Environmental degradations resulting from human activities have become a source
of concern in our rural and urban areas. Paramount among these environmental
challenges is heavy metal pollution. Elevated concentration of trace metals
as a result of human activities have been recorded since ancient times (Nriagu,
1996) and mostly associated with areas of intensive industrial activities.
However, excessive release of toxic heavy metals into the roadside environment
and the associated health implications has become apparent and has developed
into a global phenomenon (Li et al., 2005).
Lenntech Water Treatment and Air Purification (2004)
defined heavy metal as any metallic element that has a relatively high density
and is toxic or poisonous even at low concentration. Their pollution of the
environment, even at low levels and the resulting long-term cumulative health
effects are among the leading health concerns of the world (McCluggage,
1991). Although, heavy metals are essential for proper functioning of the
body system, their bioaccumulation at high concentration may pose health threat
to humans (Lenntech Water Treatment and Air Purification,
2004) while others like As, Cd and Pb have been reported to have no known
bio-importance in human biochemistry and physiology and consumption even at
very low concentrations can be toxic (Nolan, 1983).
Heavy metals such as Pb, Zn, Cu and Cd can damage nervous connections, cause
blood poisoning and cancer in humans.
Heavy metals are emitted from natural and anthropogenic sources. The anthropogenic
sources include industrial and automobile emissions (Peplow,
1999; UNEP/GPA, 2004). Oil spillage also contributes
to heavy metal pollution. For instance, Tanee and Kinako,
(2008) and Tanee and Akonye (2009) reported an increase
in heavy metals especially Zn and Cu in crude oil polluted soil. Automobile
emission is reported to be one of the largest sources of heavy metal to environment
(UNEP/GPA, 2004). Excessive heavy metals accumulation
in agricultural land may result in elevated heavy metal uptake by plants (Garcia
and Millan, 1998).
There are reports of heavy metal accumulation by plants grown on roadsides
in the developed countries, however, few studies have actually been carried
out on this area in developing countries (Fakayode and Olu-Owolabi,
2003; Singh et al., 2004; Chen
et al., 2005; Liu et al., 2005; Wilson
and Pyatt, 2007; Atayese et al., 2009).
This study attempts to determine the accumulation of heavy metals in the studied
plants and topsoil along some high vehicular traffic roads in Nigeria using
Eleme Local Government Area of Rivers State as a case study. It is expected
that the findings obtained from this study will widen our knowledge on the danger
of heavy metal pollution in our environment by providing information on its
spread. It may also provide information on the danger of roadside farming-a
common practice among peasant rural farmers.
MATERIALS AND METHODS
Description of the study area: The study was carried out along major
road in Eleme Local Government area of Rivers state situated in the south-south
(Niger-Delta) zone of Nigeria (Fig. 1). The area experiences
two distinct seasons-the dry and rainy seasons. The dry season is from November-March
and the rainy season from April-October. The climatic condition of the area
is characterized by high temperature, high rainfall, high relative humidity
and high sunshine.
The soil of the area ranged from well drained to moderately drain except in
the coastal areas where the soil is poorly drained. The soil is always low in
nutrient content due to the leaching of the nutrient down the soil profile by
high rainfall which is always heavy in the area. Eleme Local Government Area
is home of many heavy industries in Nigeria. Amongst them are two petroleum
refining plants, one petrochemical plant, a fertilizer plant, a shipping terminal
and other minor companies associated with refined oil storage and marketing
|| Map of Ogoni showing sampling area at Eleme
As a result of this, the area has heavy vehicular traffic volume and therefore
experiences enormous automobile exhaust emission from heavy duty and smaller
Sample collection: Three roads within the area were selected for the
study. These were Refinery Road, Akpajo-Onne Junction (along the East-West Road)
and Aleto-Eleme By-Pass Road (Fig. 2).
The first two (i.e., Refinery and Akpajo-Onne Junction roads) are heavy traffic
routes of more than 2,000 vehicles per day and Aleto-Eleme By-Pass Road is a
moderate traffic route of less than 2,000 vehicles per day. Samples were collected
in the month of October, 2009. Four sample points along each of these roads
(represented by P in Fig. 2) were located with a minimum of
one kilometre between sample points. Samples were taken at each of the sample
point at a distance of 0 m and 10 m away from the road. At each sample point,
three topsoil samples were collected at a depth between 0-5 cm using a spatula
and were thoroughly mixed to form a composite sample. These were transferred
into polyethene bags to avoid contamination from other sources, labeled with
a masking tape and taken to the laboratory for analysis.
Furthermore, two plant species samples found flourishing within 1 m by 1 m
of the sample point were also collected from each point. Panicum maximum
and Centrosema pubescens representing monocotyledon and dicotyledon respectively
were collected. According to Markert (1993), the criteria
for selection of a species as a biomonitor include (1) It should be represented
in a large number all over the monitored area and (2) There should be no identification
problems. The choice of Panicum maximum and Centrosema pubescens
met these two criteria. Aerial parts of these plants were collected using a
clean stainless pair of scissors (Okonkwo and Maribe, 2004).
The plant samples were placed in polythene bags, labeled with a masking tape
and taken to the laboratory for analysis. Plant specimens and soil samples collected
at each sample location were kept together and treated separate from others.
|| Sketch map showing sampling locations along the 3 road in
Analysis of soil samples: Soil samples collected were air-dried and
all clods and clumps were removed. Dried soil was sieved using 2 mm sieve to
remove course particles before analysis.
The following parameters were analyzed in the soil samples: soil pH, soil conductivity
and heavy metals (Cu, Pb, Zn and Cd).
Soil pH and conductivity were determined electronically using pH meter (Jenway
3015 model) and conductivity meter (HACH Ecttesr Microprocessor series model),
respectively. Fifty gram of dried soil was placed in 500 mL capacity beaker
containing 50 mL of distilled water (i.e., 1:1). The mixture was thoroughly
stirred and allowed to stand for 10 min before pH and conductivity values were
recorded when the figure on the meters were constant.
One gram (1 g) of the dried soil sample was placed in 100 mL beaker and 3 mL
of perchloric acid and 5 mL of nitric acid were added. The mixture was allowed
to stand for 15 min before digestion by gently heating at low temperature on
a hot plate and allowed to cool for 5 min. The digest was then filtered into
50 mL standard flask. The filtrate was analyzed for heavy metals using Atomic
Absorption Spectrophotometers (AAS) BUCK scientific 200A model.
Analysis of plant samples: The plant samples were first rinsed with
distilled water and oven-dried at 100°C for 48 h. The plant materials were
ground to fine powder. One gram of the powder was digested as described above
and analyzed for heavy metals using Atomic Absorption Spectrophotometer (AAS).
Statistical analysis: Data obtained were subjected to statistical analyses
using Analysis of Variance and Standard Error Mean (SEM) using Microsoft excel
package version 2007. Least significant difference (p = 0.05) was used to separate
Table 1 showed the results for soil chemical parameters (soil
pH and conductivity) and heavy metals content (Cu, Pb, Zn and Cd).
The pH of the studied area was generally low (i.e., acidic). The results for
pH ranged from 4.08±0.2 4.36±0.07. There was no significant difference
between roads and distance away from the road.
Conductivity was generally high and there was no significant difference (p
= 0.05) between the roads and distance from the road for the three roads.
The heavy metal content (Cu, Pb, Zn and Cd) showed similarity
with soil pH and conductivity, especially in terms of significant differences.
Copper (Cu) content of the 3 roads was in the range of 1.55±0.37 mg kg-1
(10 m distance on Aleto by-pass road) to 4.25±1.38 mg kg-1
(0 m distance on refinery road). Lead (Pb) content in the area was higher than
copper content (2.69±0.74-6.92±2.51 mg kg-1). Zinc
(Zn) recorded the highest values among the 4 heavy metals analyzed in the range
of 4.07±0.87-11.69±3.03 mg kg-1. Zinc was significantly
lower (p = 0.05) at the Aleto By-Pass Road than the other two roads. Similar
result was obtained for soil cadmium (Cd) in which the soil cadmium was significantly
higher at 10 m distance on Onne-Akpajo and Refinery roads than the Aleto by-pass
Generally, soil pH, conductivity, Cu and Pb concentrations were proportional
to the distance from the road, with higher values at the roadside than 10 m
from the road. A reverse trend was observed in Zn and Cd showing higher values
in the 10 m distance from the road than the roadside especially in the Refinery
and Onne- Akpajo roads.
Table 2 showed heavy metal contents in the two plants (Panicum
maximum and Centrosema pubescens) sampled along the three roads.
Zinc content in Panicum maximum was in the range of 43.28±1.9-73.16±6.5
mg kg-1. A significantly higher level of Zn was observed at Onne-Akpajo
|| Soil Physico-chemical and heavy metal content along the three
roads in Eleme
|Mean±SEM with different superscript between columns
means significant difference (p = 0.05)
|| Heavy metal accumulation in plants along the 3 major roads
|Mean±SEM with different superscript represent significance
difference @ p = 0.05)
|| Acceptable range of the parameters studies in soil
The concentration of Zn decreases with increase in distance away from the road
in Panicum maximum. Similar trend was obtained in Centrosema pubescens
in which the Zn content was generally higher at the roadside (i.e. 0 m distance)
than 10 m distance for the 3 roads with the highest value recorded at 0 m distance
at Onne-Akpajo Road. Panicum maximum had the highest Pb content at 0
m at Onne-Akpajo road while Centrosema pubescens showed the highest level
at Refinery Road. The two plants showed low level of Pb at Aleto by-Pass Road.
The copper (Cu) content in Panicum maximum at 0 and 10 m of the three
roads were low with no significant difference between them. However, Centrosema
pubescens recorded higher Cu values (13.46±1.2- 18.89±1.8
mg kg-1) than Panicum maximum (9.26±3.0- 17.10±3.6
Table 3 showed the recommended range of the physico-chemical
parameters and heavy metal contents in soil. It is observed that pH in the study
site in the range of 4.08- 4.36 were below the recommended range of 5.5-6.5
indicating high acidity of soil; while conductivity of 137.5 -215.0 μs
cm-1 were higher than the recommended range of 8-30 μs cm-1.
The high heavy metals concentrations were within the acceptable ranges.
A comparative analysis of the result obtained from this study and other works
done elsewhere in Nigeria is shown in Table 4. Results showed
that the concentration of heavy metals obtained from this study was lower than
those obtained from other studies such as Yauri, Lagos-Badagry Road and Osogbo
Roads. The only exception was Pb in which the result showed that the soil Pb
concentration of 2.69-6.92 mg kg-1 was higher than the range obtained
at Lagos-Badagry road (0.25-4.24 mg kg-1).
Result showed that Pb concentration in the plant (136.68-278.4 mg kg-1)
was above the acceptable maximum (recommended) range (0.2-3 mg kg-1)
while the other two metals (Cu and Zn) were within the recommended limit (Table
Human activities such as industrialization, commercialization and urbanization
have adversely increased the amount of heavy metals in our environment (Nriagu,
1996). Result showed that the soil pH values of the sampled area were generally
low (i.e., acidic). Similar result (i.e., low pH) has been reported by Abii
and Nwosu, (2009) and Tanee and Albert (2011). Low
pH is a characteristic of the tropical soil. The study area is also an industrial
area that has heavy vehicular traffic with the emission of gaseous and particulate
matters into the atmosphere which dissolves in atmospheric moisture and fall
to the soil in rain. There is the possibility that some anions may remain on
topsoil with the increase of free radicals, thereby resulting in reduced soil
pH. The low pH value is an indication of pollution since most plants and microorganisms
perform best at pH close to neutrality (Atlas and Bartha,
1992). The observed low pH could also account for the high conductivity
recorded in the area.
Results showed that heavy metal concentrations in the soil decreased with increase
in distance from the road with exception of Zn and Cd on Onne-Akpajo Road and
Refinery Road. This might be as a result of the heavy automobile traffic especially
heavy duty vehicles characteristic of these roads. Habashi
(1992) and Fuller (1974) have reported undesirable
and unnatural concentration of lead (Pb) in air, water, soil and vegetation
particularly near heavily plied automobile free-ways. These range of heavy metals
are among the wide range of heavy metals found in fossil fuel which are either
emitted into the environment as particles during combustion or may itself be
transported in air and contaminate soil (Yahaya et al.,
2010). This is in line with the report of UNEP/GPA, 2004
- that combustion and traffic are among the sources of heavy metals into the
Zn was found to have the highest concentration in the soil in the study area,
followed by Pb. This is understandable since tyre wears released zinc (Kabata-Pendias
and Pendias, 1984). The higher level of Pb might be from the deposition
from automobile exhaust since most petroleum fuel contains tetraethyl lead as
antiknock (Lenntech Water Treatment and Air Purification,
Results also showed higher concentration of heavy metal in plants than soil
in all the 3 roads sampled. This is an evidence of bioaccumulation or biomagnification
of the heavy metals in the plants. When soil is polluted with heavy metals,
the metals are taken up by plants and consequently accumulate in their tissues
(Trusby, 2003). Animals that feed on these plants also
accumulate these metals in their tissues (Horsfall and
Spiff, 1999; Peplow, 1999). These may become lethal
to the top carnivores.
The concentration of Zn was higher in Centrosema pubescens than in
Panicum maximum and Pb was higher in Panicum maximum than Centrosema
pubescens while Cu was higher in Centrosema pubescens than Panicum
maximum. This suggests that plants have different ability of accumulating
particular heavy metals.
It was also observed that the Pb concentrations in the plants were very high
when compared with other metals. This suggests that the plant might have absorbed
lead (Pb) from other sources apart from the soil. It is possible that the Pb
emitted into the atmosphere from the automobile exhaust penetrates through the
intercellular spaces of the leaves and accumulate in the plant.
It is observed that the results obtained from this study in terms of heavy
metal accumulation in the soil are similar to other studies done elsewhere in
Nigeria. This showed that major roads in Nigeria have similar levels of heavy
The high level of Pb accumulation in the plants along the roadsides is an indication
of Pb pollution of the roadside plants in which if consumed by humans might
have deleterious effect and even death due to biomagnifications.
The 3 roads showed different levels of heavy metal contaminations. The level
of contamination was more pronounced in the plants than in the soil. Therefore,
economic plants should not be cultivated along roads with heavy traffic especially
along the roadsides so as to avoid heavy metal toxicity in man and animals.
Proper biomonitoring of the environment should be done as often as possible
so as to enlighten the public on the dangers of heavy metal pollution.