Soil pollution assessment was carried out in seven oil producing areas in Rivers State, and also in Nnamdi Azikiwe University, Awka which served as the control area. Five composite soil samples from five different locations were randomly collected in each local government area (L.G.A.) with oil deposits, at a depth of 1 metre using soil auger. The samples were air-dried, ground, sieved and analyzed for pH, electrical conductivity, moisture content and heavy metals (cadmium, chromium, lead and nickel), using standard procedures. The data were subjected to statistical analysis using mean and bar graphs, and the results obtained were compared with those from the control area and DPR Standard. One way analysis of variance (ANOVA) was also done to establish the statistical difference between the study areas and the control area at 0.05 confidence level. The mean values of the parameters analyzed ranged as follows; pH: 4.25 - 6.48, electrical conductivity: 2.45 - 10.60 μS/cm, moisture: 0.60% - 0.90%, cadmium: 0.08 - 0.58 mg/kg, chromium: 2.86 - 7.91 mg/kg, lead: 0.22 - 9.79 mg/kg and nickel: 0.44 - 5.08 mg/kg. The control values were; pH: 6.04, electrical conductivity: 92.00 μS/cm, moisture: 0.80%, cadmium: 0.09 mg/kg, chromium: 8.72 mg/kg, lead: 1.04 mg/kg and nickel: 0.89 mg/kg. The results revealed that the pH levels of the soils were acidic and most of the values were within the DPR limit. Electrical conductivity and moisture contents of the soils were low and also below the control value and DPR standard. Heavy metal concentrations in the soils exceeded the control values but were below DPR limit. ANOVA results showed that there was a significant difference (p < 0.05) between chromium concentration in the assessed soils and the control soil. The results obtained revealed that the examined areas in Rivers State were not yet adversely polluted in terms of oil exploration and exploitation activities. Therefore, periodic monitoring of the project areas is strongly recommended to prevent the buildup of these contaminants beyond limits of maximum tolerance.
Soil is an important natural resource for agriculture and industrial development of a nation. Its importance which cannot be over-emphasized can be seen in human, plant and animal survival as well as micro-biological organism. The significance for an in-depth study of the physical and chemical components of soils has been greatly highlighted by Ukpong [
Oil exploration has continued to play a dominant role in the Nigerian economy, ranging from generation of foreign exchange to serving as a source of energy to run the nation’s economy. The advent of oil exploration and exploitation activities resulted in increased pollution of the Niger Delta environment, through sabotage, accidental spillages, leaks from producing wells, storage tanks, gathering lines, pipelines, flow stations, refineries and industrial dump sites.
The environmental consequences of oil pollution on the inhabitants of Niger Delta Region are enormous. Oil production gives rise to the degradation of arable soils in the Niger Delta area, which create negative impact on the physical, chemical and biological properties of the soil [
Agriculture forms the most dominant economic activities in the Niger Delta Region, yet farmlands are lost to non-agricultural use annually at alarming rates due to oil spill. According to Onwurah [
Soil pollution in Niger Delta Region is receiving worldwide attention yet information regarding this is very scanty. Apart from the data collected by some individuals and corporate bodies at scattered locations, there is no comprehensive database on the pollution status of these areas. Therefore, this study was conducted in order to examine the pollution levels of soils in the oil producing areas of Rivers State. Specific objectives of the study are to assess the physico-chemical properties and heavy metal contents of the soils, and also compare the values obtained with those obtained from the control area and with the acceptable limits of environmental agency [
Rivers State is one of the thirty-six States of Nigeria located in South-South geopolitical zone of Nigeria. The State is divided into twenty-three local government areas. Its capital, Port Harcourt is economically significant as the leading supplier of wealth to the nations. Rivers State is famous for its vast deposits of crude oil and natural gas, generating over 60% of the country’s output of crude oil [
Seven oil producing local government areas in Rivers State were selected for this study as shown in
L.G.A. | Sampling Points | Latitude | Longitude |
---|---|---|---|
Ahoada West | 1 | 5˚3'14"N | 6˚37'59"E |
2 | 5˚4'17"N | 6˚40'32"E | |
3 | 5˚3'56"N | 6˚40'15"E | |
4 | 5˚5'44"N | 6˚40'28"E | |
5 | 5˚5'25"N | 6˚38'20"E | |
Eleme | 6 | 5˚5'9"N | 6˚35'13"E |
7 | 5˚4'22"N | 6˚37'0"E | |
8 | 5˚5'53"N | 6˚37'8"E | |
9 | 5˚4'47"N | 6˚37'24"E | |
10 | 5˚4'31"N | 6˚38'45"E | |
Emohua | 11 | 4˚56'24"N | 6˚52'11"E |
12 | 4˚56'5"N | 6˚51'41"E | |
13 | 4˚53'30"N | 6˚53'60"E | |
14 | 4˚54'18"N | 6˚51'55"E | |
15 | 4˚54'39"N | 6˚51'29"E | |
Etche | 16 | 4˚60'50"N | 7˚3'52"E |
17 | 4˚58'12"N | 7˚3'10"E | |
18 | 4˚58'46"N | 7˚5'23"E | |
19 | 4˚58'35"N | 7˚3'37"E | |
20 | 4˚59'2"N | 7˚7'16"E | |
Ikwere | 21 | 5˚4'7"N | 6˚57'4"E |
22 | 5˚1'60"N | 6˚53'40"E | |
23 | 5˚3'51"N | 6˚55'47"E | |
24 | 5˚4'15"N | 6˚55'26"E | |
25 | 5˚4'33"N | 6˚56'54"E | |
Ogba-Egbema | 26 | 5˚23'37"N | 6˚39'42"E |
27 | 5˚22'10"N | 6˚37'18"E | |
28 | 5˚20'58"N | 6˚37'31"E | |
29 | 5˚20'8"N | 6˚37'24"E | |
30 | 5˚20'26"N | 6˚39'0"E | |
Oyigbo | 31 | 4˚50'3"N | 7˚7'56"E |
32 | 4˚52'34"N | 7˚8'19"E | |
33 | 4˚52'13"N | 7˚6'44"E | |
34 | 4˚53'23"N | 7˚8'30"E | |
35 | 4˚53'57"N | 7˚8'6"E |
government area were chosen for sampling. The control sample was selected from Nnamdi Azikiwe university (NAU) Awka, Anambra state, which is a none oil producing area.
Five composite soil samples (20 samples) from five different locations were randomly collected in each local government area with oil deposits, avoiding areas such as areas under construction, areas that are shut down, areas whose oil wells have dried up and riverine areas. For each collecting site, four soil samples were taken and combined together to form a representative sample of that location. The control samples were randomly collected from Nnamdi Azikiwe University premises in the same manner, for comparison with the oil producing areas. All soil samples were collected at a depth of 1 meter, using hand-driven soil auger. The collected soil samples were put in labeled plastic cans and conveyed to the laboratory for further processing. The sample collection was carried out biweekly for a period of six months.
The collected soil samples were air-dried to a constant weight for three days, ground with porcelain mortar and pestle, and sieved through a 500 µm stainless sieve. The soil samples (500 g) were securely packaged in clean, dry, closed plastic cans, individually labelled and taken to the laboratory for analysis of various parameters.
The soil analyses were carried out in the laboratory following appropriate specific standard procedures for each parameter. The parameters analyzed include:
The pH of the samples was measured using the pH meter (HI 8424 HANNA Romania) at 1:2 soil/water ratio. The soil sample (20.0 g) was weighed into 250 mL beaker, using Sartorius weighing balance (AE200 METTLER, Switzerland). 40 mL of deionized water was added to it and the mixture was stirred thoroughly with a glass rod for proper mixing. The mixture was allowed to settle for 45 minutes. The reading was obtained by immersing the pH probe into the settled mixture. The pH meter was calibrated using standard buffer solution at pH 4 and pH 7 before taking measurements
The conductivity of the samples was measured using a conductivity meter (HI 98303 HANNA, Romania) at 1:2 soil/water ratio. The soil sample (20.0 g) was weighed into 250 mL beaker, using Sartorius weighing balance (AE200 METTLER, Switzerland). 40 mL of deionized water was added. The mixture was thoroughly stirred with a glass rod and was allowed to settle for 45 minutes. The reading was obtained by immersing the conductivity probe into the settled mixture.
The moisture content was determined gravimetrically. The soil sample (10.0 g) was weighed into a ceramic crucible of known weight. This was placed in the drying oven set at 105˚C and was allowed to dry for three hours until constant weight was obtained. The sample was allowed to cool in a desiccator for 30 minutes. The sample was reweighed and the difference in weight (expelled moisture) was calculated. The percentage moisture was deduced as follows;
% moisture = A − B A X 100 1 (1)
where
A = Weight of the sample before drying;
B = Weight of the Sample after drying.
The samples were digested using aqua regia, which is 3:1 mixture of hydrochloric acid and trioxonitrate (V) acid. Soil sample (5.0 g) was weighed into a digestion flask. 15 mL of hydrochloric acid, 5 mL of trioxonitrate (V) acid and 30 mL of deionized water, were added to the weighed sample. The total volume was heated on a heating mantle in the fume cupboard for two hours until the volume reduced to about 15 mL. The digest was then cooled and about 30 mL of deionized water was added. This was filtered through a 42 Whatman filter paper into 50 mL volumetric flask and was made up to the mark with deionized water. The filtrates of the digested samples were then analyzed for heavy metals.
The concentrations of the heavy metals (cadmium, chromium, lead and nickel) were determined using flame atomic absorption spectrophotometer (A6600 GBC AVANTA PM, Australia) at wavelengths of 228.8 nm, 357.9 nm, 217.0 nm and 232.0 nm respectively. Standards of cadmium, chromium, lead and nickel solutions were made from each of the heavy metals stock solutions. The standard samples of the metals to be analyzed were injected into the AAS instrument to obtain their absorbance from which the calibration graphs of each metal were plotted using measured absorbance and the corresponding concentration. The filtrates of the digested samples were then injected into the AAS instrument and the absorbance measured was used to determine the concentrations of the metals present, from the calibration curves. The result was given in mg/kg.
The data generated were analyzed using descriptive statistics. Tables, bar charts, mean and standard deviation were computed to show the concentration levels of contaminants in the soil samples. One way analysis of variance (ANOVA) was used to determine the statistically significant difference (p < 0.05) between the study areas and the control area, using computer aided SPSS Statistical Program.
The pH analysis results of soils of the surveyed local governments in Rivers State are shown in
Results obtained (
Local Govt. Area | pH | EC (µS/cm) | Moisture (%) | Cd (mg/kg) | Cr (mg/kg) | Pb (mg/kg) | Ni (mg/kg) |
---|---|---|---|---|---|---|---|
Ahoada West | 5.02 ± 1.92 | 3.61 ± 1.51 | 0.80 ± 0.57 | 0.08 ± 0.05 | 6.71 ± 1.23 | 2.87 ± 1.42 | 5.08 ± 1.42 |
Eleme | 5.98 ± 1.63 | 2.85 ± 1.39 | 0.90 ± 0.50 | 0.58 ± 0.39 | 4.59 ± 0.48 | 2.73 ± 1.83 | 0.44 ± 0.62 |
Emohua | 6.48 ± 2.74 | 5.55 ± 3.07 | 0.70 ± 0.22 | 0.11 ± 0.10 | 5.37 ± 0.87 | 1.83 ± 0.94 | 1.20 ± 0.20 |
Etche | 5.50 ± 1.01 | 10.60 ± 2.16 | 0.90 ± 0.39 | 0.12 ± 0.07 | 5.12 ± 1.63 | 1.58 ± 0.57 | 1.39 ± 0.56 |
Ikwere | 5.68 ± 1.69 | 2.45 ± 0.99 | 0.70 ± 0.43 | 0.08 ± 0.13 | 3.01 ± 1.17 | 0.22 ± 0.12 | 1.11 ± 0.61 |
Ogba-Egbema | 4.78 ± 1.79 | 2.81 ± 1.89 | 0.60 ± 0.42 | 0.08 ± 0.05 | 2.86 ± 1.12 | 3.56 ± 1.09 | 0.92 ± 0.49 |
Oyigbo | 4.25 ± 1.78 | 7.30 ± 2.50 | 0.70 ± 0.29 | 0.16 ± 0.16 | 7.91 ± 1.44 | 9.79 ± 3.63 | 2.01 ± 0.64 |
Unizik | 6.04 ± 1.84 | 92.00 ± 20.00 | 0.80 ± 0.50 | 0.09 ± 0.08 | 8.72 ± 2.34 | 1.04 ± 0.85 | 0.89 ± 0.41 |
State | Standard | ||||
---|---|---|---|---|---|
Parameters | Range | Mean ± SD | Control | (DPR17) | P-value |
pH | 4.25 - 6.48 | 5.38 ± 1.83 | 6.04 ± 1.84 | 5.50 - 6.50 | 0.413 |
Electrical Conductivity (µS/cm) | 2.45 - 10.60 | 5.02 ± 3.39 | 92.00 ± 20.00 | 300 - 5000 | 0.225 |
Moisture Content (%) | 0.60 - 0.90 | 0.76 ± 0.39 | 0.80 ± 0.50 | 13 - 26 | 0.834 |
Cadmium (mg/kg) | 0.08 - 0.58 | 0.17 ± 0.23 | 0.09 ± 0.08 | 0.8 | 0.517 |
Chromium (mg/kg) | 2.86 - 7.91 | 5.08 ± 2.03 | 8.72 ± 2.34 | 100 | 0.002 |
Lead (mg/kg) | 0.22 - 9.79 | 3.23 ± 3.30 | 1.04 ± 0.85 | 85 | 0.538 |
Nickel (mg/kg) | 0.44 - 5.08 | 1.74 ± 1.60 | 0.89 ± 0.40 | 35 | 0.206 |
Rivers State, mainly Ahoada west, Ogba-Egbema and Oyibo, which can adversely affect soil fertility and crop production in these areas.
The electrical conductivity analysis results of soils of the assessed local governments in Rivers State are displayed in
The mean soil electrical conductivities of the investigated areas in Rivers increased from 2.45 µS/cm to 10.60 µS/cm (
The results of moisture content analysis of soils of the investigated local government areas in Rivers State are indicated in
be as a result of oil aerations and high rate of evaporation due to gas flaring, thereby increasing the moisture deficit in the soil. The resultant effect could lead to delayed seed germination, reduction in plant growth and in the population of hydrocarbons degrading microbes.
The results of cadmium analysis in the soils of the examined local government areas in Rivers State are shown in
In Rivers, the mean concentration of cadmium ranged from 0.08 mg/kg to 0.58 mg/kg as described in
and
The results of the analysis of chromium in the soils of the assessed local government areas in Rivers State are depicted in
The results of the analysis of lead in the soils of the studied local government areas in Rivers State are described in
In
mechanic workshop into the surroundings soils; while the cleansing action of rainfall may be responsible for the minimum value found in Ikwere (0.22 mg/kg). From the data obtained (
The results of the analysis of nickel in the soils of the inspected local government areas in Rivers State are outlined in
The average levels of nickel observed in the studied locations in Rivers (
The State means for the parameters determined in the soils of the investigated areas in Rivers State are shown in
may be ascribed to their similarity in soil properties.
In
In addition,
More so, the State mean chromium value for Rivers (5.08 mg/kg) was predominantly lower than the control (8.72 mg/kg) and the set standard of DPR (100 mg/kg), as illustrated in
In
Similarly, the assessed soils from Rivers showed a State average nickel concentration of 1.74 mg/kg, which exceeded the control area (0.89 mg/kg) but was below DPR limit (35 mg/kg), as indicated in
Generally, the results showed a very low concentrations of all the heavy metals analyzed, compared to DPR limit (
The test result obtained from the soil analysis of the oil bearing areas in Rivers State shows that the soils were acidic and contained low amount of organic matter. Nearly all the physicochemical properties examined were altered by oil pollution since most of the values obtained were far below the recommended DPR Standard. This in turn implies low agricultural productivity and reduced source of livelihood in the affected areas. However, the heavy metals status of the soils were observed to be very low and also below DPR permissible limits. Their effect on soil quality would therefore be insignificant. ANOVA results showed that there was significant difference (p < 0.05) between chromium concentration of the studied soils and the control soils. It can be concluded that the assessed areas in Rivers State, were not yet adversely polluted in terms of oil exploitation and exploration activities. Based on of these findings, periodic monitoring of the study areas is highly recommended to prevent the buildup of the contaminants beyond limits of maximum tolerance.
The authors declare no conflicts of interest.
Onwuka, C., Eboatu, A.N., Ajiwe, V.I.E. and Morah, E.J. (2021) Pollution Studies on Soils from Crude Oil Producing Areas of Rivers State, Niger Delta Region, Nigeria. Open Access Library Journal, 8: e7583. https://doi.org/10.4236/oalib.1107583