Evaluation of Environmental Hazards Resulted from Human Activities and Natural Radioactivity in Tuban Delta in Yemen

The natural background radiation comes from three main sources: cosmic radiation, radiation from terrestrial sources and radioactivity in the body. Most of the background terrestrial radiation a raises from Uranium and Thorium series and 40K. In addition to the natural background radiation, there are several other sources of human exposure, these sources are: diagnostic radiology, therapeutic radiology, use of isotopes in medicine, radioactive waste, fallout from nuclear weapon tests, and occupational exposures from nuclear reactors and accelerations.


Introduction
The natural background radiation comes from three main sources: cosmic radiation, radiation from terrestrial sources and radioactivity in the body. Most of the background terrestrial radiation a raises from Uranium and Thorium series and 40 K. In addition to the natural background radiation, there are several other sources of human exposure, these sources are: diagnostic radiology, therapeutic radiology, use of isotopes in medicine, radioactive waste, fallout from nuclear weapon tests, and occupational exposures from nuclear reactors and accelerations.
Factors influencing the behavior of radionuclides in soils are mainly the chemical properties of the radioelement and the characteristics of the soil, including mineral composition, organic matter content, and chemical reaction milieu [1]. Other factors also affecting the behavior of radionuclides in soil are rainfall amounts, temperature, and soil management. Finally, the pH value is an important parameter controlling the kinetics of elements in soil and the kinetics of radionuclides as well. In order to understand the mobility of radionuclides in soil, it is important to study the inorganic and organic composition of soils. The presence of an inorganic matter (clay minerals and oxides) can cause processes of sorption and complexation. On the other hand, the biological activity can increase radioelement mobility. This paper deals with the evaluation of environmental hazards resulted from human activities and natural radioactivity in soil samples collected from Tuban Delta, Yemen.

Study area
The Republic of Yemen is located in the southern sector of the Arabian Peninsula. Yemen's land is covered with rocks whose ages date back to an era prior to the Cambrian. Some Cambrian rocks even date to a time before that era (that is, about 3 billion years ago). Geologically speaking, Yemen composes part of the Arabian Shield [2]. Tuban Delta is located in the southern part of Yemen. It extends along Lehg and Aden Governorates, the study areas shown in Figure 1.
The Yemen Trap series and flood basalts occur through the Upper Cretaceous and Lower Tertiary in the northwest part of the study area to the north of the study area, the Yemen Trap series is very extensive and thick [3]. It associated with the flood basalts which are a swarm of granitic and pegmatitic dikes that strike NW-SE. Several individual dikes with lengths exceeding 50 km [4].

Climate
The study area climate is hot and arid, becoming semi-arid further inland over the Tuban catchment. Rainfall amounts are low and are highly variable, both really and temporally. Both study area and catchment lie south of the northern tropic and are subjected to monsoonal and Mediterranean weather influences. As will be seen 1ater, most study area, and catchment rainfall occur from March to May, and in larger amounts from June to September. The underlying weather systems are different during each of these two seasons [5]. Table 1 shows some records of temperature and relative humidity. It also displays the mean of sunshine and run of the wind for months

Chemical analysis
• A particle size distribution was carried out by the international Pipette method [6] by using sodium hexametaphosphate as a dispersing agent.
where A Ra , A Th and A K are the activity concentrations of 226 Ra, 232 Th and 40 K, respectively.
The representative level index, Iγ, is defined by the following equation: where A Ra , A Th and A K are the activity concentration of 226 Ra, 232 Th and 40 K, in Bq kg -1 , respectively.
The restriction on building materials for gamma radiation is based on a dose range of 0.3-1 mSv year-1 [10]. For the representative level index Iɤr ≤ 2 corresponds to a dose criterion of 0.3 mSv year-1, while Iɤr ≤ 6 corresponds to 1 mSv year-1 in the report of the European Commission [11]. The total absorbed dose rates D, at 1 m above the ground level, to the population, can be calculated from activities of terrestrial radionuclides according to the following formula [12].
D=0.462 A Ra +0.604 A Th +0.042 A K (5) where: A Ra , A Th and A K are activity concentrations of 226 Ra, 232 Th and 40 K, in Bq kg -1 , respectively.
In addition, the annual effective dose rates indoors (E) (measured in mSv y -1 ) was calculated assuming a value of 0.7 Sv/Gy for the conversion factor from absorbed dose in air to the annual effective dose received by adults and a 0.8 factor for the indoor occupancy [13]. The formula used is: E (µSv y -1 )=d (nGy h -1 ) × 24 h × 365.25 d × 0.8 × 0.7 Sv/G y × 10 -6 (6) The annual effective dose of a member of the public by 0.3 mSv y -1 at the most, which is the excess gamma dose to that received outdoors. Within the European Union, doses exceeding 1 mSv y -1 should be taken into account from the radiation point of view (EC 2000).

Results and Discussion
Physicochemical properties of soil samples Textural classes of the studied soil samples: Textural analysis has been performed to classify the studied soils into their main types [14]. Table 2 shows the textural analysis of some soil samples collected from the study area. These types are the clay loam, silt clay, loam and silty clay loam. The clay loam is the main soil type in the investigated area, which occupies most Tuban delta.
• A calcium carbonate content was measured by using Collin's Calcimeter method [7].
• A saturation soil paste extract was analyzed for determining Ece and soluble ions, as well as, soil pH was measured in the soil water suspension 1:2.5 [8].
• An organic matter content was determined by using the modified Walkley and the black method as described by Jackson [8].
• The cation exchange capacity (CEC) and the exchangeable sodium % (ESP) were determined by using sodium acetate and ammonium acetate of pH 7, respectively [7].

Sampling and sample preparation for gamma spectrometry
A total of 30 surface soils samples were collected from the study area. After collecting the stones, the inorganic materials were removed, then the samples were dried in low-temperature oven at 105°C for 48 hours to ensure that moisture is completely removed. The samples were crushed and grounded to a fine grain size of about 1 mm and sieved in order to remove the big sizes. The samples were sealed in polyethylene containers of 100 cm 3 and left for one month, before measuring them by gamma spectrometer, to allow secular equilibrium between 226 Ra and its decay products.

Radioactivity measurements
One of the most valuable techniques for low-level radioactivity measurements is gamma ray spectrometer. The various systems, consisting of semiconductor detectors coupled to multichannel analyzers, provide for a rapid simultaneous measurement of many radionuclides in the same sample.
In this study, each sample was measured by using HPGe with 35% efficiency and 1.9 keV resolution. The spectrum was collected and analyzed by using computer software called Maestro software. These measurements processes were carried out in Egyptian atomic Energy Authority Laboratory. The spectrometer was calibrated for efficiency and energy.

Calculation of the radiological parameters
The most widely used to evaluate the activity concentration of materials containing 226 Ra, 232 Th, and 40 K is the radium equivalent activity (Raeq). Radium equivalent activity can be calculated from the following relation [9].
where A Ra , A Th and A K are the activity concentration of 226 Ra, 232 Th and 40 K in Bq kg -1 respectively.
The external hazard index, H ex , is calculated by using mathematical expression for the purpose of estimating hazards caused by gamma radiation that naturally occurs. The external hazard index, Hex, is defined by some scientist [9] as: where A Ra , A Th and A K are the activity concentrations of 226 Ra, 232 Th and 40 K in Bq kg -1 respectively. The value of this index must be less than unity for the radiation risk from building material to be negligible. The internal exposure to radon and its daughter products is quantified by the internal hazard index, Hin, which is defined by Beretka and Mathew [9] as follows:  Total salinity: Several classification systems were elaborated to set up an adequate standard for covering the measured EC of the soil saturation extract into salinity classes. The approximate limits of salinity classes were defined in the [14], as in Table 2. As shown in the Table 3, three samples fall in the slightly saline class, and seven samples fall in strongly saline class.
Organic matter and calcium carbonate contents: The organic matter content in samples under study is extremely high (exceeding 6.1 g/kg) in all of the studied samples. The organic matter ranges from 1.40 to 2.75%. Also, data in the Table 3 where the concentration of the cations is in meq/l. It indicates that the degree to which cation exchange reactions occur in the soil. As sodium replaces Ca and Mg on clays and colloids, it causes the expansion or "swelling" of clay particles and leads to a reduction in permeability and hardening of the soil. SAR should remain low enough so that Na occupies a low portion of the exchange complex. The structure and drainage properties of most soils remain good when SAR values remain below 10 [15]. The quality classification of soil samples, based on SAR value is given in Table 3. Table 3 shows that the soil samples from the Tuban delta are as follows: 4 samples were low sodium hazard, 3 samples were medium, and 3 samples were high sodium hazard.
Cationic and anionic composition: Table 3 shows the cationic and anionic composition of some soil and rocks samples, the soluble cations are often dominated at the studied samples with Ca2+, Mg2+, and Na+, while K+ is the least soluble cation. For the study soil Na+>Ca2+>Mg2+. The predominance of Na+, Ca2+, and Mg2+ is a true reflection of the involvement of a marine origin in soils formation.
Regarding the anionic distribution, CO3 2is absent in all of the studied samples. The distribution of the other anionic followed the descending order: Cl ->SO4 2->HCO 3 for all studied samples.

Radioactivity concentrations
The activity concentrations (Bq kg -1 ) of the natural radioactivity due to 238 U series ( 226 Ra), 232 Th, and 40 K of soil samples are given in Table 4 Table 4, it can be concluded that the mean activity concentrations of 238 U( 226 Ra) for uncultivated soil samples are higher than the mean activity concentrations of the farm soil samples. This may be due to human activities such as using oil, and natural gas to produce electric energy in this area [16]. These activities can enhance the local activity concentrations. On the other hand, farm soils have been cultivated and irrigated, water helps to remove the radionuclides and minerals from the surface soil and transports them to the underground level. Also, in the farm soil, plants have been taken up radionuclide from soil, and this is expressed as soil-to-plant transfer factors.
The mean values of activity concentrations of 40 K and 232 Th in the farm soil samples treated with an organic and urea fertilizer are higher than the mean values of activity concentrations of 40 K and 232 Th in the uncultivated soil samples and the farm soil samples treated with urea fertilizer. This is may be due to using an organic fertilizer which contains a high percentage of K element, which leads to enhance the activity concentrations of 40 K in the soil.
From the Table 5, we noticed that the activity concentrations of 137 Cs showed in some uncultivated soil samples (2, 3, 4, 8 and 9). Anthropogenic radionuclide 137 Cs transport from one place to another on the earth with air and the rainwater helps to remove radioactivity from the troposphere and transport it to the earth's surface [1]. Tables 2 and 5 we noticed the increase of activity concentrations of series ( 226 Ra) and 232 Th with the increase of the percentage of clay in the soil. This is may be due to the increase of clay minerals, montmorillonite, and kaolinite. Also, clay soil helps to catch the soluble minerals and radionuclides from water higher than another type of soils like sand and silt. The clay particles are smaller than 2 µm in diameter when sand and silt soils have a particle size, from 50 µm to about 2 mm, from 2 to 50 µm, respectively.

From the
The results for the radiological parameters such as the radium equivalent activity, the external and internal hazard index, gamma index, the absorbed dose rate in air, and the annual effective dose of the studied samples are presented in Tables 3-5. All results of radiological parameters and calculation doses were lower than the allowed maximum value reported in UNSCEAR [17].
To compare the natural radioactivity of soil under study with the Yemeni soils in other areas, [18] study the natural radioactivity of 40 K, 226 Ra, 232 Th in soil samples collected from Abyan Delta in Yemen, who found the activity concentrations of 226 Ra, 232 Th, and 40 K for soil irrigation by the flood water samples were 34.9, 84.5, and 1232.7 Bq kg -1 , respectively, and for the soil samples irrigation by groundwater were 30.24, 63.64, and 1096 Bq kg -1 , respectively, which reveals that the activity concentrations of the soil samples from Abyan delta are nearly  Geologically, this means that the origin of soil for delta Tuban and delta Abyan is different. El-Mageed et al. [19] study the activity concentrations of 40 K, 226 Ra, and 232 Th in soil samples collected from around Juban town in Yemen, the study shows that the average activity   concentrations of these radionuclides were higher than the activity concentrations of the soil samples under the study. Also, El-kamel et al. [20] study the activity concentrations of 40 K, 226 Ra, and 232 Th in the soil samples collected from Alsalamia-Al-Homira area in Abyan governorate. The study shows that the activity concentrations of 40 K, 226 Ra, and 232 Th for clay and sandy soil were much higher than the activity concentrations of the soil samples under study.  where: *farm surface soil irrigation by flood water; **farm surface soil irrigation by groundwater; 1 farm soil treated with organic and urea fertilizer; 2 Farm soil treated with urea Also, the mean activity concentrations of 226 Ra, 232 Th, and 40 K for the samples under study were compared with similar studies in other countries and the results are given in Table 6. From Table 7, the average activity concentration of 232 Th for the samples under the study are lower than that reported by other countries except in case of Egypt reported by (Ahmed and El-Arabi [21], Jordan (ma'an) reported by Saleh and