|Year : 2019 | Volume
| Issue : 3 | Page : 112-118
Natural radionuclide transfer from soil to plants in high background areas in Oyo state, Nigeria
Augustine Kolapo Ademola
Department of Physical Sciences, Bells University of Technology, Ota, Ogun State, Nigeria
|Date of Submission||11-Jul-2019|
|Date of Decision||02-Aug-2019|
|Date of Acceptance||27-Aug-2019|
|Date of Web Publication||06-Nov-2019|
Augustine Kolapo Ademola
Department of Physical Sciences, Bells University of Technology, P.M.B 1015, Ota, Ogun State
Source of Support: None, Conflict of Interest: None
The activity concentrations of naturally-occurring radioactive materials present in soil, vegetables, and food crops and their transfer factor (TF) in some high background areas of northern parts of Oyo state, Nigeria, were measured using gamma spectrometry. The average concentration of226Ra,228Ra, and40K in soil is 25.3 ± 7.1, 26.2 ± 5.0 and 381.8 ± 16.0 Bq/kg, respectively in Agbaaru; 25.4 ± 3.1, 39.5 ± 3.8, and 401.9 ± 25.4 Bq/kg in Abuja; 26.5 ± 3.3, 39.2 ± 7.4, and 394.9 ± 18.6 Bq/kg in Arget; and 26.5 ± 3.5, 38.8 ± 2.5, and 389 ± 18.6 Bq/kg in Igbeti. The soil-to-vegetables TF for226Ra,228Ra, and40K is 0.123, 0.058, and 0.215, respectively, in Agbaaru; 0.065, 0.040, and 0.185 in Abuja; 0.093, 0.035, and 0.243 in Arget; and 0.080, 0.050, and 0.265 in Igbeti. Furthermore, the TF from soil to food crops is within the recommended value. Therefore, consumption of these vegetables and food crops will not pose any radiation health concerns to the consumers and the general population.
Keywords: Food crops and vegetables, marble-mining area, natural radionuclides, Oyo-North division, transfer factor
|How to cite this article:|
Ademola AK. Natural radionuclide transfer from soil to plants in high background areas in Oyo state, Nigeria. Radiat Prot Environ 2019;42:112-8
|How to cite this URL:|
Ademola AK. Natural radionuclide transfer from soil to plants in high background areas in Oyo state, Nigeria. Radiat Prot Environ [serial online] 2019 [cited 2019 Nov 22];42:112-8. Available from: http://www.rpe.org.in/text.asp?2019/42/3/112/270441
| Introduction|| |
Assessments of natural environments are of high importance because every living organism is exposed to ionizing radiation. These radiations comprises cosmic radiation from the outer space, terrestrial background radiation from the earth crust, building materials, artificial radionuclides, and all other things from the soil, such as food and water. Studies on natural radioactivity revealed that about 80% of the total radiation to which a person is exposed during 1 year is due to natural radiation. Naturally-occurring primordial radionuclides have been present in the environment since the formation of the earth. The continuous increase of radionuclides in the environment may be attributed to urbanization which involves the use of phosphate fertilizer in agriculture, mining and mineral operations, and production of building materials, especially one that contain gypsum. Mining and mineral processing have been identified as among the main natural sources of exposure to radiation, especially when the ore body contains the primordial radionuclides in significant amounts. These natural radioactive sources are the largest contributor of the radiation doses received by humanity.
Radioactivity in the terrestrial environment is bound to the components of the soil. Their transportation to plants is possible via root uptake and deposition of dust on plant leaves and to humans through inhalation, breathing, and soil ingestion. Soil–plant–human route had also been recognized as a major pathway for the transfer of radionuclides to human beings. The uptake of radionuclides from soil to plant is characterized by transfer factor (TF) which is defined as the ratio of radionuclide concentration in plant to soil per unit mass., This parameter is necessary for environmental transfer models, which are useful in the prediction of radionuclide concentration in agricultural crops for the estimation of dose impact to human being.
The study area, Oyo-North division, is one of the materially blessed parts in Nigeria. The area is naturally endowed with varieties of mineral deposits; tantalite and aquamarine in Komu, Ofiki, Agbaaru, and Sepeteri; iron ore in Iseyin; and marble and dolomite in Igbeti and Ado-Awaye, which contributed enormously to the economic progress of the region. These stones are of granite type and may contain a relatively high concentration of natural radioactivity. This area is also known as the food basket of Oyo state because the land is rich and is suitable for agricultural purposes. Farming is the major occupation of the inhabitants. With mining activities ongoing in these areas, there is the possibility of high background radiation, which may be transferred to food crops cultivated in the areas through root uptake and deposition of dust on plant leaves and stems.
Several researches have been carried out on the transfer of naturally-occurring radionuclides from soil to plants in many parts of the world,,,,, but very sparse in Nigeria., Therefore, there is need to study in these areas to assess the possible radiation exposure due to mining on the population and also to determine the soil-to-plant TF of some food crops and vegetables planted and consumed in the study areas. The data obtained from this study will be useful for the assessment of public dose rate and also the reference data that will help in providing solutions to environmental changes due to mining activities in Nigeria. The objectives of this paper are measurement of the activity concentrations of radium (226 Ra), radium (228 Ra) and potassium (40 K) in soil, food crops, and vegetables collected from the study areas; estimation of hazard indices in the representative samples; and determination of soil-to-plant transfer from the measurement.
Geology of the study area
Oyo-North division is endowed with varieties of raw materials and minerals that are well known in the international market. The area is covered by Precambrian igneous and metamorphic rocks referred to as basement complex. The crystalline basement complex is pervaded by pegmatites which are host for gemstones, tantalite, and varieties of mineral deposits. The older granites have resulted in smooth domed inselbergs, particularly in areas around Iseyin, Okeho, Igboho, Saki, Ogbooro, Oyo, Igbeti, and the greater parts of Oyo North. The major rock unit in the study area is the undifferentiated meta-sediments in addition to granite, granite-gneiss, and porphyritic granite. However, syenites are common and well exposed within and around Saki and Okeho, while there is occurrence of several pegmatite veins, as intrusion into the undifferentiated crystalline basement rocks, most of which are in gemstones, especially tourmaline. Certain groups of meta-sediments which include the quartz feldspathic biotite schists, quartzites, and marble are found in abundant in areas around Saki West, Kajola, Olorunsogo, and Itesiwaju, local government areas.
| Materials and Methodology|| |
Farmlands within 2–3 km distance to the mining sites were sampled in mining areas of Agbaaru (Iwere-Ile), Abuja (Komu), Arget (Ofiki), and marble mines (Igbeti). Soil, food crops (yam, cassava, and maize), and vegetables (tomato, okra, wild lettuce [yanrin], and coriander leaf) samples were collected from farmlands in the study area. Plant samples were collected from an area of about 1 m2 within the sample locations, and top soil layers together with plants (stem and root) were removed, packed, and labeled. A total of 84 soil samples, 36 food crop samples, and 48 vegetable samples were collected for the study. Soil samples were collected at a depth of 10–15 cm.
Food crops and vegetable plant samples were washed with fresh water to remove dust, mud, and other impurities from the samples.
Yam and cassava samples were peeled and dried in air. All the food samples were oven-dried at 110°C for 16 h to obtain constant weight. The residue was ground into fine powder and weighed into a plastic container.
Vegetable samples were dried separately for 4 days in an electrical oven at 110°C for 24 h to obtain constant dry weight. The samples were then placed in a furnace at a temperature of 350°C for 24 h to ash. The cooled ash sample was ground to powder, weighed into containers, and sealed tightly. The wet weight, dry weight, and ash weight of food crops and vegetables used for radionuclides counting and their ratio are presented in [Table 1].
|Table 1: Wet weight, Dry weight, Ash weight and their ratio in the food crops and vegetables|
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For soil samples, extraneous materials (roots, stones, and gravel) were removed. Samples were well mixed and dried in an oven at 110°C for 4 days to obtain a constant weight. Afterward, the samples were crushed, mixed thoroughly, and sieved with 2-mm sieve.
A mass of 200 g of the prepared soil, cassava, and yam was placed in a plastic container with cap and sealed hermetically with adhesive tape for 30 days to allow secular equilibrium between214 Bi and208 Tl with238 U and232 Th, respectively, for soil samples. For vegetables, 50 g of ashed samples were placed into the plastic container and sealed for secular equilibrium between238 U (226 Ra) and232 Th (228 Ra) and their respective progenies in vegetables and food crops before measurement. The prepared samples were then measured by a gamma-ray spectrometer.
Activity determination of radionuclides
The gamma-ray spectrometer was used to determine the activity of the radionuclides,238 U,232 Th,228 Ra, and40 K in the samples. Sodium iodide (NaI (Tl)) detector of 7.6 cm × 7.6 cm manufactured by Bicron with a Canberra multi-channel analyzer was used to record the gamma-ray spectra. The detector was shielded by approximately 10-cm thick lead on all four sides and about 5-cm thick on top to reduce background radiation due to cosmic ray components.
The spectrometer was tested for its linearity and then calibrated for energy using gamma sources supplied by the International Atomic Energy Agency, Vienna. The detection efficiency calibration of the system was carried out using a reference standard gamma source prepared by Rocketdyne Laboratories, Canoga Park, CA, USA, which is traceable to a mixed standard gamma source (No. 48722–356) by Atlantic Inc., Atlanta, GA, USA. The detector assembly has a resolution of ~8% at 0.662 MeV of137 Cs.
Radiometric measurements were undertaken for qualitative and quantitative determination of different radionuclides present in the sample. The background radiations (which are the additional radioactivity in the environment) were corrected for by first obtaining the background count with the empty container of similar geometry for 36,000 s and then subtracted from the gross count. From the counted spectra, the activity concentrations of226 Ra,232 Th,228 Ra, and40 K in the samples were determined using a computer program (Genie 2K, Canberra Industries Inc., USA). The concentration of226 Ra was determined by a 1.764 MeV gamma ray of214 Bi. The gamma-ray energy of 2.614 MeV from208 Tl was used to determine the activity concentration of232 Th,228 Ra was determined from gamma-ray energies of its daughter228 Ac (911.07 keV), and a gamma ray of 1.460 MeV of40 K was used to determine the concentration of40 K in the samples.
TF is a useful parameter for a radiological assessment and is defined as the steady-state concentration between one physical situation and another. The soil-to-plant TF measured the transfer of radionuclides from soil to plant taken through the plant roots. The TF in the samples was calculated using equation 1.,
| Results and Discussion|| |
Activity concentration of radionuclides in study areas
The activity concentrations of radionuclides analyzed in the soil, vegetable, and food crops as well as their transfer ratios are presented in [Table 2], [Table 3], [Table 4], [Table 5]. [Table 2] presents the activity concentrations of radionuclides in soil, vegetables, and food crops in Agbaaru (Iwere-Ile). The activity concentrations of226 Ra in soil samples from Agbaaru are below the world average of 35 Bq/kg.226 Ra ranged from 20.5 ± 1.2 to 32.1 ± 3.0 Bq/kg with average of 25.3 ± 7.1 Bq/kg. The activity concentrations of226 Ra in vegetables and food crops from the site ranged from 1.9 ± 0.6 (coriander leaf) to 4.3 ± 1.2 Bq/kg (wild lettuce) in vegetables and from 3.2 ± 0.8 (maize) to 6.0 ± 1.2 Bq/kg (yam) in food crops. The average activities are, respectively, 2.8 ± 0.8 and 3.7 ± 0.9 in vegetables and food crop. The values are lower than what was in Delta state, Nigeria. The228 Ra activity concentration in the soil ranged from 18.0 ± 2.1 to 35.6 ± 2.4 Bq/kg, with an average concentration of 26.2 ± 5.0 Bq/kg. In vegetables,228 Ra ranged from 1.3 ± 0.7 (coriander leaf) to 2.5 ± 0.8 Bq/kg (wild lettuce), with an average activity concentration of 1.7 ± 0.3 Bq/kg. In food crops, it ranged from 1.8 ± 0.4 Bq/kg in maize to 3.0 ± 0.6 Bq/kg in cassava, with an average concentration of 2.5 ± 0.7 Bq/kq. The activity concentration of40 K is highest in all the samples compared to other radionuclides. It is higher than the world average of 400 Bq/kg in some points at the site in soil. It ranged from 296.5 ± 12.1 to 420.1 ± 25.0 Bq/kg, with an average concentration of 381.8 ± 16.0 Bq/kg which is lower than the value obtained in Nigeria, and also world average. The activity concentration of40 K in vegetables ranged from 61.6 ± 18.1 in coriander leaf to 100.8 ± 14.1 Bq/kg in wild lettuce, and in food crops, it ranged from 84.6 ± 3.5 in maize to 160.4 ± 9.2 in yam. The average concentrations of40 K in vegetables and food crops are 85.3 ± 12.0 and 128.5 Bq/kg, respectively.
|Table 2: Activity concentration of natural radionuclide in plants; corresponding soil samples and transfer factors in Iwere-Ile (Agbaaru area)|
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|Table 3: Activity concentration of natural radionuclide in plants; corresponding soil samples and transfer factors in Komu (Abuja mining area)|
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|Table 4: Activity concentration of natural radionuclide in plants; corresponding soil samples and transfer factors in Ofiki (Arget mining area)|
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|Table 5: Activity concentration of natural radionuclide in plants; corresponding soil samples and transfer factors in Igbeti (MOLAP mining area)|
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The activity concentration of radionuclides in the soil, vegetables and food crops in Abuja mining site (Komu) is presented in [Table 3]. The activity concentration of226 Ra in soil ranged from 20.1 ± 1.2 to 30.5 ± 2.8 Bq/kg. The average activity concentration of226 Ra is 25.4 ± 3.1 Bq/kg which is lower than the world average. The concentration of228 Ra in soil samples is higher than the world average in many points in the site. It ranged from 34.0 ± 2.6 to 43.3 ± 1.8 Bq/kg with an average of 39.5 ± 3.8 Bq/kg which is relatively close to the world average of 40 Bq/kg. Potassium (40 K) concentrations in the soil samples ranged from 365.7 ± 28.3 to 420.5 ± 28.4 Bq/kg, with average concentration of 401.9 ± 25.4 Bq/kg.40 K concentrations in most of the soil samples are higher than the world average. The226 Ra concentration in vegetable and food crops in the site ranged from 1.2 ± 0.6 in okra to 2.0 ± 0.3 in coriander leaf and from 2.9 ± 0.7 in maize to 7.0 ± 0.9 Bq/kg in yam. The average concentrations are 1.6 ± 0.2 and 5.6 ± 1.1 Bq/kg, respectively, in vegetables and food crops.228 Ra concentrations in vegetables ranged from 1.2 ± 0.2 in coriander leaf to 1.9 ± 1.0 Bq/kg in wild lettuce, with an average concentration of 1.5 ± 0.6 Bq/kg. In food crops, it ranged from 1.4 ± 0.3 Bq/kg in maize to 3.1 ± 0.6 Bq/kg in cassava. The average concentration of228 Ra in food crops is 2.4 ± 1.6 Bq/kg. The average concentration of40 K in vegetables is 73.2 ± 5.6 Bq/kg, and in food crops, it is 143.8 ± 5.6 Bq/kg. All the values of40 K concentrations in vegetables and food crops from Abuja mining site are lower than the world average and what was obtained in Delta state. The activity concentration of natural radionuclide in plants and soil samples in Arget mining areas (Ofiki) is presented in [Table 4].226 Ra concentration in the soil samples from the site ranged from 18.1 ± 1.5 to 40.5 ± 3.6 Bq/kg. The average concentration is 26.5 ± 3.3 Bq/kg which is lower than the world average of 35 Bq/kg. The228 Ra concentration in the soil samples is higher than the world average of 40 Bq/kg in most soil samples in the site. The average concentration of228 Ra in soil is 39.2 + 7.4 Bq/kg. Further,40 K concentration in the soil samples is higher than world average in some samples, and it ranged from 350.0 ± 22.1 to 442.4 ± 18.8 Bq/kg, with an average of 394.9 ± 18.6 Bq/kg. In vegetables, the activity concentration of226 Ra,228 Ra, and40 K ranged from 1.6 ± 0.4 (coriander leaf) to 2.6 ± 0.2 (wild lettuce), 1.1 ± 0.7 (okra) to 2.0 ± 1.0 (coriander leaf), and 46.1 ± 0.8 (tomato) to 160.2 ± 18.6 (wild lettuce) Bq/kg, respectively. The average concentrations in the vegetables are, respectively, 2.0 ± 0.6, 1.5 ± 0.8, and 96.8 ± 11.0 Bq/kg for226 Ra,228 Ra, and40 K.
The activity concentrations of natural radionuclides in vegetables in marble-mining site in Igbeti ranged from 1.4 ± 0.6 Bq/kg in okra to 2.2 ± 0.8 Bq/kg in wild lettuce for226 Ra, 1.1 ± 0.3 in okra to 3.0 ± 1.1 Bq/kg in wild lettuce for228 Ra, and 48.2 ± 1.8 Bq/kg in tomato to 140.1 ± 8.6 Bq/kg in wild lettuce for40 K. The average concentrations of radionuclides in vegetables in the site are respectively 1.9 ± 0.7, 1.9 ± 0.5, and 99.7 ± 6.2 Bq/kg for226 Ra,228 Ra, and40 K. Concentrations of226 Ra,228 Ra, and40 K in food crops are, respectively, 6.0 ± 1.1, 9.8 ± 1.8, and 4.0 ± 1.6; 2.8 ± 1.1, 2.6 ± 1.2, and 1.4 ± 0.4; 160.6 ± 6.3, 180.2 ± 18.5, and 100.3 ± 15.8 Bq/kg, respectively, in yam, cassava, and maize. The average concentrations of226 Ra,228 Ra, and40 K in food crops are, respectively, 6.6 ± 1.4, 2.3 ± 0.9 and 147.0 ± 14.2 Bq/kg. These values are lower than the world average. In soil samples, some samples contained relatively high concentration of226 Ra,228 Ra, and40 K. They ranged from 20.1 ± 4.2 to 35.2 ± 2.5 Bq/kg for226 Ra; from 30.8 ± 2.6 to 44.2 ± 2.4 Bq/kg for228 Ra; and from 300.6 ± 12.5 to 444.1 ± 23.1 Bq/kg for40 K. The average concentrations of226 Ra,228 Ra, and40 K in the soil samples are 26.5 ± 3.5, 38.8 ± 2.5, and 389.6 ± 18.6 Bq/kg, respectively, as presented in [Table 5]. It is observed that the activity concentration of radionuclides was highest in cassava (food crops) and wild lettuce (vegetable). This may be due to the long root of wild lettuce and long size of cassava tuber which penetrate deep into the soil, thereby absorbed more natural radionuclides than others observed in this study. Similarly, the result was also reported in Delta state, Nigeria.
The TF values of226 Ra,228 Ra, and40 K in vegetables and food crops from the four sites studied are presented in [Table 2], [Table 3], [Table 4], [Table 5].
In Agbaaru, TF of226 Ra in vegetables ranged from 0.07 to 0.21 with an average of 0.123 [Table 2]. Highest value of TF was found in wild lettuce while the lowest was found coriander leaf. In food crops, the ratio of226 Ra ranged from 0.13 in maize to 0.21 in cassava with an average of 0.177. The TF of226 Ra in Abuja (Komu) ranged from 0.05 (tomato) to 0.08 (wild lettuce) with average of 0.065 [Table 3]. In food crops from the site,226 Ra ranged from 0.14 in maize to 0.26 in cassava with average ratio of 0.21. The lowest TF of226 Ra in Arget (Ofiki) was from tomato (0.06) and the highest is from wild lettuce (0.12) in vegetables, while in food crop, the minimum is 0.09 in maize and the maximum TF is 0.20 in cassava [Table 4]. The average values of TF in both vegetables and food crops are 0.093 and 0.150, respectively. In [Table 5], TF of226 Ra in marble mines (Igbeti) shows that the lowest value was 0.06 from okra and the highest was 0.10 from tomato and wild lettuce. The average TF in vegetable is 0.08. In food crops, it ranged from 0.14 in maize to 0.32 in cassava with average factor of 0.21. The results of TF from the four sites are higher than 0.066 obtained in vegetables in Alexandra region in Egypt but lower than 0.404 obtained in northwestern part of Dakar. The values obtained in this study lies within the range of 3.0 × 10−3–4.3 × 10−1 for vegetables and 2.6 × 10−4–1.9 × 10−1 for tubers. The TF of226 Ra is 1.9 × 10−4–8.3 × 10−3 for maize grains in all the sites, except in Arget (Ofiki).
The TF of228 Ra in vegetables in all the four sites are greater than the recommended range of 1.8 × 10−5–7.6 × 10−5. The average values of TF of228 Ra in vegetables in the sites are, respectively, 0.058, 0.040, 0.035, and 0.050 in Agbaaru (Iwere-Ile), Abuja (Komu), Arget (Ofiki), and marble mines (Igbeti) [Table 1], [Table 2], [Table 3], [Table 4]. For food crops, the average TFs in all the sites are higher than the recommended values of 1.9 × 10−6–5.0 × 10−5 for maize grain and 2.9 × 10−6–3.5 × 10−5 for tubers. The average values of TF of228 Ra obtained in the study is presented in column 9 in [Table 2], [Table 3], [Table 4], [Table 5]. Higher value was also obtained in Northwestern part of Dakar (0.388).
The TF for40 K in vegetables ranged from 0.15 in coriander leaf 0.25 in wild lettuce in Agbaaru [column 10, [Table 2]. The average TF in this site is 0.215. In [Table 3], column 10, the TF of40 K in Abuja (Komu) site ranged from 0.08 in tomato to 0.28 in wild lettuce with average factor of 0.185. The TF of40 K ranged from 0.11 in tomato to 0.45 in wild lettuce in Arget (Ofiki) site [Table 4], column 10]. The average value of TF is 0.243. In marble-mining areas in Igbeti, the value of TF of40 K is lowest in tomato (0.12) and highest in coriander leaf (0.40). The average TF of40 K in vegetables in the site is 0.265. The values of TF obtained in all the four sites for vegetables are lower than the factor of 0.32 recommended by the International Atomic Energy Agency, 2010. The TF obtained in coriander is higher than the recommended value.
The value of40 K in food crops in Agbaaru is 0.390 in yam, 0.540 in cassava, and 0.201 in maize [Table 2], column 10, rows 9–11]. The average TF of40 K in the food crops in the site is 0.377 [Table 2], column 10, row 12]. Further, in [Table 3], Transfer Ratio (TR) of40 K in yam is 0.35, in cassava 0.41, and in maize 0.31 with an average of 0.356 [Table 3], column 10, rows 9–12].
The value of TR of40 K in Arget (Ofiki) site is 0.47 in yam, 0.43 in cassava, and 0.21 in maize [Table 4], column 10, rows 9–11] with an average of 0.37 [Table 4], column 10, row 12]. In marbl-mining areas in Igbeti, the TF of40 K in yam is 0.38, in cassava is 0.43, and in maize is 0.28 [Table 5], column 10, rows 9–11] with an average value of 0.363 [Table 2], column 10, row 12]. The average TF obtained in the four sites is lower than the recommended ratio of 2.7 for tuber and 1.7 for maize. The TF obtained for the three radionuclides analyzed in this study is lower than the average value of TF obtained in Delta state, Nigeria, in food from oil and gas-producing areas.
| Conclusions|| |
The activity concentrations of radionuclides in soil samples, vegetables, and food crops and their TF in some high background areas in northern parts of Oyo state, Nigeria, were measured using gamma spectrometry with NaI (Tl) detector. The average concentrations of226 Ra,228 Ra, and40 K in soil samples from the four sites studied are within the limit of world average in all the sites, except for40 K in Abuja (Komu) which is slightly higher. In addition, the average concentrations of226 Ra,228 Ra, and40 K in vegetables and food crops from the four sites were, respectively, lower than the values obtained in a similar research in Delta state, Nigeria. It was observed that wild lettuce has the highest concentration of radionuclide among vegetables in all the four sites. Also is cassava among the food crops analyzed in the four sites but lower than the recommended limit.
The soil-to-plant TF for226 Ra,228 Ra, and40 K measured in vegetables in the four sites studied are higher than the values obtained in food crops. Since the activity concentrations of radionuclides in the soil, food, and vegetables samples analyzed were within the range of world average and also the TFs within the recommended ratio, consumption of these vegetables and food crops may not pose any radiation hazard to the consumers and the general population. It is therefore recommended that similar research be carried out at interval to detect the concentration of radionuclides in soil as well as in food crops and their TFs calculated for radiological and dosimetric purposes as the use of fertilizer in farming is on the increase worldwide which may affect the quality of the products adversely in terms of radionuclides migration into the food crops.
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Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]