Measurements of the elemental and radionuclide concentrations of environmetal and geological samples from selected areas of Kibwezi District, Kenya

Abstract

Assessments of human exposure to the natural background radiation in Kenya have not being exhaustive in terms of geographical coverage, the media and the parameters used. There is need to have representative data and information on the activity concentrations of natural radionuclides and elemental concentrations so as to assess the status of the environment. Measurement of gamma emitting radionuclides in soil and rock samples in selected areas of Kibwezi district was carried out using gamma ray' spectrometry technique. The respective photon energies are used to identify which particular radionuclide is present in the sample material. The intensities of the radiation arc used to evaluate the respective concentration of the sample. NaI (Tl) detector coupled with a computer based gamma ray spectrometer was used for qualitative and quantitative analysis of gamma emitting radionuclides.

The averages concentrations of 238U, 232Th and 40K measured in the samples collected in this study are 130.6.±38.7 BqKg-l, 137.9±39.7 BqKg-l and 1120.l±245.2 BqKg-l respectively. It is observed that the activity concentrations are above the global average values. As a measure of radiation hazard to the general public, the absorbed dose rate in air at a height of 1 m above the ground surface was estimated. The calculated radiation absorbed dose from the different sampling points, ranges from 95.4±3.2 nGyh-1 to 300.4±5.5 nGyh-1 with an average of 193.2±44.5 nGyh-l. The mean value is higher than the worldwide average of 60 nGyh-l. The effective dose rates were calculated for human exposure to the gamma radiations from the Radionuclides; 238U, 232Th and 40K in the soil samples, and were found to be in the range (0.23±0.01- 0.74±0.0l) mSv ii, which is below the ICRP limit of 1 mSv il for members of the general public.

Further studies on radionuclide concentrations in water and biota are suggested for estimation of risks associated with natural radioactivity in the environment. Energy Dispersive X-Ray Fluorescence (EDXRF) using a Si(Li) detector, was used to identify heavy metals in the samples and quantify the amount of those metals present to ultimately determine the elemental composition of the samples. The elemental analysis of the samples show that the levels of Pb (32.8±2.8) ~g/g and Zn (l46±9.8) ug/g were higher in the soil samples from KS2 (Yumbuni) and KS9 (Kathekani) hill respectively. The levels of Th are (12.5±0.3) ug/g and (11.4±0.8) ug/g in the soil and rock samples respectively, while it is below detection limit (7.75±0.20) ug/g in most of the other samples.