One of the unintended impacts of crude oil exploration is the pollution of agricultural land and aquatic resources when there is accidental oil spillage. The environmental effect of this has resulted in untold hardship to communities within oil producing regions all over the world and has been the main reason for conflicts in the regions. Nigerian Niger Delta region has been devastated by oil pollution and gas flaring, and both the government of Nigeria and the multinational oil producing companies have been searching for solutions for the recovery of oil polluted soil. Among the various possible methods of recovery of crude oil contaminated soil, bioremediation is most environmentally friendly.

A promising bioremediation method is the use of crops with phytoremediation capabilities. Cowpea (Vigna unguiculata) which belongs to the family of legumes is one of such crops. In addition to their nutritional value, legumes can be used in the cleaning up of crude oil contaminated soils through different mechanisms such as; biodegradation in the rhizosphere, plant uptake, translocation, transformation, and phytovolatilization. This research was aimed at evaluating the phytoremediation capabilities of cowpea towards the recovery of crude oil contaminated soil in the Niger Delta region of Nigeria.

Five (5) plastic buckets with 1000g of soils were contaminated with crude oil at different levels; 0 ml, 30 ml and 40 ml. The remaining two buckets contained 30 ml crude oil + 0.5g Indole acetic acid (IAA) and 40 ml crude oil + 0.5g IAA respectively. The Indole Acetic (IAA) - a growth hormone was introduced to the contaminated soil to improve performance. Cowpea was grown in the soil and the germination and height of the crop were monitored. The presence of heavy metals in the soil and plant were measured.

Results show that the heights of cowpea plants in the control experiment were greater than those of plants in 30 ml crude oil + IAA, while the plants in 30 ml crude oil only being the least. Atomic absorption spectroscopy analysis of the plant samples indicated cowpea plants in 40 ml crude oil + IAA to have the highest accumulation of heavy metals. EDXRF results of the soil samples presented the soil with 40 ml of crude oil to have the highest accumulation of heavy metals. These results indicate that cowpea has phytoremediation capabilities. Indole- 3-acetic acid has the potential of facilitating the rate of phytoremediation of contaminated soil.
Thus, it is concluded that cowpea can be used to clean-up crude oil contaminated farm lands in the Niger Delta, Nigeria. 

1.1 Background
Bioremediation refers to the use of biological mechanisms to destroy, transform, or immobilize environmental contaminants so as to protect potential sensitive receptors. (Bioremediation Discussion Group, 2006). Two remediation techniques are; Ex situ and In situ remediation techniquesEx situ remediation techniques involve taking out the soil from the subsurface in order to treat it. In situ remediation techniques encompass leaving the soil in its original place and bringing the biological mechanisms to the soil. Earlier thermal, chemical and physical treatment methods have been unsuccessful in eliminating the pollution problem because those approaches only move the pollution to a new phase such as air pollution. Bioremediation technology, which leads to degradation of pollutants, may be a worthwhile and environmentally beneficial substitute that could produce an economic profit (Iranzo et al., 2001.)

In Niger Delta, Nigeria, increased petroleum exploration has led to an unprecedented accidental release of crude oil polluting the land and water sources of the area. Furthermore, illegal tampering of well heads, flow lines, pipelines, manifolds and flow stations have added to the total amount of crude oil entering the environment. With recurrent reports of oil spillage in Niger Delta, there is a need for a cost effective method for remediation of crude oil contaminated soil. Crude oil affects the physicochemical properties of the soil such as temperature, structure, nutrient status and pH when spilled on land (Udeh et al., 2013).

In Nigeria, contamination of soil and water by crude petroleum and refinery products is a rising problem as oil mining and refining activities increase steadily (Vidal, 2010). This problem results particularly in the loss of fertility of agricultural lands and death of plants, including crops, in the oil producing areas of Nigeria. Soil contamination by crude oil spills is a widespread environmental problem requiring cleaning up of the contaminated sites (Bundy et al., 2002). The most common sources of petroleum contamination are the disposal of oil....

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