Natural radioactivity in marine sediment of Khor- Abdulla Northern west of the Arabian Gulf

This work presents measurements of natural radionuclides ( 238 U, 226 Ra, 232 Th and 40 K) in the sea sediment using gamma spectroscopy. The sediment samples were collected from coastal and deep water using special equipment for this purpose. This work was performed in Khor-Abdulla, northern west of the Arabian Gulf to establish the baseline data level for naturally occurring radionuclides in the study area and will be useful for tracking and assessing any accentual pollution in the marine environment in the region. The average values of 232 Th, 238 U, 226 Ra, 40 K are 5.6 Bq/kg, 7.2 Bq/kg, 44.4 Bq/kg and 293.9 Bq/kg respectively. It is found that,the mean radium equivalent Ra eq and effective dose rates are 84.5 Bq/kg and 0.046 mSv/y respectively and they are well below the recommended limit of international committees.


INTRODUCTION
The study of gamma concentration emitted from natural radioactive materials in sea sediment is becoming a subject of great interest. There are many radioactive materials in this sediment, for example the natural 238 U, 226 Ra, 232 Th , 40 K and 137 Cs (NORM). The latest one, some time comes from TEROM fallout from nuclear power station accident. Radium-226 and Radium-228 with their progenies are exist in sediments with level depend on the source of water which they are underneath. The progenies of natural radioactive series, potassium and other radionuclides first appear in the lithosphere level depositing on the soil and rocks, then washed and drained through rivers and finally end up in the estuary and entering marine environment [1][2][3]. The sediment may be regard as a sing of many natural or manmade materials which pass through the various aquatic chemical and biological operations on the earth's surface [4]. The fate of the sediment depends on many complex factors, such as: their size, date of creation and hazard. Parts of marine sediments are from deposited of radionuclides on the earth's surface, then washed by rain or human activities and drained through rivers to the nearby sea or ocean [5]. Predicting the amount of radionuclide concentration in the marine sediment is an important factor in the researches on the marine biota. This is usually based on the studystate approach used in risk assessment models, which is assumes biogeochemical equilibrium between the radioactivity concentration in water and in marine organism, through the concentration ratio [6].
The main objective of this study in to determine natural radionuclides activity concentration in sediment samples distributed in deep water and coastal sediments of Khor-Abdulla. Gamma ray spectroscopy instrument (NaI) was used in this work. This estuary and its coast are very important environmental and economic, due to the lack of information about the level of radionuclide in it.

AREA OF STUDY
The study area was conducted in Khor-Abdulla (KA) which is a semi-enclosed, has a funnel (repressive) shape in the first lower-part with a width of 17 km and with a narrow channel end in the upper -part, with a width of 6.5 km. The eastern coast has a mild decline relative to the heavy decline of the western coast (Kuwait coast). So, the international navigation channel is closer to Kuwait coastal, as shown in Figure 1. The average depth is 10m in most areas. The longitudinal hub (bevel) of the channel is 40 km in the direction of the Arabian Gulf with a width between 6 to 17 km [7]. Salinity value between 32‰ to 38‰ is regarded as a saline lagoon. The tidal system is the same as in the northern part of the Gulf, which is named as semidiurnal; the tidal range value is 2 m to 3 m at spring tide. The maximum value of the surface current, in the downstream, is greater than its like in the root, in both phases (spring and neap tide). The reversible situation is in the upstream and the value was 1.5m/s in the root [8].

Sample preparation
Twenty eight sediment samples used for this study were collected from the Iraqi part of the lagoon. The samples were collected using Sample Grab equipment, shown in Figure 2. The sediments samples were separated from the contamination materials and air-dried at room temperature for a week, then dried to 100 o C, milled and sieved through 0.2 J u n e 04, 2 0 1 5 mm. The dried samples were put inside Marinelli beakers. The beakers were sealed, gas-tight and stored for four week for secular equilibrium.

Gamma ray spectroscopy
Gamma ray spectroscopy used in this work was 3x3 NaI(Tl) scintillator detector with an energy resolution (FWHM) of 2.2 keV for the 0.662 MeV reference transition for 137 Cs. The ready measuring Marinelli beakers are placed on the detector end cap. Both samples and the detector crystal are surrounded by a cylindrical 5cm thickness of lead to suppress the background radiation. The USX software, supplied by manufacturer and multichannel analyser emulation software, data acquisition and online spectrum display was used. Measurement with empty Marilelli beaker, under identical condition, is also perform to estimate the background radiation in the laboratory.

Calculations
The specific activity concentration in Bq/kg of a radionuclide i and for a photo-peak at energy E is given by [9,10] [Bq/kg] where is the net peak counts, ε (Eγ) is the absolute full energy peak efficiency of the detector at this particular gamma-ray energy, t is the counting life time, the number of gammas per disintegration of this nuclide for transition at energy E (branching ratio) and M the mass in kg of the measured sample. Under assumption that the secular equilibrium reached between 232 Th, 238 U and their decay products, the measurement of gamma ray concentrations has been done.
Radium equivalent activity (Raeq) is used to assess hazards associated with materials that contain 226 Ra, 232 Th and 40 K in Bq/kg. which is, determined by assuming that 370Bq/kg of 226 Ra or 260 Bq/kg of 232 Th or 4810 Bq/kg of 40 K produce the same γ dose rate. The Raeq of a sample in (Bq/kg) can achieve using the following relation [10]; (2) The external hazard (Hex) is hazard evaluation of outdoor gamma radiation. The prime objective of this index is to limit the radiation dose to the admissible permissible dose equivalent limit around 1mSvy −1 . In order to evaluate this index, one can use the following relation [11] (3) This model takes into consideration that the external hazard which is caused by gamma-rays corresponds to a maximum radium-equivalent activity of 370 Bq/kg for the soil.
In order to assess the radiological impact of the investigated radionuclides in the soil and sediment sample, the gamma dose received by an adult must be considered. This value is published in UNSCEAR 2000 and UNSCEAR 1993, to be 0.7 SvGy -1 for environmental exposure to gamma rays of moderate energy. The annual effective dose equivalent is given by the following equation [12,13]; J u n e 04, 2 0 1 5 where (5) The outdoor occupancy factor consider to be about 0.2 and the world average annual effective dose equivalent (AEDE) from outdoor or indoor terrestrial gamma radiation only is 0.560 mSv/year.  In order to assess the health effects from the activity of radionuclides present in the sediment samples, this activity converted to single quantity termed as external hazard index (Hex). The limit of this index must not exceed unity. Table 2, contains the gamma activity concentration external hazard index, which ranged from 0.034 up to 0.498 with arithmetic mean value 0.209 the average value is less than the world average value 0.66. Table 2 also presents the annual effective dose equivalent due to the external hazard index. The calculated values of annual effective doses rate ranging from 0.007 mSv/y to 0.107 mSv/y, with average value equal to 0.046 mSv/y which is less than the world average value of 0.48-0.56 mSv/y recommended by (UNSCEAR2000). J u n e 04, 2 0 1 5  232 Th the results of the present work almost equal to the value of neighbouring country (Kuwait). In case of 238 U, the value of this work is in the range of all countries. Potasium-40 measured this work is closed to the value of Kuwait. The hazard indices measured in the present work are closed to the range of other countries and less than the worldwide limit. J u n e 04, 2 0 1 5

CONCLUSIONS
The sediment, are successfully analysed for radioactive elements, namely 238 U, 232 Th, 226 Ra and 40 K. The results clearly show that the levels of radioactivity of gross sediments do not exceed the limits set by the EPA and are within the range of nearby countries. The data obtained in this study will serve as baseline data for the proper assessment of radiation exposure of the dwellers. A comparison of the present measurements and the results of many closed or far countries reviled a general agreement with their results. Good correlation exists between in selected samples between uranium and radium.