Assessment of Heavy Metals Contents and Natural Radioactivity Site in Kwale.

This study was initially formulated specifically to provide data of radioactivity and elemental content of soils from two regions; Nguluku and Maumba, in Kwale District, that are earmarked for Titanium mining project.  However, while radioactivity levels for both Nguluku and Maumba regions were assessed; elemental analysis for heavy metals was only carried out for Maumba.  A total number of fifty samples were analyzed for heavy metals using energy dispersive x-ray fluorescence (EDXRF) system which consists of a radioisotope cadmium-109 source and a silicon-lithium drifted (Si(Li)) detector.  Radioactivity levels of the samples were determined using a high purity germanium (HpGe) detector.


Iron and titanium were observed as the major elemental constituents of the Maumba soils with concentration levels of 1.21% and 1.57%, respectively.  Zirxonium and manganese levels were also found to be high, with mean levels of 1193.3 mg/g and 822.2 mg/g respectively.  Niobium concentration level was found to vary between (13.77 – 79.24) mg/g with a mean of 31.81 mg/g in these samples.  These levels were found to be lower than those reported earlier for Mrima Hill soil samples.  Concentrations distribution of titanium and zirconium in the samples were found to have a strong correlation of r=0.97.


Activity concentrations of the three major primordial radionuclides – thorium-232, uranium-238 and potassium-40 – in the fifty samples from Maumba and seven samples from Nguluku were assessed.  High contributions from Th-232 and U-238 determined as 72.0 and 50.2 Bq/kg in Maumba and 178 and 162 Bq/kg in Nguluku soil samples.  These levels are much higher than the world average of 25 Bq/kg.  Contribution from K-40 was found to be negligible in all the samples analysed.


Elemental concentrations of titanium, iron, zirconium and niobium were found to correlate significantly with the activity concentration levels of radionuclides in the Th-232 and U-238 series.


Using an occupancy factor of 0.2, annual effective dose to an adult due to gamma rays, in air 1 m above the ground was estimated to be 156 mSv in Nguluku.  This level exceeds the world average effective dose (<70 mSv) by a factor of two.  Results for Maumba samples have been presented for three sub-regions – Miembeni, Maumba Central and Maumba ya Chini.  The annual effective dose levels for the three sub-regions are 84, 44 and 23 mSv respectively.




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