Environmental Evaluation and Review of Rehabilitation of Soils After Oil Spills in Niger Delta

Very heavily polluted soils can be easily reclaimed by applying scientific principles but very light pollutions of the order of 1% or below which in actual fact may be beneficial to plant growth can be left for natural rehabilitation within 6 months and much higher concentrations of the order of 5% can easily be taken care of by farmers within the same period simply by the aeration effected by hoeing or cultivating the land. From the agricultural and scientific points of view, therefore, oil pollution is just a temporary setback to farming processes and just how temporary is determined by the level of pollution. Field work was carried out to review and identify the potentials and challenges facing the cleanup exercise. The results revealed that a measurement of the physical compaction revealed the critical value > 1.6g cm-3 at the impacted areas, while, the point of spill and areas near water course were scaled as firm and extremely compacted respectively. On the other hand, the drainage pattern showed dominance of capillary pores on the surface to much aeration pores as it increased with depth. The value of permeability (5.49cmhr-1) on the 0-15cm soil depth met the rapid discharge limit and acceptable range for ease of remediation process. Whereas, the lowest permeability value of 0.36cmhr-1 representing 143% reduction from subsurface recorded at depth greater than 60cm was attributed to irregular (tortuous) pathways that increased with depth. Soil depths beyond 60cm were seen to consistently superior to other depths for low permeability.


Introduction
The soil is undoubtedly the most important basic natural resource of any nation. It is an irreplaceable natural resource and thus the need to conserves it. Therefore oil pollution very often generates a lot of emotional outburst [1]. Concerns over the effects of increasing oil spillage on land, streams, rivers and wetlands have mounted with time since oil prospecting started in Nigeria. Whatever the means of spillage, oil spillages resulting from oil well blow-outs and spillages from pipeline where oil is transported under high pressure are usually of a much higher magnitude especially if the spillage occurs is in a remote area where the spillage is not detected for a long time [2]. Oil has been considered as something foreign to the soil and the apparent longstanding adverse effects of oil from pipeline leaks or spillages from oil locations on crops and soils is of great concern to soil conservationist [3]. There is thus need to quickly rehabilitate oil polluted soils and get them back to normal. The impression has often been given that once oil heavily pollutes the land, it remains indefinitely. Our field and laboratory studies have revealed that certain changes that occurred on the soils' physical properties which control the bio-chemicals are attributable to the action of the rehabilitation in the Rumekpe's soils after oil spill. Looking at these soils from the mechanistic and agricultural standpoints, the investigation aimed at assessing the extent to which the total concentration of fluid (water, air and oil) enters the soil from the surface as a function depends of the distribution of the soils' physical characteristics down the soil profile.

Description of study area
The study was conducted in Port Harcourt. Port Harcourt is located in latitude 4o49'N and longitude 6o59'E in southeastern Nigeria, on the Bonny River, in the Niger Delta ( Figure 1). The city is a leading port of the country, and a major industrial center. Port Harcourt has prospered as the regional headquarters for the petroleum industry. It has a large petroleum refinery and storage facilities. The local fishing industry is also important to the city's economy. Located in the city are the University of Port Harcourt (1975) and Rivers State University of Science and Technology (1971). While the Ijaw people are dominant in the region, the city itself is ethnically diverse. The port was established by the British in 1915 and serves eastern Nigeria and parts of northern Nigeria [4].

Climate
Climatic condition of the area is characterized by two seasons, wet or rainy and dry season. The wet prolongs with a mean annual rainfall of 200mm-300mm. the dry season commences from November lasting till February of march with an annual mean temperature of 26°C and 30°C, highest temperature are experienced between January and February which coincides with over head of the sun with relative humidity varying from 70% to 80%. The area is characterized by growth of shrubs, herbs and medium to tall trees, native vegetation, grasses and sometimes referred to as "derived savanna" [4]. They are highly susceptible to accelerated erosion. After three years of rehabilitation of soils after oil spill, bulk and core samples were collected at three (3) stratified areas christened: point of spill area on the right of way (Q-S), impacted points (H-P) and area close to water course (A-G) ( Figure 2) two sampling depths of 0-15 and 15-30cm were properly demarcated. Samples were chained custody and properly preserved as described by [5] and a total of one hundred and forty-four (144) samples were collected and analyzed for various soil attributes.

Sample preparation and statistical analysis
Soil samples collected for particle size distribution analysis were properly air dried, crushed with plastic mortar and then made to pass through a 2mm laboratory sieve then analyzed using Bouyoucos Hydrometer Method as described by [6]. Soil samples collected for aggregate stability were air dried and then made to pass through a 4mm laboratory sieve and then analyzed using (Wet serving Technique) [7] Soil samples for Saturated Hydraulic Conductivity (K sat ) collected were saturated in water for 24hrs and then analyzed using (Constant-Head soil core method) [8]. Soil attributes were statistically analyzed with computer soft ware and results were graphically presented.

Particle size distribution
The range of particle size distributions in the soils tends to show an overall means of 67.22, 10.93 and 21.84% for sand, silt and clay respectively, at the points of spill. The adjacent control points recorded a typical range of total sand in soils of 53.3-3.3% with a mean of 63.3% and 10.2 (M4)-36.7% (P4-3) of clay with a mean of 26.7%. Silt content was consistently 10% in the control area. Rather than being one type or the other, these soils are a combination of sand, silt and clay with clear trend of high concentrations in sandy soils and low conductivity of water in soils with larger clay content.
On the other hand, most of the impacted areas had average permeability rating ranging from moderately rapid (19-36.1cm day -1) to moderately slow (4.9-17.3cm day -1 ) permeability. Theoretically, the ratio of the height of fluid above the soil to the bulk density is the driving force, changes noticed on this field are brought about by soil texture and bulk density under test. It can be observed that permeability rate increases at increase in sand size and lower bulk density.

Conclusion
Findings in this study showed that the areas with high bulk density and very slow permeability revealed the direction where more rehabilitation work is expected. From agricultural and scientific points of view, oil pollution is just a temporal setback to farming processes and just how temporal is determined by the level of pollution and cleanup exercise. It must be stated, however, that during this period of temporary setback the inhabitants of the area or landowners may suffer a lot of inconveniences economically in terms of destruction and deprivation of their land from farming. A programmer of rapid rehabilitation and monitoring is necessary.