Southwestern Nigeria has vast resources of dimension stones such as granites,
granodorite, gneisses, amphibolites and many more which possesses colours and
structures that impart a particular aesthetic appearance when cut and polished.
The rocks are also rich in silica and are of high quality and give rise to high
strength concrete (Akpokodje, 1992). They compared well
with BS (1975) acceptance limits for absorption (<3%), bulk density (>2.60
g cm-3) aggregate impact value (<30%) (Harvey
et al., 1974; Collins, 1988; Hudec,
1980) which make these rocks quarrable for construction purposes. Most of
the granites and its varieties are widespread in Abeokuta, Odeda, Akiode, Ajebandele,
Gbokutaru areas of Ogun state; Okeigbo, Idanre, Ondo, Akure areas of Ondo state;
Igbo Ora in Oyo state and Ado-Ekiti, Ikere all in Ekiti state where they appear
massive and extensive (Jones and Hockey, 1964). This
naturally occurring and abundant resource has great values that can be harnessed
for the development of these states. Granite when used as cutstones or dimension
stones are considered by many as the premium material for beauty and durability
in institutional and monumental constructions. Granite as cutstones can be used
in flagging, roofing slates and mills stock slates. They can be used as curbing
and paving blocks and in laboratory furniture and sinks. They have been used
to line tube mills for grinding one or other materials. However, the only notable
usage of these dimension stones till date is its exploitation by the people
in the quarry business as aggregates in small scale and monumental constructions
especially in the nearby city of Lagos the economic nerve centre of Nigeria.
The resultant effects of quarry activities is the extensive devastation of the
environments in terms of deforestation, destruction of nearby farmlands with
stone relics, gaseous pollution from the use of explosives, release of toxic
metals into the surrounding environments. The natural radiations from these
granitic bodies and other geological formations are other sources of environmental
hazard (Fernandez et al., 1992; UNSCEAR,
2000; Doveton and Merriam, 2004). In this study,
measurements of radiations from various rock aggregates, surface and subsurface
soils within and around a typical quarry sites, Stone Bridge quarry were carried
out. The effects of aggregate sizes, soil distribution (i.e., surface and subsurface)
and proximity of the quarry site on the radiation concentrations were assessed
and the possible health implication also inferred.
MATERIALS AND METHODS
Study area: The study area, Stone Bridge quarry is found at 22 km Lagos-Ibadan expressway, Oyo state, Southwestern Nigeria. It is situated on longitude (9°22' and 9°26') and latitude (7°98' and 8°08'E) and is accessible by network of asphalt and untarred graded roads. It has an undulating topography with iselberg of rocks that are quarried by Stonebridge, Ratcon, WASSI and many other quarries.
The area is tropical in nature with two climatic seasons viz: wet season which
begins in March/April and ends in October with a break in August and the dry
season which begins in November and end in March (Oguntoyinbo
et al., 1983). The soils of the area is generally lateritic with some
clay intercalation while the geology of the area is essentially crystalline
basement complex with dominant rock suites being granite gneisses (Rahaman,
1988). This rock which belongs to the Late Phase Biotitemuscovite has been
observed to be essentially biotitic in composition and occur separately or in
juxtaposition with some muscovitic bends (Jones and Hockey,
1964) in the study area.
Sample collection: On the site measurements of radiations from different
rock aggregate sizes (Quarry rock dust, 12.7, 19 and 44.4 mm of black and white
colour, 9.6 mm coarse aggregate etc.) were carried out at the two phases of
the Stonebridge quarry. The in situ measurements of the radiations from
the surface of the soil was done directly in an undisturbed manner while the
measurement of the radiation from the subsurface soils was carried out directly
inside the manually dug pits each of which is 0.85 m wide and either 1.5 or
2.0 m deep. Measurements of the radiations were achieved using rpi rad-monitor
model, a portable digital radiation meter; inspector 06250 (S.E International,
Inc., USA). The radiation meter which is optimized to detect low levels α,
β, γ and X-ray radiations, measures radiation parameters in units
of activity which was converted to dose rate and exposure rate. The meter consists
of a halogen-quenched Geiger Muller tube detector with mica window of density
1.5-2.0 mg cm-2 and 3500 cpm/mR/h reference to Cs-137. The meter
has an accuracy of ±15%. The measurements were carried out by positioning
the radiation meter at the targeted sample (rock aggregates, surface and subsurface
soil samples) located at varying distance from the quarry phase (s) established
by Geographical Positioning System (GPS). For each measurement, the background
radiation level was recorded. At each point, a sample of 10 measurements were
taken and the mean value considered. The background reading was then deducted
from the mean value to obtain the actual mean radiation levels emitted by each
sample type. Measurements of the activity and exposure rate were carried out
in units of count per minute (cpm) and milli Roentgew per hour (mR/h), respectively.
The activity was further converted to dose equivalent rate by a conversion factor
of 32240 cpm = 100 mSv h-1 as specified by the equipment manufacturers.
This was achieved assuming an average of 8 working hours by the farmers and
the quarry reserchers a day for 6 days of a week (excluding Sundays). The result
was then compared with the dose reference of 0.02 mSV week-1 for
protection against ionizing radiation (ICRP, 1992).
Quality assurance procedures: The precaution taken in order to ensure quality assurance include viz: standardization of the measuring equipment before usage, multiplicity of measurement for each sample type (n = 6 for radiation measurements for each sample type). The knob was turned to return the meter to zero after each measurement.
Data analysis/conversion: The generated data were converted to nGy h-1 using the relation 1.0 rad = 1.0x10-2 Gy the results are presented as means and standard deviations while the bar chart illustrations were carried out to determine the significant relationships between the radiations from different sample types.
The results obtained in this study are display of the average radiation levels
emitted by each sample type with their standard errors even at varying distances
away from the quarry. The errors were estimated using the standard error method.
The results shown are for the measured parameters of activity (cpm), exposure
rate (mR h-1) and equivalent dose rate (mSv week-1). The
results of the radiation measurement obtained from the quarry sites and their
corresponding various aggregate sizes are shown in Table 1
while measured radiations in both the surface and subsurface soil samples are
shown in Table 2 and 3. These also show
other valuable parameters such as the coordinates where each sample was collected
and their corresponding altitudes. In order to have better evaluation of the
different level of radiations emitted from different sample types, a plot of
exposure rate against different locations and sample types weres shown in Fig.
1 and 2.
The study area is on a relatively high elevation with altitudinal values in
the range of between 128 and 166 m above sea level. The highest rock exposed
at the quarry phase is about 4 m high above ground surface. The surface radiations
at the quarry phase was 120 cpm with an exposure rate of 4.34±0.01x10-2
mR h-1 value and a dose eqviualent of 1.80±0.03x10-3
mSv week-1. Among the aggregates, the highest measured radiation
was 138 cpm with an exposure rate of 6.20±0.01x10-2 mR h-1
and a dose eqviualent of 2.16±0.03x10-3 mSv week-1
obtained from the freshly quarried rock boulders (i.e., coarse aggregates) followed
by 132 cpm with exposure rate of 4.77±0.02x10-2 mR h-1
and dose equivalent of 1.99±0.01x10-3 mSv week-1
obtained from the previously quarried rock boulders (i.e., coarse aggregates).
Among the sizeable aggregates, ½ inch aggregates has the highest radiation
value of 126 cpm with exposure rate of 4.56±0.03x10-2 mR h-1
and dose equivalent of 1.89±0.02 x10-3 mSv week-1
while <½ inch aggregates (i.e., quarry dust) has radiation value of
90 cpm with an exposure rate of 3.25±0.02x10-1 mR h-1
and dose equivalent of 1.35±0.05x10-3 mSv week-1.
Investigations have shown that levels of radiations vary considerably based
on rock types and also in the types of radioisotopes.
||Radiation measurements of rock aggregates from the quarry
||Radiation measurement of surface soil of quarry environment
||Radiation in subsurface soil of quarry environment
||Comparison of exposure rate of surface and subsurface soil
from dug pits
||Comparison of exposure rate for different rock aggregate sizes
In particular high radiation levels from natural radionuclide have been associated
with granitic and silicic igneous rocks like those in the study area (Brimhal
and Adams, 1982). It is therefore possible that the measured radiation from
the study areas are from the natural radionuclides like 238U,
232Th and 40K which are the radioisotopes associated with igneous
Similarly, the radiation levels generally ranged between 42 cpm with an exposure rate of 1.51±0.01x10-2 mR h-1 and dose equivalent of 6.33±0.01x10-4 m Sv week-1 and 120 cpm with an exposure rate of 4.34±0.02x10-2 mR h-1 and dose equivalent of 1.80±0.03x10-3 mSv week-1 in the surface soils.
The values of radiations in the subsurface soil range from 48 cpm with an exposure rate of 1.70+0.02x10-2 mR h-1 and dose equivalent of 7.23±0.03x10-4 mSv week-1. The higher values of radiations in the recently quarried coarse aggregate rocks at the quarry phase compared with the previously quarried coarse aggregate is suggestive of the fact that the radionuclide are resident in the parent rock materials. Also the relatively higher values of the radiations in the coarse rock materials compared with the subsurface and surface soils confirms that the natural sources of the radiations in the soils is from the underlying bedrocks.
Igneous rocks of plutonicvolcanic origin are known to be widespread in occurrence.
This rock type may be older granite member of the Basement Complex rocks. In
the study area, the rocks have been sub grouped by Rahaman
(1988) include Migmatite, gneissquartzite complex; slightly migmatized to
nonmigmatized metasedimentary and metaigneous rocks; Chanockitic, gabbroic and
dioritic rocks; older granite suite; metamorphosed and unmetamorphosed calcalkaline
volcanic and hyperbasal rocks and unmetamorphosed diorite dykes, basic dykes
and syenite dykes. These rock types are known to be associated with elevated
levels of naturally occurring radionuclides (Kline and Mose,
1990; Scott, 1988; Piller and
Adams, 1962; De Jong et al., 1994; Richardson,
These radionuclides are unstable atomic nucleus (Z>84) that decays spontaneously
to emit natural radiations. The three radiations viz gamma (γ), beta (β)
and alpha (α) are indications of the presence of radioisotopes in the material.
These (three) radiations have their own characteristics and possess different
frequencies and wavelengths. These rocks have been observed to be more radioactive
than the metamorphic rocks (Brimhal and Adams, 1982).
The high value of radiations shown in Table 1 (120 cpm) obtained
in the miscovitic coarse aggregates (rocks with dark and white bands) compared
with the values 108 cpm obtained in the biotitic coarse aggregates (i.e., rocks
with dark bands) in this study may implies that rocks of multiple colours (dichromic)
absorbs and reflects radiation than rock of single colour (i.e., monochromic).
It is also likely that aggregate size distribution has effects on radiation
levels since the finest quarry dust has the least radiation (90 cpm). This implies
that fine grain aggregates allow the easy escape of radiations than coarse aggregates
thereby giving shorter period of hazard compared with coarse aggregates.
The values of radiation are relatively higher in the subsurface soils that are closer to the parent bedrock materials vertically than the overlining surface soils This is supported by the absorbed radiation dose (nGy h-1) and annual effective dose (μS year-1) calculated as shown in Table 2 and 3.
Therefore, there is a relationship between the radiations from bedrock and
the subsurface soil materials next to it while higher values of radiations in
soils from some pits (i.e., p1-4) also indicates that the concentration level
of radiations in soil may be controlled by particle size distribution. This
finding confirms the fact that radionuclide distribution in soils is influenced
by several factors and the physicochemical characteristics of the soils such
as texture, porosity (Stricker et al., 1994;
Mortredt, 1991; Morton and Evans,
1996). In this study, the radiation effects can only be felt only laterally
within the few radius of meter away from the quarry phase while vertically there
is close relationship in the radiation values of the bedrocks and the proximate
subsurface soils than the distant surface soils. The bar chart illustrations
of the measured radiations indicate higher values of radiations in the subsurface
soil compared with the surface soil. The obtained values of radiations dose
rate and equivalents in this study (101.4-148.8 and 4.2-5.9 mSv year-1)
are generally higher when compared with the occupational dose rate (20 mSv year-1)
and public dose equivalent (1 mSv year-1), respectively for the rocks
and soils. This may signifies some health problem to the people around the quarry.
The environmental impacts of radiations depend on the type and amount of a particular
radiation. However, all forms of radiation constitute danger to biological tissues.
The amount of damage of ionizing radiation to biological tissue is α>γ>β.
The health effects varies with level of exposure at an exposure of 70 rem it
can results in vomiting and hair loss at the exposure of 100 rem it leads to
hemorrhage while exposure rate of between 400-2000 rem will constitute death.
This is so because the normal exposure to ionizing radiation is <1 rem year-1.
Besides, human exposure to radiations may increase if they live in houses or
buildings constructed with aggregate materials having radiation doses above
normal background value in the area.
The high radiation in the aggregate rocks sizes from the studied Stonebridge quarry is an indication that the quarried parent rock materials contains radioisotopes. The high radiations in the subsurface soils confirm the presence of radionuclides/radioisotopes in the underlying parent rock materials. There is therefore the possibility of radiation emitting radionuclides in most of the houses built from the various rock aggregates which are widely used in the numerous constructions in and around the study area.
Similarly, the possibility of exposure of the quarry workers and the farmers to different degree of radiations in the area. It is necessary to ascertain the radiation levels in and around the quarries and the different rock aggregate sizes before supply to the people that use these aggregates in construction works. This will definitely reduce the exposure of every stakeholder in quarry activities and aggregates to radionuclide radiations.