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Investigation of Natural Radioactivity Levels and Evaluation of Radiation Hazards in Residential-Area Soil Near a Ras Tanura Refinery, Saudi Arbia

One of the oldest oil refineries in the Middle East is located south of Ras Tanura in eastern Saudi Arabia, Arabian Gulf. Naturally occurring radionuclides may be associated with the extraction processes of oil and gas. Therefore, the activities concentrations of 238U, 226Ra, 232Th, 40K, and 137Cs in soil samples near Ras Tanura refinery were measured using the gamma ray spectrometric technique. The mean activity concentrations of the radionuclides 238U,226Ra,232Th, 40K, and 137Cs were 39.0±4.8 Bq/kg, 23.2±1.4 Bq/kg, 7.73±1.2 Bq/kg, 278±9.8 Bq/kg, and 1.42±0.5 Bq/kg, respectively. Radiation hazard indices were calculated to evaluate the radiological risk for the public and environment. The results show that the mean values of radium equivalent activity (Raeq) and gamma absorbed dose rate (D) in air, annual effective dose equivalent (E), and external radiation hazard indices for analyzed samples were lower than the recommended limit by the United Nations Scientific Committee on the Effect of Atomic Radiation. This study is the first to evaluate the radiological impacts in the investigation area. Accordingly, the present study could be considered a baseline for natural radioactivity levels in soil near a refinery area in Ras Tanura.

Therefore, measuring 137 Cs levels in soil is also necessary in the environment as such data would serve as baseline data. Its presence in soil would be an indicator that the area in question might have received some fallout radioactivity in the past [3].
Radioactive materials that occur naturally (NORM) and expose people to radiation occurring widely. NORM is an acronym for naturally occurring radioactive material, which includes long-lived radioactive elements (e.g., uranium, thorium, and potassium) found in the environment [4]. NORM wastes concentrate and accumulate in tubing and surface equipment in the form of scale and sludge [5]. In the exploration and extraction processes of oil and gas, the natural radionuclides 238 U, 235 U, and 232 Th, as well as the radium-radionuclides 223 Ra, 224 Ra, 226 Ra and 228 Ra, and 210 Pb, etc., are brought to slurry surfaces and may contain levels of radioactivity above the surface background [4,6]. Several studies have been conducted on the concentrations of naturally radionuclides in the environment in different regions throughout the world [7][8][9][10][11][12][13].
Radionuclides present in the environment may enter the human body through air, food, and water, thereby adversely affecting human health at higher radiation dose levels. Humans and other living organisms are continuously exposed to ionizing radiation from natural and anthropogenic sources that are present everywhere in the environment [8,[14][15][16][17].
The levels of background radiation vary significantly from area to area, and hence radiation doses received by the general public [18]. In Saudi Arabia there is a lack of scientific information concerning naturally occurring radioactive material wastes in the environment around the refinery area and petroleum industries. Therefore, knowledge about the distribution pattern of both anthropogenic and natural radionuclides plays an important role in minimizing the expected health risks to the general public. The present work deals with the measurement of specific activities of 238 U, 226 Ra, 232 Th, 40 K, and 137 Cs, calculation of the radium equivalent activity, external and internal hazard indices, absorbed dose, and annual effective dose equivalent for Ras Tanura soil.

Study Area
The city of Ras Tanura is located on the Arabian Gulf in Eastern Province of Saudi Arabia and its geographical coordinates are 26°38'38"N and 50°9'33"E. Seawater surrounds the city on three sides. The tide rise ranges from 1.5 to 2 m during the year. Its area is about 290 km² and a population is 74,000 inhabitants. Wind is strong and blowing dust during the year -especially in summer. Ras Tanura is one of the most important cities in Saudi Arabia because of the presence of one of the largest and oldest oil refineries in the Middle East, which began operating in 1945. Also, the study area contains a gas plant and two ports for shipping oil. West and south of the city, the coast is mainly composed of sand, mud, sediment, and sludge, while the east coast is composed of sand and marine sediments. In general, the soil in Ras Tanura is sandy and composed of calcareous soil that varies in depth, thickness, and composition. A number of spring waters are located to the west.

Sampling and Sample Preparation for Gamma Spectroscopy
Thirty four soil samples were collected from different locations in Ras Tanura city nearby oil refinery and around residential area (Table 1, Fig. 1). All samples were bulked as a single sample and dried in an oven at 70ºC for 24 hours. After drying the samples and removing stones, leaves and foreign particles, soil samples were crushed and prepared for radiation counting by sieving through 0.2 mm mesh. Each sample was packed into 62.172 cm 3 standard size can and tightly sealed and stored for one month to acquire secular equilibrium between 226 Ra and its progenies. Reference material was packed into the same standard size beaker for efficiency calibration [19].

Experimental Setup
A hyper-pure germanium detector (HPGe), coaxial type, p-type with relative efficiency of 20% was used. The resolution (FWHM) at 122 keV, 57 Co is 0.934 keV, and at 1332 keV, 60 Co is 1.79 keV. 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 4,960 Bq/kg. The energy transitions of the 226 Ra daughters ( 214 Pb and 214 Bi) were used to develop the efficiency calibration curve [19][20]. A 4 th -degree polynomial fitting was performed to reach the best R 2 value (= 0.983).

Gamma-Spectrometric Analysis
The gamma transmissions used for activity calculations are 214 Pb with energies 295.2 keV and 351.93 keV, and 214 Bi with energies 609.31 keV, 1120.29 keV, and 1764.49 keV for 226 Ra ( 238 U) series. For 232 Th, the specific activity concentration was determined using the gamma lines 338.40 keV and 911.07 keV for 228 Ac and 583.14 keV for 208 Tl. The average values were calculated. In the case of 40 K and 137 Cs, the specific activity concentrations were estimated directly by their gamma lines of 1460.75 keV and 661.7 keV, respectively [19]. The software used for analysis and reducing the gammaray spectra was Quantum Gold version 4.04.00. The minimum detectable activity (MDA) for each isotope ( 226 Ra, 232 Th, 40 …where BG is the background count below the peak of interest, ε is absolute efficiency, I γ is gamma line intensity, and t is the counting time in seconds. The MDAs for 226 Ra, 232 Th, 40

Activity Concentrations
The activity concentrations of naturally occurring radionuclides ( 238 U, 226 Ra 232 Th, 40 K, and 137 Cs) were determined in soil samples from Ras Tanura city, Saudi Arabia. The statistics of the activity concentrations expressed in Bq/kg are presented in Tables 1 and 2. The activity concentrations of 238 U ranged from 19.0±4.4 to 62.8±7.7 Bq/kg, with a mean value of 39.0±4.8 Bq/kg. The activity concentrations of 226 Ra ranged from 8.68±0.6 to 156±3.6 Bq/kg, with a mean value of 23.2±1.4 Bq/kg. The highest activity concentration of Radium was found to be 156 ± 3.6 Bq/kg for soil sample S18, which was higher than the allowed limit of 30 Bq/kg [22]. The site of this sample is located in an area composed of sandy soil, marine sediments, and mud. The high value of radium in sample 18 may be attributed to the near of the sample collection site from the sludge area, where the NORM waste is concentrated and accumulates in the environment in the form of sludge and scale. Sludge area may contain a level of radioactivity above the surface background, where radium is the most important naturally occurring radionuclide content in sludge [23][24].
The activity concentrations of 232 Th ranged from 2.76±0.2 to 12.8±1.8 Bq/kg with a mean value of 7.73 Bq/kg. The concentrations of 40 K and 137 Cs ranged from 108±5.4 to 446±12 Bq/kg and from below the MDA to 3.83±0.6 Bq/kg, respectively. The obtained results show that the mean values of 40 K concentrations in soil samples were lower than the range of the world average 370 Bq/kg [22]. The activity level of 137 Cs was found to be lower than the minimum detection limit in 44% of soil samples. The variations in activity concentrations of 226 Ra, 232 Th, and 40 K in soil samples are based on the nature of its geological formations in the study area. Otherwise, naturally occurring radioactive materials encountered in oil and gas exploration, development, and production operations originate in subsurface formations that may contain radioactive materials such as uranium, thorium, and their daughter products [25]. This can be brought to the surface and contribute in the surface background. In addition, some weather phenomena such as rainfall and direction of wind may contribute to the distribution of radionuclides in the study area during the year.
The low values of the standard deviations obtained for the three radionuclides show the relative uniformity in the distribution of the radionuclides within the study  area. The distribution also reflects uniformity in the geology of the study area. The relative contribution to dose due to 40 K was 80%, and 238 U was 11%, followed by the contribution due to 226 Ra and 232 Th as 7% and 2%, respectively, while 137 Cs do not contribute to the dose rate (Fig. 2) Th, and 40 K, respectively, for the world average concentrations published by the United Nations Scientific Committee on the Effects of Atomic Radiation [22], it is noted that the obtained results in most samples are lower than the recommended reference levels.
The activity concentration measured in this study was compared with those from local and other countries (Table 3) [7,26,[30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45]. The values of 226 Ra activity concentration in our study are lower than the values in similar studies in Yemen, Hungary, Yunnan, China, Turkey, Thailand, Algeria, Egypt, Pakistan, Iraq, and Jordan, while it is little higher than that of Sudan, Nigeria, and Malaysia, and within the range of India. The mean value of 232 Th activity concentration in the study area was less than those of all compared countries. Similarly, the mean value of 40 K activity concentration in the study area was less than the values of all countries except Nigeria, Jordan, Sudan, and Malaysia. However, the mean values of 226 Ra, 232 Th, and 40 K in the present study are within the range of the local studies from Qassim (north), Albaha (south), and Riyadh (middle). Variations of radioactivity levels in soil locally and in other countries worldwide may depend upon geological and geographical conditions of the area and the presence of NORM, or the applications of phosphate fertilizers in agricultural soil [19,26]. Some of these countries have a geological structure that differs from the geology of Ras Tanura. Soils from Sudan, Yemen, and Nigeria contain volcanic sediments, while soils from Jordan and Egypt are composed mainly of coarse sand and sandy loam mixed with limestone, marble, and gravel. Similarly, the geology of the study area in Hungary and Yunnan is sandstone, clay rocks, and a few tens of kilometers are granitic stock, while the geological nature of Iraq is similar to Ras Tanura geology along the Arabian Gulf.

Radium Equivalent Activities (Ra eq )
The Gamma-ray radiation hazards in soil due to the specified radionuclides 226 Ra, 232 Th, and 40 K can be assessed using different indices. Radium equivalent activity, Ra eq , is the most widely used as a radiation   hazard index associated with the three radionuclides of 226 Ra, 232 Th, and 40 K. Ra eq is a weighted sum of activities of these radionuclides based on the estimation that 370 Bq/kg of 226 Ra, 259 Bq/kg of 232 Th, and 4810 Bq/kg of 40 K produce the same γ-ray dose rates. Ra eq is given by the following equation [19,22]: Ra eq (Bq/kg) = A Ra + 1.43A Th + 0.077A K (2) …where A Ra , A Th , and A K are the activity concentrations (Bq/kg) of 226 Ra, 232 Th, and 40 K, respectively. When the value of Ra eq is 370 Bq/kg, this corresponds to 1 mSv/y. The radium equivalent concept allows a single index or number, which is a widely used hazard index to describe the gamma output from different mixtures of radium, thorium, and potassium in the soil samples from different locations. The calculated values vary from 21.6 (sample No. 1) to 204 (sample No. 18) Bq/ kg with an average of 62.1 Bq/kg (Table 4). These values are lower than the permissible maximum value of 370 Bq/kg [22].

Absorbed Dose Rate (D)
The absorbed gamma dose rate D (nGy/h) in air at 1 m above the ground surface for the uniform distribution of radionuclides were calculated based on guidelines provided by [22]: D (nGy/h) = 0.462 A Ra + 0.604A Th + 0.0417 A K ( 3) …where A Ra , A Th , and A K are the activity concentrations (Bq/kg) of 226 Ra , 232 Th, and 40 K, respectively, in the samples. The absorbed dose rate expresses the received dose in the open air from the radiation emitted from radionuclide concentrations in environmental materials. Also, it is the first major step for evaluating health risk and is expressed in Gray (Gy). The calculated total absorbed dose rates of samples are tabulated in Table 3. It is observed that the total absorbed dose rate resulting from 226 Ra, 232 Th, and 40 K ranges between 10.5 (sample 1) to 96.4 (sample 18), and the average value was 29.3, which was lower than the limits as recommended by the International Commission on Radiological Protection, 55 nGy/h [27]. The highest value in site No. 18 is slightly higher than the world average. This value is due to the significant amount of radium, which reflects the highest value of absorbed gamma dose rate in soil sample 18. The location of this soil sample was observed close to the sludge area, and consists of sand mixed with a large amount of suspended materials which are thought to be re-sedimented.   Table 4. Radium equivalent activity (Ra eq ), air absorbed gamma radiation dose rate (D), annual effective dose equivalent outdoor (E out ), annual effective dose equivalent indoor (E in ), internal radiation hazard index (H in ), external radiation hazard index (H ex ), and representative level index (I γr ) for soil samples.

Annual Effective dose Equivalent (E)
The annual effective dose equivalent (E) was calculated from the absorbed dose by applying the dose conversion factor of 0.7 Sv/Gy with an occupancy factor of 0.2 for outdoor and occupancy factor of 0.8 for indoor [22]: Equation (4) simplifies into E out = D × 1.226 × 10 -3 (mSv/y) E in = D (nGy/h) × 8760 ( h/y) × 0.7 (Sv/Gy) ×10 -6 × 0.8 (5) Equation (5) simplifies into E in = D × 4.905 × 10 -3 (mSv/y) To estimate annual effective doses, account must be taken of (a) the conversion coefficient from absorbed dose in air to effective dose and (b) the indoor occupancy factor. The average numerical values of those parameters vary with the age of the population and the climate at the location considered. In the UNSCEAR report, the committee used 0.7 Sv/Gy for the conversion coefficient from absorbed doses in the air to the effective dose received by adults and 0.8 for the indoor occupancy factor, i.e., the fraction of time spent indoors and outdoors is 0.8 and 0.2, respectively [28]. The resulting worldwide average of the annual effective dose is 0.48 mSv, with the results for individual countries being generally within the range of 0.3-0.6 mSv. For children and infants, the values are about 10% and 30% higher in direct proportion to an increase in the value of the conversion coefficient from absorbed doses in air to effective doses [22].
As shown in Table 4, the annual effective dose rate for the outdoors ranged between 0.013-0.118 mSv/y with a mean value of 0.038 mSv/y, while the annual effective dose rate for indoors ranged between 0.052-0.743 mSv/y with a mean value of 0.148 mSv/y. The corresponding world average value is 0.48 mSv/y, of which 0.07 mSv/y is from outdoor and 0.41 mSv/y from indoor exposure [22]. Therefore, the study area is still in the zones of normal radiation levels, which can be considered as less of a threat to the environment as well as human health. The variations of the average values of radium equivalent (Ra eq ), absorbed dose rate (D), annual effective dose for outdoor E out , and annual effective dose for indoor E in in the different locations are due to the distributions of radionuclides in soil samples.

External and Internal Hazard Indices (H)
The external exposure to gamma rays in the study area is called external hazard index, or H ex . This index is given by the following equation [20]: In addition to H ex , internal exposure to radon and its short-lived products are also hazardous to the respiratory organs. The internal exposure to radon and its daughter products is quantified by the internal hazard index (H in ), which is given by the following equation [22]: …where A K , A Ra , and A Th are the activity concentrations ((Bq/kg) of the specific radiation) of 226 Ra , 232 Th, and 40 K, respectively. The maximum value of H ex was found to be less than unity corresponds to the upper limit of Ra eq (370 Bq/kg). If the maximum concentration of radium is half that of the normal acceptable limit then H in will be less than 1. The prime objective of this index is to limit the radiation dose to dose equivalent limit of 1 mSv/y [27]. Gamma Index deals with the assessment of excess gamma radiation originating from the present soil. The calculated values of H in are ranged from 0.082 to 0.974 with an average of 0.235, which is lower than the recommended limit. H ex values vary from 0.058 to 0.552 with a mean value of 0.156, and all values are less than the critical value of unity. As shown in Table 4, the internal hazard index is greater than the external hazard index at all sampling sites.

Representative Level Index (I γr )
The level of gamma radiation hazard due to the specified radionuclides of 226 Ra, 232 Th, and 40 K were assessed by another index called representative level index, or I γr .The representative level index, I γr , must be less than unity to keep the radiation hazard insignificant for the public. The following equation was applied to calculate I γr for soil samples under investigation [29]: I γr = A Ra /150 + A Th /100 + A K /1500 ≤ 1 (8) …where A Ra , A Th , and A K are the specific activities of 226 Ra, 232 Th, and 40 K in Bq/kg, respectively. I γr varies from 0.162 to 1.43, with a mean value of 0.453 as shown in Table 3. The values of representative level index for all soil samples were lower than unity, with the exception of the sample from site No. 18. However, the average value of representative level index (I γr ) is less than unity.

Conclusions
In the present study, the activity concentrations for 238 U, 226 Ra, 232 Th, 40 K, and 137 Cs were measured in 34 soil samples collected from Ras Tanura city, Saudi Arabia, near an oil refinery and around a residential area using gamma-ray spectrometry. The measurements of all samples under investigation showed that the mean values of radioactivity levels were lower than the world average for 238 U, 226 Ra, 232 Th, 40 K and 137 Cs. The maximum value of 226 Ra in soil was 156±3.6 Bq/kg,