Radiation Exposure Levels Associated With Construction Sand From Tharaka-Nithi County In Kenya

Natural radioactivity levels and radiation risks due to construction sand mined from Tharaka region of Tharaka-Nithi County in Kenya has been measured. The region is mostly rocky with majority of rocks being volcanic rocks and sedimentary rocks of volcanic origin; the area is occupationally characterized by sand mining for construction purposes within the County and also in some parts of the neibouring Meru and Embu Counties. The sand is mined along the river banks and also gullies that carry sand from withered rocks within the county. High concentrations of 238U, 232Th and 235U are associated with soils that are developed from acidic magmatic rocks (igneous rocks).The goal of this study was to analyze the level of radiation exposure associated with construction sand from Tharaka region.  The activity concentration levels of natural radionuclide (uranium, thorium and potassium) in the sand samples were analyzed using NaI(TI) gamma ray spectrometer. The associated radiological parameters (radium equivalent activity, dose rates, external hazard and internal hazard indices) were calculated based on the activity obtained. The results of the naturally occurring radionuclide were as follows; 40K ranged from 32 ± 2 to 2662 ± 122 Bqkg-1 with a mean value of 1069 ± 46 Bqkg-1, 238U ranged from 19 ± 1 to 269 ± 13 Bqkg-1 with a mean value of 98 ± 4 Bqkg-1, 232Th ranged from 11 ± 1 to 114 ± 7 Bqkg-1 with a mean value of 53 ± 3 Bqkg-1. The mean values of activity concentration exceeded the world averages of 400 Bq/kg, 33 Bq/kg, 45 Bq/kg for K-40, U-238 and Th-232 respectively. Sand mined in Tharaka South from the gullies originating from Kijege hills and Ntugi hills were found to have elevated activity concentration especially uranium and thorium while that from Tharaka North was found to have high levels of potassium and low levels of thorium and uranium. Generally most of the samples contained elevated values of activity concentration of potassium. The hazard indices were computed and used to determine the suitability of the sand for building in the context of radiological hazard posed. Radium equivalent activity varied from 50 ± 3 to 461 ± 20 Bqkg-1 with a mean value of 256 ± 13 Bqkg-1, only four samples exceeded the recommended limit of 370 Bq/kg. External and Internal indices varied from 0.14 ± 0.01 to 1.23 ± 0.05 with a mean value of 0.69 ± 0.04 and 0.19 ± 0.02 to 1.80 ± 0.06 with a mean value of 0.95 ± 0.05 respectively. The outdoor absorbed dose rate calculated ranged from 22.79 ± 1.39 nG/h to 221.57 ± 9.52 nG/h with a mean of 120.99 ± 6.07 nG/h while indoor absorbed dose rate ranged from 31.15 ± 1.88 nG/h to 308.77 ± 13.20 nG/h with a mean of 166.67 ± 8.31 nG/h, the mean values were above the world averages of 54 nG/h and 84 nG/h respectively.  The mean annual effective dose rate indoor and outdoor annual effective dose was 0.61 ± 0.06 mSv and 0.30 ± 0.02 mSv which was slightly above the world averages of 0.41 and 0.07 respectively but below the maximum dose constraint of 1 mSv/y to the public set by ICRP, 2005. Construction sand from the region can therefore be used without posing any danger to the public.