Natural radioactivity levels in rocks and sand from Tyaa River in Mwingi, Kitui County

Abstract

The variability of background radiation from primordial radionuclides has led to the exposure of human beings to radiation doses of different ranges. Prompt or delayed harmful effects as a result of radiation exposure have been reported in areas with enhanced levels of radioactivity. The residents of Kitui County near Tyaa River and the surrounding regions heavily mine sand and rocks from its riverine for construction purposes despite the scanty information on the potential harm in case the latter has elevated levels of radioisotopes. Conducting a radiometric survey was therefore imperative as the area neighbors Machakos and Thika regions whose bedrock comprises of granites and silic rocks whose mineral accessories is similar to those found in Uranium-Thorium rich rocks. A total of 56 samples of raw building materials (sand and rocks) each weighing 500 g were randomly sampled along the riverine during the consecutive rainy and dry seasons and prepared for analysis. The 238U (226Ra), 232Th and 40K radionuclides present in geological samples were identified and quantified from spectra recorded using HPGe gamma-ray spectrometry. The spectral analysis reports an overall mean activity concentration of 226Ra, 232Th, and 40K of 33±1.7 Bq/Kg for 226Ra, 55±2.8 Bq/Kg for 220Th, and 812±40 Bq/Kg for 40K in sand samples which surpasses the world’s average of 35 Bq/Kg, 30 Bq/Kg and 400 Bq/Kg respectively. A similar analysis of rock samples reveals a mean of 21±2.5 BqKg-1, 49±2.5 BqKg-1 and 782±39 BqKg-1 for 226Ra, 232Th and 40K respectively. The average corresponding dose rates and radiological hazard indices were within the safety limits recommended in ICRP reports. The higher activity concentration of 226Ra, 232Th, and 40K was reported for samples collected during the dry season and as such the seasonal changes had a significant effect on the natural levels of radioactivity in sediments from this river.