Petroleum contamination of Sombreiro River in Akuku-Toru Local Government Area Rivers State, Nigeria, revealed by Chemical Fingerprinting of Aliphatic Hydrocarbons

Water samples were collected from the lower section of Sombreiro River in Akuku Toru local government area (L.G.A.) of Rivers State, Nigeria and analyzed for aliphatic hydrocarbons using gas chromatographyflame ionization detector (GC-FID) after extraction and clean up. GC identified well resolved aliphatic hydrocarbons from C 9 to C 40 which showed similar bimodal n-alkane distributions and unresolved complex mixtures (UCM). Pristane was observed as the most prominent aliphatic hydrocarbon and dominant over phytane in all the samples. Common aliphatic hydrocarbon diagnostic ratios used for fingerprinting and interpreting chemical data were determined. These include carbon preference index (CPI) from 0.70 to 0.82; Pr/Ph from 1.85 - 2.17; Pr/nC 17 from 1.67 - 1.87; Ph/nC 18 from 0.94 - 1.07 and cross plot.The results reveal the following: (1) crude oil contamination of the lower section of Sombreiro river in Akuku Toru L.G.A. of Rivers State; (2) the same crude oil spilled in the river; (3) the spilled oil is Niger Delta in origin, derived from mixed marine and terrigenous organic matter deposited in an oxic environment; (4) the spilled oil was slightly degraded, its chemical composition has not undergone significant alteration. Keywords : Petroleum, Aliphatic hydrocarbon; Gas chromatography,  Fingerprinting, Correlation,

Spills and leaks of crude oil and its refined products cause widespread contamination of the environment. It can occur due to accidents, from industrial releases, or by-products of commercial or domestic uses (Ou et al., 2004 andMille et al., 2007). Crude oil is composed of a complex mixture of hydrocarbons classified in bulk groups of saturates, aromatics, resins and asphaltenes (Wang et al., 2006). The saturate include aliphatic and cycloalkanes which consist of hydrocarbons up to C40 (Rudzinski and Aminabhavi, 2000). Aliphatic hydrocarbons (AHCs), normal and isoalkanes, are the predominant class of chemical compounds in most crude oils and refined products (Tissot and Welte, 1984). Their distribution and compositionin petroleum is unique and used for chemical fingerprinting of petroleum spills (Wang et al., 2006). Chemical fingerprinting of the AHCs can provide information on the biological sources of organic matter which generated the petroleum, the condition of deposition at that time, the age of the petroleum source rock, the relative maturity, and the degree of microbial biodegradation (Osuji, 2011). They can also provide reliable environmental forensic information used for the determination of the source of unknown oil spilled, even for biodegraded oils, correlation, differentiation and subsequent monitoring of the affected ecosystems (Peters et al., 2005). Samples collected from contaminated environments are usually analyzed for the AHCs by gas chromatography (GC) and the composition data obtained are used for characterization by chemical fingerprinting techniques (Wang et al., 1999;Sun et al., 2009). Diagnostic tools of AHCs generally employed for chemical fingerprinting related environmental forensic investigations include carbon preference index (CPI), ratios of pristane/phytane (Pr/Ph), pristane/nheptadecane (Pr/nC17) and phytane/n-octadecane (Ph/nC18) (Wang et al., 1999;Seki et al., 2006). Simoneit et al. (1990) used aliphatic ratios of nC15-35/nC16, ΣnC15-20/ΣnC21-35 and nC31/nC19 for assessing petroleum contamination.The Niger Delta is located in Southern Nigeria, between longitude 5° -9° E and latitude 4° -6° N, on the Atlantic coast of West Africa. It is endowed with enormousnatural resources, especially hydrocarbons (crude oil) which is the mainstay of Nigeria's economy. River Niger splits into rivers, forming deltas, via which it drains into the Atlantic Ocean. Sombreiro river is one of the major rivers of the Niger Delta. The Sombreiro river flows from Egbema area of Imo State into Rivers State through a number of communities and empties into the Atlantic Ocean (Iyama et al., 2014).This study describes the chemical fingerprinting and data interpretation technique used to reveal hydrocarbon

Study area:
The study area is the lower section of Sombreiro River located in Akuku Toru local government area (L.G.A.) of Rivers State in Niger Delta, Nigeria. Akuku-Toru L.G.A. is typically riverine and endowed with abundant oil and gas reserves. Sombreiro River is one of the outlets through which the River Niger drains into the Atlantic Ocean. It is mainly freshwater and predominantly occupied by rural communities that depend on the water for drinking, bathing, washing of clothes and fisheries (UNDP Report, 2006). Other activities carried out in the river include washing and fermentation of cassava, defecation and the dumping of refuse and abattoir. These activities result in the discharge of various pollutants into the aquatic environment, threatening the health of the local dwellers and damaging the quality of the environment (Abowei and Sikoki, 2005).
Sampling: Water samples were collected from four (4) points of the lower section of the Sombreiro River located in Akuku Toru L.G.A. of Rivers State between longitude 6° 46′39.4″ E to 6° 48′ 28.2″ E and latitude 4°34′46.8″ to 4° 40′04.7″ N. The water samples, one (1) litre each, were collected using a dark glass bottle. The sample bottles were sealed, labelled appropriately, transported to the laboratory and stored in the refrigerator prior to analysis.
Sample extraction and cleanup: Petroleum hydrocarbons were extracted from the contaminated water samples according to the separatory funnel liquid-liquid extraction EPA Method 3510 (EPA, 1996). 50 ml each contaminated water sample was poured into a separatory funnel and equal volume of hexane added. The separatory funnel was agitated for 30 minutes with a mechanical shaker and the mixture left to stand for two hours to allow for maximum separation of the aqueous and organic phase. The organic phase was concentrated by evaporating hexane under a stream of nitrogen at 40 °C. Cleanup was achieved by transferring the concentrated extracts onto the top of a glass column (25 cm x 1 cm) packed with activated silica and plugged with glass wool at the bottom. n-hexane was poured into the column to elute the saturates, which contains the aliphatic hydrocarbons. The eluent was concentrated by evaporating the solvent at room temperature in the laboratory.
Gas Chromatography (GC) analysis: Detailed aliphatic hydrocarbon analysis was performed on each sample using an Agilent 7890B gas chromatography (GC) system equipped with a HP-5 silica capillary column 30 m x 320 μm ID and 0.25 μm film thickness, flame ionization detector (FID). The clean-up concentrates were dissolved in hexane and 1μL of each was injected into the GC capillary column with the aid of a G4513A automatic liquid sampler (ALS), using the splitless injection mode. Hydrogen was used as carrier gas at a linear velocity of 30ml/minand the oven temperature was programmed from 40 °C to 260 °C at 10 °C/min with a 5 mins hold at 40 °C and a 20 mins hold at 260 °C.

RESULTS AND DISCUSSION
Gas chromatography (GC) analysis of all the samples showed well-resolved peaks of aliphatic hydrocarbons from C9 to C40 (Fig. 1). Petroleum oil and its refined products can be identified by their GC carbon range profile (Wang, 2009). The aliphatic hydrocarbon range of the water samples was considerably wider than that of refined petroleum products but similar to crude oils. This suggest the source of contamination of Sombreiro River in Akuku Toru L.G.A. was crude oil.
The GC profiles of the water samples showed bimodal n-alkane distributions ( fig. 1). GC fingerprints characterized by the dominance of the C13 -C20 nalkanes reflect marine sources for crude oils, while the dominance of the C25 -C35 n-alkanes are associated with terrestrial sources (Zhu et al., 2005). All the water samples showed the abundance of n-alkane in the C13 -C20 and C25 -C35 range. These bimodal distributions suggest the spilled crude oil in Sombreiro River was derived from mixed marine and terrestrial sources. The rise above the baseline of the sample chromatograms, unresolved complex mixtures (UCM), was observed ( fig. 1). UCM consist of hydrocarbons connected at one or more branched points which are not resolved into peaks during gas chromatography. It is a strong evidence for petroleum hydrocarbon contamination and degradation (Peters and Moldowan, 1993). Pristane (Pr) and phytane (Ph) are the most common isoprenoids in crude oils (Tissot and Welte, 1984). In Niger Delta crude oils, pristane is characteristically dominant over phytane (Stout et al., 2007;Manilla and Onyema, 2008). From fig. 1, it was observed that pristane was the most prominent aliphatic hydrocarbon and was dominant over phytane in all the water samples ( fig. 1). This suggest a similar crude oil of Niger Delta origin as the source of contamination of Sombreiro River. The carbon preference index (CPI) is the ratio of odd to even number n-alkanes between C23 and C34 and is used to indicate the source of aliphatic hydrocarbons (Didyket al., 2000). CPI values close to 1 indicate crude oil source values below unity suggesting degraded crude oils (Maioli et al., 2010;Onyema et al., 2013). Higher CPI values represent oiled sediment with increased terrigenous input, while values from 4 to 10 indicate land plant sources (Kennicutt et al., 1987;Jeanneau et al, 2006). From table 1, the CPI values from 0.70 to 0.82 were below unity. This indicated the crude oil which contaminated the waters of Sombreiro river was degraded. nheptadecane (nC17), pristane (Pr), n-octadecane (nC18) and phytane (Ph) are aliphatic hydrocarbons commonly used for characterization in oil spill studies. Each pair of compounds elute almost simultaneously from the gas chromatograph. Ratios derived from these hydrocarbons are Pr/Ph, Pr/nC17 and Ph/nC18 (Table 1). Pr/Ph ratio is a commonly used indicator of diagenetic condition during organic matter deposition. Values below 1 indicate anoxic deposition, between 1 and 3 reflect oxic deposition and above 3 indicate terrigenous source (ten Haven et al., 1988).   (1985) stated that with increasing maturity, n-alkanes are generated faster than isoprenoids, but during biodegradation, they are degraded faster than the isoprenoids leading to increased ratio values. Ratio values of Pr/nC17 and Ph/nC18 for Niger Delta crude oils are 1.35 and 0.66 but for significantly degraded oils 3.55 -3.78 and 2.45 -2.61 respectively (Onyema et al., 2013). For the water samples from Sombreiro river, ratio values of Pr/nC17from 1.67 -1.87 and Ph/nC18 from 0.94 -1.07 suggest the spilled oil was slightly degraded. Shungunam (1985) used the cross plot of Pr/nC17 and Ph/nC18 ratios to provide information on source, diagenetic conditions, maturation and biodegradation (Fig. 2).

Fig 2:
Cross plot of Pr/nC17 versus Ph/nC18 ratios for water samples from Sombreiro River Figure 2 suggest the spilled oil which contaminated Sombreiro River was derived from mixed marine and terrigenous sources deposited in a suboxic to oxic environment. The cross plot also showed the spilled oil was degraded in the order: AK-A2> AK-A3> AK-A4 >AK-A1.
Conclusion: Gas chromatography analyses of water samples from Sombreiro River in Akuku Toru L.G.A. of Rivers state identified aliphatic hydrocarbons from C9 to C40. Pristane was the most prominent aliphatic hydrocarbon and dominant over phytane. Bimodal nalkane distributions, ratios of CPI, Pr/Ph, Pr/nC17, and Ph/nC18 suggest slightly degraded crude oil derived from mixed marine and terrigenous sources deposited in a suboxic to oxic environment. The results revealed a typical Niger Delta crude oil contaminated the Sombreiro River.