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. 2023 Apr 15;9(4):e15508. doi: 10.1016/j.heliyon.2023.e15508

Preliminary investigation of polycyclic aromatic hydrocarbons (PAHs) concentration, compositional pattern, and ecological risk in crude oil-impacted soil from Niger delta, Nigeria

Oluwabamise L Faboya a,∗, Samuel O Sojinu b, Joseph O Otugboyega c
PMCID: PMC10161696  PMID: 37151689

Abstract

Crude oil contamination could serve as an important source of polycyclic aromatic hydrocarbons (PAHs) in the environment. Determining the concentration and distribution of PAHs and their ecological risk could provide clues for appropriate remediation. The present study investigated the concentrations, composition pattern, and ecological risk of PAHs in crude oil-contaminated soil collected from the Niger Delta, Nigeria. The concentrations of Σ29PAHs and 16 priority US-EPA PAHs (Σ16PAHs) in the soil ranged from 24230.68 to 40845.32 ng/g (average: 29953.47 ng/g) and 7361.66–14141.49 ng/g (average: 9819.96 ng/g), respectively. The concentrations of Σ16PAHs US-EPA in all the studied locations far exceeded the safety value of 1000 ng/g set by the soil quality guidelines of Switzerland and above which is regarded as being highly contaminated for Agricultural soils of Poland. The percentage composition of 2-, 3-,4-, 5-, and 6-ring in the soils were 33.69%, 56.31%, 9.47%, 0.52%, and 0.02% of the total PAHs, respectively, indicating the predominance of low molecular weight PAHs typical of petrogenic origin. Ecological risk assessment indicated a high risk to the soil biota and ecosystem in the studied soil samples as presented by individual PAH and total PAHs. 2-, 3- and 4-ring PAHs in the soil contributed significantly to the ecological risk burden in the soils. This research work provides useful information on the ecological risk associated with crude oil contamination and consequently would assist the government in formulating precise, targeted, and effective remediation measures for soil contaminated with crude oil.

Keywords: Polycyclic aromatic hydrocarbons, Ecosystem, Oil pipeline, Remediation, Niger Delta

1. Introduction

Polycyclic aromatic hydrocarbons (PAHs) are a group of persistent organic pollutants consisting of two or more fused aromatic rings without heteroatoms [1,2]. The occurrence of PAHs in the environment is of concern globally because the PAHs are carcinogenic, mutagenic, and toxic to humans [3,4]. PAHs containing four-to seven-ring are extremely mutagenic and carcinogenic while two- or three-ring PAHs are considered less mutagenic but with the possibility of high toxicity [5]. Many PAHs have been associated with breast, lung, colon cancers, and neuro-, reproductive, and developmental toxicities [6]. PAHs are released into the environments mainly through pyrolytic and petrogenic sources. The pyrolytic PAHs are considered to originate principally from incomplete combustion of fossil fuel and other organic materials while petrogenic PAHs are derived primarily from crude oil and allied products.

Globally, environmental pollution resulting from crude oil and allied products has become a major concern. At least 0.08–0.4% of the internationally produced oil has been estimated to be spilled into the marine ecosystem as pollutants [7]. Nigeria is the 6th largest producer of crude oil in the world and has recorded the highest number of oil spill incidents since 1970 [8]. The Niger Delta region is home to the country’s approximately 37 billion barrels of reserve crude oil and 180 trillion cubic feet of natural gas [9]. It also accommodates major oil refineries and many other manufacturing industries. The region has an intricate and widespread system of pipelines running across the region, thereby leading to huge volumes of oil spill incidences [10]. An estimated average of 1.5 million tons of oil has been spilled into the Niger Delta ecosystem over the past 50 years. Amounts that are 50 times higher than the estimated volume of Exxon Valdez spilled oil in Alaska in 1989 [10]. Ahiamadu et al. [11] reported that the national oil spill detection and response agency (NOSDRA), quoted approximately 7581 crude oil spill incidents between 2008 and 2018 in the Niger Delta region of Nigeria. The region is credited with the largest un-remediated oil-polluted areas in the Gulf of Guinea [8,12,13]. Crude oils are often seen floating on water surfaces and are usually dispersed to shorelines by wind and wave actions, thus affecting the soil environment [14]. Apart from oil spills emanating from pipeline failure or vandalism, other sources of oil spills in the region may be oil well blow-outs, seepages, tanker accidents, and intentional damage to operational facilities [15,16]. These activities have led to the release of different pollutants, including heavy metals and various organic pollutants like PAHs into the environment [11,[17], [18], [19], [20], [21]]. There is a high possibility of PAHs and other organic pollutants building up along the food chain in the Niger Delta. The majority of the residents of the region engage in fish farming; thereby making aquatic dwellers readily available for human consumption. In addition, the region’s location on the globe with several rivers directly emptying into the Atlantic Ocean (Fig. 1) has necessitated the need to monitor the pollution status of the region.

Fig. 1.

Fig. 1

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Map of the study area showing the sampling locations along the oil pipeline and oil flow station in Owaza, Abia State, Nigeria modified after [21].

Soils contaminated with PAHs can pose serious health challenges to humans, livestock, and wildlife, as well as an Ecological risk to the soil microbes, soil functions, plants, the quality of air, and aquatic life [22]. The composition of the PAH mixtures in the environment depends on their sources [22]. However, the original composition of PAHs mixtures could be altered by post-emission transport, transformation, and other processes which are in turn controlled by compound properties, soil properties, vegetation, and other ambient conditions [[22], [23], [24]]. This transformation may practically influence PAHs source identification, abundance, spatial distribution, and risk impact assessment.

Studies on the abundance, sources, composition patterns, and health risk of PAHs in anthropogenic impacted soils from the oil-producing Niger Delta region have been reported by previous authors [11,20,[25], [26], [27]]. However, some of these studies mainly collected samples from the host communities and adjoining communities to the flow station and producing oil field. Moreover, for the current study, samples were collected from locations along the pipeline conveying the crude oil. The choice of these locations was necessitated owing to the frequent pipeline vandalisation by miscreants. Therefore, for a better understanding of PAHs distribution and their immediate ecological risk as a result of crude oil contamination, this study investigated the concentration, distribution, and ecological risk of PAHs in crude oil-contaminated soil in the Niger Delta of Nigeria.

2. Materials and methods

2.1. Sample collection and preparation

Twelve (12) composite oil-contaminated representative soil samples were collected from 3 different locations around oil facilities (L1- N 04°58'49.3, E 007°10'31.5; L2- N 04°58'48.4, E 007°10'32.8; L3 – N 04°58'47.5, E 007°10'34.0) in Owaza, Abia State, Nigeria (Fig. 1) The soils were sampled at 25 × 25 m2 sampling plots with 5–10 sub-samples [28] at a depth of 0–30 cm each. The collected soil samples were wrapped with aluminium foil and then placed in pre-cleaned polyethylene bags after an incident of oil spillage in the area in November 2012. The total bulk mass of soil sample collected from each sample location was roughly 1 kg. The soils were air-dried at room temperature, ground with a mortar, sieved (212-μm), and kept at 4 °C prior to chemical analysis. 20–50 g of finely powdered soil samples were soxhlet extracted using pre-extracted thimbles, with 200 mL of dichloromethane (DCM) for a minimum of 48 h. Surrogate standards (naphthalene-d8, acenaphthene-d10, phenanthrene-d10, chrysene-d12, and benzo [g,h,i]perylene-d12) were added prior to extraction to determine the accuracy of the employed methods. Activated copper was added to remove inorganic sulphur in the extract. The concentrated extracts were fractionated by a glass column packed with silica-alumina (2:1) column chromatography into aliphatic hydrocarbons and PAHs fractions by successive elution with 20 mL n-hexane and 70 mL n-hexane/dichloromethane (7:3 v/v), respectively. The PAH fraction was further concentrated with a rotary evaporator at 30 °C to approximately 1 mL, transferred to a 1.5 mL vial, and blown down to 0.5 mL under a gentle stream of nitrogen.

2.2. Instrumental analysis

GC-MS analyses were performed on a Shimadzu Model 2010 GC-MS (Shimadzu, Japan) with an HP-5MS fused silica column (30 m × 0.25 mm i. d., 0.25 μm film thickness) at the State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, China. Ultrapure Helium was used as a carrier gas. The mass spectrometer was operated in electron impact mode (70 eV). The samples were injected in splitless mode at 100 °C with a purge of 1 min after injection. The following temperature program was used for PAH analysis: the initial temperature was 60 °C and heated to 200 °C at 5 °C/min, then to 250 °C at 2 °C/min and further heated to 280 °C at 10 °C/min (held for 20 min) and finally heated to 290 °C at 10 °C/min (held for 5 min). Identification of the target PAHs was based on the comparison of their GC-retention times and mass spectrum with appropriate reference standards, while data acquisition was made in the selective ion monitoring (SIM) mode. Quantification was performed with a conventional internal calibration method.

2.3. Quality assurance and control

All the chemicals used in this study were of analytical grade with high purity (98+ %). The standards with 98+% purity were purchased from Accustandard Incorporated, New Haven, the USA. Appropriate blanks (Field, laboratory, spiked, and replicate) samples were analysed along with field samples [29]. Quantification was performed using the internal calibration method based on eight-point calibration curves for individual compounds. The average recoveries of the surrogate standards from all the blanks ranged from 65.5% to 99.6%. The average recoveries of the same surrogate standards from the soil samples ranged from 70.5% to 100.5%. All the spiked have surrogate recoveries in the range of 75.0%–120%, the matrix also recorded surrogate recoveries in the range of 72.5%–110.9%.

2.4. Risk quotients

The risk quotients (RQ) method is a useful tool when characterising the risk of PAHs to surrounding organisms and ecosystems [2,30,31]. The RQ method can be used to assess the ecosystem risk of ∑PAHs sensitively and accurately [2]. RQ of PAHs in the soil was calculated using the following equation [2]:

RQ = CPAH/CQV (1)

where CPAH is the concentration of each individual PAH in the soil and CQV is the corresponding quality value of each individual PAH in the soil [32]. Thus, the RQ (NCs) and RQ (MPCs) values could be respectively obtained based on the equation as follows [2]:

RQ(NCs) = CPAH/CQV(NCs) (2)
RQ(MPCs) = CPAH/CQV(MPCs) (3)

where CQV(NCs) represents the quality value of negligible concentrations (NCs) and CQV(MPCs) maximum permissible concentrations (MPCs) as reported by Cao et al. [2]. MPCs are the concentration of pollutants above which the risk of adverse effects is considered unacceptable to ecosystems while NCs are the concentrations in the environment below which the occurrence of individual PAHs has no adverse effects [33].

RQ∑PAHs (NCs) and RQ∑PAHs (MPCs) values are calculated by adding RQ(NCs) and RQ(MPCs) of each single PAH [2].

RQ∑PAHs (NCs) = ∑RQi(NCs) (RQi(NCs)≥ 1) (4)
RQ∑PAHs (MPCs) = ∑RQi(MPCs) (RQ(MPCs)i ≥ 1) (5)

2.5. Statistical analyses

Data analyses were performed with Origin (Pro) version 2021b (OriginLab Corporation, Northampton, MA, USA). The average concentrations of the total PAHs for all the locations were compared using the Bonferroni t-test (pairwise multiple comparisons). Differences between locations were considered statistically significant when p < 0.05.

3. Results and discussion

3.1. PAHs concentration and composition pattern

The concentrations of 29 target PAHs, the sum of which is defined as ∑29PAHs, determined from the oil-contaminated soil are shown in Table 1. The ∑29PAHs in the soils ranged from 24230.68 to 40845.32 ng/g with an average value of 29953.47 (Table 1). High concentrations of PAHs were detected in all the locations with location 3 having the highest value. A high abundance of n-alkanes ∑(nC14 to nC29) with an average concentration value of 24664 μg/g had been previously observed from the same soil samples [21]. This could be attributed to the volume of the spilled oil, and also an indication of little effect of environmental degradation on the impacted soil. The highest value recorded in location 3 may be due to relatively high volume of oil discharged into the area and its closeness to the flow station (Fig. 1). Since most international regulatory bodies and several studies in the literature usually refer to the concentration of the 16 priority US-EPA PAHs in their reports [34], the concentrations of 16 priority US-EPA PAHs (Σ16PAHs) are also indicated in Table 1 for comparison. The Σ16PAHs values in the soil samples ranged from 7361.66 to 14141.49 ng/g (average: 9819.96 ng/g). PAHs pollution levels in soils had been characterised as not contaminated (<200 ng/g), weakly contaminated (200–600 ng/g), contaminated (600–1000 ng/g), and heavily contaminated (>1000 ng/g) [35], according to the classification system suggested for Agricultural soils of Poland. Also, the concentrations of Σ16PAHs far exceeded the precautionary value of 1000 ng/g set by the soil quality guidelines of Switzerland [36]. Based on these classifications, the Niger Delta soil could be classified as heavily contaminated with PAHs. The very high concentrations recorded in this study could be attributed to intense exploration and exploitation activities with incessant pipeline leakages occasioned by vandals in the area. It should also be noted that there is continuous gas flaring in the sample locations as a result of the exploration activities which further increased the PAHs loads in the collected samples. The PAHs displayed noticeable distribution patterns, with 2-, 3-,4-, 5-, and 6-ring accounting for 33.69%, 56.31%, 9.47%, 0.52%, and 0.02% of the total PAHs, respectively (Fig. 2a and b), indicating the predominant of 3- ring PAHs over 2-, 4-, 5- and 6- ring PAHs. The percentage contribution from 6-ring PAHs is insignificant and thus, could not be clearly displayed in the pie chart (Fig. 2b). Such distributions have been attributed to contamination resulting from petrogenic sources [37].

Table 1.

Average concentration (ng/g dry wt.) of polycyclic aromatic hydrocarbons (PAHs) in Niger Delta soil.

Compound Abbreviation L1 (n = 4) L2 (n = 4) L3 (n = 4) tAvg
Naphthalene Nap 140.30 ± 11.1 155.47 ± 6.3 242.03 ± 2 179.27 ± 4.6
2-Methylnaphthalene 2-MNap 458.05 ± 35.5 367.44 ± 12.6 765.27 ± 6.9 530.25 ± 15.1
1-Methylnaphthalene 1-MNap 271.12 ± 19.4 236.33 ± 8.17 452.52 ± 4.4 319.99 ± 7.8
Biphenyl Bi 158.47 ± 4.9 117.18 ± 0.5 194.41 ± 1.5 156.69 ± 2.3
2,6-Dimethylnaphthalene 2,6-DMNap 2340.27 ± 114.3 1871.5 ± 0.5 3872.32 ± 29.9 2694.70 ± 59.1
Acenaphthylene Ace 173.00 ± 9.2 145.91 ± 41.8 291.07 ± 2.4 203.33 ± 21.1
Acenaphthene Ac 407.02 ± 13 344.39 ± 3.7 514.74 ± 4.9 422.05 ± 5.1
2,3,5-trinaphthalene 2,3,5-TNap 6044.43 ± 108.2 4988.48 ± 0.3 5740.29 ± 37.7 5591.07 ± 54.8
fluorene Fl 828.47 ± 13.6 532.44 ± 1.0 1310.70 ± 1.2 890.54 ± 7.2
phenanthrene Phe 2661.64 ± 26.5 1636.11 ± 6.4 4325.46 ± 21.9 2874.40 ± 10.5
anthracene Ant 2628.14 ± 26.2 1633.57 ± 7.4 4316.34 ± 21.1 2859.35 ± 9.7
2-phenanthrene 2-MPhe 3164.07 ± 25.3 2555.54 ± 12.6 5563.49 ± 19.1 3761.03 ± 6.4
1-methylphenathrene 1-MPhe 3111.99 ± 23.9 2491.4 ± 6.1 5402.14 ± 18.5 3668.51 ± 9.1
2,6-dimethlyphenanthrene 2,6-DMPhe 1803.60 ± 10.2 2508 ± 3.4 4159.96 ± 1.72 2823.85 ± 4.5
fluranthrene Flu 139.51 ± 5.1 587.79 ± 1.5 523.79 ± 1.52 417.03 ± 2.1
Pyrene Pyr 120.96 ± 4.2 587.79 ± 1.8 371.75 ± 1.73 360.17 ± 1.4
11H-benzo(b)fluorine 11-BbF 41.34 ± 1.8 1092.2 ± 6.7 381.92 ± 3.17 505.15 ± 2.5
benzo(a)anthracene BaA 160.81 ± 12.6 389.29 ± 22.4 994.26 ± 4.64 514.79 ± 8.9
chrysene Chr 94.50 ± 1.8 1936.51 ± 4.3 1088.25 ± 4.54 1039.75 ± 1.5
benzo(b)fluoranthene BbF 6.45 ± 3.0 9.48 ± 3.7 64.59 ± 1.73 26.84 ± 1.0
benzo(k)fluoranthene BkF 6.38 ± 3.0 9.38 ± 3.7 6.91 ± 1.71 7.56 ± 1.0
benzo(e)pyrene BeP 6.16 ± 3.8 7.8 ± 4.1 64.45 ± 2.75 26.14 ± 0.7
benzo(a)pyrene BaP 5.25 ± 4.8 6.57 ± 5 66.04 ± 0.63 25.95 ± 2.5
perylene Per 8.69 ± 2.5 17.27 ± 10.3 66.35 ± 2.43 30.77 ± 4.5
9,10-diphenylanthracene 9,10-DPha 1.64 ± 0.9 1.84 ± 0.5 25.36 ± 0.43 9.61 ± 0.3
dibenzo (a,h)anthracene DahA 0.8 ± 0.9 0.32 ± 0.2 16.7 ± 1.5 5.94 ± 0.7
benzo (ghi)perylene BghiP 0.34 ± 0.5 0.00 8.90 ± 0.04 3.08 ± 0.3
indeno (123-cd) pyrene IcdP 0.85 ± 0.9 0.47 ± 0.04 13.78 ± 0.11 5.03 ± 0.5
coronene Cor 0.16 ± 0.2 0.21 ± 0.02 1.53 ± 0.04 0.63 ± 0.1
∑16 US-EPA PAHs 7361.66 7956.73 14141.49 9819.96
∑29PAHs 24784.41 24230.68 40845.32 29953.47
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Data are reported in the format average (mean ± standard deviation), tAvg is the mean concentrations of each target at all the studied locations.

Fig. 2.

Fig. 2

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(a) Relative distribution pattern of individual PAHs in the soil samples; (b) Pie chart of percentage contribution of the four major PAH groups (2-, 3-, 4-, and 5-ring) to the total PAHs in the soil samples.

The mean values of the PAHs obtained from the different locations (L1, L2, and L3) (Fig. 3) are statistically compared. The results show significant differences among the locations, however, the difference in the means is not significant at p = 0.05 level between L1 and L2. This suggests that the locations have received different input of PAHs from crude oil contamination.

Fig. 3.

Fig. 3

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Average of total 29 PAHs concentrations in each of the sample locations. Values represent Mean ± SD (n = 29). Means that do not share a letter are significantly different (p < 0.05).

3.2. Comparison with PAH levels from other studies

PAH concentrations in the soils were compared with reported values from other parts of the globe (Table 2). Sojinu et al. [20] and Adedosu et al. [38] have previously reported PAHs concentrations that ranged from 23.8 to 120 and 7.40–78.3 ng/g, respectively (Table 2) in similar soil samples from the region, although, their soil samples for instance, in Sojinu et al. [20] were collected within the host communities where oil facilities were installed and not along pipeline pathways as it is the case in the present study. The concentrations of PAHs obtained in the present study were found to be higher than those reported from other regions of the world with the exception of samples from chemical plant areas in Australia (Table 2). This may reflect the quantity of spilled hydrocarbons in the area under study. The present samples have higher concentrations of PAHs when compared with global distribution patterns of PAHs in soils from oil exploration areas (Fig. 4). The Niger Delta is heavily affected by anthropogenic influences resulting from agriculture, deforestation, and, especially, oil exploration activities. With respect to the latter, the environment of the Niger Delta is intensely damaged by oil spills and contamination by industrial by-products as well as intense natural gas flaring. In early August 2011, the United Nation Environmental Programme (UNEP) released a report on oil spills in Ogoni land in the Niger Delta, Nigeria. The report showed that it will take up to 30 years to fully recover from the devastation done by oil spills in the area, even though oil industry operations have been suspended in the area since 1993.

Table 2.

PAHs concentrations in soils from the Niger Delta and from different regions of the world.

Location Source type No. of PAHs ∑PAHs (ng/g) References
Uzbekistan Industrial area 29 41–2670 [22]
China Riverbank soils 16 36.9–378 [2]
United States Highway 14 3000a [44]
United Kingdom Motorway 7 20000a [45]
China Urban soils 16 467–5470 [46]
Canada Highway 17 1400a [47]
Australia Chemical plant 18 300–79000 [48]
Niger Delta, Nigeria Gas plant area 16 7.40–78.3 [38]
Niger Delta, Nigeria Close to oil installation 28 23.8–120 [20]
Niger Delta, Nigeria Close to oil installation 29 29953.47a This study
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a

Mean values.

Fig. 4.

Fig. 4

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PAH concentrations in Nigeria (the present study) compared with those reported from other regions of the world. The data for the Shetland Island, Fladen Ground, and Global Range were obtained from Refs. [[41], [42], [43]].

Benzo(a)pyrene (BaP) is the most carcinogenic PAH in the present study with concentrations that ranged from 5.25 to 66.04 ng/g and with an average value of 26 ng/g (Table 1). These values are higher than those reported in soils from communities hosting oil installations in the same region by Sojinu et al. [20] (0.09–2.05 ng/g) and Adedosu et al. [38] (0.02–2.98 ng/g). However, a relatively comparable average value (22 ng/g) was reported from the city of Chiang Mai in Thailand [39]. Hashmi et al. [40] observed relatively higher BaP values between 5 and 270 ng/g with an average concentration value of 55 ng/g from the industrial zones of the Korean Peninsula. This present study shows that the Niger Delta soils have received significant input of BaP contamination when compared with previous studies from the region.

3.3. Ecological risk assessment

The ecological risk of sixteen listed PAHs by the United State Environmental Protection Agency (USEPA) as priority pollutants in the soil samples is assessed in this present study using the risk quotients method. The average values of RQ (NCs) and RQ (MPCs) in the studied soil samples are listed in Table 3. RQ values are interpreted as suggested by Cao et al. [2], where RQ (NCs) < 1.0, indicates that the contamination of individual PAH present in the soil has a very low ecological risk and might be of insignificant concern. RQ (NCs) ≥ 1 and RQ (MPCs) < 1 indicate the pollution of individual PAH is at moderate risk, while RQ (MPCs) ≥ 1 suggests much more severe contamination and high ecological risk. The RQ∑PAHs(NCs) and RQ∑PAHs(MPCs) values seem to accurately reflect the pollution levels and signify the overall or all-inclusive risk posed by ∑PAHs. RQ∑PAHs (NCs) ≥ 1, <800, and RQ∑PAHs(MPCs) = 0 represent low risk; RQ∑PAHs(NCs) ≥ 800 and RQ∑PAHs(MPCs) = 0, moderate risk1; RQ∑PAHs(NCs) < 800 and RQ∑PAHs(MPCs) ≥ 1, moderate risk2; while RQ∑PAHs(NCs) ≥ 800 and RQ∑PAHs(MPCs) ≥ 1 of high risk.

Table 3.

Average values of RQ(NCs) and RQ(MPCs) of sixteen US-EPA PAHs in soils from Niger Delta, Nigeria.

NCs(ng/g)a MPCs(ng/g)a RQ (NCs) (ng/g) RQ (MPCs) (ng/g)
Nap 1.4 140 128.05 1.28
Ace 1.2 120 169.44 1.69
Ac 1.2 120 351.77 3.52
Fl 1.2 120 742.12 7.42
Phe 5.1 510 563.61 5.64
Ant 1.2 120 2382.79 23.83
Flu 26 2600 16.04 0.16
Pyr 1.2 120 300.14 3.00
BaA 3.6 360 143.00 1.43
Chr 107 10700 9.72 0.10
BbF 3.6 360 7.46 0.07
BkF 24 2400 1.11 0.01
BaP 27 2700 0.96 0.01
DahA 27 2700 0.22 0.00
IcdP 59 5900 0.09 0.00
BghiP 75 7500 0.04 0.00
∑16PAHs 4815.25 47.81
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a

Cao et al. [2].

As shown in Table 3, the RQ (NCs) values are >1 for Nap, Ace, Ac, Fl, Phe, Ant, Flu, Pyr, and BaA in the soil samples, and RQ (MPCs) values are all >1. This indicates that this individual PAH presented a high risk for the biota and ecosystem in the area. The RQ (NCs) values for Chr, BbF, and BkF are <1, indicating a moderate risk to the environment. This assessment shows that most of the low molecular PAHs pose a high risk to the environment. Although these PAHs are less mutagenic and carcinogenic, their high ecological risk calls for serious attention. Meanwhile, PAHs such as BaP, DahA, IcdP, and BghiP pose no ecological threat. However, the average values of RQ∑PAHs (NCs) (4815.17) and RQ∑PAHs (MPCs) (47.81) are greater than the minimum threshold values of 800 and 1.0 respectively, indicating a high risk of ∑PAHs to the biota and ecosystem in the study area.

A plot of relative % values of RQ(NCs) for individual PAHs in the soil samples is shown in Fig. 5. 3-ring PAHs contribute to the major environmental risk burden and anthracene provides the largest percentage. This is followed by 2-ring PAHs, and then 4-ring PAHs. It is also noteworthy that Pyr contributes significantly to the ecological risk burden in the soils among the 4-ring PAHs. Therefore, in the event of crude oil spillage, the class of PAHs that require immediate de-contamination are the lower molecular weight PAHs.

Fig. 5.

Fig. 5

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A plot of relative % values of RQ(NCs) for individual PAH in soils from the Niger Delta.

4. Conclusion

The average concentrations of Σ29PAHs Σ16PAHs in the investigated crude oil contaminated soil samples were 29953.47 ng/g and 9819.96 ng/g, respectively. The concentrations of 16 priority US-EPA PAHs (Σ16PAHs) far exceeded the threshold values set by the soil quality guidelines of Switzerland and Poland and classified the area under study as heavily polluted. The very high concentrations recorded in this study could be attributed to intense oil exploitation activities and incessant pipeline leakages occasioned by vandals in the areas. The percentage composition of 2-, 3-,4-, 5-, and 6-ring in the soils were 33.69%, 56.31%, 9.47%, 0.52%, and 0.02% of the total PAHs, respectively, indicating the preponderance of low molecular weight PAHs over high molecular weight PAHs. The average values of RQ∑PAHs (NCs) (4815.17) and RQ∑PAHs (MPCs) (47.81) are greater than the minimum threshold values of 800 and 1.0 respectively, indicating a high risk of ∑PAHs to the biota and ecosystem in the study area. 2-, 3- and 4-ring PAHs in the soils contribute significantly to the ecological risk burden in the soils. Thus, in the event of crude oil spillage, the immediate decontamination of the low molecular weight PAHs must be considered. Based on the enormous quantity of the PAHs determined in the study area and the high ecological risk of low molecular weight PAHs, preventative and remedial measures should be conducted.

Declarations

Author contribution statement

Oluwabamise L. Faboya: Conceived and designed the experiments; Performed the experiments; Analysed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper. Samuel O. Sojinu: Performed the experiments; Analysed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper. Joseph O. Otugboyega: Analysed and interpreted the data; Wrote the paper.

Data availability statement

Data will be made available on request.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Declaration of interest’s statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.

Acknowledgment

We are deeply grateful to the anonymous reviewers whose positive comments helped to improve the original version of this article.

The authors also wish to thank Prof. Eddy Zeng of the State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Guangzhou, China for granting the use of his laboratory facilities and equipment for this study.

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