The inevitable and disastrous consequence of crude oil pollution for the biotic and abiotic components of the ecosystem has been a major source of concern to the government and people living in oil producing and industrialized countries. This had led to ethnic and regional crises in the Niger Delta region that generated significant tension between them and the multinational oil companies operating in the region (Vidal, 2010).Crude oil exploration, production and transportation in the Niger Delta region have increased tremendously since its discovery in Nigeria in 1956 and has become a veritable source of economic growth and the main stay of the Nigerian economy (Okoh, 2006).The global scale of oil production is staggering and its demand is in the order of 3.25 x 109 tones or 3.8 x 1012 liters per year and much of it is transported thousands of kilometers before it is used (Prince and Lessard, 2004).
Crude oil is a complex mixture of organic compounds including volatile aromatic fractions and less volatile aliphatic fractions. The main constituents of crude oil are the elements hydrogen (10 – 40%) and carbon (83 – 87%). Various types of crude oil contain small quantities of sulphur, nitrogen, oxygen and trace metals such as vanadium, nickel, iron and copper which are not usually found in refined petroleum (Atlas and Bartha, 1973). Individual chemical composition of each crude petroleum however, depends on its origin and location and has a unique mixture of molecules which defines its physical and chemical properties. Crude oil has been part of the biosphere for millennia and has been used since ancient times in one form or the other and has risen in importance due to rise in commercial aviation, invention of internal combustion engines and the increasing use of pesticides, fertilizers and plastics which are mostly made from oil (Okoh, 2006).
Soil is an extremely complex, dynamic and living medium, formed by mineral particles, organic matter, water, air and living organisms. It establishes the interface between earth, air and water and performs many vital functions. The importance of soil for the survival of plants has become apparent due to numerous services it renders, ranging from filtration of ground water, removal of pathogens, degradation of organics, recycling of nutrients on which agriculture thrives and provision of raw materials for industries which are of economic value. Human activities such as the production, transportation, storage and sometimes vandalization of oil facilities accidentally release large quantities of crude oil and its fractions to marine and terrestrial environments thereby posing a long term threat to the soil and the services it renders (Blum, 1997).
Crude oil is a fossil fuel derived from ancient fossilized organic material. The fossilization processes include the initial process of diagenesis and the final or completion process called catagenesis. The initial process of diagenesis occurs at temperatures at which microbes partially degrade the biomass and result in dehydration, condensation, cyclisation and polymerization of the biomass. Subsequent burial under more sediments at higher temperature and pressure allows catagenesis to complete the transformation of the biomass to fossil fuel by thermal cracking and decarboxylation (Prince and Lessard, 2004).
1.1 CRUDE OIL POLLUTION OF THE ECOSYSTEM
The release of crude oil and its fractions into the natural environment has adverse ecological impacts on both terrestrial and aquatic ecosystems. In Nigeria, it has been reported that, annually an estimated quantity of 2,105,393 barrels of oil was spilled on land, coastal and offshore marine in the Niger Delta region between 1976 and 1990 (Kontagora, 1991). The impact of oil exploration and exploitation on the environment is one of the inevitable consequences of industrialization and economic development in Nigeria (Osuji and Onojake, 2006). In aquatic ecosystem, crude oil floats and blocks out sunlight, thus initiating the death of phytoplanktons and seaweeds, which are at the base of the aquatic food chain, thereby starving organisms that depend on them. Crude oil also has become one of the most frequently detected underground water pollutant caused by leakages from underground tanks, pipelines and other components of crude oil distribution (Kharoune et al., 2001; Gwendoline, 2010). Soil soaked with crude oil loses fertility and initiates environmental degradation and ecological succession. Crude oil pollution changes the composition of soil microorganisms and alters the physicochemical properties of the soil rhizosphere which affects plant growth and development (Gesinde et al., 2008; Ebere et al., 2010).