Modeling Soil Organic Carbon in Crude Oil Contaminated Soil in the Niger Delta

Oil spillage is a major environmental threat in south south of Nigeria where most of oil exploration and exploitation activities takes place. If this goes on unchecked or poorly managed, it would lead to total annihilation of the ecosystem. The objective of this study is to develop a model for the prediction of potassium depletion in the crude oil contaminated soil with time using regression analysis. Each sample containing 10kg of soil was artificially polluted with 0.5, 1.0, 1.5, 2.0, and 2.5 liters of crude oil (Bonny Light).  The potassium concentrations were determined using standard methods. Results shows that the concentration of residual potassium in the soil for all the volumes of crude oil introduced into the soil decrees significantly with time when compared to values obtained for the control samples. The concentration of residual concentration in the control soil sample was about three times higher than the concentration obtained for other samples. The results obtained from the derived model were very close to the experimental value. The model is suitable for determining potassium content in crude oil polluted site.


I. INTRODUCTION
Globally there is a growing concern over environmental pollution and its management.The three major areas of environmental pollution include water, air and land.One of the major causes of this environment pollution in Nigeria especially the Niger Delta region is as a result of hydrocarbon exploration and exploitation [1].This has led to the degradation of farm lands, pollution of air, surface and ground waters due to gas flaring.The natural recovery of crude oil polluted land is slow.Communities affected are denied meaningful and economic use of their lands a long time.Hence modeling soil organic carbon fate over time as a result of oil pollution has become imperative.The prediction will help to determine the level of degradation and possible bioremediation work to be carried out.A model may help to explain a system and to study the effect of different component and to make predictions about behavior.Modeling is a process of generating abstract, conceptual, graphical and or mathematical model.Reference [2]  (e-mail: nwakwasilevi@gmail.com).modeling as the act of constructing or fashioning a model of something or finding a relationship between variables.The trend in modeling is to collect existing records (data), establish relations through mathematical equations, and calibrate such equations in the way of assigning values of associated constant and adopting such equations for forecasting or prediction.Prediction takes us into the future for decision making as we examine different responses arising from changes in control variables.The panel data multiple regression analysis was chosen after considering some other engineering tools like finite element method, finite differences, neural network and Matlab due to its capacity to analyze data with several variables.It also gives the researcher a large number of data points by increasing the degree of freedom and reducing the collinearity among explanatory variables hence improving the capacity to produce the expected results in this research work.Analysis of the linear regression can be extended to cover situations in which the dependent variable is affected by several controlled variables (independent variables).In this case, the question is how soil organic carbon is affected by crude oil pollution at various levels in the soil during the duration of pollution.
The fate of soil organic carbon over time as a result of oil pollution is now a growing concern in the Niger Delta region of Nigeria.The objective of the study is to carry out a laboratory investigation using crude oil and soil samples collected from the region to determine the effect of crude oil pollution on the soil organic carbon over a period of time [6].Other authors whose publications were reviewed in respect to this research work include: [3]- [11].

Study Area
The study area is located in Owerri, Imo State and lies between latitude 5 0 22 ' 51.5 " N and longitude 6 0 59 ' 39 ' 3 " E, with an elevation of 61m.It is a humid tropical environment with average annual rainfall of 2400mm.The mean daily temperature is about 27 0 C. The geological formation in the area shows that the soils are derived from coastal plain sands called acid sands -Benin formation [12].

Modeling Soil Organic Carbon in Crude Oil
Contaminated Soil in the Niger Delta The study was carried out over a period of sixteen (16) weeks using different containers measuring 17cm (height) by 18.5cm (diameter).Samples measuring 10kg polluted soil were placed in each of the containers and exposed to the same atmospheric and environmental conditions.The soil used in the study was collected from the Federal University of Technology Owerri (FUTO) Research Farm from 15cm to 20cm depth with shovel.The soil was measured into containers and taken to the laboratory for treatment (greenhouse treatment).
The soil was air dried for two weeks and sieved through 2.0cm sieve.The soil samples labeled B, C, D, E, F, each weighing 10kg were polluted with 0.5, 1.0, 2.0, 2.5 liters of crude oil (Bony light) respectively, and thoroughly mixed on a polythene sheet and put in a labeled container.
Sample A was not polluted and was used as the control.To maintain the moisture content of the soil, 50cl of water was sprinkled on each polluted soil sample at two weeks' intervals.
The polluted samples were allowed to stay 14 days before commencement of analysis.The representative samples from (A, B, C, D, E, F) containers were taken at two weeks' intervals to the soil science laboratory of Department of Crop, Soil and Pest Management, School of Agriculture and Agricultural Technology, FUTO for analysis to determine the soil organic carbon nutrient with time at various levels of pollution with crude oil.The concentration remaining after 14, 28, 42, 56, 70, 84, 98 and 112 days' intervals were obtained.
For determination of soil organic carbon, half a gram (0.5g) of each air dried soil sample was put in a conical flask and 2.5ml of 1N K2Cr2O7 solution was added and swirled gently to disperse the sample in solution.5ml of Concentrated Tetraoxosulphate (VI) H2SO4 was added rapidly into the flask and swirled gently until the sample and reagent were mixed and finally swirled vigorously for about a minute.The flask was allowed to stand in a fume cupboard for 30min.five (5) drops of the indicator were added to FeSO4 solution until the color changes to maroon color.A blank determination was carried out to standardize the dichromate [14] organic carbon content were calculated as follows: Where    2  2 = 1  2.5    4 = 0.5  .    0.003 =  ℎ   This was repeated for various levels of crude oil pollution in the soil sample.
The panel data computer software called stata 13 version was used to obtain he regression coefficient B0, B1, B2, B3 and B4 and the model equation for the soil organic carbon using the data obtained from the laboratory.The model equation for the soil organic carbon is expressed as Where   =     0 ,   Table II shows the soil organic carbon remaining in the soil after any given time (t = 14 to 112 days), for values of soil samples with crude oil pollution volume ranging from 0 to 2.5L per 10Kg of soil.The R 2 for the determination of the proposed model is 0.9807 with a root mean square error of 0.10432 as shown in Table III.The root mean square error is small, hence the adopted model fits.The P value of 0.00 shows that there is a strong relationship between soil organic carbon and concentration of crude oil spilled at any given time.The equation for prediction of soil organic carbon fate in crude oil depleted soil is therefore  = 1.310 − 0.4276  − 0.0004  + 3.34e  2 + 1.1214√  + 0.01043.
The model was checked and adjusted using another set of experimental data.The model validation is represented in Fig. 1 and Table III respectively   Fig. 2 shows the graph of the control sample in comparison with the soil organic carbon content at various levels of crude oil pollution with time Fig. 2 present soil organic carbon contents at various levels of crude oil pollution with time.Total organic carbon contents were slightly higher than the 1.340% obtained for the control sample after the first 14 days of pollution.Organic carbon content should normally increase following the addition of such levels of carbonaceous substances by the polluted soils.The most plausible connection perhaps might be that the crude oil increased the metabolic processes that facilitated the agronomic addition of organic carbon from the petroleum hydrocarbons, thereby increasing the carbonmineralizing capacity of the microflora.
However, it is most likely that while these organisms might have been stimulated by the presence of the increased carbonaceous substances, their proliferation did not adequately cope with the business of breaking down the excess carbonaceous substrate.Perhaps due to various factors that might include the environmental conditions of weathering and climatic predispositions as well as the physico-chemical properties of the soil [12].From the study the values of total organic carbon content, when averaged over a period of time was found to be about 2 times higher than the values obtained for the control sample.The values of organic carbon content at various levels of crude oil pollution were slightly above the critical levels (1.5 -2.0%) [13] for tropical soil.This slight increase might be utilized by microorganism for their survival and not sufficient to be released for plant growth, thereby resulting to deficiencies of organic carbon content in the soil [12].

N
. L. Nwakwasi, B. C. Okoro, J. C Osuagwu, B. U. Dike, A. N. Nwachukwu and J. C. Agunwamba II.METHODS . The values indicate closeness of the predicted values with the observed values, thus confirming the validity of the model developed.

Fig. 1 .
Fig. 1.Experimental and predicted soil organic carbon over time defined

TABLE II :
THE VARIATION OF SOIL ORGANIC CARBON VALUES WITH TIME AFTER POLLUTION.

TABLE III :
REGRESSION MODEL COEFFICIENT FOR THE PROPOSED MODEL

TABLE IV :
EXPERIMENTAL AND PREDICTED VALUES FOR SOIL ORGANIC Where COV = Crude Oil Volume ED for P = Experimental Data for Organic carbon PV for p = Predicted value for Organic carbon Fig.2.Soil Organic carbon content at various crude oil levels with time