PYROGENIC POLYCYCLIC AROMATIC HYDROCARBONS IN SEDIMENTS FROM
A RESERVOIR IN NIGERIA
WANGBOJE, O.M.
DEPARTMENT OF FISHERIES, UNIVERSITY OF BENIN, P.M.B. 1154, BENIN CITY, NIGERIA.
E-mail: sojeapex@yahoo.com Tel: +234 802 3544500
ABSTRACT
The concentrations of some pyrogenic Polycyclic Aromatic Hydrocarbons (PAHs) in sediments
from the Ikpoba reservoir, Benin City, Nigeria, were evaluated by Gas Chromatographic
technique, in order to ascertain the impact of these organic pollutants on the investigated
ecosystem. The mean concentrations in µg/g wet weight of 1-methynapthalene, acenaphthylene,
acenaphthene, fluorene, pyrene, benzo(a)anthracene and chrysene were 0.013, 0.24, 0.013,
0.014, 0.014, 0.0025 and 1.36 respectively. The mean concentration of 1-methynapthalene was
not significantly different (P>0.05) between the stations while a similar trend was observed for
chrysene. The Probable Effect Concentration (PEC) for PAHs revealed that all the detected
PAHs showed a potential toxicity risk to fishes in the reservoir with the exception of chrysene.
The Hazard Quotient (HQ) values were all below unity indicating that the possibility of these
PAHs being contaminants of potential ecological concern was low. The PAH content of
sediment was attributed mainly to the incomplete combustion of fossil fuels from motor vehicles.
It was recommended that a sustained surveillance of the Ikpoba reservoir should be carried out
by the Ministry of Environment in order to ensure that PAH levels do not reach critical limits.
Key words: Polycyclic Aromatic Hydrocarbons, sediments, Ikpoba reservoir, Nigeria.
1
INTRODUCTION
Essentially, Polycyclic Aromatic Hydrocarbons (PAHs) are organic compounds that are formed
through the combination of two or more benzene (aromatic) rings. It has been recognised that
environmental contamination by these compounds has steadily increased in recent years around
the world thus raising concern amongst scientists and the public (Van Metre et al., 2000).
Members of this class of compounds have been identified as exhibiting toxic properties and for
this reason the United States Environmental Protection Agency (USEPA) has recommended their
monitoring in environmental media (National Research Council, 1997). Furthermore, it has been
recognised that PAHs are of great concern for aquatic biota because of their carcinogenic,
teratogenic, mutagenic properties and their ability to biotransfer along food webs (Wangboje,
2013). Contributing to the PAH burden of inland waters in Africa are population explosion, rapid
urbanization and industrialization (Calamari and Naeva, 1994). Polycyclic Aromatic
Hydrocarbons are lipophilic, possess low solubility and volatility hence they are able to
concentrate to harmful levels in aquatic environments through biomagnifications and
biogeochemical processes (Duke, 2008). In addition, the lipophilic nature of PAHs enable them
to easily accumulate in aquatic organisms including fin fish and shell fish thus posing potential
health risks to man who may eventually consume such contaminated food (Ndiokwere, 2004).
The physical and chemical characteristics of PAHs vary with molecular weight and PAH
resistance to oxidation, reduction and vapourization increases with increasing molecular weight
whereas the aqueous solubility of these compounds decreases. As a result, PAHs differ in their
behavior, distribution in the environment and their effects on biological systems (United States
Department of Health and Human Services, 2002). It has been reported that sediment dwelling
organisms are more likely to be adversely affected by PAHs as the sediment acts as a sink for
2
organic pollutants including PAHs (Environment Australia Technical Report, 1999). Polycyclic
Aromatic Hydrocarbons (PAHs) never occur as individual compounds in the environment as
they occur as a mixture of many other PAHs which may compound their toxicological effects on
the environment (Mottier et al., 2000). The main sources of PAHs in the environment include
forest fires, natural petroleum seepage, combustion of fossil fuel, coal burning and the use of oil
for cooking and heating. Other sources include domestic and industrial waste waters and sewage
(Dhananjayan and Muralidharan, 2012). Although the extent of accumulation of PAHs in
sediment is affected by such environmental factors as organic matter, structure and particle size
distribution, microbial population, presence of toxic compounds, the physical and chemical
properties of the PAHs, they may often be found near emission sources (Wilson and Jones,
1993). The present study was conducted on the Ikpoba reservoir which is located within Benin
City, Edo state, southern Nigeria. The reservoir was constructed for the supply of drinking water
to inhabitants of the City and environs and also serves as a source of valuable fisheries resources.
The reservoir is impacted by effluents from surrounding communities which has necessitated the
conduct of several ecotoxicological studies (Oronsaye et al., 2010; Wangboje and Oronsaye,
2012; Wangboje and Ekundayo, 2013; Wangboje and Oronsaye, 2013). However, the
aforementioned studies dwelt exclusively on heavy metals. This study was thus designed to
investigate the PAH profile of the reservoir using sediment as the test environmental matrix in
order to fill the gap in knowledge regarding the contaminated status of this ecosystem. Sediments
are repositories for both organic and inorganic contaminants and are thus essential components
of ecological risk assessments as they can indicate long-term impact of chemical contaminants in
aquatic ecosystems.
3
MATERIALS AND METHODS
The study area
The study area is gridlocked between Latitude 6.5° N and Longitude 5.8° E in Benin City,
Nigeria and falls within the Tropical Rain Forest Belt. Details of the study area have been
described extensively by Wangboje and Ekundayo (2013) while the geomorphologic
characteristics of the same area have been described in detail by Asuen and Oronsaye (1990).
Sediment samples were collected with an Eckman grab sampler from four stations established
within the reservoir namely, Okhoro station, Midpoint station, Low lift pump station and Ekiuwa
station. The Okhoro and Ekiuwa stations are adjacent to the Okhoro and Ekiuwa communities of
the City respectively while the Midpoint station is approximately equidistant between the
Okhoro and Low lift pump stations. It is at the Low lift pump station that water is pumped out
for treatment by the State Urban Water Board. Sediment samples were wrapped in aluminum foil
to avoid contamination and thereafter placed in labeled black polythene bags. The bags were
placed in an ice chest and transferred to the laboratory within 24 hours where they were deep
freezed at -40 C. Sediment samples were collected fortnightly in January, March, May, July and
September, 2006.
Analysis of Sediment for Polycyclic Aromatic Hydrocarbons (PAHs)
Analysis of sediment samples for Polycyclic Aromatic Hydrocarbons (PAHs), was done
employing standard procedures as recommended by the American Public Health Association
(1998). The following specific procedures were applied.
4
Soxhlet Extraction
Fifteen (15) grams of thawed sediment sample was homogenized in a mortar with 15 g of Na
SO4 until a completely dry homogenate was obtained. The apparatus consisted of a 250 ml round
bottom flask, condenser and extractor tube, seated in a temperature controlled heating mantle.
A Fisher brand rotary evaporator (rotovap) was used for evaporating the extracts to the desired
concentrations. The homogenate was carefully transferred into the extraction thimble and placed
in the extraction chamber of a Soxhlet extraction unit. Soxhlet extractions were carried out using
180 ml dichloromethane for 15 hours. The extract was concentrated in the rotovap.
Post – Extraction Cleanup
The extract was concentrated to 1 ml and loaded onto a silica gel column. The silica gel column
was prepared by loading an activated silica gel onto a chromatographic column to about 5cm.
1ml of anhydrous NaSO4 was added to the column and conditioned with methyl chloride. 1 ml of
concentrated extract was loaded into the column to which was added 10 ml of methyl chloride.
Prior to analysis, 0.5 µg of four internal standards were added to each of the sample extract and
the volume reduced to 1ml.
Analysis
The final extracts were packed into 2 ml Gas chromatography vials and analyzed with a Gas
chromatograph/ Mass spectrometer system (Buck Scientific Model 910).The Gas chromatograph
was equipped with a fused silica capillary column and helium was used as the carrier gas. The
column head pressure was maintained at 10 psi to give a flow rate of 1ml/minute. The column
temperature was initially held at 70o C for 4 minutes, ramped to 300 o C at a rate of 10 o C /minute
then held at 300 o C for 10 minutes. The mass spectrometer was used in electron ionization mode
5
and all spectra were acquired using a mass range of 50 – 400 MHz and Automatic Gain Control
(AGC). Concentrations of PAHs were expressed in µg/g wet weight.
Determination of Total Organic Matter in Sediment
The total organic matter in sediment was determined through percentage loss on ignition as
described by Agboola (1986). One (1) g of sediment sample was oven dried and put into a preweighed crucible. The crucible was placed in a muffle furnace (Carbolite CWF 1300 Series) and
the temperature was allowed to rise slowly to 55o C and left for 2 hours. The crucible was
transferred to a desiccator, cooled to room temperature and reweighed. The % loss on ignition
was calculated from the weight loss during combustion using the equation below.
Loss on ignition (%) = Weight loss (g) x 100/ Oven dry weight (g)
Computation of Hazard Quotient (HQ) for Polycyclic Aromatic Hydrocarbons
The Hazard Quotient (HQ) as expressed below indicates the possibility of a contaminant being
an ecological risk or a contaminant of potential ecological concern (Purchase, 2000).
HQ= Measured concentration of contaminant/Toxicity reference value or selected screening
benchmark.
HQ ≥ 1 = Possibility of ecological risk indicated or a contaminant of potential ecological concern
Computation of Probable Effect Concentration (PEC) for Polycyclic Aromatic
Hydrocarbons
The Probable Effect Concentration (PEC) for PAHs as expressed below was computed based on
the procedure as described by Ingersoll et al., (2002).
PEC = Concentration of PAH in sediment/ % Total Organic Matter in sediment (TOM)
PEC Quotient (PECQ) = Concentration of PAH in sediment/ PEC for PAH
6
Mean PECQ = Sum of Individual Quotients/Number of PECs evaluated
Where mean PECQ>PEC = Toxicity is indicated
Computation of Total Polycyclic Aromatic Hydrocarbon values in sediment
In order to compare the total Polycyclic Aromatic Hydrocarbon content at the different stations,
the Total PAH value of individual PAHs at each station was computed as expressed below
(Usero, et al., 1996).
P1 + P2 + P3................+ Pk = Total PAH for a particular station.
Where: P1, P2, P3.......Pk are the individual PAH values at the station.
Statistical Analysis
Data generated from the study were subjected to one-way Analysis of Variance (ANOVA) to
determine significant differences between mean values of PAHs at the stations at 5 % level of
significance. Significant means were separated with the Duncan Multiple Range Test. A
Genstat® computer software (Version 8.1 for Windows) was used for statistical analysis.
RESULTS
As presented in Table 1, in January 2006, the mean concentrations of Polycyclic Aromatic
Hydrocarbons (PAHs) in sediment ranged from 0.0024 µg/g for benzo(a)anthracene at the
Midpoint station to 1.781 µg/g for chrysene at the Low lift pump station. There were no
significant differences (P>0.05) in the mean concentrations of 1-methynapthalene between
stations. As shown in Table 2, in March 2006, the mean concentrations of PAHs ranged from
0.0023 µg/g for benzo(a)anthracene at the Midpoint station to 1.528 µg/g` for chrysene at the
Okhoro station. There were no significant differences (P>0.05) in the mean concentrations of 17
methynapthalene and chrysene between stations. As presented in Table 3, in May 2006, the mean
concentrations of PAHs ranged from 0.0016 µg/g for benzo(a)anthracene at the Ekiuwa station
to 1.451 µg/g` for chrysene at the Low lift pump station. There were no significant differences
(P>0.05) in the mean concentrations of 1-methynapthalene, acenaphthene and chrysene between
stations. As shown in Table 4, in July 2006, the mean concentrations of PAHs ranged from
0.0013 µg/g for benzo(a)anthracene at the Midpoint station to 1.345 µg/g` for chrysene at the
Okhoro station. There were no significant differences (P>0.05) in the mean concentrations of 1methynapthalene, acenaphthene, fluorene and chrysene between stations. As presented in Table
5, in September 2006, the mean concentrations of PAHs ranged from 0.0011 µg/g for
benzo(a)anthracene at the Midpoint station to 1.234 µg/g` for chrysene at the Low lift pump
station. There were no significant differences (P>0.05) in the mean concentrations of 1methynapthalene, acenaphthene, fluorene and chrysene between stations. The mean PECQ was
greater than the individual PECs for the PAHs with the exception of chrysene as shown in Table
6. The highest mean concentration (1.36 µg/g) amongst the PAHs in the reservoir was recorded
for chrysene while the lowest mean concentration (0.0025 µg/g) was recorded for
benzo(a)anthracene (Fig. 1). In this study, the HQ values ranged from 0.000005 for
benzo(a)anthracene to 0.00272 for chrysene (Fig. 2). The Total PAH values in sediment ranged
from 1.84 µg/g at the Ekiuwa station to 2.32 µg/g at the Low lift pump station (Fig. 3).
8
Table 1: Mean Polycyclic Aromatic Hydrocarbons (PAHs) concentrations (µg/g ) in sediment
among stations in January, 2006.
PAHs
Okhoro
Midpoint
Low lift
Ekiuwa
station
station
pump
station
station
1-methynaphthalene
0.016a
0.013a
0.015a
0.0142a
Acenaphthylene
0.275a
0.243b
0.254a
0.248b
Acenaphthene
0.016a
0.014b
0.0165a
0.0141b
Fluorene
0.0186a
0.015b
0.0183a
0.0154b
Pyrene
0.0181a
0.0155b
0.0175a
0.0152b
Benzo(a)anthracene
0.0044a
0.0024b
0.0042a
0.0026b
Chrysene
1.775a
1.632b
1.781a
1.539b
Means with similar superscripts on the same row are not significantly different at 5% level of significance.
9
Table 2: Mean Polycyclic Aromatic Hydrocarbons (PAHs) concentrations (µg/g ) in sediment
among stations in March, 2006.
PAHs
Okhoro
Midpoint
Low lift
Ekiuwa
station
station
pump
station
station
1-methynaphthalene
0.017a
0.015a
0.015a
0.0144a
Acenaphthylene
0.268a
0.241b
0.255a
0.232b
Acenaphthene
0.015a
0.013b
0.0166a
0.0135b
Fluorene
0.0177a
0.011b
0.0176a
0.0132b
Pyrene
0.0176a
0.0143b
0.0174a
0.0146b
Benzo(a)anthracene
0.0036a
0.0023b
0.0039a
0.0024b
Chrysene
1.528a
1.437a
1.512a
1.521a
Means with similar superscripts on the same row are not significantly different at 5% level of significance.
10
Table 3: Mean Polycyclic Aromatic Hydrocarbons (PAHs) concentrations (µg/g ) in sediment
among stations in May, 2006.
PAHs
Okhoro
Midpoint
Low lift
Ekiuwa
station
station
pump
station
station
1-methynaphthalene
0.013a
0.014a
0.011a
0.0141a
Acenaphthylene
0.254a
0.228b
0.246a
0.232b
Acenaphthene
0.014a
0.013a
0.013a
0.0135a
Fluorene
0.0162a
0.012b
0.0164a
0.014b
Pyrene
0.0165a
0.0135b
0.0156a
0.0131b
Benzo(a)anthracene
0.0033a
0.0018b
0.0035a
0.0016b
Chrysene
1.426a
1.314a
1.451a
1.414a
Means with similar superscripts on the same row are not significantly different at 5% level of significance.
11
Table 4: Mean Polycyclic Aromatic Hydrocarbons (PAHs) concentrations (µg/g ) in sediment
among stations in July, 2006.
PAHs
Okhoro
Midpoint
Low lift
Ekiuwa
station
station
pump
station
station
1-methynaphthalene
0.011a
0.012a
0.013a
0.013a
Acenaphthylene
0.232a
0.217b
0.225a
0.211b
Acenaphthene
0.013a
0.011a
0.010a
0.012a
Fluorene
0.015a
0.013a
0.014a
0.011a
Pyrene
0.0143a
0.0124b
0.0136a
0.0126b
Benzo(a)anthracene
0.0025a
0.0013b
0.0027a
0.0014b
Chrysene
1.345a
1.267a
0.679a
0.894a
Means with similar superscripts on the same row are not significantly different at 5% level of significance.
12
Table 5: Mean Polycyclic Aromatic Hydrocarbons (PAHs) concentrations (µg/g ) in sediment
among stations in September, 2006.
PAHs
Okhoro
Midpoint
Low lift
Ekiuwa
station
station
pump
station
station
1-methynaphthalene
0.017a
0.011a
0.015a
0.012a
Acenaphthylene
0.233a
0.237a
0.237a
0.225a
Acenaphthene
0.011a
0.011a
0.012a
0.013a
Fluorene
0.014a
0.012a
0.012a
0.013a
Pyrene
0.015a
0.013b
0.014a
0.011b
Benzo(a)anthracene
0.0022a
0.0011b
0.0023a
0.0012b
Chrysene
1.224a
1.231a
1.234a
1.218a
Means with similar superscripts on the same row are not significantly different at 5% level of significance.
13
Table 6: Probable Effect Concentration (PEC) for Polycyclic Aromatic Hydrocarbons
PAHs
Conc.
% TOM*
PEC
PECQ
(µg/g)
1-
Mean
PECQ
0.013
0.57
0.02
0.65
0.64
Acenaphthylene
0.24
0.57
0.40
0.60
0.64
Acenaphthene
0.013
0.57
0.02
0.65
0.64
Fluorene
0.014
0.57
0.02
0.70
0.64
Pyrene
0.014
0.57
0.02
0.70
0.64
Benzo(a)anthracene
0.0025
0.57
0.004
0.62
0.64
Chrysene
1.36
0.57
2.26
0.60
0.64
Methylnaphthalene
*Mean TOM value for the study period (Station means: Okhoro: 0.57; Midpoint: 0.56; Low lift
pump: 0.58; Ekiuwa: 0.55).
14
Fig. 1: Mean Polycyclic Aromatic Hydrocarbons (PAHs) concentrations (µg/g ) in sediment of Ikpoba reservoir for study period
15
Fig. 2: Hazard Quotient (HQ) values for Polycyclic Aromatic Hydrocarbons in sediment of Ikpoba reservoir
16
Fig. 3: Total Polycyclic Aromatic Hydrocarbon content in sediment by station for the study period
17
DISCUSSION
The United States Environmental Protection Agency (USEPA), has categorized seven PAH
compounds as probable human carcinogens viz; benzo(a)anthracene, benzo(a)pyrene,
benzo(b)fluoranthene, benzo(k)fluoranthene, chrysene, dibenz(a,h)anthracene and indeno(1,2,3cd)pyrene (USEPA, 1992). In this study, two PAHs belonging to this class (benzo(a)anthracene
and chrysene) were detected in sediment. Benzo(a)anthracene and chrysene can exhibit positive
genotoxicity in humans as reported by the International Agency for Research on Cancer of the
World Health Organization (WHO, 1998). The highest mean concentrations of PAHs was
recorded for chrysene while the lowest mean concentration was recorded for benzo(a)anthracene
across all the sampled months. This trend was observed in both the dry months (January and
March) and wet months (May, July and September).This finding is a clear indication of a
generally higher influx of chrysene into the reservoir and on the other hand a lower influx of
benzo(a)anthracene during the study period. Possible entry routes of PAHs into aquatic media
include run-off from catchment areas and atmospheric/aerial deposition (Ndiokwere, 2004). In
the case of 1-methynapthalene, the mean concentration of this PAH was not significantly
different (P>0.05) between the stations across the sampled months indicating that there was a
steady input of this particular PAH into the reservoir during the study period. A similar trend was
observed for chrysene. It was observed that the lowest mean concentrations of
benzo(a)anthracene were all recorded at the Midpoint station while the highest mean
concentrations of chrysene were generally recorded at the Low lift pump and Okhoro stations.
This finding would imply that the Midpoint station is obviously least impacted by
benzo(a)anthracene while the Low lift and Okhoro stations are more impacted by chrysene. In
addition the total PAH values computed for this study revealed that generally the Ekiuwa station
18
was least impacted by PAHs while the Low lift pump station was more impacted. This finding
may be attributed to the higher organic matter content of sediment at the Low lift pump station
which could have served as a repository for PAHs. This assertion is based on the fact that PAHs
being organic pollutants have a natural affinity for organic matter in sediment as corroborated by
Page et al., (1999).The Probable Effect Concentration (PEC) for PAHs revealed that all the
detected PAHs presented a potential toxicity risk to fishes in the reservoir as the mean PECQuotient exceeded the individual PEC for the PAHs with the exception of chrysene. Chrysene
therefore does not present an immediate risk to fishes in the reservoir in terms of toxicity. The
Hazard Quotient (HQ) values computed in this study, were all below unity indicating that the
possibility of these PAHs being contaminants of potential ecological concern is low but could
change if present PAH concentrations in sediment increase. The World Health Organization’s
(WHO) recommended background concentration of 0.5 µg/kg for benzo(a)pyrene in food
samples was used for computing HQ values in this study. All the mean values of PAHs obtained
in this study fell below the aforesaid WHO benchmark (WHO, 2004), indicating that the present
levels of these PAHs in sediment are still low enough not to cause serious impact on man who
may consume fish containing these organic pollutants. This assertion is based on the fact that
sediments are capable of releasing domiciled PAHs into the overlying water column which may
eventually become bioavailable to fish and other aquatic organisms (Adams et al., 1992). The
PAH profile of the sediment revealed that the mean concentrations of PAHs ranged from 0.0025
µg/g for benzo(a)anthracene to 1.36 µg/g for chrysene. In comparison, Duke (2008), reported
mean concentrations of PAHs in sediment from the Ekpan creek in Warri, Delta State, Nigeria,
ranging from 0.0447 mg/kg for acenaphthene to 1.295 mg/kg for benzo(a)anthracene and
attributed the PAH content of sediment to pyrogenic sources because phenanthrene, anthracene,
19
flouranthene, pyrene, chrysene and benzo(a)anthracene were detected. In another study Okafor
and Opuene (2007), reported mean concentrations of PAHs in the sediment of Taylor creek in
Bayelsa State, Nigeria, ranging from 0.00146 to 0.001610 mg/kg for anthracene and attributed
their finding to anthropogenic impact. In the United States of America, Wicker and Gantt (1994),
reported mean concentrations of PAHs in sediment of the Lower Pamlico river, North Carolina,
ranging from 0.01 mg/kg for benzo(b)fluoranthene to 0.05 mg/kg for fluoranthene and attributed
the PAH content of the river to effluents from a phosphate mining site in the vicinity. The PAH
content of sediment in Ikpoba reservoir is thus attributed to pyrogenic sources as evidenced by
the presence of fluorene, pyrene, chrysene and benzo(a)anthracene. This may be attributed
mainly to the incomplete combustion of fossil fuels especially by motor vehicles as the reservoir
is located within the busy Benin metropolis which is characterized by heavy vehicular traffic.
CONCLUSION
The study detected the presence of some pyrogenic Polycyclic Aromatic Hydrocarbons (PAHs)
in sediment of the Ikpoba reservoir, which included two probable human carcinogens i.e.
benzo(a)anthracene and chrysene. The ecological risk indices employed in this study revealed
that presently, the threat posed by the PAHs is low. However, caution must be exercised by way
of a sustained surveillance of the Ikpoba reservoir by relevant regulatory authorities such as the
Ministry of Environment in order to ensure that PAH levels do not reach critical limits. Such
surveillance should be carried out on other environmental media such as water and bioindicators
in order to ascertain the actual and potential impacts of these organic pollutants on this
ecosystem.
20
ACKNOWLEDGEMENTS
I am most grateful to the University of Benin for part-sponsorship of the study. I wish to thank
Management of the Edo State Environmental Laboratory for assisting with the analysis of
sediment samples.
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