Table 1 shows the occurrence of PAHs with their concentrations in crude oil contaminated soil samples from Ekpan town. The PAHs are composed of two to six fused rings compounds, which include those PAHs considered as carcinogens by the US environmental protection agency. The occurrence of these PAHs is attributed to transport of crude oil and its spill around NNPC refinery and NNPC petrochemical industry located in the town. The most abundant of PAHs were napththane with concentration ranging from 9.74 mg/kg to 17.2 mg/kg. Benzo(b)fluoranthene, indeno (1,2,3,cd) pyrene and dibenzo(a,h)anthracene had the lowest concentrations of 0.44 mg/kg, 0.13 mg/kg and 0.08 mg/kg for SA, SB and SC respectively. The relative occurrences were 15.4-17.1% (two rings PAHs), 33.3-38.8% (three rings PAHs), 37.2-40.2% (four rings PAHs), 6.9-7.1% (five rings PAHs) and 0.6-1.6% (six rings PAHs) of the total PAHs. This implies that the concentrations of PAHs with 5-6 rings were relatively low in all soil samples.
The total PAHs concentrations were 112 mg/kg, 67.3 mg/kg and 56.8 mg/kg for SA, SB and SC respectively (Table 1). Surprisingly, PAHs concentrations in soils from this town were much higher in magnitude than what was reported (0.395 ± 0.190 mg/kg) as PAH concentration in a conventional farming site in Orgeval catchment of early industrialized region in France [28
]. PAHs concentrations in soils from Ekpan town were compared with some environmental quality criteria for PAHs in soil released by the Netherlands Ministry, Canadian Councils of Ministers of Environment, British Columbia Ministry of Environments, for assessing risk of contaminated sites. The necessity for employing these standards stems from the fact that such environmental standards for PAHs in soil are not established yet in Nigeria.
The significance difference between PAHs concentrations measured and 0.1 mg/kg of PAHs set as the Canadian soil quality criterion for agricultural use [29
] may possibly be due to pollution generated by crude oil exploration in Ekpan town. This implies that the soil is polluted with PAHs compounds and may pose risk to human health if cultivated for agricultural purpose. The inhabitants of Ekpan town may be predisposed to high risk of cancer due to long-term exposure to such compounds through bioaccumulation in the food chains. PAHs have very high lipid solubility and hence may quickly be absorbed into the gastrointestinal tract of humans [30
]. More hydrocarbons were found in Ekpan soil contaminated possibly by spills when compared with soils from the control site (Botanical garden) as far as in Ibadan, which is usually uncontaminated.
Figures 2-5 give overview of studies on incubating contaminated soil with P. sajor-caju
for 2-, 4- and 6 weeks respectively. PAHs were potentially biodegradable with the fungus. Significant reductions in all the PAHs concentration were observed with increase in incubation periods. The ranges of the extent of degradation in PAHs concentrations, between the incubation periods of two weeks to six weeks were indicated in the figures.
As a way to evaluate possible sources, the ratios of some PAHs compositions regarded as molecular indices were used to distinguish the natural and anthropogenic PAHs in soils [32
]. The use of these molecular indices depends on the fact that PAHs distribution is dependent on temperature [33
]. The PAHs distribution is governed by thermodynamic properties during low temperature processes such as catagenesis of organic matter leading to the formation of petroleum. On the other hand, their distributions are governed by kinetic properties during high temperature processes such as pyrolysis of organic matter [32
The relative kinetic and thermodynamic stabilities of different PAHs molecules during their formations in petroleum source rocks under different heating or cooling conditions and by their comparable environmental fate were related to their distribution in substrates. So, the molecular indices were derived by evaluating the ratios of phenanthrene/anthracene (Phe/Ant) within the 3-ring PAHs group and fluoranthene/pyrene (Flu/Pyr) within the 4-ring PAHs [35
]. Phenanthrene is much more stable than anthracene so that at low temperature molar fraction of phenanthrene produced is higher than that of anthracene. This might possibly explain higher levels of phenanthrene than that of anthracene as reflected in Figure 3. Consequent upon this, biodegradation of phenethrane by P. sajor-caju
in terms of reduction in concentration (21.5-35.7%) was low compared to that obtained for the biodegradation of anthracene (35.9-47.9%).
Baumard et al. [36
] proposed Phe/Ant ratio<10 and Flu/Pyr ratio>1 as quantitative indication of PAHs coming from pyrogenic source (such as petroleum combustion through a flare stack) and Phe/Ant ratio>15 and Flu/Pyr ratio<1 as indication of PAHs originating from petrogenic source (such as oil spill contamination). The Phe/Ant ratios in this study for all the samples ranged from 1.11 to 1.59 and Flu/Pyr ratios ranged from 0.70 to 0.78, indicating that there were both pyrogenic and petrogenic influences or input of PAHs on Ekpan soil samples. This implies that there is a natural leakage of oil related hydrocarbon in this study area. The basic concept of soil sorption of organic molecules suggests that the sorption of hydrophobic organic molecules is related to the organic matter derivable from organic carbon content of the soil [37
organic content varied from 3.28% to 5.80%. The results of regression analysis conducted to determine the relationship between the total PAHs concentrations and the soil organic contents showed positive exponential relationship with r2
of 0.8560. The relationship implies that high amounts of PAHs mainly occur in soil with high organic content.