1. POPOOLA, Samuel Olatunde,. OYATOLA, Opeyemi Otolorin
Spatial distributions and contamination risk assessment of some trace metals from bottom sediments in Lagos Lagoon.
POPOOLA, Samuel Olatunde,. OYATOLA, Opeyemi Otolorin
Department of Physical and Chemical Oceanography Nigerian Institute for Oceanography and marine research, PMB,12729,Lagos, Nigeria,
INTRODUCTION
METHODOLOGY
Sediment samples were collected from twelve sampling stations with the aid of van-veen grab. The choice of sampling stations was influenced by documented anthropogenic activities in the adjourning environments .Samples were collected from May to July 2014, on a monthly basis and kept air dried in black polyethylene bags, under room temperature. The air dried samples were further disaggregated and sieved to remove animal shells and plant roots from the sediments. The sieved sediments (5grams) were leached with aqua regia (Nitric/Hydrochloric acid, ratio1:3),using standard digestion procedure (APHA, 1998). Trace metal contents were analysed with Argillent 200 A model, Atomic Absorption Spectrophotometer (AAS).
The city of Lagos harbors over 75% of the industrial outfits. These industries include; textile industries, chemical and paint industries, breweries and bottling companies, metal industries shipyards, plastic and petrochemical factories, paper mills and sawmills. In addition to these industries are recreational and tourist centers built around the lagoon front at several locations in the metropolis. The dense huge population, coupled with the industrialized nature of Lagos, result in the generation of huge volumes of wastes on a daily basis. These industries discharge untreated wastes into several drainages and canals that litter the metropolis. The dumped wastes eventually find their way into the Lagos Lagoon. The Lagos lagoon can thus be described as a depository of last resort for all the solid and liquid wastes generated in the adjoining land area (Olatunji and Abimbola., 2010).Many of these wastes were potential sources of higher density trace metals (density>5g/cm–3) contamination that have the potential to impact the balance of the aquatic ecosystem. Moreover, benthic biota or other aquatic organisms can ingest metal particles from contaminated water, thereby resulting in metals accumulation in their respective tissues and ultimately entering the food chain.
Hence, a continuous assessment of contamination risks of trace metals in Lagos Lagoon is crucial.
Fig 6: showing the sediments geoaccumulation index values
Figure 2: Showing the mean concentration of trace
metals in the stations
Conclusion
Contamination factor: The contamination factor modified by Hakanson(1980) showed the following classes: CF <1,low contamination,1<CF<3,moderate contamination and 3<CF<6,considerable contamination. The CF of the study area are: Zn (2.89) > Pb (0.66) >Mn (0.43) > Cu (0.30) >Ni (0.12) > Fe (0.24) > Cr (0.06). Zinc metal falls within the moderate to considerable contamination ratio(figure3).
Sediments are heterogeneous mixtures that include mineral phases (Fe and Mn oxides) and detrital organic matter. Contaminants may bind to these phases by adsorption and co-precipitation. Element mobility is controlled by both the properties of the binding phase and binding mechanism. In assessing the impact of trace metal pollution on estuarine, coastal, and marine environment, various reference and quantification methods have been used by several authors. However, for the purpose of this study; CF, Lgeo, EF, DC and PLI assessment was used. On the basis of the calculated contamination factor and enrichment factor, it can be affirmed that the analysed sediments of the selected stations in Lagos Lagoon are: moderately contaminated and severely enriched by Zn, minor to moderately enriched by Pb and Mn (evidence of anthropogenic source, especially at the Iddo and Majidun sediments). However, Cu, Ni and Cr exhibit more of background enrichment (evidence of crustal sources).The geoaccumulation factor’s benchmark confirmed that the analysed sediments are; unpolluted to moderately polluted by Pb, ,Zn, Cu Cr, Ni, Mn and Fe in descending order.
Nonetheless, the overall toxicity assessment of the study area (PLI and DC) affirmed a close to background value and a synthetic low toxic effect.
Plate 3-4: (3)Showing the sediment digestion processes,(4)showing the sediments filtration processes
STATISTICAL ANALYSIS
Data were subjected to several sediment quality index ratios using the Microsoft Excel descriptive tool, 2010 ,matlab and software statistical 7.
The following contamination risk indices were used to assess the toxicity potentials of the sediments:
Contamination Factor (CF). This is calculated as the ratio between the average metal concentration in sediments and the background concentration levels.
Cf = C / Cn
Cf = contamination factor; C = mean concentration of each metal in the sediments; Cn=background value.
Degree of Concentration (DC) is the sum of the contamination factors of all the elements examined. According to Hakanson (1980), degree of contamination were calculated using the overall degree of contamination, mDC = Σ Cf .
Enrichment Factor (EF) = T _sample_/ Fe sample_
c T (background) / Fe (background)
Where T sample is average trace element concentration in sediment sample,
T background is trace element concentration in the background.
Geoacumulation index
OBJECTIVES
The objectives of the present study are the following: (i) estimating trace metals (Cr, Zn, Cu, Mn, Pb, Ni) and major element (Fe) concentrations and contamination levels in bottom sediments from selected stations in Lagos Lagoon, (ii) describing the possible sources and distribution pattern of contaminants in bottom sediments of Lagos Lagoon, and (iii) Assess the risk and toxicity rankings of trace metals associated with the sediments, in accordance with standard geochemical benchmark.
Figure 3: showing the sediments contamination factor plot
STUDY AREA
According to Hakanson, 1980, olatunji and Abimbola, 2010, and Tomlinson etal, 1980, DC values of ≤ 8, represent a low degree of contamination, while, 8 ≤ DC ≤16, represent a moderate contamination. However, PLI (the assessment of the overall toxicity of the study area) PLI ≤1, represent close to background concentration, while, PLI >1, represent a progressive pollution. The degree of contamination values for the study area is 4.7; while the pollution loads index values is This fall within a low contamination and close to background value.
The Lagos lagoon (3°10’E and 3°45’ E and 6°15N and 6°36’N) is a part of the continuous system of lagoons and creeks that are found along the coast of Nigeria from the border with the Republic of Benin to Niger-Delta. The geology of the Lagos area is dominated by a continuous and monotonous repetition of clayey and sandy horizons. These horizons show some lateral continuation in some places but in most parts, these lithology pinches out (Olatunji and Abimbola, 2010). Twelve stations were purposely selected based on earlier works of past researchers like:Ajao,1996. and Don-Pedro e tal on the: sources, types and pollutants load in the Lagos Lagoon
REFERENCES
Where CHm is the average measured concentration of trace metal in the sediment,
Bhm is geochemical background value and 1.5 is the background matrix
correction in factor due to lithogenic effects
Abrahim, G.M.S.and Parker, R.J.(2008). Assessment of heavy metal enrichment factors and the degree of contamination in marine sediments from Tamaki Estuary, Auckland, New Zealand.Environmental Monitoring and Assesment.13(6).pp
Ajao, E.A. (1996) .Review of the state of pollution of the Lagos Lagoon. NIOMR Tech. Paper No. 106.
APHA. (1998). American Public and Health Association Standard method for the examination of water and waste water. 20thEdn.,NewYork.
Don-Pedro,K.N,.Oyewo,E.O and Otitoloju,A.A.(2004). Trend of heavy metal concentrations in Lagos Lagoon Ecosystem, Nigeria. West African journal of Applied Ecology, 5, pp
Hakanson, L. (1980). An ecological risk index for aquatic pollution controls, a sediment logical approach. Water Res.14, (2), pp 975.
Olatunji,A.S.and Abimbola, A.F. (2010). Geochemical Evaluation of the Lagos Lagoon Sediments. World Applied Sciences Journal 9 (2): , 2010.
Taylor, S.R. and McLennan, S.M.(1995).The geochemical evolution of the continental crust. Reviews of Geophysics , 3,
Tomlinson, D.L,.Wilson, J.G., Harris, C.R. and Jeffney , D.W. (1980). Problems in the assessment of heavy metals levels in estuaries and the formation of pollution index. Helgol. Wiss. Meeresunters, pp33, 566.
Turekian, K.K. and Wedepohl, K.H. (1961).Distribution of elements in some major units of the earth’s crust. Geological Society America Bulletin, 72, 175 – 192. )
Pollution load index (PLI
Discussion
For the purpose of inorganic pollution assessment in aquatic sediments, the above methods have been used by various researchers: The compilation of the average continental shale (Turekian & Wedepohl, 1961) as background value or, average continental crust abundances (Taylor & McLennan, 1995). An alternative approach is to use the metal content found in deeper sediment samples as reference backgrounds values (Abraham & Parker, 2008).
However, the average continental shale was adopted in this study as the background value.
Table 1:Mean concentration of the selected stations and average shale concentration
Fig4: Showing the sediments pollution load index and degree of contamination
concentrations
Enrichment Factor. EF was used to assess the level of contamination and the possible anthropogenic impact in sediments of the selected stations in Lagos Lagoon. EF values in the ranges 0.5 to 1.5 suggested that the trace metals sources might be entirely from crustal materials or natural weathering processes, while EF values > 1.5 suggested that a significant portion of trace metal was delivered from non crustal materials or non natural weathering processes (Olatunji and Abimbola, 2010).
The EF values of the study area are in the order of: Zn (11.62) >Pb (2.67) > Mn(1.74) >Cu(0.30) >Ni(0.12) >Cr(0.06).This shows that Mn, Pb are minor-moderately enriched, while Zn falls within the severely enriched classification (figure5).
Fig 1: Map of Lagos Lagos Lagoon showing the sampling stations
Mean concentration
(Mg/kg)
Average shale content(ppm)
Ni(Mg/kg)
9.38 80
Mn(Mg/kg)
367.75
850
Pb(Mg/kg)
13.27 20
Zn(Mg/kg)
259.78 90
Cu(Mg/kg)
14.94 50
Cr(Mg/kg) 6
100
Fe(Mg/kg)
46000
ACKNOWLEDGEMENT
The authors sincerely appreciate the support of Mrs Oshisanya Khaphilah Ibironke of the department of physical and chemical oceanography and Mrs. Ajani Gloria Ekaete of the department of Biological oceanography in the course of sampling collection.
Plate 1-2: (1)Showing the dredging activities at Ibeshe station (2)Anthropogenic activities at Ikorodu port.
Fig 5: showing trace metal enrichment