RADIOLOGICAL RISK ASSESSMENT OF 238 U , 232 Th AND 40 K IN THE TOP SOILS OF AHERO PADDY FIELDS OF KISUMU COUNTY, KENYA

ABSTRACT


I. INTRODUCTION
The origin of radionuclides of Uranium 238 U, Thorium 232 Th and Potassium 40 K dates back to the formation of the Earth [1].The three radionuclides are found in significant concentrations in various environmental media such as water, soil sediments, plants and foodstuffs [2] and their contents in the soil are directly related to the weathered bedrock.The radionuclides of 238 U, 232 Th and 40 K forms a major source of radiation exposure to the largest group of human population [3].The radionuclides along with essential nutrients may be absorbed from the soil via the plant roots and transported to other parts of the plant.When they get in edible parts of the crop; they can cause internal exposure to human beings [4].
It is worth noting that 238 U and 232 Th are radiotoxic elements if they exceed permissible levels whereas 40 K is both radiotoxic and nutritionally important [5].Humans and their foodstuffs are exposed to various types of radiations that originate from primordial, cosmogenic, terrestrial and natural decay series radionuclides [6].Human beings ingest and inhale radionuclides via consumption of food, water and air respectively. 238U, 232 Th and 40 K and their numerous progeny are the common radionuclides available in food [7].An amount of eighty three percent annual effective dose is experienced by individuals due to natural decay series radionuclides, whereas sixteen percent is contributed by primordial 40 K and the remaining one percent is due to anthropogenic radionuclide.Soil to plant and plant to human beings is one of the foremost corridors for transmission of radionuclides [8].It should be noted that rice is one of the main food consumed by the Kenyan population both in the rural and urban areas.Consequently, human exposure owing to the ingestion of radionuclides via consumption is global concern [9].Whichever is the mode of exposure, it's a fact that ionizing radiation is detrimental to human health [10].
Human activities such as Agricultural practices have continued to make significant additions to the radioactivity levels of the soil.The use of inorganic fertilizers to replenish both micro and macro elements lost and other agrochemical inputs are associated with the release and subsequent accumulation of natural and artificial radionuclides in the agricultural soils and nearby water sources [11].The discharge from machines used in production of rice also adds to the overall radionuclide concentration of the soils.The large scale rice farming in Ahero paddy fields is not void of use inorganic fertilizres, agrochemicals, untreated water and mechanical implements.These operations have consequential effects on radioactivity levels of the soil and the whole farming environment [11].Notwithstanding the economic benefits to the community and government; the knowledge of natural radioactivity due to 238 U, 232 Th and 40 K was important in order to qualify the radiological safeness of the soils as well as categorizing the radiological hazards to the farmers and the general public.

II. 1 STUDY AREA
The study was done in Ahero paddy fields of Muhoroni SubCounty of Kisumu County, Kenya whose population is 151799 (2019 census).It is located on latitude 00 °9´´S and longitude 34 °56 ´´E and at an altitude of 1160 m above sea level.The other crops grown here includes soy beans, maize and tomatoes but on a small scale.The source of water for irrigation in this paddy fields is from River Nyando [12].The soils in the paddy fields are suitable for irrigation of rice due to their low percolation rates.The main rock types that surround the Ahero paddy fields are granites and granodiorites on the north and south while on the eastern and north western are phonolites.

II.2 SAMPLE COLLECTION AND PREPARATION
The top soil samples within the depths of 15 -20 cm were collected from three paddy fields where rice had been cultivated and from the control site field.
The rice fields were demarcated as field 1, field 2, field 3 and field 4. The field 4 (control site) was 1 km away from the fields 1, 2 and 3. Five soil samples were collected from each of the fields 1, 2 and 3 while two samples were collected from field four (control site).The samples were collected using the manually constructed hand auger and trowel.The top most layers of soil were cleared first to get rid of pebbles and roots from the soil.In each field, five [5] soil samples were collected from the three fields 1, 2 and 3.The samples were then put in containers properly labeled to avoid mix up.The samples were transported to the laboratory and spread on prewashed and labeled polythene mats in an open floor for two weeks for the samples to dry.In order to achieve a constant weight, the samples were manually pulverized using a mortar and pestle and then allowed to pass through a 2.00 mm sieve (< 2.00 mm particles were used).Soil samples from uncultivated land (for 2 years) at about 1 km from the fields which served as a control site was also collected and prepared in the same way.170 g of each sample from the fields was weighed in to cylindrical plastic containers of uniform geometry which were soaked in dilute Sulphuric acid and then rinsed with distilled water to avoid external contamination of samples.The containers were properly labeled and hermetically sealed for a minimum of 30 days to allow for the radioactive secular equilibrium to be reached between parent and daughter radionuclide before embarking on gamma counting.

II.3 GAMMA RAY SPECTROSCOPY
Each sample was placed in a NaI(Ti)  ray spectrometer that was shielded to prevent stray radiations.The system included an oxford PCA-P multichannel analyzer (MCA) card and its software for spectral acquisition and analysis.The gamma ray spectrometer was calibrated using certified samples of 238 U, 232 Th and 40 K.The peaks of corresponding to 232 Th (2615 KeV), 1460 KeV ( 40 K) and 1765 KeV ( 238 U) were considered for the respective activity concentrations.Each sample was put in the NaI(Ti) detector for measurement for a period of 30000 seconds.Distilled water was also put in the detector to provide background measurement.

II.4 DETERMINATION OF ACTIVITY CONCENTRATIONS OF THE RADIONUCLIDES
The spectra for 238 U, 232 Th and 40 K were obtained using the peaks as follows: 1765 KeV ( 214 Bi), 2615KeV (Ti) and 1460 KeV (( 40 K).The activity concentration was computed using equation 1 in Bq/kg [13].
Where Ai is the activity concentrations of the i th radio nuclide in Bq/kg -, ε is the efficiency of the detector at the energy of the i th radionuclide, Nci is the net counts of the i th radionuclide in the corresponding photo peak after background subtraction, Ƴ  is the emission probability of the i th radionuclide, m is the mass of the sample in kg and t is the counting time.

II.5 ESTIMATION OF ABSORBED DOSE RATE (ADR)
The radiation absorbed dose rate, ADR was estimated for radiation risk assessment to quantify the amount of radiation energy that may be deposited per unit time on a potentially exposed person [11].The absorbed dose rate was calculated using the activity concentration and the conversion factors [14].The conversion factors for 238 U, 232 Th and 40 K were 0.462, 0.604 and 0.0417 respectively.Equation 2shows the equation used in computing ADR [15].
Where A u , A Th and A k are activity concentrations of 238 U, 232 Th and 40 K in Bqkg -1 respectively [16].

III.1 ACTIVITY CONCENTRATIONS
It can be noted that the mean activity concentrations of the three radionuclides for field 1 were 32.63 ± 1.63 Bq/kg for 238 U, 104.69 ± 5.20 Bq/kg for 232 Th and 75.00 ± 3.26 Bq/kg for 40 K.The field 1 is the one in which rice had already been transplanted and the rice seedlings were a month old and some fertilizer had been applied.The activity concentration of 238 U was higher than the world permissible level of 45 Bq/kg and although that of 238 U was below the world permissible limit of 33 Bq/kg; it was still high.Their high concentrations can be attributed to the phosphatic fertilizers that had been applied apart from the geology of the place characterized by underlying granitic rocks.The activity concentrations of 40 K were below the world permissible limit of 420 Bq/kg [17].The average concentrations of the radionuclides from field 2 where transplanting was being done were 16.97 ± 0.84 Bq/kg, 68.03 ± 3.40 Bq/kg and 70.31 ± 3.51 Bq/kg for 238 U, 232 Th and 40 K respectively.It can be noted from these values that it's only 232 Th that had higher activity concentrations above the world acceptable limit of 45 Bq/kg; this can be attributed to either underlying rocks of the field or River Nyando where water for irrigation originates that contains this radionuclide.
The average concentrations of the radionuclides from field 3 (post harvesting) where harvesting had be done and ploughed were 28.92 ± 1.44 Bq/kg, 91.73 ± 4.58 Bq/kg and 122.60 ± 6.13 Bq/kg for 238 U, 232 Th and 40 K respectively.The activity concentrations of 238 U and 232 Th had higher values than the world permissible limits.At this stage, top dressing had been done twice and this may also have added to increased activity concentrations.
The field 4 that had not been ploughed for two years and was a control field unfortunately also recorded higher concentrations of 232 Th.The average concentrations of the radionuclides were 29.74 ± 1.48 Bq/kg, 121.11 ± 6.05 Bq/kg and 87.51 ± 4.37 Bq/kg for 238 U, 232 Th and 40 K respectively.This field had been cultivated continuously previously.The continuous use of inorganic phosphatic fertilizers whose origin is from rocks that contains high concentrations of Uranium and Thorium accumulates in the soils increasing their activity concentrations.
Although some of the activity concentrations for 238 U and all for 232 Th were higher; they were below the 1000 Bq/Kg for both radionuclides [18], thus the soils are not hazardous to the human population interacting with them.
Graphical representation of the activity concentrations for all the samples in this work are as shown in Figure 2.

Figure 2 :
Figure 2: Graphical representations of activity concentrations of the radionuclides in this study.Source: Authors, (2022).