Determination of the Level of Naturally Occurring Radioactive Nuclides from Soil Samples in Kikuyu, Kiambu County, Kenya

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Wangari, Gitahi Agnes
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Kenyatta University
Human population is exposed to ionizing radiation from radionuclides. Radionuclides that occur naturally and other forms of radiation from materials such as water and vegetation releases ionizing radiation that can be a risk to human population. Radiation from these sources, referred to as background radiation, often goes un-noticed and therefore is not always easy to control. Additionally, human undertakings such as exploration/mining, construction of roads or rails and agricultural activities may introduce significant amounts of NORM in the atmosphere which may lead to enhanced levels of natural radiation levels. Exposure to such high radiation levels can lead to health problems like cancer, and it is for this reason that international bodies such as International Commission on Radiation Protection (ICRP) have proposed guidelines and recommendation such as reference levels aimed at reducing NORM-induced risks. This study looks at concentration levels of NORM in top level soils around Kikuyu subcounty in Kiambu County. 20 samples of these soils were sampled randomly from 4 different sampling cites namely Ondiri, Kidfarmaco, Kalro and Alliance girls high school grounds. The study area was selected based on the most resent constructed Southern bypass whose erosion landed to the Ondiri Swamp.At Kidfirmaco there were caves that exist and of late the population around them has increased rapidly. At Kalro and Alliance girls, there are large scale farming areas where most residents in Kikuyu purchase their foodstuff from. The samples were dried in an oven at 1000℃ for 24 hours to completely remove the moisture. Each sample with a mass of about 300g was packed in the special polythene plastic containers which were tightly sealed. The containers were kept for 30 days for radionuclides to reach secular equilibrium. The samples were analysed using NaI(Tl) gamma-ray scintillator to determine their content of 232Th, 238U and 40K and the associated risk indices. The concentration level of 40K, 238U and 232Th in the samples were 1363.87±88.46 Bq/kg , 172.16±58.75 Bq/kg , 275.56±21.86 Bq/kg respectively while the the radium equivalent activity, external and internal hazard indices were 498±25 Bq/kg, 1.85 ±0.093,1.15±0.058 respectively. The average dose rate was estimated as 279.9 nGy/h and annual effective dose rate 0.68mSv/y. While this dose rate is less than the ICRP reference level of 1 mSv/y for the general public, the risk indices were found to be above the ICRP reference level. This may be due to the high levels of U and Th in the samples relative to the world average levels of 33 Bq/kg and 45 Bq/kg respectively. I recommend more study to be done on rocks near the railway and the road construction and also explore the use of organic fertilizer at Alliance girls and KALRO
A Thesis Submitted in Partial Fulfillment of the Requirements for the Award to the Degree of Master of Science in the School of Pure and Applied Sciences of Kenyatta University, May, 2022
Determination, Level, Naturally Occurring, Radioactive Nuclides, Soil Samples, Kikuyu, Kiambu County, Kenya