Oil spills in the Niger Delta region of Nigeria has occurred frequently since oil extraction started in the 1950's. The oil spills originates from facilities and pipelines, leaks from ageing and abandoned infrastructure and from spills during transport and artisanal refining of stolen oil under primitive conditions. It is estimated that spills in Nigeria amount to 100 000 to 200 000 tons per year and have been doing so for almost 60 years. While a number of reports have been written about the Niger Delta and the civil unrest in this area during the last decades, very few scientific reports with actual data regarding the extent of the contamination has been published. This paper describes the contamination of sediments and water in a part of the Niger Delta, which has been particularly hard to assess for decades: Ogoniland. It does not discuss the origin of the oil spills.
During 2010, the United Nations Environment Programme (UNEP) conducted an extensive environmental assessment of Ogoniland. The assessment was conducted at the request of the Nigerian government. During the assessment, drinking water samples were taken in wells and sediment and surface water samples were collected from streams, ponds and wetlands in and around Ogoniland from April to November. The levels found in the more contaminated sites are high enough to cause severe impacts on the ecosystem and human health. Extractable Petroleum Hydrocarbons (EPHs) reached levels of up to 7420 μg/l in surface water and drinking water wells show up to 42 200 μg/l. Benzene levels were measured up to 9000 μg/l, which is more than 900 times the WHO guidelines. EPH concentrations in sediments were up to 17 900 mg/kg. Polycyclic Aromatic Hydrocarbon (PAH) concentrations in sediments reached 8.0 mg/kg in the most contaminated sites. The impacts of this pollution were obvious to be seen, with large slicks of crude oil visible in the water and large areas of mangroves suffocated by oil. However, most sites did not show extremely high levels of EPH and PAH concentrations.
Although the natural conditions for degradation of petroleum hydrocarbons are favorable with high temperatures and relatively high rainfall, the recovery of contaminated areas is prevented due to the chronic character of the contamination.
Oil spills have occurred repeatedly for decades in the Niger Delta and large parts of the land and wetlands are chronically affected by oil spills. Due to the influence of the tides and at times floods in connection with rains, spilt oil is rapidly distributed over large areas and remobilized with rising tides. The oil originates from leaking pipelines, wellheads and flow stations, transportation and loading of oil, illegal tapping of the wells and from artisanal refining. As a result of the contamination of oil in mangroves and wetlands as well as on land, oil has penetrated into soils down to several meters and has contaminated ground waters over large areas. This has resulted in the contamination of water wells as a particularly serious concern from a human health perspective (Ana, Sridhar, & Bamgboye, 2009; Mmom & Arokoyu, 2010; Moffat & Lindén, 1995; UNEP, 2011).
The Niger Delta has been the scene of a long series of conflicts, two of the most serious being the Biafra War in 1967-1970, and the Ogoni Uprising in the 1990's. The violence has centered around the cities of Warri in Delta State and Port Harcourt in Rivers State (Human Rights Watch, 2005; Idemudia, 2009; UNEP, 2011). As a result of these conflicts, oil companies have for decades found it difficult to carry out oil production in parts of the Niger Delta. One such area is Ogoniland, east of Port Harcourt. This area consists of four Local Government Areas (LGAs): Eleme, Tai, Gokana and Khana with a total population of around 830 000 (UNEP, 2011).
During 2009-2011, at the request of the Federal Government of Nigeria, the United Nations Environment Programme (UNEP) carried out a survey of the nature and extent of oil pollution in Ogoniland. The assessment covered contaminated land, ground and surface water, sediments, vegetation, air pollution, public health, industry practices, and institutional issues. The assessments were made in collaboration with a number of partners in the region including experts from Rivers State University of Science and Technology, Nigerian Government, agencies at national, state and local government levels, traditional rulers and various community groups. An additional objective was to determine appropriate remediation measures to rehabilitate contaminated sites to the level of international standards. The full report including the results from all the assessments and the recommendations regarding rehabilitation and remediation was published in 2011 (UNEP, 2011). The results presented in the present paper were collected as a part of the UNEP assessment and focus on the petroleum hydrocarbon contamination of surface waters and sediment.
The sampling strategy was developed in collaboration with local experts from the Fisheries and Aquatic Environment Department of Rivers State University of Science and Technology. It should be pointed out that the security situation in Ogoniland at the time of the investigation was better than it had been for decades. Despite this, the fieldwork had to be carried out under stringent security conditions and at times sampling could not be carried out at all or was restricted in time and space.
A total of 41 sites in the four LGAs were visited for sampling of surface water during 2010 (see Figure 1). Due to security and logistical challenges, it proved to be impossible to sample at the same time each tide cycle. Sediment samples were taken at 37 sites, in conjunction with surface water sampling (see Figure 1). All samples were taken, stored, shipped and analyzed according to industry standards. ALcontrol Laboratories in Chester, UK conducted the analyses. Please refer to (Lindén & Pålsson, 2013) for details regarding methodology and analyses.
The results of the water analysis show clearly elevated concentrations at most of the sampling sites. Surface water levels of EPH from the streams and rivers averaged 773 μg/l, but ranged from below the quantification limit to 7 420 μg/l. PAH levels in water samples were generally below the quantification limit with a maximum of 2,0 μg/l (see Table 1).
Water samples taken in the local drinking wells used by communities in Ogoniland varied dramatically in the concentrations of EPHs (Lindén & Pålsson, 2013; UNEP, 2011). Many samples particularly from hand-dug wells in Tai showed levels ranging from 11 to 154 μg/l. Bore wells sometimes showed high levels of EPHs: the highest concentrations recorded were 20 000 to 42 200 μg/l. Furthermore, in bore wells in Eleme very high levels of benzene was also noted: up to 9 280 μg/l. The World Health Organization (WHO) guideline value for benzene in drinking water is 10 μg/l (WHO, 2011).
The presence of oil in creeks and water bodies were obvious as sheen on the surface or as oil slicks of considerable size (see Figure 2). From the ground it was difficult to assess the size of these slicks but over-flights showed oil more or less covering every surface of water for several square kilometers.
The results clearly show elevated concentrations of EPHs in all four LGAs. The highest concentrations were found in stations in Gokhana, where EPH concentrations ranged from 279 to 17 900 mg/kg (see Figure 3). In Eleme, one station showed 9 680 mg/kg (see Figure 4) and in Khana two stations showed 2 150 to 4 520 mg/kg (see Figure 5). In Tai, levels reached 12 000 mg/kg at the station with the highest concentration (see Figure 6). Please see (Lindén & Pålsson, 2013) for detailed numbers.
The concentrations of PAH in sediments are slightly above the quantification limits in most of the samples from the four LGAs. The exceptions are two sites in Gokhana (see Figure 3) and one in Tai (see Figure 6) where high concentrations were found. The PAH concentrations in Gokhana were up to 8 pg/kg, while 7 pg/kg was found in the most contaminated site in Tai.
The results of the water analysis show clearly elevated concentrations of EPH in most places where samples were taken. Concentrations of several hundred μg/l EPHs in rivers and creeks cause impacts to sensitive organisms and in waters with several thousand μg/l, even very tolerant organisms are likely to disappear (NRC, 2003). Moffat and Linden (Moffat & Lindén, 1995) reported similar concentrations of dissolved petroleum hydrocarbons in surface waters as those reported here as common concentrations in the Niger Delta in the 1990's. However, much higher concentrations were reported locally, for example Ibiebele (Ibiebele, 1986) reported concentrations of 53 000 to 62 700 μg/l in refinery waste water from the Niger Delta.
The concentrations found in drinking water are several thousand times above the Nigerian water standard of 3 μg/l and are clearly not suitable for consumption. The very high levels of the known carcinogen benzene in several wells in Eleme LGA, illustrates the threats to human health from the contaminated water in the area. The WHO guideline value for benzene in drinking water is 10 μg/l and the results from the wells show levels that are more than 600 times higher – in one case nearly 900 times the WHO guideline value (WHO, 2011).
The concentrations found in sediments over much of the area indicate clearly elevated levels. Concentrations of one hundred or several hundred mg/kg were found in Khana, Tai and Eleme. Concentrations of several thousand mg/kg were found in most of the stations in Gokhana and some of the stations in the other LGAs. However, in several stations in Tai and Khana, the concentrations were lower than what might have been expected considering the extensive surface contamination. The reason for this may be that oil concentrations may be very significant at the surface but due to relatively calm conditions only a small fraction of the oil is emulsified and distributed to the sediments. A large number of studies of petroleum hydrocarbon concentrations in more or less chronically contaminated freshwater and seabed sediments have been carried out around the world. The results have been summarized for example by the National Research Council in the USA(NRC, 2003). The studies show that sediment concentrations of EPH in areas far from urbanized coastal areas are often in the range of twenty to fifty mg/kg. Concentrations in the range of fifty to several hundred mg/kg are often found in coastal sediments where anthropogenic activities are intensive such as in densely trafficked ship channels, near marinas and similar areas. Near direct point sources of oil contamination, such as water-cooled oil refineries and oil terminals or areas that are exposed to large volumes of oil due to spills the concentrations may be thousand to several thousand mg/kg. A study in Iko River estuary (Nigeria) showed EPH of between 6,1 to 35,27 mg/kg in the sediment and in contaminated areas near an oil field in Akwa Ibom State (Nigeria) the sediment levels were up to 372,4 mg/kg (Essien, Eduok, & Olajire, 2011). Following the Deepwater Horizon incident in the Gulf of Mexico in 2010 one study (Zengel & Michel, 2013) showed concentrations of EPH from 60 000 - 87 000 mg/kg could be found in heavily oiled marsh soil. Following the Prestige oil spill in northern Spain in 2002, surveys showed total hydrocarbon concentrations in the sediment between 3,7 and 787 mg/kg during and after the spill (Juanes et al., 2007).
Compared with levels from other parts of the world, the PAH concentrations found in the present investigation are typical for areas under influence from intensive anthropogenic activities. The elevated levels found in Gokhana and Eleme are comparable to those expected to be found in shallow water sediments in coastal areas with intensive industrial activities and very close to urban areas (Jinshu, Richardson, Shouming, & Jianhua, 2004; Soclo, Garrigues, & Ewald, 2000). For example, in contaminated coastal sediments along the coast of Lebanon, levels up to 174 mg/kg were found in 2007 (Lindén & Rust, 2008). The concentrations of PAHs in heavily contaminated industrial sites in Calabar (Nigeria) varied from 1.8 to 334 mg/kg (Nganje, Edet, & Ekwere, 2006). About 10 years after the Gulf War, the concentrations of total PAHs in the shoreline sediments in Saudi Arabia were still high, up to and over 100 mg/kg (Bejarano & Michel, 2010).
The contamination of oil in large parts of Ogoniland is significant, particularly in Gokhana LGA. Fresh oil could be seen drifting on surfaces as a film or sheen almost everywhere. However, spills have occurred over extensive periods of time throughout Ogoniland. Evidence of this is the extensive area of dead mangrove vegetation covered with deposited asphalt tar. A further indication that the contamination has a very long history is that in many places the mangrove consisted of only dead trunks and thicker branches, while all thinner branches and twigs had been lost over time due to various degradation processes (see Figure 7).
This study is the first systematic investigation of the oil contamination in Ogoniland, Nigeria. It was part of a comprehensive UNEP-lead investigation to assess the environmental conditions in this part of the Niger Delta, where local opposition to the government and oil company operations have been particularly strong. The study showed extensive oil contamination of rivers, creeks and ground waters. The spills have caused extensive damage to the mangroves, where large areas of the vegetation have died. The concentration of oil in streams and creeks was up to 7 420 μg/l, in sediments from the bottom of creeks, streams and other water bodies up to 17 900 mg/kg. In drinking water wells up to 42 200 μg/l of petroleum hydrocarbons was found, about 14 000 times the Nigerian standard for drinking water. Benzene concentrations were up to 9 000 μg/l, more than 900 times the WHO guidelines. PAH concentrations in sediments reached 8,0 mg/kg in the most contaminated sites. Although the natural conditions for degradation of petroleum hydrocarbons are favorable with high temperatures and relatively high rainfall, the recovery of contaminated areas is prevented due to the chronic character of the contamination. The present investigation has shown very extensive contamination of mangroves, creeks and rivers, particularly in Gokhana. Even if the pollution were to stop, the fact that mangroves and wetlands have been so heavily impacted indicates that a recovery of the affected areas is a matter of many years, perhaps decades.
A large number of people made this investigation possible. We wish to particularly acknowledge Dr. Muralee Thummarukudy, Mr. Michael J. Cowing, Dr. Babu Gopinathan, Mr. Thorsten Kallnischkies, Dr. Donna Vorhees and Mr. Yves Barthelemy of the UNEP team and Dr. I. I. Kakulu Rivers State University of Science and Technology.
Prof. Nsirimah, Drs. S.N. Deekae, A.D.I. George, U.U. Gabriel and K. Wokoma of the Rivers State University of Science and Technology were instrumental in the planning of sampling sites. Mr. Andrew Akhighu of the UNEP team assisted in the fieldwork.