Evaluation of Status of Heavy Metals Pollution of Sediments in Qua-Iboe River Estuary and Associated Creeks , South-Eastern Nigeria

Sixteen bottom sediment samples collected from Qua-Iboe River estuary and associated creeks were analyzed for Cd, Cr, Cu, Fe, Pb, Zn, Ni, pH, Organic carbon (orgC),and grain size in order to assess the current pollution status in sediment of the study area. Concentration data were processed using Pearson correlation analysis. Sediment pollution assessment was carried out using Enrichment factor, Geo-accumulation index and Modified degree of contamination. The calculated enrichment factor showed that the sediment was enriched with Cd, Zn, Cu and Pb. The results of geo-accumulation index (Igoe) indicated that sediments are unpolluted with Fe, moderately polluted with Cr, Cu, Pb, strongly polluted with Cd and extremely polluted with Ni. This was attributed mainly to oil contaminating wastes and metal scraps. The results of the modified degree of contamination (mCd) revealed that the sediment of Qua Iboe estuary and associated creeks fall between 8 ≤ mCd ≤ 16 indicating very high degree of contamination.


Introduction
The pollution of aquatic environment (especially estuaries) by heavy metals has been a source of serious concern to government regulatory agencies, environmentalist and the public at large (Manahan, 1991).This is particularly important because estuaries are rich in nutrient and as a result mothers varieties of fishery resources.Heavy metals play important roles in our society as most of them are vital raw materials in most industries.As trace elements, some heavy metals (e.g.Cu, Se, and Zn etc.) are essential in the maintenance of some metabolic activities in human bodies.However, at certain concentrations they become toxic.They are natural components of the earth's crust with large variations in concentration.They cannot be degraded nor destroyed due to their persistence in the environment.Their distribution in aquatic environment has been evidenced in human health effects and aquatic life disruptions due to long term exposure and bioaccumulation (Dahilia, Apodaca, Emerson, Tui, & Allyn, 2003).Marine sediments are the ultimate sinks of pollutants in the marine environment and it constitutes an important medium for scientific research.Like soils in the terrestrial environment, marine sediments in the aquatic ecosystem are the sources of substrate nutrients and become the basis of support to living aquatic organisms (Abdullah, Sidi, & Aris, 2007).The enrichment of metal in a sink is shown mainly by an increase in their concentrations in the bottom sediment.Their occurrence in the environment results primarily from anthropogenic activities.Also, natural processes, such as weathering of rocks and volcanic activities play a significant role in the enrichment of heavy metals in water bodies (Forstner & Wittmann, 1981;Forstner & Wittmann, 1983;Nriagu, 1989).
Heavy metals accumulate in sediments through complex physical and chemical adsorption mechanisms depending on the nature of the sediment matrix and the properties of the adsorbed compounds (Maher & Aislabie, 1992;Leivouri, 1998;Ankley et al., 1992).Several processes enhance the association of heavy metals with solid phase such as direct adsorption by fine-grained inorganic particles of clays, adsorption of hydrous ferric and manganic oxides which may in turn be associated with clays, adsorption on natural organic substances, which may also be associated with inorganic particles and direct precipitation as new solid phases (Gibbs, 1973).The dissolution and adsorption processes are influenced by several physicochemical parameters such as: pH, dissolve → 1mm → 0.50mm → 0.11mm → 0.050 mm → Pan (clay).100 grams of each sample was placed on the arranged laboratory test sieve, and inserted into the sieve shaker machine for 10 minutes.After this duration, particles that passed through were retained on the standard set of sieves of various sizes and were measured for the weight percentage of particles.The procedure was repeated for each sample and the grain size % was calculated using the formula as given by Wikipedia (2013) and modified as given below: (1)

Sample Digestion and Analysis of Metal Ions
1 g of sediment sample was digested with a solution of concentrated HClO 4 (2 ml) and HF (10 ml) to near dryness.Subsequently, a second addition of HClO 4 (1 ml) and HF (10 ml) were made and the mixture was evaporated to near dryness.Finally, HClO 4 (1ml) was added and the sample was evaporated until white fumes appeared.The residue was dissolved in concentrated HCl and diluted to 25 ml prior for heavy metals analysis using Atomic Absorption Spectroscopy (Model: SpetrAA B65).The total organic carbon content was evaluated using the Walkey and Black titration method (Walkey & Black, 1934).The sediment pH was determined using JENWAY 370 pH meter.A buffer solution with pH 7.0 was added to a beaker and the pH electrode was then inserted, the pH meter was calibrated to a pH of 7.0.20 g of each sample was placed in a beaker; 50ml of distilled water was added to each sample and stirred for 30 minutes before inserting the probe into the system.Also, 50 ml of filtered water samples were placed in beakers after which the pH probe was inserted and the values were then read off from the electronic meter attached to the probe and data obtained recorded (Bascomb, 1994).

Results and Discussion
The total organic carbon (orgC) in sediments of Qua Iboe River estuary and associated creeks ranged from 0.05% to 1.36%, with an average of 0.38% during the dry season and 0.08% and 1.03% with an average of 0.28% during the wet season (Table 1).The higher values observed during dry season may be as a result of high anthropogenic activity during this season.Also, co-precipitation with carbonate minerals is another important source of organic carbon (Fortsner & Wittmann, 1983;Alloway, 1990).pH values ranged from 6.71 to 9.69 with an average of 7.82 during dry season and 5.99 to 7.67 with an average of 6.59 during wet season.The higher concentrations of Cd, Cr, Cu, Fe, Ni, Pb, Zn, and Hg during this season were attributed to lower pH levels in the sediments.The lower pH values may have been from run-off from bush land areas which particularly introduces tannic acid (tannins) which are found naturally in leaves which also account for a tea-like colour of the seas (Barnes, Meyer, & Freeman, 1998).The descriptive statistics of pH, orgC (%), sand (%), silt (%), clay (%) and heavy metals content (mg/g) in sediment of Qua-Iboe estuary and associated creeks during wet and dry seasons.
The percentage of sand fraction ranged from 18.62% to 75.32% with an average of 43.06%, while silt fraction ranged from 20.42% to 49.68% with an average of 33.08% and the percentage fraction of clay ranged from 4.22% to 45.28% with an average of 24.07%.
From the data presented on Table 1, it is observed that both physical and chemical parameters vary between seasons (wet and dry), and these may be as a result of differences in anthropogenic inputs due to intense seasonal variations, influence of tides and salt water intrusion (Asuquo, 1998;Ekwere et al., 1992)    There was positive correlation between Fe and pH (r = 0.22), Hg (r = 0.28) but strong positive correlations with Silt (r = 0.54), Clay (r = 0.72), Pb (r = 0.77) and Zn (r = 0.83), strong positive correlations between Pb and Clay (r = 0.68), Zn (r = 0.81), Zn with Clay (0.91) and Hg (r = 0.54), Ni with Sand (r = 0.66).Significant correlation among the variables indicates that there are linear relationships between the parameters.It is well established that organic matter content is an important controlling factor in the abundance of heavy metals (Rubio, Nombela, & Vilas, 2000).The poor correlation between Cd, Cr, Cu, Fe, Zn, Pb, Ni, Hg, and organic carbon, sand, silt fractions indicates that organic carbon and silicates are not the main geochemical carriers of the metal in sediments of the study area (Chatterjee et al., 2006).
Apparent difference of the anthropogenic inputs of heavy metals from the geogenic sources is important in evaluating the extents of heavy metal pollution.Enrichment factor (EF), Geo-accumulation index (Igeo), Modified degree of contamination (mC d ) was used to assess and interpret the pollution status of the estuary in different stations.Several kinds of refractory metals such as Al, Fe, Mg, Ti, Sc, Li and Cs have been used to normalize the grain size effect for heavy metal concentrations in sediments (Habes & Nigem, 2006;Baptista Neto, Smith, & McAllister, 2000;Schiff & Weisberge, 1999).In this study iron was used as a conservative tracer to differentiate natural from anthropogenic components.Although Fe and the heavy metals in the sediments showed discrepancies depending on the type of elements, significant correlations from Pearson correlation analysis were observed.Heavy metal concentrations were normalized to Fe to account for differences in grain size and mineralogy and then normalized by background values from the study carried out by Ekwere et al. (1992) who studied the geochemistry of sediments in Qua-Iboe estuary and associated creeks, to assess the anthropogenic input of metals in the study area.The advantage of using enrichment factor (EF) analysis is that it is possible to establish a contamination guideline.This technique has been well applied in several studies to assess metal contamination in marine sediments ( Enrichment factor is a convenient measure of geochemical trends and is used for comparison between areas.It is applied widely in sediment geochemical studies (Abraham, 1998;Soto-Jimenez & Pacz-Osuna, 2001;Kamau 2002;Qu, Chen, Yang, & Lu, 1993;Kehing, Pinto, Moreira, & Malm, 2003;Barakat et al., 2012).
According to Ergin, Saydam, Basturk, Erdem, and Yoruk (1991), the metal enrichment factor (EF) is defined by the equation below: is the ratio of metal and Fe concentration of the sample, and Background Fe

M
is the ratio of the metal and Fe concentration of the background value.
The formula below was applied to the studied heavy metals in the study to assess the anthropogenic and lithogenic contributions: corresponds to the average background ratio.The anthropogenic heavy metals can be estimated by the formula shown below: Birch ( 2003) divided contamination into different categories based on EF values.EF<1 demonstrates "no enrichment", 1<EF<3 is "minor enrichment", EF=3-5 is "moderate enrichment", EF=5-10 is "moderately severe enrichment", EF=10-25 is "severe enrichment", EF=25-50 is "very severe enrichment" and EF>50 is "extremely severe enrichment".The enrichment of heavy metals in sediments of Qua Iboe River estuary and associated creeks is shown in Table 3.
The factor obtained for the studied area revealed that there were extreme enrichment of cadmium in all the stations during dry and wet seasons, severe enrichment of zinc, minor to moderate enrichment of lead, minor enrichment of copper, minor enrichment of nickel, no enrichment of chromium, mercury and iron.This is similar to work reported by Joseph (2002) in sediments of Port-Reitz creek, Mombasa; Rezaee, Saion, Yab, Abdi, and Riyahi (2010) in sediments cores from South China Sea; Habes and Nigem (2006), in bottom sediments of Wadi The result of enrichment factor (EF) for both wet and dry seasons in the study area.
According to these researchers, possible enrichment of Cd in bottom sediments was attributed to anthropogenic inputs from fertilizers and pesticides used in agricultural activities.Manaf, Samah, and Zukki (2009) reported that domestic wastes is the primary source of the generation of solid wastes as a result the high concentration of Cd in Malaysia and its coast.
Sediment bacteria may also assist in the partitioning of cadmium from water to sediments.Studies indicate that concentrations of cadmium in sediments are at least one order of magnitude higher than in the overlying water.
Results from the mean geo-accumulation index (Equation 5. and Table 4) reveal the following trend Ni > Cd > Zn > Cr > Cu > Pb > Hg >Fe (Figure 7).Based on Muller (1979) classification (Table 3), marine sediment for geo-accumulation of metals, Ni belongs to class 6 (extremely polluted), Cd belongs to class 4 (strongly polluted), Zn belongs to class 2 (moderately polluted), Cr, Cu, Pb, and Hg belongs to class 1 (unpolluted to moderately polluted) and Fe belongs to class 0 (background concentration) making it a suitable normalizer for sediments of the study area.The geo-accumulation is important because on a weight per square meter basis, the uppermost superficial sediments serve as the largest heavy metal sinks in marine ecosystem.Once these heavy metals accumulate in sediments, they tend to pose threats to aquatic life as a result of re-suspension into the water column from geochemical cycling, bioaccumulating in benthic organisms that feed on substrate nutrient and also biomagnified through the aquatic food web.where sample C and background C respectively refer to the mean concentration of a pollutant in the contaminated sediments and the pre-industrial "baseline" sediment or average shale.The numeric sum of the K specifies contamination factors which express the overall degree (Hakanson, 1980) of sediment contamination (C d ) using the following formula: The C d is aimed at providing a measure of the degree of overall contamination in surface layers of the sediment.Furthermore, all n species must be analyzed in order to calculate the correct C d for the range of classes defined by (Hakanson, 1980).The modified formula is generalized by defining the degree of contamination m Cd as the sum of all the contamination factors Cf for a given set of estuarine pollutants divided by the number of analyzed pollutants.The modified equation for a generalized approach to calculating the degree of contamination is given: where n is the number of analyzed elements and (i) is i th element (or pollutant) and Cf is formula to calculate M cd .
It allows the incorporation of as many metals as the study may analyze with no upper limit.Table 5 shows the model for classifying estuarine sediment.
- Modified degree of contamination using pristine values (Ekwere et al., 1992) for heavy metals in bottom sediments from Qua-Iboe River estuary and associated creeks, South Eastern Nigeria.
The earlier indices (EF and Igeo) gave diverse classification of sediment quality of the study area.The modified degree of contamination has an advantage over other indices and provides a measure of the overall degree of contamination of all the chemical parameters in each sampling station.Based on Hakanson classification (Equation 8 and Table 5), Ikot Iwang shows an ultra high degree of contamination during both seasons, Iwoachang is moderately contaminated from the wet season results but highly contaminated from the dry season resuts, Ukpenekang shows low degree of contamination, Atasi moderately polluted while Mkpanak shows low degree of contamination during both seasons, Douglass Creek is moderately polluted, Egerton Port and Stubbs Creek show very high degree of contamination.However, on the average, the sediments of Qua Iboe River estuary and some associated creeks were found to be highly contaminated during both seasons.
Figure 3. Modified degree of contamination

Conclusion
Identification and quantification of heavy metal sources, as well as their enrichment in marine sediments are important environmental scientific issues.Descriptive statistics showed that the mean concentrations of Cr, Cu, Fe, Pb, Ni, and Hg were lower than marine sediment quality standard (MSQS) but the mean concentration of Cd and Zn were significantly higher than MSQS.There were significant higher concentrations of heavy metals during the dry season than the wet season.Analysis of the data using Pearson correlation matrix showed significant correlation between the metals, Fe and Clay suggesting that Fe is the best chemical normalizer for other metals.Different metal assessment indices were applied in order to interpret the sediment's quality of Qua Iboe River estuary and associated creeks.All the EF's values of Cr, Fe and Hg were less than 1.5.However, the EF's values for Cd, Cu, Pb, Zn, and Ni were greater than 1.5 signifying greater percentages from anthropogenic inputs.The sources of pollution were mainly high surface runoffs, domestic effluents from the coastal dwellers and possibly from the oil exploration in the area.The high Igeo values for Ni may be attributed to oil spill and the slow water movement along the creeks.With regards to an overall degree of contamination as proposed by Hakanson (1980) sediment quality equation, the sediments of Qua Iboe River estuary and associated creeks fall between 8 ≤ mCd < 16 indicating high degree of contamination.
Asuquo, F. E. (1998).Water pollution monitoring of Great Kwa River at .In proceedings of the 1 st conference of Nigeria water and sanitation, Port Harcourt.

Figure 2 .
Figure 2. Seasonal variations in geo accumulation index of heavy metals in sediments of the study area

Table 1 .
Descriptive statistics Apparently, the mean values of Cd and Zn were higher than marine sediment quality standards (MSQS) of 5.1 and 410 ppm respectively.The lower concentrations of these metals in sediment during wet season may be attributed to the lower pH levels dissolving these metals into the water column.

Table 3 .
The result of enrichment factor (EF)

Table 4 .
Muller's classification for geo-accumulation index