MEASUREMENTS OF HEAVY METALS AND NATURAL RADIOACTIVITY
LEVELS IN SOILS AROUND THE TITANIUM MINING SITE IN KWALE
DISTRICT.
By
DOUGLAS NDIRANGU MAINA
Abstract
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 g/g and 822.2 g/g respectively. Niobium
concentration level was found to vary between (13.77 – 79.24) g/g with a mean of 31.81 g/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, uranium238 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 Sv in Nguluku. This level exceeds the world
average effective dose (<70 Sv) 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 Sv respectively.
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