Human Exposure and Associated Risks Due to Natural Radioactivity and Heavy Metals In Ortum, West Pokot County, Kenya
Felix Omonya, Wanjala
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The background radiation in air, the activity concentration of selected radionuclides in soil and rock samples, the radon and thoron concentration in selected mud houses and elemental concentration of heavy metals in soil and water samples from Ortum, West Pokot County in Kenya was determined in this study. Ortum was chosen due to the presence of granitic and silicon rocks which are associated with high levels of background radiation. The activity concentration of 238U, 232Th and 40K in soil was determined using the High Purity Germanium detector (HPGe) and the average values were 40 ± 3 Bq/kg, 56 ± 4 Bq/kg and 425 ± 19 Bq/kg respectively which is within the world average range. The activity concentration of 238U and 232Th in soil samples reduced with increasing depth while that of 40K increased slightly with increase in depth. The average activity concentration of 238U, 232Th, and 40K in soil samples was higher than in the rock samples implying that the soils originate from other regions. The average outdoor absorbed dose rate in air at 1 m above the ground was found to be 112 ± 30 nGy/h which is almost double the world average value of 60 nGy/h. The average Raeq was 153 ± 49 Bq/kg which is less than the limit of 300 Bq/kg and the external hazard indices (Hex) and the internal hazard index (Hin) were 0.41 ± 0.13 and 0.52 ± 0.16 respectively which is below the limit values of unity (>1). This implies that soil and rocks in Ortum poses low radiological risk and they can therefore, be used for construction of houses, industrial and agricultural purposes. The average radon and thoron concentration in mud houses was determined using RADUET detector and found to be 40 ± 19 Bq/m3 and 54 ± 30 Bq/m3 respectively which is below the ICRP recommended lower and upper limit of 100 Bq/m3 and 300 Bq/m3 respectively. The elemental concentration of Ni, Cu, Zn, Pb, K, Ca, Fe, Ti, Mn, Rb, Sr, Zr and Nb in soil was determined using the Energy dispersive X-Ray Fluorescence Spectrometer (EDXRF) and found to be below the WHO recommended limits. The mean concentration of trace elements Pb, Zn and Cu in soil samples reduced with increasing depth while that of Ni increased with increasing depth. The Geoaccumulation Index (Igeo), Potential Ecological Risk Index (Ei) and synthesized potential ecological risk index (Er) were evaluated and found to be -0.40, 4.92 and 19.69 respectively. The results show that soil from Ortum is moderately polluted and the risk associated with exposure to heavy metals in soil is low. The concentration of Ni, Cu, Pb, Zn, Ag, Al, As, Ba, Ca, Cd, Co, Cr, Fe, K, Mg, Mn, Mo, Na and Se in water samples was determine using Agilent-5100 Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES). The elemental concentrations in water samples from the two rivers was found to be lower than the WHO permissible limits, except for calcium (Ca) which was higher than the permissible levels in borehole water. Hence, water from rivers in Ortum is unpolluted and fit for use except for borehole water which has high calcium levels. The lifetime cancer risk due to background radiation (LTCRBR) and elemental pollution in water (LTCREP) was found to be 1.47 x 10-3 and 1.92 x 10-6 respectively which is within the recommended safe limits. The lifetime cancer risk due to exposure to background radiation evaluated using RESRAD programme for a resident farmer in Ortum was found to be 0.011 or 1.1%. This implies that cancer risk due to exposure to background radiation in Ortum is low.