Geochemical Fingerprinting pf Oil-Impacted Soil and Water Samples In Some Selected Areas in the Niger Delta

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

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.

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].

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.

Figure 3

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.

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.