Oil exploration and exploitation has uplifted Nigerian economy leading to rapid
development but the impacts of oil exploration and exploitation on the environment
are receiving less attention (Idodo-Umeh, 2002). One of
the major anthropogenic sources of heavy metal enrichment in terrestrial habitats
of oil producing areas of Niger Delta of Nigeria is the frequent spills of crude
oil on land and gas flaring (Idodo-Umeh, 2002). Nigerian
crude oil is known to contain heavy metals such as Al, Zn, As, Ba, Fe, Pb, Co,
Cu, Cr, Mn, Ga, Sb, Ni and V (Unpublished data). Toxicity of ingested heavy
metals has been an important health issue for decades (LeCoultre,
2001). High levels of accumulation of heavy metals from soil by common garden
vegetables has been reported by many environmental researchers (Boon
and Soltanpour, 1992; De Pieri et al., 1997;
Xiong, 1998; Cobb et al., 2000; LeCoultre,
2001). Some species of Brassica (cabbage) are high accumulators of
heavy metals in the edible parts of the plants (Xiong, 1998) and this can be
an important exposure pathway for people who consume vegetables grown in heavy
metal contaminated soil (LeCoultre, 2001).
Certain plants can accumulate heavy metals in their tissues and uptake increases
generally in plants that are grown in areas with increased soil contamination
with heavy metals and therefore, many people could be at risk of adverse health
effects from consuming common garden vegetables cultivated in contaminated soil
One of the major anthropogenic sources of heavy metal enrichment in terrestrial habitats in Niger Delta of Nigeria is oil spillage. Oil spillage and dumping of petroleum effluents on land are common phenomena. Gas flaring also contributes to heavy metal contamination of soil.
The aims of this study are to:
||Determine the levels of heavy metal concentrations in soils
impacted with petroleum and non-petroleum activities in relation to accumulation
of heavy metals in cassava tubers and plantain fruits obtained from both
||Compare the results obtained to internationally accepted limits
||Highlight the effects of consuming such contaminated tubers
MATERIALS AND METHODS
The study was conducted in November 2007 at two sites of cultivated land
in Olomoro, Isoko South Local Government Area of Delta State, Nigeria (Fig.
1). The area marks the geological boundary of the Sombreiro-Warri formation
and the meader belts of the upper deltaic plains of the Niger Delta (Short
and Stauble, 1967). The Sombreiro-Warri formation has been described by
Allen (1965) as older sands of the Niger Delta comprising
massive, generally fine to medium grained and fairly sorted but consolidated
The two study sites fall within the tropical climate characterized by rainy
and dry seasons (Ofune, 1979; Hare
and Carter, 1984).
Site 1 is a farm land by the side of petroleum activities. It lies between latitude 5o 29 29N and longitude 6°C 10 23 E. There are two flow stations, gas compressor and many oil wells nearby. The site lies by a tarred road and traffic is relatively low. Cultivated plants included plantain, cassava and maize.
Site 2 lies between latitude 5°C, 25 31.1 and longitude 6°C 08 4N. It is a farmland in which cassava and plantain are cultivated. It lies by a tarred road. There are few inhabited houses. The site is opposite a secondary school and is about 5.30 km away from site 1. Traffic is relatively low and petroleum activities are completely absent.
Collection of Samples
Four random composite soil samples were collected from 20 cm depth into
clean polythene bags and kept in a cooler.
Cassava Tubers and Plantain Fruits
Three cassava tubers were uprooted manually while three plantain fruits
were plucked from three parent plants at both sites. All samples were kept in
a cooler and carried to laboratory where they were preserved in refrigerator
at a temperature of less than 4°C until the analysis were carried out.
Soil Sample Preparation
Soil samples were homogenized with a clean glass rod and oven-dried at 85°C
to constant weight. Any lump present was broken up with a clean glass rod in
order to expose the inside for drying and all plant materials were removed.
|| (A) Map of Nigeria showing Delta State; (B) Map of the study
area showing River Areba
The oven dried soil sample was thoroughly ground into powdery form and sieved
through 650 μm stainless sieve.
Extraction and Analysis of Heavy Metals
Ten grams of the sieved dry soil was weighed into acid-washed 250 mL polythene
extraction bottles and 100 mL of extraction solution (0.05 M HCL and 0.0125
M HNO3) was added and shaken for 1 h on a mechanical shaker. The
solution was filtered through Whatman No 42 filter paper. Blank samples were
prepared using the same procedure with deionised water instead of soil solution.
Analysis of Cd, Zn, Cu, Pb, Mn, Co, Ni, Cr and V was carried out with Unican
929 atomic absorption spectrophotometer fitted with solar software. Mercury
and As were analyzed by Cold Vapour Technique (Goldwater,
Extraction and Analysis of Total Petroleum Hydrocarbons
Five grams of the soil was weighed into a clean 50 mL glass vial with a
teflon cap. One gram of anhydrous sodium sulphate was added to remove moisture
and the mixture was thoroughly mixed with a clean spatula. Twenty milliliter
of tetrachloroethylene was added to the mixture, corked and properly shaken
to ensure thorough mixture. The cork was removed and the mixture was placed
in a water bath sonicator and sonicated for 30 min. The mixture was filtered
through a glass wool. Additional 10 mL of tetrachloroethylene was added and
the process was repeated as above to extract any minute oil left. One gram of
deactivated florosil was added to remove all polar compounds (that might interfer
with TPH determination) and filtered through a 0.45 μm solvent resistant
membrane filter to remove all particulate matter. Tetrachloroethylene was scanned
as a blank. The filtered solution was analyzed using Fourier Transformed Infrared
(FTIR) Spectrophotomer fitted with Winfirst software and values were expressed
in mg kg-1. Tetrachloroethylene was scanned as a blank.
Extraction and Analysis of Heavy Metals in Plantain Fruits and Cassava Tubers
Plantain and Cassava
The plantain and cassava samples were brought out of refrigerator and kept
in clean polythene bags and allowed to attain room temperature. The epicarps
of plantain fruits and piliferous layers of cassava tubers were carefully removed.
The epicarps and mesocarps of plantain, piliferous layers and cortex of cassava
were cut into pieces and oven dried at 85°C to constant weight. The dried
samples were ground into powdery form and labeled.
One gram of each ground sample was weighed into 100 mL beaker. Five milliliter concentrated nitric acid and 2 mL perchloric acid were added and heated in a fume cupboard to almost dryness. Then, 10 mL of deionised water was added and the solution was properly, stirred and filtered with Whatman filter paper No. 42. Blank samples were prepared in the same procedure with deionised instead of plantain or cassava sample. The filtrate of each sample was aspirated into the flame of AAS along with standard solution.
The Total Petroleum Hydrocarbons (TPH) value at site 1 was 5251.99 mg kg-1 compared to the below detection level in Site 2. All heavy metals showed higher values in Site 1. Cadmium (Cd), Ni, As and Hg were below detection levels in both sites while in Site 2, Cd, Pb, Ni, Cr V, As and Hg were also below detection levels. All heavy metals detected recorded higher values in site 1 soil than site 2 soil (Table 1).
The order of abundance of heavy metals in petroleum impacted soil was Fe>Zn>Mn>Cu>Co>V>Pb while in non-petroleum soil it was Fe>Zn>Mn>Cu>Co.
Heavy metals below detection levels both in epicarp and endocarp were Cd, Cr, V, As and Hg. In both sites, Zn was not detected in the mesocarp while Ni was not detected in the epicarp but bioaccumulated in the mesocarp. Higher values were recorded in epicarp than in endocarp in samples of both sites. In both samples, Fe was the most abundant heavy metal both in epicarp and endocarp (Table 2).
Table 3 shows the values of heavy metals in cassava. Cadmium, Cr, V, As and Hg were below detection levels in samples from both sites. The values of other heavy metals were higher in the piliferous layer than cortex except Zn, Pb and Ni that were higher in the cortex than piliferous layer. In Site 1, Cu had equal value in the piliferous layer and cortex.
The concentrations of all the metals, except iron were higher in the mesocarp and epicarp of plantain fruits from the contaminated site than the levels found in fruits from control site. Cassava tubers from the contaminated site also had higher concentrations of heavy metals in the piliferous layer and cortex, except zinc which was slightly higher in the piliferous layer of tubers from the control site (Fig. 2a-d).
|| Concentrations of TPH and heavy metals (mg kg-1)
|| Accumulation of heavy metals in epicarp and mesocarp of plantain
|| Accumulation of heavy metals in piliferous layer and cortex
of cassava tubers
||Comparison of heavy metal accumulation in the mesocarp and
epicarp of Plantain fruits and the piliferous layer and cortex of Cassava
tubers; (a) Mesocarp, (b) Epicarp, (c) Piliferous and (d) Cortex
Heavy metals have been found in food crops and have a potential health hazards
to man through the dietary pathway in Nigeria (Obiajunwa
et al., 2001). The Total Petroleum Hydrocarbons (TPH) at Site 1 (area
of petroleum activities) was 5251.99 mg kg-1 while the below detection
level was recorded for site 2 (an area of non-petroleum activities). This high
value could be attributed to the operation of petroleum activities. Flow stations,
gas compressor, giant electric generators, flaring of gas and numerous oil wells
are associated with site 1, an abandoned dry burrow pit probably receiving wastes
arising from petroleum activities. Rain water runoff could also have carried
spilled crude oil and petroleum effluents to the burrow pit during the rainy
season. The absence of petroleum hydrocarbons in site 2 (Grammar School Area)
which is 5.30 km away from site 1 is a confirmation of petroleum activities
being responsible for the value recorded at site 1. Pollution of the groundwater
with petroleum hydrocarbons and heavy metals is a possibility since the soil
of Olomoro is mainly composed of sand (Allen, 1965).
The soil also lacks clay layer above the aquifer (Egwebe,
Petroleum renders the soil infertile, burns vegetation and kills useful soil
organisms (Idodo-Umeh, 2004). The bioaccumulations of heavy
metals in plantain fruits and cassava tubers were generally higher at site 1,
than at site 2. This is expected since any area near petroleum activities has
higher level of pollutants including heavy metals (Woodward
and Riley, 1983). Uptake of heavy metals is increased in plants that are
grown in areas with increased soil concentrations of heavy metals (LeCoultre,
2001). Heavy metal concentrations were generally higher in the epicarps
of plantain than in the mesocarps at both sites 1 and 2 except Ni in site 1
and Cu in site 2. This phenomenon could be explained in two ways. It seems,
the epicarp is a storage site or is the route through which metals enter the
mesocarp since it is exposed to the atmosphere. The atmosphere is a major source
of metal pollution (Forstner and Wittman, 1981; National
Water and Soil Conservation, 1985). Cr, V, As and Hg were below detection
levels in both plantain fruits and cassava tubers, indicating non-absorption
and contamination from anthropogenic source.
In cassava, the values of heavy metal bioaccumulated both in piliferous layer
and mesocarp were higher at site 1 than at site 2 except Ni that recorded higher
values at site 2, suggesting that absorption and bioaccumulation depend upon
availability of metals. Metals tend to bioaccumulate in animals and plants (Melville
and Burchett, 2002). Anthropogenic origin of petroleum wastes, crude oil
and continuous gas flaring in site 1 could have possibly enriched the soil with
heavy metals than site 2.
Heavy metal uptake from soil is generally increased in plants that are grown
in areas with increased heavy metal concentrations (LeCoultre,
2001). Anthropogenic sources of heavy contaminants such as burning of fossil
fuel, oil spills and use of fertilizers are likely the cause of higher heavy
metals contaminants in soil (Woolson, 1973; Boon
and Soltanpour, 1992; Manahan, 1994; Ademoroti,
1996; Cobb et al., 2000; LeCoultre,
2001). Many investigators have reported that some common garden vegetables
accumulate high levels of heavy metals from soil (Garcia
et al., 1979; Xiong, 1998; Cobb et al.,
2000; LeCoultre, 2001). Species of some Brassica
(cabbage) are hyperaccumulators of heavy metals in the edible tissues of
the plant (Xiong, 1998) and thus creating a pathway for heavy metal accumulation
for people who consume the vegetables and fruits grown in heavy metal contaminated
soil (LeCoultre, 2001).
The values of Zn (3.39), Pb (1.17-2 .44) in the epicarp of plantain fruits and Zn (4.385-5.955) in the cortex and piliferous layer respectively and Pb (1.75) in the cortex of cassava were higher than the values recorded in soils, indicating hyperaccumulation. Therefore, plantain fruits and cassava tubers can be used as bioindicators of soil pollution.
The levels of heavy metals in farm soil are generally not analyzed before planting
and therefore consumption of contaminated fruits, seeds or tubers is common
(LeCoultre, 2001). Heavy metal concentrations in tree fruits
are very low even when grown on contaminated soils (ISHS,
2005). However, the levels of Fe (4.865 mg kg-1) and Mn (0.65
mg kg-1) in mesocarp of plantain fruits and Fe (3.92 mg kg-1)
and Mn (1.395 mg kg-1) in the cortex of cassava grown in the soils
impacted with petroleum activities call for serious concern as these heavy metals
exceeded WHO (1984) maximum acceptable limits for food.
Although the value of Pb (1.75 mg kg-1) in the cortex of cassava
in site 1 (petroleum impacted soil) was below WHO limit (2 mg kg-1)
for food, threat to Pb poison is likely.
Garri which is a product of cassava is consumed daily by Nigerians in large
quantities. Therefore, Pb concentrations could be cumulative. Pb can easily
cross the placenta and damage the foetal brain and may also cause development
of autoimmunity, in which a persons immune system attacks its own cells
leading to diseases such as rheumatoid arthritis, diseases of kidneys, nervous
system and circulatory system (Casarett and Doull, 1996).
The populations most affected by consumption of contaminated farm products are
pregnant women and young children (Boon and Soltanpour,
1992; LeCoultre, 2001). Neurological disorders, central
nervous system destruction and cancers of various body organs are some of the
reported effects of heavy metal disorders (Luckey and Venugopal,
1978; Forstner and Wittmann, 1981; Manahann,
1994; Casarett and Doull, 1996; Van
Vuren and Nussey, 1999). Low birth weight and severe mental retardation
of newborn children have been reported in some cases where pregnant mothers
ingested toxic amounts of heavy metals (Mahaffey et al.,
1981). Soils contaminated with Pb have been reported to decrease the growth
and yield of plants (Balba et al., 1991).
This study is a pioneer work on heavy metal accumulation by edible food crops
in this part of the country ravaged by oil pollution activities. All heavy metals
showed higher values in petroleum impacted soil than non-impacted soil. The
heavy metal concentrations in cassava tubers and plantain fruits grown in petroleum
impacted soil were significantly higher than the levels recorded in the same
crops grown in non-impacted soils. The levels of Fe and Mn in mesocarp of plantain
fruits and Fe and Mn in the cortex of cassava grown in the soils impacted with
petroleum activities call for serious health concern as these heavy metals exceeded
WHO (1984) maximum acceptable limits for food. The reported
hyperaccumulation of heavy metals in plantain fruits and cassava tubers indicates
the potential of these plants as bioindicators of soil pollution. Further studies
on the socio-economic and health status of the neighbouring communities will
throw more light on the overall impact of petroleum activities.