Evaluation of naturally occurring radioactive materials and toxic heavy metals in the surface soil around Serule prospective mining area in Botswana

Human exposure to Naturally Occurring Radioactive Materials (NORMs) and toxic

heavy metals has been a concern for the entire history of mankind since they are com monly found in the environment. Their increased levels are a result of anthropogenic

activities such as mining. A-cap Resources Ltd, now A-cap Energy Ltd company dis covered that there were large uranium deposits in the underlying rocks of Serule. It

was given licensing to start mining in 2016 with a lease of about 144 km2

. Uranium

mining results in large disruptions of surface soils which causes radionuclide and metal

contamination thus affecting soil quality and human and environmental health. As a

result, this work focused on evaluating NORMs and toxic heavy metal concentrations

in the surface soil around Serule prospective mining area as well as their health risk

to members of the public. From a depth of about 15 cm, 29 soil samples were ran domly collected using an auger and analysed for natural radionuclides and selected

toxic heavy metals using a High-Purity Germanium (HPGe) detector and Inductively

Coupled Plasma Mass Spectrometer (ICPMS), respectively. A control sample was

also collected and analysed in the same manner. All measurements obtained were

compared to the control and reference standards.

The HPGe detector with a relative efficiency of 60 % and a 2.0 keV resolution was

used to identify and measure activity concentrations of radionuclides found in samples.

Average activity concentrations of 226Ra, 238U, 232Th, 40K and 210Pb in areas with high

activity of 238U were 1126.26 ± 6.49, 498.21 ± 10.01, 71.39 ± 1.27, 284.18 ± 21.74 and

1490.20 ± 28.77 in Bq/kg, respectively. These activities were higher in comparison

to the world average values as reported by UNSCEAR (2000), where average activity

concentrations of 226Ra, 238U, 232Th and 40K were 32, 33, 45 and 420 in Bq/kg, respec tively. The world average activity concentration value of 210Pb is still not published.

iv

The radiological risk to members of the public in the study area was calculated using

radiological parameters from UNSCEAR (2000). Areas with high activity of 238U had

higher concentrations of radiological parameters in comparison to the world average,

areas with low activity of 238U and the control. This showed that areas with high

activity of 238U pose a radiological risk to residents.

Following an acid digestion of method 3051A, toxic heavy metals were analysed using

an Agilent (7500 series) ICP-MS. This was used to determine mass concentrations of

these heavy metals. In areas with high activity of 238U, toxic heavy metal concentra tions decreased in the order Cr > Pb > As > Cd > Hg with values 101, 26.4, 9.56,

0.133 and 0.00475 in mg/kg, respectively. These concentrations were compared to the

maximum permissible limits of World Health Organisation (WHO) guidelines, where

the maximum permissible limits for Cr, Pb, As and Cd were 10, 100, 20 and 3 in

mg/kg, respectively. The maximum permissible limit for Hg is not yet published. Cr

was the only toxic heavy metal greater than the recommended World Health Organisa tion (WHO) value for areas with high activity of 238U. Using these concentrations, the

total non-carcinogenic risk for adults and children was determined and results showed

that they both have a close to zero non-carcinogenic risk due to toxic heavy metals

in the study area. Toxic heavy metals from all locations of the study area showed

a carcinogenic risk that was within the allowable limit of 1 × 10−4

, therefore, the

carcinogenic threat to residents due to toxic heavy metals is negligible.