1. bookVolume 67 (2020): Issue 4 (December 2020)
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Geokemični kazalniki z nafto nasičenih vzorcev zemljin in vode na nekaterih izbranih področjih delte reke Niger

Online veröffentlicht: 08 May 2021
Seitenbereich: 209 - 219
Eingereicht: 17 Feb 2021
Akzeptiert: 19 Feb 2021
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License
Format
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Erstveröffentlichung
30 Mar 2016
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Povzetek

Z več kot petdesetimi leti raziskovanja in pridobivanja nafte na območju delte reke Niger narašča stopnja degradacije okolja zaradi onesnaževanja z ogljikovodiki. Namen raziskave je slediti virom razlitij nafte in porazdelitev onesnaževal v izbranih skupnostih v delti reke Niger z uporabo geokemičnih pristopov. Skupno je bilo odvzetih 16 vzorcev, od tega 10 vzorcev z nafto nasičenih zemljin iz globine 30 cm ter 6 vzorcev vode, od tega dva iz vrtin, dva iz jame ter dva iz površinske vode (en vzorec iz reke in en iz deževnice). Z detektorjem plamenskega ioniziranja s plinskim kromatografom Agilent 7890B (GC-FID) je bila izvedena identifikacija in kvantifikacija alifatskih ogljikovodikov (AH) in policikličnih aromatskih ogljikovodikov (PAH). Identificirani so bili AH z vključujočim pristanom (pristane) in fitanom (phytane) skupaj s 17 PAH. Vrednosti AH in PAH v vzorcih vode se gibajo med 0.13 mg/l do 5.78 mg/l in 0.09 mg/l do 1.109 mg/l. Vrednosti AH in PAH v vzorcih zemljine se gibajo med 22.52 mg/kg do 929.44 mg/kg in 10.544 mg/kg do 16.879 mg/kg.

Ključne besede

Introduction

The Niger Delta is one of the major hydrocarbon provinces of the world, with an estimated reserve of about 23 billion barrels of oil and 183 trillion cubic feet of natural gas with ongoing exploration in the province for over 50 years [1]. Much of the oil industries located within this region have contributed immensely to the growth and development of the nation.

However, oil exploration activities have rendered the Niger Delta region one of the most severely degraded ecosystems in the world [2]. Crude oil spills are common in the region with an estimated total of over 7,000 oil spill accidents reported over 50 years [3]. Studies have shown that the quantity of oil spilt over this period amounts to 9–13 million barrels, which is equivalent to 50 Exxon Valdez spills [4].

These spills occur through equipment failure, operational mishap, haulage, oil bunkering and/or vandalisation of pipelines leading to the destruction of aquatic and terrestrial flora and fauna of the Niger Delta region [5].

Geochemical or Oil fingerprinting is one of the ways of assessing and evaluating petroleum pollution. It involves the analysis of the released oil with gas chromatography (GC) and measurement of the hydrocarbon compound contents [6]. From the qualitative method (visual comparison of chromatograms) as well as quantitative determination of polycyclic aromatic hydrocarbons (PAHs) diagnostic ratio, n-alkane distribution and statistical analysis of data obtained are used for source identification and interpretation of chemical data from oil spills. An assessment and evaluation of hydrocarbon pollution are therefore essential to curb the growing rate of environmental degradation in the region as well as its social, economic and health impacts. This assessment includes; determination of sources, characterisation, distribution, and fate of organic pollutants such as PAHs and aliphatic hydrocarbons (AHs) in the Niger Delta. The objective is to evaluate the AH and PAHs which are said to be source-specific.

Location and geology of the study area

The study area lies within the Niger Delta region between latitudes 5°37′00″E–5°47′00″E and longitudes 5°53′00″N–6°02′30″N (Figure 1) and cuts across Sapele and Ethiope West Local government, Delta State, Nigeria. Stratigraphically, the Niger Delta consists of three formations, notably; Akata Formation, which is the oldest unit and constitutes under compacted shales, turbidites and silts. This is overlain by the paralic Agbada Formation, made up of alternating sequences of sandstone and shale which contains most of the hydrocarbon reservoirs in the basin while the youngest unit is the Benin Formation, which is made up of continental sands [7]. The area is characterised by an even topography. It is situated in the tropics and experiences a fluctuating climate characterised by rainy and dry seasons. The area is drained by minor rivers which are tributaries of the major River Ethiope with a dendritic pattern.

Figure 1

Geological map of the Niger Delta region showing the study area (modified after Geological Map of Niger Delta [8]).

Materials and methods
Sampling and sample preparation

The field study involved the collection of soil and water samples from selected points as shown in Figure 2. A total of sixteen samples made up of ten crude oil-impacted soils taken at a depth of 30 cm and six water samples (two from boreholes, two from burrow pits plus and two from surface water – one from a river and the other from rain harvest as control) were collected. The water and soil samples were collected in clean, well-labelled glass jars and aluminium foils, respectively, and taken to the laboratory for analyses. Due to the relatively high volatility and instability of AHs and PAHs, the soils were not prepared using conventional soil preparation techniques such as grinding and sieving. However, the soil samples were dried by mixing the samples with 5 g of anhydrous sodium sulfate.

Figure 2

Map of study area showing the sample points (insert: map of Nigeria showing the Niger Delta region).

Analytical methods

Organic pollutants were separated from the soil and water samples using an ultrasonic extraction and a separatory funnel, respectively. The extracts were fractionated into the AH and PAH fractions by eluting with n-hexane and dichloromethane, respectively. The identification and quantification of AHs and PAHs were performed with an Agilent 7890B gas chromatography flame ionisation detector (GC-FID). The gas chromatographic column has a detection limit of 0.01 ppm. Separation occurs as the constituents of the vapour partition between the gas and liquid phases and oven temperature was programmed from 60°C to 180°C. Identification of analytes was done by comparing the retention time of an individual compound to that of a reference standard.

Results and discussion
Concentration of AH and PAH

The results of the AH and PAHs in this study are shown in Table 1. The concentrations of the AHs and PAHs found in the studied samples are low when compared with values from other areas in the Niger Delta (Table 2). However, in this study, the concentrations are higher than the regulatory limits given by the United Nations Environment Programme (UNEP) [9].

Results of concentration of the AHs and PAHs present in the soil and water samples

Sample name Sample medium AHs PAHs
Css_1 Soil 37.59 mg/kg 16.88 mg/kg
Css_2 Soil 25.70 mg/kg 11.66 mg/kg
Css_3 Soil 34.43 mg/kg 14.72 mg/kg
Css_4 Soil 22.52 mg/kg 14.77 mg/kg
Css_5 Soil 929.44 mg/kg 14.54 mg/kg
Css_6 Soil 79.55 mg/kg 15.91 mg/kg
Css_7 Soil 36.85 mg/kg 13.11 mg/kg
Css_8 Soil 34.86 mg/kg 10.54 mg/kg
Css_9 Soil 44.73 mg/kg 12.15 mg/kg
Css_10 Soil 41.93 mg/kg 15.81 mg/kg
Cbw_1 Water 0.22 mg/l 0.09 mg/l
Cbw_2 Water 0.13 mg/l 0.29 mg/l
Pw_1 Water 5.78 mg/l 0.86 mg/l
Pw_2 Water 5.14 mg/l 1.11 mg/l
Sw_1 Water (control) 0.61 mg/l 0.17 mg/l
Sw_2 Water 2.08 mg/l 0.86 mg/l

AH, aliphatic hydrocarbon; PAHs, polycyclic aromatic hydrocarbons.

Comparison of AH and PAH present in the studied samples with those found in some other areas in the Niger Delta and some regulatory standards

Sample medium Reference AHs (mg/l) PAHs (mg/l)
Contaminated soil Present study 22.52–929.44 10.54–16.88
Olawoyin et al. [14] 7,878.8–76,510.9 31.4–132.0
Adedosu et al. [12] 575.96–1,202.47 7.40–78.30
Udoetok and Osuji Leo [24] 77.64–3,946.58 8.16–3,756.81
United Nations Environment Programme (UNEP) [9] 10 No limit
Department of Petroleum Resources (DPR) [15] No limit 1.00
United States Environmental Protection Agency (USEPA) [10] No limit 1.00

Borehole Present study 0.13–0.22 0.09–0.29
Olawoyin et al. [14] No limit 119.90–450.58
Ibezue et al. [29] 0.03–0.422 0.002–0.007
WHO 0.0002 0.0002
Department of Petroleum Resources (DPR) [15] No limit 0.1

Surface water Present study 0.61–2.08 0.17–0.86
Inyang et al. [30] 2.5–183.0 No limit
European Union Environmental Protection Agency (EUEPA) [25] 0.3 No limit
Department of Petroleum Resources (DPR) [15] No limit 0.0001
WHO No limit 0.05

Contaminated water Present study 5.14–5.78 0.86–1.11
Inyang et al. [30] 2.5–183.0 No limit
European Union Environmental Protection Agency (EUEPA) [25] 0.3 No limit
WHO No limit 0.05

AH, aliphatic hydrocarbon; PAH, polycyclic aromatic hydrocarbon.

Occurrence, distribution and sources of PAHs

The distribution of seventeen priority PAHs in the water and soil samples in the study area is presented in Table 3. The main PAH pollutants in the studied areas were found to be Chrysene, Acenaphthene, Methylnaphthalene, Naphthalene, Anthracene, Benzo(g,h,i) perylene, Fluorene, Indeno(1,2,3-cd)perylene and Phenanthrene. It is important to note that the sum of the PAHs in the contaminated soil samples is 10.54–16.89 times higher than the standard level (1 mg/kg) of heavy [10]. The level of PAH pollution in the control sample (Sw-1) is very low as compared with those from the other samples studied. The spatial distribution of PAHs in this study is shown in Figure 3 and indicates a predominance of three-ring PAHs which suggests recent deposition according to Jiao et al. [11]. The abundance of three-ring PAHs in the study area is in agreement with studies of some oil-polluted sites in the Niger Delta [12]. The four-ring PAHs are also abundant and they indicate the persistence of high molecular weight (HMW) PAHs in the environment. According to Li et al. [13], petrogenic sources are those PAHs derived from petroleum spills while pyrogenic sources are generated by incomplete combustion of fossil fuel such as coal, crude oil and natural gas plus biomass. Diagnostic ratios such as Phenanthrene/Anthracene, Fluorene/Pyrene, Benz(a)pyrene/Chrysene, Naphthalene/Acenaphthene, Anthracene/(Phenanthrene + Anthracene), Fluoranthene/(Fluoranthene + Pyrene), Benzo(a)anthracene/(Benzo(q)anthracene + Chrysene), Indeno(1,2,3-cd)perylene/(Indeno (1,2,3-cd)perylene + Benzo(g,h,i)perylene) and low molecular weight (LMW) hydrocarbon/HMW hydrocarbon have been utilised in deducing the source of pollution [18, 20, 23, 26, 28]. From the source diagnostic indices as presented in Table 4, most PAHs in the study area are from petrogenic sources with a minor contribution from pyrogenic sources.

Occurrence and spatial distribution of PAHs in the soil and water samples

PAHs Water samples (mg/l) Soil samples (mg/kg)


Cbw_1 Cbw_2 Pw_1 Pw_2 Sw_1 Sw_2 Css_1 Css_2 Css_3 Css_4 Css_5 Css_6 Css_7 Css_8 Css_9 Css_10
Nap BDL 0.04 0.098 0.043 BDL 0.046 0.814 0.819 0.809 0.758 1.228 1.004 0.913 0.807 0.746 0.788
Mnap BDL BDL 0.066 0.176 BDL 0.064 1.814 1.007 1.356 1.106 3.249 1.548 1.264 1.093 1.235 1.350
Acep BDL BDL 0.056 0.037 BDL 0.066 3.352 1.148 1.666 0.683 1.459 2.311 1.672 1.340 1.740 1.830
Ace BDL 0.07 0.081 0.087 BDL 0.073 1.404 1.437 1.420 1.558 3.020 1.536 1.401 1.410 1.376 1.433
Fl BDL BDL 0.036 0.173 BDL 0.042 0.768 BDL 0.697 1.216 1.085 0.959 0.706 0.697 0.769 0.771
Phe BDL 0.06 0.061 0.048 BDL 0.057 0.985 1.041 1.032 1.027 0.174 1.509 0.961 0.803 0.801 0.958
Ant BDL 0.01 0.010 0.133 0.166 0.017 0.364 0.274 0.375 0.744 0.111 1.632 0.336 0.222 0.233 0.265
Flu BDL 0.05 0.046 0.040 BDL 0.057 0.924 0.906 1.156 0.841 0.088 0.301 1.027 0.759 0.747 0.927
Pyr BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL 0.146 0.250 BDL BDL BDL BDL
BaA BDL BDL 0.133 0.050 BDL 0.015 0.299 BDL 0.313 0.707 0.259 0.134 0.292 0.124 0.104 0.269
Chr 0.02 0.05 0.205 0.137 BDL 0.033 1.217 0.296 0.758 1.265 0.108 0.483 0.620 0.241 0.491 0.082
BbF 0.02 0.01 0.010 0.042 BDL 0.105 1.592 0.728 0.506 2.668 0.085 0.447 0.501 0.108 1.149 0.066
BkF 0.02 0.01 0.018 0.010 BDL 0.018 0.860 0.320 0.199 0.434 0.269 0.169 0.393 0.157 0.242 0.248
BaP 0.02 BDL BDL 0.025 BDL 0.015 0.252 0.215 0.883 0.228 0.091 0.366 0.167 0.413 0.203 0.283
DahA BDL BDL 0.026 0.012 BDL 0.014 0.202 0.211 0.290 0.194 0.065 0.151 0.359 0.516 0.263 0.295
InP 0.01 BDL BDL 0.038 BDL 0.114 0.912 1.895 0.407 0.665 0.562 0.396 0.598 0.659 0.182 3.374
BghiP BDL BDL BDL 0.060 BDL 0.126 1.119 1.362 2.852 0.673 2.545 2.722 1.899 1.193 1.869 2.870
Total 0.09 0.3 0.856 1.109 0.166 0.863 16.879 11.657 14.720 14.768 14.543 15.917 13.108 10.544 12.151 15.807
Mean 0.005 0.02 0.050 0.065 0.010 0.051 0.993 0.688 0.866 0.869 0.855 0.936 0.771 0.620 0.715 0.930

Ace, Acenaphthene; Acep, Acenaphthylene; Ant, Anthracene; BaA, Benzo(a)anthracene; BaP, Benzo(a)pyrene; BbF Benzo(b)fluoranthene; BDL, Below Detection Limit; BghiP, Benzo (g, h, i) perylene; BkF, Benzo(k)fluoranthene; Chr, Chrysene; DahA, Dibenzo (a, h) anthrace; Fl, Fluorene; Flu, Fluoranthene; InP, Indeno (1, 2, 3-cd) perylene; Mnap, 2-Methylnaphthalene; Nap, Naphthalene; Phe, Phenanthrene; Pyr, Pyrene.

PAH diagnostic ratios of the studied soil and water samples in comparison with that of standard references (after Tobiszewski and Namieśnik [21])

PAH diagnostic ratio Value range Source Reference Value ranges from studied samples Inferred source
ΣLMW/ΣHMW <1 Pyrogenic Zhang et al. [26] 0–2.45 Petrogenic/pyrogenic
>1 Petrogenic
Fl(Fl + Pyr) <0.5 Petroleum Emissions Ravindra et al. [20] 0–0.4 Petroleum emissions
>0.5 Diesel Emissions

Ant(Phe + Ant) <0.1 Pyrogenic Pies et al. [18] 0–0.21 Petrogenic/pyrogenic
>0.1 Petrogenic

Flu(Flu + Pyr) <0.4 Petrogenic Fossil fuel Combustion Grass, wood, coal combustion De La Torre-Roche et al. [27] 0–0.4 Petrogenic/mixed source of fossil fuel and combustion
0.4–0.5
>0.5

BaA/(BaA + Chr) 0.2–0.35 Coal combustion Akyüz and Çabuk [23] 0–0.31 Coal combustion/petrogenic
>0.35 Vehicular emission
<0.2 Petrogenic Yunker et al. [28]
>0.35 Combustion

InP/(InP + BghiP) <0.2 Petrogenic Yunker et al. [28] 0–0.23 Petrogenic/petroleum combustion
0.2–0.5 Petroleum Combustion
>0.5 Grass, Wood, Coal Combustion

ΣLMW/ΣHMW, the sum of low molecular weight hydrocarbon/the sum of high molecular weight hydrocarbon.

The concentrations of AHs in the soil and water samples

AHs Water samples (mg/l) Soil samples (mg/kg)


Cbw_1 Cbw_2 Pw_1 Pw_2 Sw_1 Sw_2 Css_1 Css_2 Css_3 Css_4 Css_5 Css_6 Css_7 Css_8 Css_9 Css_10
C8 BDL BDL BDL 0.11 BDL 0.51 12.76 1.13 0.59 0.30 2.38 BDL 1.32 1.44 1.77 9.42
C9 BDL BDL 0.02 0.01 BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL
C10 BDL BDL 0.81 0.17 BDL 0.01 BDL BDL BDL BDL 0.94 BDL BDL BDL 0.29 0.59
C11 BDL BDL 0.08 0.04 BDL 0.04 1.63 BDL BDL BDL 11.06 BDL BDL BDL BDL 0.53
C12 BDL BDL 0.18 0.11 BDL BDL BDL BDL BDL BDL 6.35 BDL BDL 0.06 BDL 0.04
C13 BDL BDL 0.23 0.18 BDL 0.14 2.43 0.36 0.46 BDL 11.24 BDL BDL 0.18 1.28 3.06
C14 BDL 0.02 0.33 0.23 0.03 0.01 0.53 0.77 0.66 0.42 47.91 0.09 BDL 0.17 0.57 0.56
C15 BDL BDL 0.31 0.23 BDL BDL BDL BDL BDL 0.06 48.04 0.88 0.13 0.29 BDL BDL
C16 BDL 0.04 0.33 0.25 0.02 0.01 0.85 0.19 0.12 0.69 29.37 3.90 0.31 0.39 0.95 0.89
C17 BDL BDL 0.31 0.25 BDL BDL BDL BDL BDL BDL 9.39 6.82 BDL BDL BDL BDL
C18 BDL 0.03 0.28 0.23 0.01 0.04 0.45 0.10 0.73 0.60 17.20 7.94 0.13 BDL 0.93 0.64
C19 BDL 0.01 0.21 0.20 0.02 BDL BDL 0.27 0.22 0.13 16.14 7.08 BDL 0.19 0.14 BDL
C20 BDL 0.02 0.22 0.28 0.03 0.05 0.39 1.18 0.65 0.78 24.05 7.78 1.24 1.36 1.65 0.74
C21 BDL BDL 0.20 0.19 BDL BDL BDL BDL BDL 0.12 15.49 6.19 0.04 BDL BDL BDL
C22 BDL BDL 0.26 0.22 0.02 0.02 1.37 0.57 0.28 BDL 15.23 5.27 1.49 1.71 0.45 0.17
C23 0.01 BDL 0.26 0.17 BDL BDL 0.13 BDL BDL 0.24 11.69 4.35 BDL 0.42 BDL BDL
C24 0.01 0.02 0.36 0.23 0.03 BDL BDL BDL 0.60 1.62 19.32 4.16 0.38 0.34 BDL 0.05
C25 0.02 BDL 0.30 0.17 BDL BDL 0.40 BDL BDL 0.19 13.90 3.17 BDL BDL BDL BDL
C26 0.02 BDL 0.12 0.16 BDL 0.04 0.73 0.66 1.01 0.74 12.47 2.75 1.40 1.14 1.15 0.96
C27 0.02 BDL 0.14 0.14 BDL BDL 0.59 BDL BDL 0.31 10.58 2.44 BDL 0.19 BDL BDL
C28 0.01 BDL 0.11 0.12 BDL 0.03 0.86 0.30 0.71 0.54 18.87 1.93 0.96 1.18 0.80 0.68
C29 BDL BDL 0.08 0.10 BDL BDL 0.24 BDL BDL BDL 30.88 1.19 BDL BDL BDL BDL
C30 BDL BDL 0.08 0.05 BDL BDL BDL BDL BDL BDL 29.12 0.29 BDL BDL BDL BDL
C31 BDL BDL 0.08 0.08 BDL BDL 1.06 BDL BDL BDL 15.14 1.11 0.63 0.32 BDL BDL
C32 BDL BDL BDL BDL BDL 0.11 BDL 2.47 4.62 0.72 4.53 BDL BDL 0.44 5.40 3.79
C33 BDL BDL BDL 0.31 BDL 0.40 5.05 8.38 12.62 2.20 13.12 BDL 3.23 4.24 13.73 10.84
C34 BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL 15.04 3.88 3.66 BDL BDL BDL
C35 BDL BDL BDL BDL 0.07 BDL BDL BDL 11.16 BDL 6.12 BDL BDL BDL BDL BDL
C36 0.13 BDL BDL 0.75 0.03 0.02 8.12 7.66 BDL 6.97 11.78 BDL 9.81 7.43 12.83 11.78
C37 BDL BDL 0.15 BDL 0.26 0.65 BDL BDL BDL BDL 5.90 BDL 9.99 11.26 BDL BDL
C38 BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL 2.27 BDL BDL BDL BDL BDL
C39 BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL
C40 BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL
PRISTANE BDL BDL 0.27 0.16 BDL BDL BDL BDL BDL 5.32 392.91 6.70 BDL BDL BDL BDL
PHYTANE BDL BDL 0.04 0.02 0.08 BDL BDL 1.67 BDL 0.56 61.00 1.64 2.12 2.10 BDL BDL
Pr/nC17 0 0 0.90 0.63 0 0 0 0 0 0 41.84 0.98 0 0 0 0
Total 0.22 0.13 5.78 5.14 0.61 2.08 37.59 25.69 34.43 22.52 929.44 79.55 36.85 34.86 44.73 41.94

AH, aliphatic hydrocarbon.

The AH diagnostic ratios (after Sojinu et al. [22])

Sample CPI OEP
Cbw_1 0.29 1.81
Cbw_2 0.06 0
Pw_1 0.76 0.82
Pw_2 0.71 0.79
Sw_1 2.14 0
Sw_2 1.47 0
Css_1 0.44 0.21
Css_2 0.60 0
Css_3 2.46 0
Css_4 0.24 0.27
Css_5 0.85 0.72
Css_6 0.87 0.94
Css_7 0.68 0.006
Css_8 1.09 0.31
Css_9 0.57 0
Css_10 0.48 0
Mean 0.86 0.37

AH, aliphatic hydrocarbon.

Figure 3

The spatial distribution of the PAH rings in the water and soil samples. PAH, polycyclic aromatic hydrocarbons.

Normal alkanes and isoprenoids distribution and sources

Although some components of the PAHs and AHs in the study area have been degraded, the majority of the other components still persist in the environment which may affect groundwater, rivers and soils. This may be injurious to both human and animal health. Some sources of the PAH and AH studied are pyrolytic, i.e. from combustion/bush fire occasioned by explosion of oil tankers, oil installations, leakages from oil pipes and pipelines explosion during oil bunkering or pipeline vandalism. All these have bearing on agriculture, water supply settlement and the biodiversity within the study area.

Cancer risk assessment

PAHs are known to be injurious to health. The eight PAHs typically considered as possible carcinogens are Benzo(a)anthracene, Chrysene, Benzo(b)fluoranthene, Benzo(k)fluoranthene, Benzo(a)pyrene, Dibenzo(a,h)anthracene, Indeno(1,2,3-cd)pyrene and Benzo(g,h,i) perylene. In particular, Benzo(a)pyrene has been identified as being highly carcinogenic. The World Health Organization (1993) revealed that Benzo(a)pyrene concentration of 0.7 μg/l corresponds to an excess lifetime cancer risk of 10–5. The BaP-equivalent (BaPE) is used as a way to access carcinogenic risk due to the contamination by PAHs. The BaPE not only includes the risk due to BaP but also calculates all of the carcinogenic PAHs, where each of the PAH is weighed according to its carcinogenicity in relation to the carcinogenicity of BaP, which is measured by 1. This index can be calculated with this equation [17]; BaPE = BaP + (BaA*0.06) + (BkF*0.07) + (BbF*0.07) + (DahA*0.06) + (InP*0.08). BaPE ranged from 0 mg/l to 0.042 mg/l and 0.22 mg/kg to 1.16 mg/kg in the water and soil samples, respectively. The highest value of BaPE in the samples is in Css_3, hence indicating that PAHs at this sample point have high carcinogenic effects.

Conclusion

The prevalence of petrogenic-derived PAHs was confirmed in the studied samples. AHs in both media originated from both petrogenic and biogenic. The AHs are products from both terrestrial and marine inputs. The pollution level of the study area is high as compared with USEPA, DPR and WHO standards which poses health hazards. However, the values are lower compared with other areas in the Niger Delta. The PAH and AH diagnostic ratios have proven to be useful in tracking pollution emission sources and have helped in assessing the level of degradation of oils in impacted soils and water.

Figure 1

Geological map of the Niger Delta region showing the study area (modified after Geological Map of Niger Delta [8]).
Geological map of the Niger Delta region showing the study area (modified after Geological Map of Niger Delta [8]).

Figure 2

Map of study area showing the sample points (insert: map of Nigeria showing the Niger Delta region).
Map of study area showing the sample points (insert: map of Nigeria showing the Niger Delta region).

Figure 3

The spatial distribution of the PAH rings in the water and soil samples. PAH, polycyclic aromatic hydrocarbons.
The spatial distribution of the PAH rings in the water and soil samples. PAH, polycyclic aromatic hydrocarbons.

Results of concentration of the AHs and PAHs present in the soil and water samples

Sample name Sample medium AHs PAHs
Css_1 Soil 37.59 mg/kg 16.88 mg/kg
Css_2 Soil 25.70 mg/kg 11.66 mg/kg
Css_3 Soil 34.43 mg/kg 14.72 mg/kg
Css_4 Soil 22.52 mg/kg 14.77 mg/kg
Css_5 Soil 929.44 mg/kg 14.54 mg/kg
Css_6 Soil 79.55 mg/kg 15.91 mg/kg
Css_7 Soil 36.85 mg/kg 13.11 mg/kg
Css_8 Soil 34.86 mg/kg 10.54 mg/kg
Css_9 Soil 44.73 mg/kg 12.15 mg/kg
Css_10 Soil 41.93 mg/kg 15.81 mg/kg
Cbw_1 Water 0.22 mg/l 0.09 mg/l
Cbw_2 Water 0.13 mg/l 0.29 mg/l
Pw_1 Water 5.78 mg/l 0.86 mg/l
Pw_2 Water 5.14 mg/l 1.11 mg/l
Sw_1 Water (control) 0.61 mg/l 0.17 mg/l
Sw_2 Water 2.08 mg/l 0.86 mg/l

Comparison of AH and PAH present in the studied samples with those found in some other areas in the Niger Delta and some regulatory standards

Sample medium Reference AHs (mg/l) PAHs (mg/l)
Contaminated soil Present study 22.52–929.44 10.54–16.88
Olawoyin et al. [14] 7,878.8–76,510.9 31.4–132.0
Adedosu et al. [12] 575.96–1,202.47 7.40–78.30
Udoetok and Osuji Leo [24] 77.64–3,946.58 8.16–3,756.81
United Nations Environment Programme (UNEP) [9] 10 No limit
Department of Petroleum Resources (DPR) [15] No limit 1.00
United States Environmental Protection Agency (USEPA) [10] No limit 1.00

Borehole Present study 0.13–0.22 0.09–0.29
Olawoyin et al. [14] No limit 119.90–450.58
Ibezue et al. [29] 0.03–0.422 0.002–0.007
WHO 0.0002 0.0002
Department of Petroleum Resources (DPR) [15] No limit 0.1

Surface water Present study 0.61–2.08 0.17–0.86
Inyang et al. [30] 2.5–183.0 No limit
European Union Environmental Protection Agency (EUEPA) [25] 0.3 No limit
Department of Petroleum Resources (DPR) [15] No limit 0.0001
WHO No limit 0.05

Contaminated water Present study 5.14–5.78 0.86–1.11
Inyang et al. [30] 2.5–183.0 No limit
European Union Environmental Protection Agency (EUEPA) [25] 0.3 No limit
WHO No limit 0.05

Occurrence and spatial distribution of PAHs in the soil and water samples

PAHs Water samples (mg/l) Soil samples (mg/kg)


Cbw_1 Cbw_2 Pw_1 Pw_2 Sw_1 Sw_2 Css_1 Css_2 Css_3 Css_4 Css_5 Css_6 Css_7 Css_8 Css_9 Css_10
Nap BDL 0.04 0.098 0.043 BDL 0.046 0.814 0.819 0.809 0.758 1.228 1.004 0.913 0.807 0.746 0.788
Mnap BDL BDL 0.066 0.176 BDL 0.064 1.814 1.007 1.356 1.106 3.249 1.548 1.264 1.093 1.235 1.350
Acep BDL BDL 0.056 0.037 BDL 0.066 3.352 1.148 1.666 0.683 1.459 2.311 1.672 1.340 1.740 1.830
Ace BDL 0.07 0.081 0.087 BDL 0.073 1.404 1.437 1.420 1.558 3.020 1.536 1.401 1.410 1.376 1.433
Fl BDL BDL 0.036 0.173 BDL 0.042 0.768 BDL 0.697 1.216 1.085 0.959 0.706 0.697 0.769 0.771
Phe BDL 0.06 0.061 0.048 BDL 0.057 0.985 1.041 1.032 1.027 0.174 1.509 0.961 0.803 0.801 0.958
Ant BDL 0.01 0.010 0.133 0.166 0.017 0.364 0.274 0.375 0.744 0.111 1.632 0.336 0.222 0.233 0.265
Flu BDL 0.05 0.046 0.040 BDL 0.057 0.924 0.906 1.156 0.841 0.088 0.301 1.027 0.759 0.747 0.927
Pyr BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL 0.146 0.250 BDL BDL BDL BDL
BaA BDL BDL 0.133 0.050 BDL 0.015 0.299 BDL 0.313 0.707 0.259 0.134 0.292 0.124 0.104 0.269
Chr 0.02 0.05 0.205 0.137 BDL 0.033 1.217 0.296 0.758 1.265 0.108 0.483 0.620 0.241 0.491 0.082
BbF 0.02 0.01 0.010 0.042 BDL 0.105 1.592 0.728 0.506 2.668 0.085 0.447 0.501 0.108 1.149 0.066
BkF 0.02 0.01 0.018 0.010 BDL 0.018 0.860 0.320 0.199 0.434 0.269 0.169 0.393 0.157 0.242 0.248
BaP 0.02 BDL BDL 0.025 BDL 0.015 0.252 0.215 0.883 0.228 0.091 0.366 0.167 0.413 0.203 0.283
DahA BDL BDL 0.026 0.012 BDL 0.014 0.202 0.211 0.290 0.194 0.065 0.151 0.359 0.516 0.263 0.295
InP 0.01 BDL BDL 0.038 BDL 0.114 0.912 1.895 0.407 0.665 0.562 0.396 0.598 0.659 0.182 3.374
BghiP BDL BDL BDL 0.060 BDL 0.126 1.119 1.362 2.852 0.673 2.545 2.722 1.899 1.193 1.869 2.870
Total 0.09 0.3 0.856 1.109 0.166 0.863 16.879 11.657 14.720 14.768 14.543 15.917 13.108 10.544 12.151 15.807
Mean 0.005 0.02 0.050 0.065 0.010 0.051 0.993 0.688 0.866 0.869 0.855 0.936 0.771 0.620 0.715 0.930

The concentrations of AHs in the soil and water samples

AHs Water samples (mg/l) Soil samples (mg/kg)


Cbw_1 Cbw_2 Pw_1 Pw_2 Sw_1 Sw_2 Css_1 Css_2 Css_3 Css_4 Css_5 Css_6 Css_7 Css_8 Css_9 Css_10
C8 BDL BDL BDL 0.11 BDL 0.51 12.76 1.13 0.59 0.30 2.38 BDL 1.32 1.44 1.77 9.42
C9 BDL BDL 0.02 0.01 BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL
C10 BDL BDL 0.81 0.17 BDL 0.01 BDL BDL BDL BDL 0.94 BDL BDL BDL 0.29 0.59
C11 BDL BDL 0.08 0.04 BDL 0.04 1.63 BDL BDL BDL 11.06 BDL BDL BDL BDL 0.53
C12 BDL BDL 0.18 0.11 BDL BDL BDL BDL BDL BDL 6.35 BDL BDL 0.06 BDL 0.04
C13 BDL BDL 0.23 0.18 BDL 0.14 2.43 0.36 0.46 BDL 11.24 BDL BDL 0.18 1.28 3.06
C14 BDL 0.02 0.33 0.23 0.03 0.01 0.53 0.77 0.66 0.42 47.91 0.09 BDL 0.17 0.57 0.56
C15 BDL BDL 0.31 0.23 BDL BDL BDL BDL BDL 0.06 48.04 0.88 0.13 0.29 BDL BDL
C16 BDL 0.04 0.33 0.25 0.02 0.01 0.85 0.19 0.12 0.69 29.37 3.90 0.31 0.39 0.95 0.89
C17 BDL BDL 0.31 0.25 BDL BDL BDL BDL BDL BDL 9.39 6.82 BDL BDL BDL BDL
C18 BDL 0.03 0.28 0.23 0.01 0.04 0.45 0.10 0.73 0.60 17.20 7.94 0.13 BDL 0.93 0.64
C19 BDL 0.01 0.21 0.20 0.02 BDL BDL 0.27 0.22 0.13 16.14 7.08 BDL 0.19 0.14 BDL
C20 BDL 0.02 0.22 0.28 0.03 0.05 0.39 1.18 0.65 0.78 24.05 7.78 1.24 1.36 1.65 0.74
C21 BDL BDL 0.20 0.19 BDL BDL BDL BDL BDL 0.12 15.49 6.19 0.04 BDL BDL BDL
C22 BDL BDL 0.26 0.22 0.02 0.02 1.37 0.57 0.28 BDL 15.23 5.27 1.49 1.71 0.45 0.17
C23 0.01 BDL 0.26 0.17 BDL BDL 0.13 BDL BDL 0.24 11.69 4.35 BDL 0.42 BDL BDL
C24 0.01 0.02 0.36 0.23 0.03 BDL BDL BDL 0.60 1.62 19.32 4.16 0.38 0.34 BDL 0.05
C25 0.02 BDL 0.30 0.17 BDL BDL 0.40 BDL BDL 0.19 13.90 3.17 BDL BDL BDL BDL
C26 0.02 BDL 0.12 0.16 BDL 0.04 0.73 0.66 1.01 0.74 12.47 2.75 1.40 1.14 1.15 0.96
C27 0.02 BDL 0.14 0.14 BDL BDL 0.59 BDL BDL 0.31 10.58 2.44 BDL 0.19 BDL BDL
C28 0.01 BDL 0.11 0.12 BDL 0.03 0.86 0.30 0.71 0.54 18.87 1.93 0.96 1.18 0.80 0.68
C29 BDL BDL 0.08 0.10 BDL BDL 0.24 BDL BDL BDL 30.88 1.19 BDL BDL BDL BDL
C30 BDL BDL 0.08 0.05 BDL BDL BDL BDL BDL BDL 29.12 0.29 BDL BDL BDL BDL
C31 BDL BDL 0.08 0.08 BDL BDL 1.06 BDL BDL BDL 15.14 1.11 0.63 0.32 BDL BDL
C32 BDL BDL BDL BDL BDL 0.11 BDL 2.47 4.62 0.72 4.53 BDL BDL 0.44 5.40 3.79
C33 BDL BDL BDL 0.31 BDL 0.40 5.05 8.38 12.62 2.20 13.12 BDL 3.23 4.24 13.73 10.84
C34 BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL 15.04 3.88 3.66 BDL BDL BDL
C35 BDL BDL BDL BDL 0.07 BDL BDL BDL 11.16 BDL 6.12 BDL BDL BDL BDL BDL
C36 0.13 BDL BDL 0.75 0.03 0.02 8.12 7.66 BDL 6.97 11.78 BDL 9.81 7.43 12.83 11.78
C37 BDL BDL 0.15 BDL 0.26 0.65 BDL BDL BDL BDL 5.90 BDL 9.99 11.26 BDL BDL
C38 BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL 2.27 BDL BDL BDL BDL BDL
C39 BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL
C40 BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL
PRISTANE BDL BDL 0.27 0.16 BDL BDL BDL BDL BDL 5.32 392.91 6.70 BDL BDL BDL BDL
PHYTANE BDL BDL 0.04 0.02 0.08 BDL BDL 1.67 BDL 0.56 61.00 1.64 2.12 2.10 BDL BDL
Pr/nC17 0 0 0.90 0.63 0 0 0 0 0 0 41.84 0.98 0 0 0 0
Total 0.22 0.13 5.78 5.14 0.61 2.08 37.59 25.69 34.43 22.52 929.44 79.55 36.85 34.86 44.73 41.94

PAH diagnostic ratios of the studied soil and water samples in comparison with that of standard references (after Tobiszewski and Namieśnik [21])

PAH diagnostic ratio Value range Source Reference Value ranges from studied samples Inferred source
ΣLMW/ΣHMW <1 Pyrogenic Zhang et al. [26] 0–2.45 Petrogenic/pyrogenic
>1 Petrogenic
Fl(Fl + Pyr) <0.5 Petroleum Emissions Ravindra et al. [20] 0–0.4 Petroleum emissions
>0.5 Diesel Emissions

Ant(Phe + Ant) <0.1 Pyrogenic Pies et al. [18] 0–0.21 Petrogenic/pyrogenic
>0.1 Petrogenic

Flu(Flu + Pyr) <0.4 Petrogenic Fossil fuel Combustion Grass, wood, coal combustion De La Torre-Roche et al. [27] 0–0.4 Petrogenic/mixed source of fossil fuel and combustion
0.4–0.5
>0.5

BaA/(BaA + Chr) 0.2–0.35 Coal combustion Akyüz and Çabuk [23] 0–0.31 Coal combustion/petrogenic
>0.35 Vehicular emission
<0.2 Petrogenic Yunker et al. [28]
>0.35 Combustion

InP/(InP + BghiP) <0.2 Petrogenic Yunker et al. [28] 0–0.23 Petrogenic/petroleum combustion
0.2–0.5 Petroleum Combustion
>0.5 Grass, Wood, Coal Combustion

The AH diagnostic ratios (after Sojinu et al. [22])

Sample CPI OEP
Cbw_1 0.29 1.81
Cbw_2 0.06 0
Pw_1 0.76 0.82
Pw_2 0.71 0.79
Sw_1 2.14 0
Sw_2 1.47 0
Css_1 0.44 0.21
Css_2 0.60 0
Css_3 2.46 0
Css_4 0.24 0.27
Css_5 0.85 0.72
Css_6 0.87 0.94
Css_7 0.68 0.006
Css_8 1.09 0.31
Css_9 0.57 0
Css_10 0.48 0
Mean 0.86 0.37

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