Materials Sciences and Applications, 2010, 1, 127-134
doi:10.4236/msa.2010.13021 Published Online August 2010 (
Copyright © 2010 SciRes. MSA
Heavy Metal Contamination of Surface Soil in
Relationship to Land Use Patterns: A Case
Study of Benue State, Nigeria
Christopher Iorfa Adamu, Therese Ntonzi Nganje
Department of Geology, University of Calabar, Calabar, Nigeria.
Received April 26th, 2010; revised May 14th, 2010; accepted June 1st, 2010.
A detailed investigation was conducted on the relationship between land use patterns and trace metal content in surface
soils of the Benue State to assess soil environmental quality. Results revealed that metals levels were generally high in
mineralized and urban soils and lower in agricultural soils whilst forest soils were lease by anthropogenic pollution.
Mineralized soils developed from weathered sulphides were rich in lead (Pb), zinc (Zn), and cadmium (Cd). Urban soils
accumulated copper (Cu), Zn, and Cd most probably from refuse dumps, gasoline combustion and farming. Agricul-
tural soils were enriched in arsenic (As) and to a lesser degree Pb and Cd originating most probably from the applica-
tion of pesticides, manure and fertilizers. A pollution index (PI) based on plant-tolerant contamination levels, indicates
that multi-element contamination in soils is low and implies that the sampled soils could be cultivated for crop produc-
tion especially away from point sources of pollution. The degree of anthropogenic pollution was high for As (80%), and
Pb (54%), moderate for Zn (47%), Cd (40%) , and low for Cu (27%). Correlations (r) are significant between Zn- Pb
(0.7), Cu-As (0.6) in mineralized soils, between Zn-Cu (0.64), Zn-Cd (0.5), Cu-Cd (0.6), in urban soils, As-Cd (0.61), in
agricultural soils and Zn-Pb (0.82) in forest soils. These distinct relationships indicate a common source or similar
geochemical control. Based on the overall evaluation, recommendation in respect of contamination, control and moni-
toring strategies as well as land use planning in the study area are presented.
Keywords: Heavy Metals, Pollution Index, Surface Soil, Land Use
1. Introduction
Soil is not only a medium for plants growth or waste dis-
posal but also a transmitter of many pollutants to surface
water, ground water, atmosphere and food. Soil pollution
may threaten human health not only through its effect on
hygiene quality of food and drinking water, but also th-
rough its effect on air quality. Little attention has been
paid to soil pollution compared to food in the past [1].
More recently, however, the impact of soil pollution on
soil functions and the biosphere has been increasingly
emphasized by the government, environmental protection
agencies and the public [2]. In particular significant ad-
vances have been made in respect of mining related
heavy metal pollution of the soil in different parts of the
world [3-7].
However, a survey of heavy metals indicates that they
do accumulate in soils in some localized areas of human
activities as compared to areas that have remained under
virgin conditions [8]. In addition to mining activities,
concentration of heavy metals in terrestrial environment
have increased significantly as a result of human activi-
ties such as emissions from thermal power stations, waste
disposal, soil amendments and vehicle traffic/road infra-
structures [9]. Other non point sources of contamination
affecting predominantly agricultural soils include inputs
such as, fertilisers, pesticides, sewage sludge, organic
manures and composts [10,11]. Some of the anomalous
accumulation may also be geology-related [12].
A knowledge of the level and distribution of heavy
metals in soils can play a key role in land use planning,
the designing of control strategies to achieve a better en-
vironmental quality as well as a key to effective mana-
gement of soil quality especially in rapidly growing area
such as Benue State. Unfortunately there is little infor-
mation on the geochemical distribution and behavior of
heavy metals in the developing countries because heavy
metals analysis is rarely carried out in routine monitoring
Heavy Metal Contamination of Surface Soil in Relationship to Land Use Patterns: A Case Study of Benue State, Nigeria
of soil quality [13].
Uncontrolled inputs of heavy metals are undesirable
because once accumulated in the soil they are generally
very difficult to remove and are potentially harmful.
Subsequent problems may include toxicity to plants, sur-
face and groundwater contamination as well as dietary
exposure to man and animals. The impact of contamina-
tion extends beyond the contaminated environment [14,
In Nigeria, like many other developing countries, more
metal-containing waste are constantly being released into
environment through urbanization as well as industrial,
mining, and agricultural activities. Heavy metals are also
emitted during high temperature processes such as oil
combustion in automobiles, electric power stations, in-
dustrial plants as well as refuse incineration [16]. The
objectives of this study include the determination of the
levels of selected heavy metals in surface soils of the
Benue state in relation to land use patterns and geology
and contamination.
2. Selection and Description of Sites
The Benue state has a rapidly growing population of over
4.2 million people and total land area of over 40,000 km2
[17]. The state is dominantly an agricultural state. More
than 70% of the land is used for agriculture while mining,
urban centres and forest reserves are also significant. The
potential inputs of heavy metals above the geogenic
background values in the state may be rel- ated to these
human activities. Therefore, four sites (Figure 1) were
selected to represent the dominant land use pattern (or
sources of heavy metals) as follows:
Mineralized soil from Arufu (Site A)
Urban soil from Makurdi (Site B)
Agricultural soil from Ugba (Site C)
From a government forest reserve (near Makurdi Site D)
Site A (Arufu) is an area well known for its Pb-Zn
mineralization in the middle Benue Trough. The area is
important mainly for the mining of Pb-Zn and associated
Ag. The site was chosen because the mineralization and
mining activities have lead to the occurrence of many
abandoned mines (galleries) and tailings scattered around
the open mines pits. These can cause the contamination
of the local environments as elements could be released
to the soil at greater rate than would occur by the natural
weathering of barren or underlying parent material. Sam-
ples collected here were termed mineralized soils.
Site B (Makurdi) is the capital city of Benue State.
Since becoming the capital city in 1976 it has witnessed
tremendous increase in human population and activities
due to abundant and expanding economic opportunities.
Of its total land area of about 200 km2, built up areas
Figure 1. Map of Benue state showing showing the study sites
Copyright © 2010 SciRes. MSA
Heavy Metal Contamination of Surface Soil in Relationship to Land Use Patterns: A Case Study of Benue State, Nigeria 129
constitute about 40% while swamps, rivers and open land
cover about 53%. As in many urban centers in develop-
ing countries, poor land use planning, lack of proper
disposal facilities for sewage and solid wastes as well as
high traffic congestion characterizes Makurdi town. The
population in the town is unevenly distributed such that
commercial, industrial, agricultural, recreational, admin-
istrative and residential areas are scattered all over the
town and these are point sources of heavy metals [16]
into the soil. Soil samples collected here were termed
urban soils.
Site C (Ugba) is a rural settlement near Zaki-Biam and
is well known for its intensive farming. As in many rural
areas, farming in Ugba is characterized by poor land-use
planning and indiscriminate application of fertilizer, bio-
cides and manures with potential risk of heavy metal
contamination [18]. Samples were collected from se-
lected farms in Ugba using soil traverse reconnaissance
data. The farms selected reflect the geochemical range of
elements of interest to evaluate the agricultural signifi-
cance of metal contamination. Samples collected from
this site were termed agricultural soils.
Site D (Forest) is a forest reserve along the Makurdi-
Lafia road that is not open to the public. The site is se-
lected to serve as control. The levels of heavy metals in
forest soils were considered as “present background val-
ues” as the original background values have changed
since due to processes of soil formation as well as natural
contributions from wind blown dust and wild fires.
3. Materials and Methods
Composite surface soil samples (0-6 cm) were collected
from the four representative sites using an auger and
stored in properly labeled polyethylene bags. They were
air-dried at room temperature (21-27˚C) for seven days
and later oven-dried at 100˚C for three hours to obtain a
constant weight. The soil samples were mechanically
ground and sieved to obtain < 2 mm fraction. A fraction
of the soil was drawn from the bulk soil (< 2 mm fraction)
and reground to obtain < 200 μm fraction using a mortar
and pestle and then digested using aqua-regia. Analysis
of the heavy metals (Zn, Cu, As, Pb, and Cd) was in du-
plicate, using atomic absorption spectrophotometer (AAS
–bulk scientific model 210) from Soil Science Depart-
ment University of Agriculture, Makurdi. The heavy me-
tals analysed (Zn, Cu As, Pb and Cd) are among those
considered most problematic in terms of environmental
pollution and toxicity [19]. The level of precision of the
method used for the analyzed metals ranged between
4. Data Evaluation
For the assessment of the heavy metal contamination in
the study sites, some quantitative indices were used to
describe the concentration trends and also to allow for
easy comparison between the determined parameters.
The indices used were anthropogenic factor (AF) and
pollution index (PI).
Anthropogenic factor (AF) reflects the degree of con-
tamination relative to the average composition of the re-
spective metal in soils or to a measured background val-
ue from geologically similar but uncontaminated area. It
is expressed as
Where Cm is the measured concentration in soil and
Bm is the background concentration of the metal, “m”.
Bm values are either taken from the literature or directly
determined from a geological similar but uncontaminated
The Pollution index (PI) has been used to evaluate the
degree of multi-element contamination. This is consid-
ered a better method of evaluation because heavy metals
contamination in the surface environment is associated
with a cocktail of contaminants rather than one element
[20]. Although a variety of PI is used by researchers, the
basic concept is the same. In this study, the PI of soils
was computed using average levels in soils tolerable to
plants growth given by [21]. The equation is as follows:
5 / 300)}(Zn / 100) / (Pb
100)(Cu / 3) / (Cd20) / {(AsPI
Taking As (mg/kg) as example. When PI values are >
1.0 the soils are considered to be contaminated by an-
thropogenic inputs.
So we relay on the methods used in the present study
for quality assessment with general applicability and va-
lidity for now.
5. Discussion of Results
5.1 Metal Concentration in Soils
The mean concentrations of As, Zn, Cu, Pb and Cd in the
various soil categories are presented in Table 1 alongside
with their general average abundance in soil compiled by
Cox [19]. Significant levels of trace elements were found
in mineralized and urban areas, with mean values of 171
mg/kg and 170 mg/kg (Zn), 70 mg/kg and 109 mg/kg
(Cu), 58 mg/kg and 45 mg/kg (Pb), 16 mg/kg and 10
mg/kg (As), and 1.8 mg/kg and 1.2 mg/kg (Cd) respec-
tively. Agricultural soils contained moderate levels of
trace metals with average values of 150 mg/kg (Zn), 67
mg/kg (Cu), 52 mg/kg (Pb), 18 mg/kg (As), and 0.8
mg/kg (Cd). Forest soils are the least enriched in trace
metals with mean values of 85 mg/kg (Zn), 54 mg/kg
(Cu), 36 mg/kg (Pb), 8 mg/kg (As), and 6 mg/kg (Cd).
The results revealed elevated levels of heavy metals rela-
tive to world mean abundance of 36-100 mg/kg (Zn), 20
-50 mg/kg (Cu), 17-30 mg/kg (Pb), 2-10 mg/kg (As), and
0.1-0.5 mg/kg (Cd), indicating that the original background
Copyright © 2010 SciRes. MSA
Heavy Metal Contamination of Surface Soil in Relationship to Land Use Patterns: A Case Study of Benue State, Nigeria
Copyright © 2010 SciRes. MSA
values of all soil categories have changed since because
of soil forming processes as well as anthropogenic inputs
(from human activities). Metals also occur in relatively
high concentrations in urban, mineralized and agricul-
tural soils as compared to the forest soils indicating that
the forest soils have suffered least anthropogenic inputs
and the metal values in the forest soil could be used as
“present background values” in soil quality evaluation.
Zinc has the highest concentration in soils with mean
values ranging from 85ppm in forest soil to 171 ppm in
urban soil compared to world mean abundance of 70 ppm.
Copper has average concentrations ranging from 54 ppm
in forest soils to 109 ppm in urban soils compared to wo-
rld mean abundance of 55 ppm. Lead has mean values of
36 ppm for forest soil, 45 ppm for urban soil, 52 ppm for
agricultural soil and 58 ppm for mineralized soil as
compared to world mean abundance value of 13 ppm.
Arsenic and Cadmium have mean values of 8 ppm and
0.6 ppm for forest soil, 10 ppm and 1.2 ppm for urban soils;
16 ppm and 1.8 ppm for mineralized soils and 18 ppm and
0.8 ppm for agricultural soils as compared to world mean
abundance values of 2 ppm and 0.2 ppm respectively.
The order of concentration of metals in soils is Zn > Cu >
Pb > As > Cd. The indications of contamination are
clearly revealed by the respective metal concentrations
with values above world mean values (Table 1).
With respect to the mean concentrations in the various
soil types, urban soils have the highest concentration of
Zn and Cu; mineralized soils have highest values for Pb,
Zn, and Cd while agricultural soils have the highest value
of As. (Table 1). The order of concentration of heavy
metals in the soil types is mineralized soils > urban soils
> agricultural soils > forest soils.
Zinc exhibits high levels of variations in all soils with
large standard deviation values (±34 to 107) while As
and Cd concentrations are least variable as indicated by
low standard of deviation values (< ±10). Other heavy
metals exhibit moderate variability with standard devia-
tion values ranging from ±20 to ±53. Large variations
imply great heterogeneity of metals in soils while low
variations show more or less homogeneous distribution
of metal in the soil. On the basis of soil type, urban soils
are more variable exhibiting highest variability for As,
Cu and Cd; followed by agricultural soils with highest
variability for Zn and Pb. Forest soils are least variable.
The order of variability is urban soils > agricultural soils
> mineralized soil > forest soils and may reflect varying
sources of anthropogenic inputs.
Correlation coefficients (r) between different heavy
metals were calculated in order to understand their co-
variation (Table 2).
Significant correlations (P 0.05) occurred between
Zn-Pb (0.64) and Cu-As (0.6) in mineralized soil, be-
tween Zn-Cu (0.64), Zn-Cd (0.5) and Cu-Cd (0.6), in
urban soils, As-Cd (0.61) in agricultural soils and Zn-Pb
(0.82) in forest soils (Table 2). Strong correlations be-
tween elements imply similar geochemical controls in the
surface environment. This is a strong indication that the
sources of these metals are related to common anthropo-
genic rather than geogenic inputs, which would have
been dependent on geochemical factors including vari-
ables such as soil properties pH, Eh, etc, which control
metal behaviour in the soil.
5.2 Assessment of Metal Contamination
Quantitative evaluation of soil quality in the study was
carried out using indices such as Anthropogenic Factors
(AF) and Pollution Index (PI). The average crustal abun-
dance [22] and levels of metals obtained from this study
were used as “background values’’ and “present background”
values in order to give a comparative idea about the qual-
ity and degree of heavy metal contam- ination of surface
soils of Benue State. Estimated values of anthropogenic
factors (AF) for the heavy metals determined in soils
samples with respect to the average crustal abundance
were generally greater than one and range from 4-9 (As),
1.2-2.4 (Zn), 1.1-2.2 (Cu), 1.8-2.9 (Pb) and 1.4-3.6 (Cd)
(Table 3).
This indicates a, 1 to 10 fold enrichment compared to
average crystal abundance. Mineralized soils have the
highest enrichment factors for Zn Pb and Cd, urban soils
are most enriched in Cu, while agricultural soils are the
most enriched in As. In all, forest soils are the least en-
riched in all analyzed heavy metals. This shows that all
Table 1. Mean ± S.D. values of analyzed heavy metals (mg/kg) in soils in parts of Benue State, Nigeria
Site Location Description N Zn Cu Pb As Cd
A Arufu Mineralized soil 40 171 ± 83 70 ± 83 58 ± 28 16 ± 5 1.8 ± 5
B Makurdi Urban soil 40 170 ± 107 109 ± 53 45 ± 31 10 ± 8 1.2 ± 8
C Ugba Agricultural soil 40 150 ± 94 67 ± 23 52 ± 35 18 ± 6 0.8 ± 3
D Makurdi Forest soil 20 85 ± 34 54 ± 31 36 ± 20 8 ± 3 6.0 ± 3
Crustal average* 70 55 13 0.2 0.5
Common range in soils** 36-100 20-50 17-30 2-10 0.5-10
N = number of samples; *Bowen, 1979 [22]; **Cox, 1995 [21]
Heavy Metal Contamination of Surface Soil in Relationship to Land Use Patterns: A Case Study of Benue State, Nigeria131
Table 2. Correlation coefficients between heavy metals of soil studied
A Zn Cu Pb Cd
A. Mineralized soils (n = 40)
As 0.31 0.60* 0.45 0.56*
Zn 0.26 0.70* 0.01
Cu 0.37 0.25
Pb 0.12
B. urban soil (n = 40)
As 0.42 0.18 0.30 0.12
Zn 0.64* –0.20 0.52*
Cu 0.06 0.56*
Pb –0.18
C. Agricultural soil (n = 40)
As 0.23 0.30 0.41 0.61*
Zn –0.20 –0.23 –0.10
Cu 0.31 0.02
PI 0.12
D. Forest soil (n = 20)
As 0.36 0.24 0.46 0.21
Zn 0.08 0.82* 0.36
Cu 0.05 0.17
Pb 0.22
Indicates significant difference at P < 0.05
Table 3. Summary of quantitative indices with respect to metal contamination in surface soil samples in parts of Benue state,
Site Description Zn Cu Pb As Cd PI
A Mineralized soil 2.4 1.4 2.9 8.0 3.6 0.6
B Urban soil 2.4 2.2 2.3 5.0 2.4 0.6
C Agricultural soil 2.3 1.4 2.6 9.0 1.6 0.6
D Forest soil 1.2 1.0 1.8 4.0 1.4 0.4
AF = Anthropogenic factor, PI = pollution index
representative soil types are enriched in heavy metals
relative to mean crustal abundance. The relative enrich-
ment in forest soils where human activities are restricted
could be attributed to general enrichment through soil
forming processes as well as deposition from air by
emissions through automobile exhaust and application of
biocides which are wide spread in Nigeria [23]. Natural
sources of heavy metals such as windblown dust and
wildfires might have also contributed to heavy metal
enrichment in forest soils. This is consistent to the asser-
tion by [24] that a certain amount of contamination of
soil through deposition from the air is unavoidable.
Quantification of the overall contamination of the soils
(Table 3, Figure 2) shows that about 75-90% As, 45-
70% Pb, 53-60% Zn, 37-72% Cd and 7.4-54% Cu are
derived form anthropogenic inputs, confirming As as the
most enriched element and Cu as the least enriched
(Figure 3). The levels of heavy metals derived form an-
thropogenic inputs (Figure 2) for the various soil catego-
ries is as follows:
Site A (mineralized soil) As (87.50%) > Cd (72%) >
Pb (65%) > Zn (60%) > Cu (54%)
Site B (Urban soils) As (80%) > Zn (60%) Cd (60%)
> Pb (55%) > Cu (54%)
Site C (Agricultural soils) As (90%) > Pb (60%) > Zn
(50%) > Cd (40%) > Cu (54%)
Site D (forest soils) As (76%) > Pb (44%) > Zn (17%)
Cd (17%) > (Pb 13%) > Cu (7.4%) (Figure 2).
These patterns reveal that the order of anthropogenic
inputs in soils of the study area is As > Pb > > Zn Cd >
Copyright © 2010 SciRes. MSA
Heavy Metal Contamination of Surface Soil in Relationship to Land Use Patterns: A Case Study of Benue State, Nigeria
Cu. In terms of soil types, the order is mineralized soil >
urban soils > agricultural soils > forest soils. Such trends
of anthropogenic contamination are consistent with the
estimated AF trends which reflects environmental con-
tamination arising form unplanned land use pattern and
mineralization in the study area.
6. Summary and Conclusions
Results of geochemical studies of surface soils in parts of
Benue state revealed an overall enrichment of heavy me-
tals relative to crustal abundance depending on the deg-
ree of anthropogenic inputs and source of heavy metals.
In general the extent and seriousness of heavy metal pol-
geogeni c
ant hropo geni c
anth ropogen i c
Figure 2. Anthropogenic contribution of heavy metals
among soil types
Zn Cu PbAs Cd
Figure 3. Distribution of input sources of heavy metals in
soil samples
lution in mineralized and urban soil were the highest am-
ong the soil categories, followed by agricultural soils. Fo-
rest soils were subjected to the least impact form anthro-
pogenic sources. Mineralized soils contained elevated
levels of Zn, Pb and Cd most probably released into the
soil form the weathering of sulphide minerals form the
mine waste. Urban soils accumulated Cu, Zn and Cd
most probably originating from commercial, domestic,
Copyright © 2010 SciRes. MSA
Heavy Metal Contamination of Surface Soil in Relationship to Land Use Patterns: A Case Study of Benue State, Nigeria133
agricultural and to a lesser extent industrial activities in
the area. Agricultural soil accumulated As and to a lesser
extent Pb and Cd most probably from the application of
biocides, animal manures and fertilizers. The elevation of
heavy metal content in the surface soils of Benue State
seems to be related to urbanization and poor land plan-
ning and use.
Furthermore, quantification of degree of pollution re-
vealed high anthropogenic contributions for As (82%)
and Pb (54%) indicating multiple sources for the heavy
metals. Also anthropogenic inputs accounts for about
50% of Zn and Cd and 30% Cu. In addition the concept
of pollution index gives important information on the
extent and degree of multi-element contamination and
can be applied to the evaluation of soils before their use
for agriculture and production of food crops.
This study provides useful data on the present con-
tamination of surface soils in relation to land use pattern
in the study area and may form the basis for the formula-
tion of pollution control and monitoring strategies as well
as land use planning.
7. Recommendation
It is important to note that the challenge of heavy metal
contamination lies in the fact that it is a by-product of de-
velopment efforts and cannot be eliminated. Hence there
is the need to continually monitor potential contaminated
areas. A thorough geo-environmental study involving co-
nstant geochemical analysis, remotely sensed data and
modeling must be a primary step in environmental moni-
toring in heavy metal contaminated areas. Monitoring is
necessary to identify possible sources and degree of hea-
vy metal contamination, select appropriate remedial mea-
sures and to manage land use in relation to mining, agric-
ulture and urban development.
Since most soil contamination in agricultural land is
associated wlth indiscriminate application of fertilizers
and biocides, the most effective control strategy would
be to educate farmers to apply precisely determined qua-
ntiies of these chemicals on their farms. This approach
will reduce the amount of contaminants entering the soil
and also save cost. Small scale, low-input farming offers
an alternative to industrial, chemically intensive agricul-
ture. Industries in the state should reduce contaminants
through proper waste management, recycling and reuse
of materials that might be discarded as waste. Prior to re-
clamation, unauthorized and unplanned cultivation of
mine wastes and other contaminated areas be prohibited
to reduce heavy metal mobility and bioavailability.
Effective land use planning involving the raising of
public awareness and commitment to contamination con-
trol, seeking a compromise between developers and con-
servationist, pre-environmental impact assessment prior
to developing a site and identification of specific uses for
designated sites is recommended.
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