Radioactivity in Surface Soils Around the Proposed Sites for Titanium Mining Project in Kenya

doi:10.4236/jep.2011.24053

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Journal of Environmental Protection, 2011, 2, 460-464

doi:10.4236/jep.2011.24053 Published Online June 2011 (http://www.SciRP.org/journal/jep)

Radioactivity in Surface Soils around the

Proposed Sites for Titanium Mining Project in

Kenya

M. K. Osoro1, I. V. S. Rathore2, M. J. Mangala3, A. O. Mustapha4

1Department of Physics, Kenyatta University, Nairobi, Kenya; 2Department of Physics, Northern India Engineering College, Babu

Banari Das University, Nairobi, Kenya; 3Institute of Nuclear Science and Technology, University of Nairobi, Nairobi, Kenya;

4Department of Physics, University of Agriculture, Abeokuta, Abeokuta, Nigeria.

Email: [email protected]

Received March 3rd, 2010; revised March 9th, 2011; accepted April 23rd, 2011.

ABSTRACT

Radioactivity measurements were carried ou t around Maumba and Nguluku villages, two of the proposed sites for tita-

nium mining in the coastal area of Kenya. Samples of surface soils were analyzed using a HPGe gamma spectrometer.

The average activity co ncentrations for 226Ra, 232Th and 40K are 20.9  7.6, 27.6  9.1 and 69.5  16.5 Bqkg–1, respec-

tively. The absorbed dose rates in air, calculated on the basis of the measured activity concentrations, range from 9.8 to

50.0 nGyh–1, with an average of 29.2 nGyh–1. These values are below the global population-weighted mean, and they

should be considered when p lanning app ropriate mon ito ring and su rveillance programmes du ring the mining op era tion,

as well as the reclamation and restoration programmes after mining.

Keywords: Radioactivity Measurements, Titanium Mining, Absorbed Dose Rates, Natural Background Radiation

1. Introduction

The ambient natural background radiation is usually of

little or no radiological concern, and it varies from one

location to another depending partly on the local geology.

Mining and other industrial activities such as oil and gas

exploration, extraction and purification of water, etc. can

enhance the natural background radiation to levels that

are not insignificant from radiation protection point of

view [1,2]. Mining results in large volumes of materials

containing natural radionuclides – the so called naturally

occurring radioactive materials (NORM). This is not

restricted to uranium and thorium ores, it is also true for

other raw materials like heavy mineral sands, phosphate

rocks, etc.

Tiomin Resources Inc., the Canadian mining company,

discovered vast deposits of titanium in the coastal area

Kenya [3]. Earlier appraisal of the areas’ mineral poten-

tial [4] shows that most of the south coastal area of

Kenya is underlain by the Duruma group consisting of

the coarse-grained Taru and the fine-grained Maji-ya-

Chumvi formations. Two of the deposits in Kwale dis-

trict; the Central Dune at Maumba and South Dune at

Nguluku constitute the Kwale titanium mining project,

with a mineral reserve of 140.8 million tones (Figure 1).

The average ore body composition is 3.48% heavy min-

erals, and the expected annual yield is 77,000 tonnes of

rutile, 38,000 tonnes of zircon and 330,000 tonnes of

ilmenite [3]. According to the mining company, the pro-

posed operations will involve clearing the vegetation,

removing and stockpiling the topsoil for further use in

rehabilitation process, excavation of the dunes using

bucket wheel excavators or scrapers and, transportation

of the ore-bearing sand to the wet plant where the heavy

minerals will be separated from sand. The final products

will be transported from the mine sites to a ship-loading

facility that will be constructed at Likoni, in Mombasa. It

is estimated that mining activities in each of the two sites

will take seven years.

The main source of surface water in the area is the

Mukurumudzi river, a perennial river that flows from

northwest to southeast and drains into the Indian ocean

near Gazi Bay. Human population in these areas is esti-

mated to be about 3,000 [5]. The project was put on hold

because local residents and other stakeholders raised se-

rious social, economic, environmental and radiological

questions, which are still being addressed. Enhancement

Radioactivity in Surface Soils Around the Proposed Sites for Titanium Mining Project in Kenya461

Figure 1. Location of Kwale Titanium Mines (KTM) and the sampling sites.

of the natural background radiation can arise from con-

tamination of groundwater by liquid effluents and leach-

ing radionuclides, contamination of land and agricultural

produce by atmospheric releases, re-use of tailings, etc.

The principal exposure pathways will be inhalation of

airborne radionuclides, ingestions of radionuclides with

food, water, etc., and external exposures to gamma radia-

tion from tailings, surface deposition, and submersion in

airborne r ad ionucli d es .

In order to assess the net radiological impact of the

mining operations, it is necessary to establish baseline

concentrations of the naturally occurring radionuclides in

relevant environmental matrices prior to the commence-

ment of the mining operation. Soil is th e ultimate sin k fo r

various kinds of contaminants and it could be an impor-

tant indicator of environmental pollution [6]. This is the

report of an independent study carried out to establish the

baseline concentrations of radionucides in the surface

soil around the proposed mining sites.

2. Materials and Methods

2.1. Soil sampling and Preparations

Soil sampling was carried out in an area of about 20 km2

and 10 km long surrounding the two mine sites (Figure

1). The area was divided into large grids around the two

villages and along the adjoining path, but the sampling

points within each grid were chosen randomly provided

they are free from large stones or roots and relatively in

the open. This is similar to the systemic/stratified random

sampling technique described in the IAEA technical

document [6]. The distance between neighbouring grid

centers is about 500 m. At each sampling point, dirt and

other extraneous (non-soil) materials were first removed

to expose the soil. Soil was then collected with hand

trowels down to 5 - 10 cm depth within a 30 × 30 cm2

area. The soil was homogenized and three samples, each

of about 1000 cm3, were packed in clean polythene bags

and labeled accordingly. Altogether 78 samples of sandy

soil were collected from 26 sa mpling points. Th e sample

preparation involve oven-drying at 110˚C, pulverizing to

fine powder, and sealing aliquots in 450 ml bottles for a

minimum of 4 weeks to establish secular equilibrium

between 226Ra and the short-lived decay products of

222Rn before gamma-ray spectrometric analysis.

2.2. Gamma-Ray Spectrometric Analysis

The samples were analyzed using a high purity germa-

nium (HPGe) detector of 30% efficiency relative to the

standard 3” × 3” NaI(Tl) detector and energy resolution

of 1.8 keV (FWHM) at the 1.33 MeV gamma line of

60Co. Detailed description of the gamma-ray spectrome-

ter setup as well as the detector calibration procedures

using the IAEA reference materials (RGU-1, RGTh-1,

and RGK-1) are presented in ea r l i e r reports [ 7-9].

Radioactivity in Surface Soils Around the Proposed Sites for Titanium Mining Project in Kenya

462

3. Results and Discussions

3.1. Activity Concentrations of Radionuclides in

Soil Samples

Figure 2 shows a typical gamma-ray spectrum of the

soils. Concentrations of 226Ra and 232Th were derived

from the gamma-lines of their respective decay products,

namely: 238 keV of 212Pb, 583 of 208Tl, and 911 keV of

228Ac for 232Th; 295 keV and 352 keV of 214Pb and 609

keV of 214Bi for 226Ra, while concentration of 40K was

derived from its 1460 keV gamma-line. There is no sig-

nificant difference between the ranges of activity con-

centrations in the soils from Nguluku and those from

Maumba (Table 1). A summary of the distributions of

the concentrations is presented in Figure 3. They are

lower than the global averages [10], and also lower than

those reported by Mustapha et al [7] for soils in other

parts of Kenya. Generally, soils derived from sedimen-

tary rocks, such as in the areas where the present study

was carried out, are expected to contain less radioactivity

compared to those derived from igneous rocks. In par-

ticular, the surface soils from the mine sites appear de-

pleted of 40K - the highest concentration recorded (114

Bqkg–1) is less than 30% of the global average (Table 1 ).

These values are comparable to some of the lowest val-

ues so far reported, e.g. from Cyprus, Egypt, Iceland, etc.

[10], and from Gazi [8] near the area being surveyed in

the present study (Figure 1).

3.2. Absorbed Dose Rate in Air

Absorbed dose rate conversion factors: 0.047, 0.462,

0.604 nGyh–1 per Bq·kg–1 of 40K, 226Ra, and 232Th, respec-

tively [10], were used to convert the concentrations of

radionuclides in the soils to absorbed dose rates,

D(nGyh–1), in air at 1m above the ground according to

the relation:









40 226232

0.047K 0.462Ra 0.604ThDA AA 



(1)

where A(40K), A(226Ra) and A(232Th) are the specific ac-

tivities (in Bq·kg–1) of 40K, 226Ra, and 232Th, respectively.

Table 2 shows the range and mean of the absorbed

dose-rate in air around the miming sites. The mean value

(29.2 nGyh–1) is less than 50% of the global popula-

tion-weighted average [10] and it is likely to be less than

the country (Kenya) average, judging from results of

similar surveys carried out in other parts of Kenya, e.g.

Mustapha et al. [7]. According to the 2000 UNSCEAR

report [10], the lowest cases of outdoor absorbed dose

rates were reported in Cyprus (18 nGyh–1), Iceland (28

nGyh–1), Egypt (32 nGyh–1), th e Netherlands (32 nGyh –1),

Brunei (33 nGyh–1) and UK (34 nGyh–1). It therefore

implies that prior to the commencement of mining opera-

tions in the proposed sites the external exposure rates due

to the natural background radiation are among the

world’s lowest. It is therefore imperative that future as-

sessment of radiological impact of the mining operations,

as well as the effectiveness of the post-mining restoration

programs should be based on the area-specific average as

documented in this report, rather than on country or

global averages. It is also recognized that distribution of

radionuclides in surface soil alone will not provide com-

plete assessment of the radiological impact of large-scale

mining operations. The pre-operational survey should be

extended to measurements of distribution of radionuclides

in other relevant media, e.g. air, water, sediments and

biota, etc. in the areas of interest. Finally, it is conceivable

that mining operations and other NORM generating hu-

man activities will continue for the foreseeable future in

different parts of Kenya. Therefore a well planned na-

tionwide environmental radioactivity survey is desirable.

Figure 2. A typical gamma-ray spectrum of the soil samples.

Radioactivity in Surface Soils Around the Proposed Sites for Titanium Mining Project in Kenya463

Table 1. Comparison of mean activity concentrations of radionuclides in soils around the titanium mining sites to other ar e as.

Activity concentration (Bq·kg–1)

Location 40K 226Ra 232Th

Nguluku 61.1 ± 13.0 21.2 ± 9.7 27.0 ± 11.8

Maumba 77.0 ± 15.0 20.6 ± 4.8 28.3 ± 5.8

Overall mean (and ranges) 69.5 ± 16.5

(31.9 - 114.1) 20.9 ± 7.6

(7.4 - 40.6) 27.6 ± 9.1

(8.4 - 43.6)

Kenya (differe nt parts) a 255.7 ± 38.5 28.7 ± 3.6 73.3 ± 9.1

Gazib 206.1 ± 26.7　 11.9 ± 1.4 10.8 ± 1.0

Global average for soilc 420 33 45

a[7], b[8], c[10]

Figure 3. Distribution of activity concentrations of radionuclides in soil samples.

Table 2. Absorbed dose-rate in air estimated from activity

concentrat ions of 40K, 226Ra and 232Th in soil.

Absorbed dose-rate in air (nGyh–1)

Location Range Mean

Proposed mi ni n g sites 9.8 - 50.0 29.2

Kenya (different parts)a 9.9 - 176.5 68.2

Global averagesb 18 - 93 59

a[7], b[10].

4. Conclusions

Pre-operational environmental radioactivity measure-

ments carried out around the proposed sites for Kwale

titanium mining project in the coastal area of Kenya re-

vealed that activity concentrations of naturally occurring

radionuclides are low in the area’s surface soils: means

of 61.1, 21.2 and 27.0 (in Bq·kg–1) for 40K, 226Ra and

232Th, respectively. The absorbed dose rates in air due to

the observed radionucides concentrations in soils are also

low, with a mean of 29.2 nGyh–1. These values are the

baseline on which the assessment of the impact of the

mining operations should be based, instead of country or

global averages. They will also be used in evaluating the

effectiveness of the land restoration program.

5. Acknowledgements

The authors than the following: Kenyatta University and

Institute of Nuclear Science and Technology (University

of Nairobi) for availing field and laboratory equipment;

the government of Kenya (through the ministries of edu-

cation and the office of the President) for authorising the

study; and the United Nations Environmental Programme

Radioactivity in Surface Soils Around the Proposed Sites for Titanium Mining Project in Kenya

464

(UNEP) – Nairobi for granting access to their library.

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