) for each isotope (²²⁶Ra, ²³²Th, ⁴⁰K and ¹³⁷Cs) in the background was calculated separately based on the sample’s weight (0.0332 kg) using the detection limit according to the formula (Currie, 1989):

To assess the radiological hazard associated with sand samples and sediments of the studied area, it is useful to calculate an index called the radium equivalent activity, Ra_eq. It can be calculated from the following relation (Boukhenfouf and Boucenna, 2011; Alshahri and Alqahtani, 2015):

The absorbed dose rate in air 1 m above the ground surface for the radionuclides (²³²Th, ²²⁶Ra, and ⁴⁰K) was computed on the basis of guidelines provided by Ahmed and El-Arabi (2005). The conversion factors used to compute the absorbed dose rates (D) in air per unit activity concentration in 1 Bq kg⁻¹ sand correspond to 0.666 nGy h⁻¹ for ²³²Th, 0.429 nGy h⁻¹ for ²²⁶Ra, and 0.0417 nGy h⁻¹ for ⁴⁰K. Therefore, D could be obtained from the following relation:

The annual effective dose rate E (mSv y⁻¹) received by the population is calculated using the following equation (UNSCEAR, 2000):

Another radiation hazard index is called the external hazard index (H_ex). This index due to external exposure to gamma rays from the studied sand and sediment samples, which must be less than unity. H_ex is defined as the following (AlZahrani et al., 2011):

3.1 Activity concentrations of ²²⁶Ra, ²³²Th, ⁴⁰K and ¹³⁷Cs in sand samples and sediment

The activity concentrations of ²²⁶Ra, ²³²Th, ⁴⁰K and ¹³⁷Cs were determined for sand samples and sediment (Tables 2–4). The activity concentrations of ²²⁶Ra ranged from 12.1 ± 1.3 to 36.3 ± 3.3 Bq kg⁻¹ with a mean value of 22.7 ± 2.8 Bq kg⁻¹ for sand samples collected from surface and from 19.3 ± 2.5 to 31.4 ± 3.4 Bq kg⁻¹ with a mean value of 23.9 ± 2.9 Bq kg⁻¹ for subsurface samples. The highest activity concentration of Radium was found to be 36.3 ± 3.3 Bq kg⁻¹ for the sample S3 which was collected from the surface at site 3. The activity concentrations of ²³²Th ranged from 9.30 ± 0.9 to 26.9 ± 5.6 Bq kg⁻¹ with a mean value of 14.8 ± 2.3 Bq kg⁻¹ for sand samples from surface and from 8.7 ± 1.0 to 17.3 ± 2.1 Bq kg⁻¹ with a mean value of 12.8 ± 1.8 Bq kg⁻¹ for subsurface samples. The concentrations of ⁴⁰K and ¹³⁷Cs for surface samples ranged from 261 ± 10 to 454 ± 17 Bq kg⁻¹ and from below the MDA to 1.38 ± 0.15 Bq kg⁻¹, respectively. Similarly, the concentrations of ⁴⁰K and ¹³⁷Cs for subsurface sand samples ranged from 358 ± 14 to 686 ± 21 Bq kg⁻¹ and from below the MDA to 0.97 ± 0.12 Bq kg⁻¹, respectively.

For sediment samples, the activity concentrations ranged from 18.3 ± 1.6 to 37.6 ± 4.5 Bq kg⁻¹ with a mean value of 26.4 ± 2.8 Bq kg⁻¹ for ²²⁶Ra, from 7.8 ± 1.3 to 25.5 ± 3.7 Bq kg⁻¹ with a mean value of 16.3 ± 2.2 Bq kg⁻¹ for ²³²Th, from 202 ± 11 to 432 ± 15 Bq kg⁻¹ with a mean value of 351 ± 15 Bq kg⁻¹ for ⁴⁰K and from below the MDA to 2.16 ± 0.25 Bq kg⁻¹ for ¹³⁷Cs.

The obtained results show that the mean values of ⁴⁰K concentrations in sand samples were higher than the range of the world average 370 Bq kg⁻¹. The activity levels of ¹³⁷Cs were found to be lower than the minimum detection limit in most of sand and sediment samples. The slight appearance of ¹³⁷Cs was observed in the measured samples which may be due to the fertilized soil in the investigated area (Boukhenfouf and Boucenna, 2011). The variations in activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in sand samples based on the nature of its geological formations and may be due to the result of the depositional processes of radionuclides and heavy minerals along the shoreline during the high tide.

The distributions of radium, thorium and potassium activities in all sand and sediment samples are given in Figs. 2–4. These figures compare between the values of this study and the allowed values in soil as considered by UNSCEAR (2000). The result showed that the highest values of potassium were found in subsurface samples.

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Figure 2

Distribution of ²²⁶Ra, ²³²Th and ⁴⁰K in surface sand samples along the beach and the allowed values by UNSCEAR (2000).

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Figure 3

Distribution of ²²⁶Ra, ²³²Th and ⁴⁰K in subsurface sand samples along the beach and the allowed values by UNSCEAR (2000).

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Figure 4

Distribution of ²²⁶Ra, ²³²Th and ⁴⁰K in sediment samples along the beach and the allowed values by UNSCEAR (2000).

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3.2 Radiation hazard indexes

The radium equivalent activity (Ra_eq), total absorbed dose rate in air 1 m above the ground (D), annual effective dose (E) and external hazard index (H_ex) were calculated for all sand and sediment samples, the mean values of the above radiation hazard indexes are shown in Table 5. The mean values of radium equivalent activity, Ra_eq, were 74.1, 78.8 and 78.1 Bq kg⁻¹ for surface sand, subsurface sand and sediment which were lower than the acceptable value of 370 Bq kg⁻¹ (UNSCEAR, 1993). The mean values of gamma adsorbed dose rate (D) in air and annual effective dose (E) for sand samples from surface were 37.2 nGy h⁻¹ and 0.04 mSv y⁻¹, respectively, and for sand samples from subsurface were 38.3 nGy h⁻¹ and 0.05 mSv y⁻¹, respectively. The mean values of gamma adsorbed dose rate D in air and annual effective dose (E) for sediment samples were 36.7 nGy h⁻¹ and 0.05 mSv y⁻¹, respectively. These results were within the estimated average global terrestrial radiation of 55 nGy h⁻¹ and the acceptable value of annual effective dose (1 mSv y⁻¹) for the public (UNSCEAR, 1993, 2000). The mean values of the external radiation hazard index (H_ex) for all samples were lower than unity. From these results, the radiological risk is insignificant due to the low values of gamma dose rates in all samples under investigation.

3.3 Comparison of activity concentrations with other studies

The mean values of the radioactivity levels were compared with the world average which are 40, 40 and 370 Bq kg⁻¹ for ²²⁶Ra, ²³²Th and ⁴⁰K, respectively, (UNSCEAR, 2000). The measurements of all sand and sediment samples under investigation showed that the mean values of radioactivity levels were lower than the world average, expect the mean values of potassium in sand samples. In addition, the specific activities of ²²⁶Ra, ²³²Th and ⁴⁰K for the analyzed samples were compared with the similar investigations in other countries (Table 6). It can be seen that, the natural radioactivity levels fall within the range of published data from other countries.