Radiation Protection and Environment

ORIGINAL ARTICLE
Year
: 2019  |  Volume : 42  |  Issue : 4  |  Page : 128--137

Environmental radioactivity levels in agricultural soil and wheat grains collected from wheat-farming lands of Koya district, Kurdistan region-Iraq


Najeba Farhad Salih, Zakariya Adel Hussein, Shalaw Zrar Sedeeq 
 Department of Physics, Faculty of Science and Health, Koya University, Koya KOY45, Kurdistan Region, Iraq

Correspondence Address:
Mr. Shalaw Zrar Sedeeq
Faculty of Science and Health, Koya University, Koya KOY45, Kurdistan Region
Iraq

Abstract

The purpose of this study is to determine the activity concentration levels of radionuclides of226Ra,232Th,40K, and137Cs in soil and wheat grain samples. A total of 72 samples of soil and wheat grains were collected in the study area. Measurements were performed using a gamma-ray spectroscopic system based on high purity germanium detector. The average activity concentrations of226Ra,232Th, and40K in soil samples were found to be 14.7 ± 0.2 Bq/kg, 19.9 ± 0.5 Bq/kg, and 329 ± 5.4 Bq/kg, respectively, and for the measured wheat grain samples their values were found to be 0.4 ± 0.09 Bq/kg for226Ra, 0.36 ± 0.1 Bq/kg for228Ra, and 109.2 ± 2.2 Bq/kg for40K. The calculated mean values of radium equivalent activity Raeq, outdoor absorbed dose rate Dout, and outdoor annual effective dose rate of the soil samples were found to be 68.55 Bq/kg, 32.5 nGy/h, and 0.039 mSv/year, respectively. Moreover, the radionuclide transfer factor from soil-to-wheat grains of naturally occurring radionuclides were found in this order40K > 226Ra > 228Ra.



How to cite this article:
Salih NF, Hussein ZA, Sedeeq SZ. Environmental radioactivity levels in agricultural soil and wheat grains collected from wheat-farming lands of Koya district, Kurdistan region-Iraq.Radiat Prot Environ 2019;42:128-137


How to cite this URL:
Salih NF, Hussein ZA, Sedeeq SZ. Environmental radioactivity levels in agricultural soil and wheat grains collected from wheat-farming lands of Koya district, Kurdistan region-Iraq. Radiat Prot Environ [serial online] 2019 [cited 2020 Feb 20 ];42:128-137
Available from: http://www.rpe.org.in/text.asp?2019/42/4/128/276924


Full Text



 Introduction



The radionuclides of terrestrial source (primordial radionuclides) are in the class of naturally occurring radioactive materials. The primordial radionuclides comprise the natural series such as238 U,232 Th, and nonseries40 K which are ordinarily long-lived and with a half-life more than 100 million years.[1],[2] Commonly, these radionuclides can be found in all the environmental elements and are present in various amounts in the soil, rocks, water, air, vegetables, animals, and human body it self.[3] In addition, they symbolize the main source of external dose, and they make an important contribution to the internal exposure due to the food and water consumption.[4] Food ingestion is the most common pathway to transfer radionuclides to people; therefore, the detection of radioactive materials is absolutely important in the process of people and environment protection.[5] Human beings are exposed to both external and internal radiation. The internal exposure comes from the intake of terrestrial radionuclides through inhalation or ingestion pathways. The inhalation exposure is related to the existence of dust particles in the air, which comprise the radionuclides from the decay series of238 U and232 Th and nonseries40 K as well.[6] The short half-life decay products of radon are the largest contributor to the inhalation exposure. The ingestion exposure is normally related to238 U and232 Th decay series as well as40 K through eating foods and drinking liquids.[7] Plants acquire the main source of terrestrial radionuclides through the roots and leaves, while humans and animals acquire radionuclides through the consumption of these plants. There are two different mechanisms for the transferring of radionuclides to plants, either through root uptake or directly through aerial deposition.[8] The absorption of radionuclides through root uptake in the plants rely on some factors such as the majority of radionuclides in the soil, soil characteristics, physiochemical properties of the soil, and the type of plant under study.[9] The levels of radionuclides in plants vary typically from a few tens of Becquerel (Bq) to several hundred of Becquerel per kilogram.[10] The radionuclide uptake from the soil by plants is ordinarily described by transfer factor (TF).[11] The soil-to-plant TF is considered as a significant parameter in the environmental safety assessment.[12] Human practices, in particular, mineral processing and uses such as phosphate fertilizer production and utilization can modify many exposures to natural radiation sources. The use of agricultural fields continually has led to the diminution of the natural elements in soils. On the other hand, due to the fact of the chronic increase in the world's population, a large quantity of food is required to comply this increase. Therefore, this has encouraged a lot of countries to widely use of phosphate fertilizers in crop production to increase their annual products. The radionuclides that exist in the fertilizers are uranium and thorium decay series as well as potassium. Besides, the concentration of radionuclides in fertilizers differs from different countries and depending on the origin of the components.[1],[13] Based on the information present in the agricultural directorate of Koya district and our meetings with the farmers working on the farming lands of Koya district, it can be said that the different types of fertilizers in great amounts are continuously used by the local farmers without being any estimation with the experts. Therefore, this research was carried out to investigate the levels of radioactivity due to the natural radionuclides of226 Ra,232 Th, and40 K in soils and wheat grains of the wheat-plantation fields of Koya district and also to estimate the radionuclide TF from soil-to-wheat grains.

 Materials and Methods



Collecting samples

The study was conducted at Koya district, which is also known as (Koysinjaq). It is situated in Erbil governorate from the south part of Kurdistan, which is called Iraqi Kurdistan region, as shown in [Figure 1]. The intensive farming of wheat is distributed at the plain of Erbil-south of Koya district.[14] Moreover, according to the annual reports of the agricultural directorate of Koya, wheat planting can be considered as a dominant agricultural activity in that district, and the largest area of agricultural lands is devoted for wheat planting. A total of 72 samples (36 agricultural soil samples of wheat-plantation fields and 36 samples of mature wheat grains obtained from the wheat plants grown in the corresponding soil of wheat plantation fields) were collected throughout the center of Koya district, and its five subdistricts (Ashti sub-district, Taq Taq sub-district, Segirdkan sub-district, Shorsh sub-district, and Siktan sub-district) within 36 villages where the local growers use a great area of land for the cultivation of wheat plant. [Table 1] shows some information about the sampling locations such as subdistricts and subregion names, sample codes, elevations, latitudes, and longitudes of all the sampling points. The sample locations are shown in [Figure 2]. The sample sites were determined using a Global Positioning System (GPS). The coordinates of each sample location were registered using a GPS navigator (model: GPS map 6030SX GARMIN). In order to make a representative sample from each location, 6 points were selected across each wheat-plantation field, and the area of each point was (2 m × 2 m). Five subsamples of soil and five subsamples of mature wheat grains grown in the identical soil in each point of the selected 6 points of a particular wheat-plantation field were collected, and then all the subsamples of a particular location were mixed up thoroughly to make a representative sample which is recommended by the International Atomic Energy Agency.[3] Because the wheat plant has an adventitious root system, it spreads in the upper layer of the soil. Therefore, the soil samples were taken to a depth of 6 cm including the surface layer.[3],[15] A reasonable quantity (about 3 kg) of soil and wheat grain samples was labeled and transferred into a polythene bag and then the samples were transported into the counting room.{Figure 1}{Table 1}{Figure 2}

Preparation of samples

Soil samples

The preparation process of all the samples was performed in accordance with the recommendations of the International Atomic Energy Agency. Soil samples were placed in the open air and dried against the sun for 3–4 days to ensure that the moisture is completely removed.[16] Then, the samples were cleaned carefully by removing wheat roots, wheat leaves, stones, gravels, and debris. After that, to ensure adequate drying, the samples were placed in an oven for 24 h at temperature 105°C.[17] Afterward, the soil samples were ground using a ball mill machine. Then, the powdered samples were passed through a (1 mm) mesh to get homogeneous samples. A very sensitive balance was utilized to measure the mass of the samples, each sample approximately 1 ± 0.02 kg. The weighted samples were placed in a plastic container, which is known as (Marinelli beakers). The beakers were tightly sealed and stored for more than 4 weeks to allow reaching the secular equilibrium of thorium - 232 and radium - 226 with their decay products.[18] Finally, after that, the samples reached radioactive equilibrium; they were transported into the counting room for radiometric measurement and analysis.

Wheat grain samples

The samples were carefully cleaned from wheat roots, wheat leaves, and any kind of debris. Then, the samples were ground using a powder grinder machine. The samples were passed through a 1 mm mesh to get homogenized samples. In order to remove moisture and for adequate drying, the samples were placed in an electrical oven at 100°C for 10 h.[19] A very sensitive balance was used to measure the mass of the dried samples, each sample about 1 ± 0.02 kg of dry weight. For measurements, the samples were packed into Marinelli beakers and tightly sealed then stored for a month to reach secular equilibrium.[20]

The technique of radiometric analysis and nuclide identification

A gamma-ray spectroscopic system with a high resolution was used to measure the levels of radioactivity of the soil and wheat grain samples due to the naturally occurring radionuclides of226 Ra,232 Th (228 Ra) (The activity concentration of228 Ra was calculated instead232 Th for the wheat grain samples[21]) and40 K and the man-made radionuclide137 Cs. The system was based on high purity germanium, p-type of vertical closed-end coaxial detector. The detector has a relative efficiency of 73.8% at 1.33 MeV for60 Co, and its resolution (full width at half maximum) was 1.18 keV at 122 keV for57 Co, and at 1332 keV of60 Co was 1.97 keV. The detector is protected from the background radiation by a lead shielding as the circulator, which is 10 cm in thickness in all dimensions. Radioactivity measurements were carried out for 36000 s[22] to measure the quantitative and qualitative determination of226 Ra,232 Th (228 Ra),40 K, and137 Cs available in the soil and wheat grain samples. The activity concentration of the interested natural radionuclides and137 Cs was calculated using equation 1. In order to investigate the background radiation due to the man-made and naturally occurring radionuclides in the environment around the detector, an empty Marinelli beaker was counted in the same manner as the samples. After background subtraction, the activities of226 Ra and232 Th (228 Ra) were determined through their daughters,[3],[19],[21],[23],[24] as shown in [Table 2]. After storing the samples for a month and under the assumption that secular equilibrium was achieved between226 Ra and232 Th and their decay products, the activity concentration of226 Ra was calculated from the average concentrations of the214 Pb and214 Bi decay products and that for232 Th (228 Ra) was calculated from the average concentrations of208 Tl and228 Ac decay products in the sample.[25]{Table 2}

[INLINE:1]

Where Iɤ is the emission probability per decay of the specific peak, ε is the absolute gamma peak efficiency for the detector at a particular photopeak, t is the counting time in seconds, and m is the mass of the sample in kilogram.

Determination of radiological index parameters

The exposure to radiation arising from the terrestrial radionuclides in soils of wheat plantation fields of Koya district and wheat grains growing on the soil of those corresponding agricultural fields can be determined in terms of some parameters as given below:

Radium equivalent activity (Raeq)

The specific activity of226 Ra,232 Th, and40 K can be represented by a single quantity (Raeq) in Bq/kg. The Raeq is the most important to assess the radiation hazards and could be mathematically calculated using equation (2) given by Mehra et al.[26]

[INLINE:2]

Where ARa, ATh, and AK are the activity concentration of226 Ra,232 Th, and40 K, respectively. It was assumed that 1 Bq/kg of226 Ra, 0.7 of232 Th, and 13 Bq/kg of40 K produces the same gamma-ray dose rate.

Outdoor absorbed gamma dose rate (Dout)

The outdoor absorbed dose rate Dout at 1 m above the ground level is determined using equation (3), postulating that the effects of the other artificial and natural radionuclides are low and could be neglected.[27],[28],[29]

[INLINE:3]

Where ARa, ATh, and AK are the activity concentration of226 Ra,232 Th, and40 K, respectively. Moreover, the conversion factor of 0.462 nGyh−1/ Bqkg−1 for226 Ra, 0.604 nGy/h/Bq/kg for232 Th, and 0.0417 nGy/h/Bq/kg for40 K was used to estimate Dout.

Indoor absorbed gamma dose rate (Din)

The primordial radionuclides are causing to indoor exposures, the conversion factors (0.92 nGy/h/Bq/kg) for226 Ra, (1.1 nGy/h/ Bq/kg) for232 Th, and (0.081 nGy/h/Bq/kg) for40 K are used to estimate the Din due to the presence of gamma-ray dose indoors using the following formula.[27],[30]

[INLINE:4]

Outdoor annual effective dose rate

The annual effective dose can be calculated from the outdoor absorbed gamma dose rate Dout using two factors; the outdoor occupancy factor (0.2) and the conversion coefficient from the absorbed dose rate (0.7). The outdoor annual effective dose rate (AEDRout) is calculated using equation (5).[1],[31]

[INLINE:5]

Indoor annual effective dose rate (AEDRin)

Indoor exposure becomes more important if the occupancy duration was taken into account, i.e., normally people stay about 80% of their time indoors.[32] Ein is an amount of dose that is taken by human beings, and it can be calculated from the indoor absorbed dose using the dose conversion factors; the conversion coefficient from the absorbed dose rate (0.7) and indoor occupancy factor (0.8) the time staying in the indoor during the year. The indoor annual effective dose rate (AEDRin) can be mathematically represented as the following equation.[31]

[INLINE:6]

Transfer factor

TF is a significant parameter for radiological assessment, and it could be defined as; the steady-state concentration ratio between one physical situation to another. A steady-state concentration ratio (Bq/kg dry weight plant to Bq/kg dry weight soil) is usually preferred to reduce the uncertainty because the quantity of radioactivity in a dry weight sample is much less variable than the quantity per unit fresh weight.[33] Radionuclide TF from soil to plants depends on the soil properties, plant type, and type of radionuclides.[25] The TF value is calculated using the following formula.[1],[34] The edible part of wheat plant (grains) was selected to measure the TF values.

[INLINE:7]

 Results and Discussion



Activity concentration in agricultural soils

The activity concentrations of226 Ra,232 Th,40 K, and137 Cs in soil samples are presented in [Table 3]. [Table 3] shows that the maximum levels of226 Ra,232 Th,40 K, and137 Cs were recorded from the sampling locations of (SS14-Takaltu, SS1-Sewasan, SS23-Darbasari bchuk, and SS33-Kamusak), respectively. In general, the radionuclides of interest, namely226 Ra,232 Th,40 K, and137 Cs were found in all the collected soil samples. The specific activity of40 K in all the agricultural soil samples was much greater than the specific activities of226 Ra,232 Th, and137 Cs. The average value of the activity concentration of40 K in the present study was lower than the recommended worldwide average value of40 K. Besides, in some locations, the activity concentration of40 K was higher than the worldwide average value (412 Bq/kg), which is recommended by the United Nations Scientific Committee on the Effects of Atomic Radiation Sources.[29] This might be due to the use of (N. P. K compound fertilizers) in the wheat-plantation fields of Koya district. Moreover, the amount of activity concentration of232 Th is almost greater than the amount of226 Ra in all the measured soil samples. The average values of activity concentrations of the natural radionuclides226 Ra,232 Th, and40 K for the agricultural soil samples in the present study were lower than the worldwide average values recommended by the United Nations Scientific Committee on the Effects of Atomic Radiation Sources[29] as 32 Bq/kg for226 Ra, 45 Bq/kg for232 Th, and 412 Bq/kg for40 K.{Table 3}

Activity concentrations in wheat grain samples

[Table 4] shows the activity concentrations of artificial137 Cs and natural radionuclides of226 Ra,228 Ra, and40 K for 36 investigated wheat grain samples. The detection limit of137 Cs was about 0.012 Bq/kg for wheat grain samples. It can be noticed that137 Cs was not detected (ND) in all the measured wheat grain samples. The228 Ra activity was below minimum detectable activity (BMDA) in samples of (WS3, WS5, WS8, WS22, WS27, WS29, WS32, and WS35), and it was ND for two samples (WS13 and WS34) as shown in [Table 4]. As for228 Ra, its concentration was ND or BMDA in some wheat grain samples, but it does not imply absolutely that the absence of228 Ra in these samples. In fact, many researchers in their studies have reported BMDA or ND for228 Ra in wheat grains, for instance.[16],[35],[36]40 K was found in all samples with a minimum value of 72.0 ± 1.5 Bq/kg (recorded in WS2-Pebazok), a maximum value of 136.1 ± 2.6 Bq/kg (recorded in WS31 - Sinawa) and with an average value of 109.2 ± 2.2 Bq/kg. Finding40 K with a high concentration was already expected because it is naturally high abundance in environmental samples.226 Ra was detected in all the samples and the value of activity concentration of226 Ra varies from 0.25 ± 0.1 Bq/kg (recorded in WS10-Ella Allah) to 0.74 ± 0.08 Bq/kg (recorded in WS15-Kani Lala) with an average value of 0.407 ± 0.097 Bq/kg. The average values of activity concentrations of226 Ra,228 Ra, and40 K for the wheat grain samples in this study were too much lower than the acceptable values recommended by the United Nations Scientific Committee on the Effects of Atomic Radiation Sources.[1] The obtained results of activity concentrations of the interested natural radionuclides had this order226 Ra 228 Ra 40 K, which is in accordance with the information presented by Changizi et al.[16] The noticeably high recorded values of40 K in the wheat grain samples within the present study is like the similar findings recorded in the related literature.[19],[22],[37],[38]{Table 4}

The assessment of radiological index parameters of soil samples

A common index which is called radium equivalent activity Raeq is used to determine the uniformity of distribution of natural radionuclides of226 Ra,232 Th, and40 K in rocks and soils with respect to radiation exposure due to these radionuclides.[1],[39] The Raeq was calculated using equation (2), and the obtained results are shown in [Table 5]. The calculated values of radium equivalent activity for the soil samples ranged from 17.5 to 90.9 Bq/kg with an average value of 68.5 Bq/kg. These values are lower than the maximum permissible limit (370 Bq/kg) which is recommended by the United Nations Scientific Committee on the Effects of Atomic Radiation Sources.[1] Furthermore, the outdoor and indoor absorbed dose rates Dout and Din (nGy/h) in air at 1 m above the ground surface due to the existence of natural radionuclides were calculated using equations (3) and (4), respectively. And also, when the soil samples were collected, the outdoor absorbed dose rate in every sampling point was measured using an isotope identifier and a gamma scout at 1 m above the ground level. Columns 3, 4, and 5 in [Table 5] give the measured values of Dout during sample collecting and the calculated values of Dout and Din for the investigated soil samples, respectively. As shown in [Table 5], there is a good correlation between the measured and calculated Dout values. The mean values of Dout (calculated) and Dout (measured) were lower than the recommended value of 57 nGy/h as given by the United Nations Scientific Committee on the Effects of Atomic Radiation Sources.[1] In addition, the Din (calculated) mean value was lower than the worldwide average value 70 nGy/h as reported by the United Nation Scientific Committee on the Effects of Atomic Radiation Sources.[40] Thus, according to the obtained results, there is no radiation exposure effect due to external radiation exposure for those people working on the wheat-plantation fields of Koya district. Moreover, to estimate the annual effective dose equivalent to be received by the people work in the agricultural lands of the study area due to soil radioactivity the equations (5) and (6) were used to calculate the AEDRout and AEDRin, respectively. The calculated values of AEDRout and AEDRin are shown in columns 6 and 7 of [Table 5]. The AEDRout and AEDRin values ranged from 0.01 to 0.052 mSv/year with an average value of 0.039 mSv/year and from 0.081 to 0.400 mSv/year with an average value of 0.304 mSv/year, respectively. The reported values of AEDRout and AEDRin in this study were lower than the worldwide average values of 0.07 and 0.41 mSv/year, respectively, as given by the United Nations Scientific Committee on the Effects of Atomic Radiation Sources and Rao.[1],[41]{Table 5}

Radionuclide transfer from soil-to-wheat grains

TF explains the absorption rate of radionuclides by the root system of the plants from the soil. The soil-to-plant TF is considered to be one of the most significant parameters required for environmental safety assessment.[22] Equation (7) was used to determine the TF from soil-to-wheat grains. The calculated TF values for the natural radionuclides of226 Ra,228 Ra, and40 K are presented in [Table 6]. The calculated values were ranged from 0.013 to 0.1 with an average value of 0.028 for226 Ra, from 0.0 to 0.126 with an average value of 0.021 for228 Ra and from 0.189 to 1.110 with an average value of 0.373 for40 K. The obtained results of TF show some variations according to the different sample cites. Radionuclide absorption from soil by plants depends on the soil characteristics which include pH content, clay content, soil texture, cation exchange capacity, dominant clay minerals, exchangeable cations, and organic matter content. In addition, the uptake of radionuclides is affected by the plant type and type of radionuclides - the radionuclide is heavy or light element.[21],[42] The soil-to-wheat grain TF s of40 K is considerably higher than those for226 Ra and228 Ra because of the high solubility of40 K in water and its high mobility in soil.[43] Soil-to-wheat grain TF of137 Cs was not observed because137 Cs was ND in all the measured wheat grain samples.{Table 6}

 Conclusions



This research aimed to measure the natural radioactivity levels in soil and wheat grain samples collected from the wheat-plantation fields of Koya district. The mean values of activity concentration of naturally occurring radionuclides in the soil and wheat grain samples were found to be lower than the worldwide average values. The existence of the fallout radionuclide137 Cs is observed in all the investigated soil samples. While no-detection for137 Cs in the wheat grain samples was confirmed. The calculated average values of radium equivalent activity, outdoor absorbed dose rate, and outdoor annual effective dose equivalent for soil and wheat grain samples were below the recommended permissible limits. The radionuclide TF s from soil-to-wheat grains for natural radionuclides of226 Ra,228 Ra, and40 K have this order40 K >226 Ra > 228 Ra. The accumulation of primordial radionuclides in the wheat grains produced from the wheat-plantation fields of Koya district does not have health risk. Overall, it was confirmed that the natural radioactivity levels in the collected soil and wheat grain samples are not at the range of health risk because all the computed radiological index parameters were below the international permissible limits.

Acknowledgments

The authors would like to thank the Department of Physics, Faculty of Science and Health, Koya University for providing research facilities.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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