IOSR Journal of Environmental Science, Toxicology and Food Technology (IOSR-JESTFT) e-ISSN: 2319-2402,p- ISSN: 2319-2399.Volume 8, Issue 8 Ver. I (Aug. 2014), PP 13-16 www.iosrjournals.org www.iosrjournals.org 13 | Page Geochemical fractionation of Heavy metals in the sediment at the point of discharge of Iyalaro stream into Lagos Lagoon I.A. Ladigbolu 1* , J. Appia 1 , K.J. Balogun 1 , and A.K. Adedeji 1 * Nigerian Institute for Oceanography & Marine Research, Victoria Island, Lagos Abstract: Sequential extraction by Tessier et al; 1979 was used to study the geochemical fractionation of the following heavy metals; cadmium, chromium, Nickel, lead and zinc in the sediments collected at the point of discharge of Iyalaro stream into Lagos Lagoon to understand their mobility and bioavailability. The highest percentage (%) of all heavy metals studied were observed in the oxidisible fraction; Cd (40.3) %, Cr (35.9) %, Pb (42.6) %, Zn (41.8) % except Ni (42.6) % which was observed at residual / lithogenous (crystal lattice or silicates bound) fraction of the sediment. High Cr (31.2) % contribution was also observed in the residual fraction, this is an indication that rock / soil weathering / formation is also playing meaningful contribution to the presence of Ni and Cr in the studied area. Sequential extraction for the speciation of elements among the geochemical phases in this study indicated that most of the metals studied were in exchangeable, carbonate bound, reducible (Fe-Mn oxides) and oxidisable (organic matter/sulphide bound) fractions which are potentially available fractions, and contributed by anthropogenic activities. The order of reducing availability of the studied heavy metals were as follow; Zn (93.2) % >Pb (81.6) % > Cd (73.1) % > Cr (68.8) % > Ni (57) %. Therefore, under favourable biogeochemical processes for remobilization of the studied metals from the sediment, the studied metals can get the water body heavily polluted, impact food chain and become an environmental risk. Keywords: Sequential extraction, geochemical fractionation, sediments, food chain. I. Introduction Sediment receives toxic elements from both natural and a wide range of anthropogenic sources and several authors have identified that in order to understand forms in which these elements exist in the aquatic biota; speciation study in the sediment is crucial. Chemical speciation of sediment is defined as the identification and quantification of the forms in which metals or elements occurs in the sediment. Metals in the sediments through speciation study can be classified into two major geochemical phases namely lithogenous (immobile) and non – lithogenous (mobile). Lithogenous or residual fraction metals are not available in long term for bio – uptake, due to strong bond which exists between these metals and crystal lattice of silicate of the sediments and soils. Therefore, they are from natural source that is rock or soils weathering while non- lithognous fractions metals are readily available in short term (Badri and Anston, 1983). Non- lithogenous (mobile) fraction can be further subdivided into exchangeable fraction, carbonate bound fraction, reducible (Fe-Mn oxides / hydroxides) fraction and oxidisable (organic matter / sulphide) fraction (Bancroft et al; 1981). Metals associated with these four mobile fractions, most of time originate from anthropogenic sources and readily available for uptakes and bioaccumulation in aquatic biota, although their availability depend on pH, redox- potential and temperature (Gambrell, 1994, Schlinder, 1991). A comprehensive knowledge of chemical speciation and total metal concentration give proper understanding of the interaction between the trace elements and sediments. It explains the environmental impact of these metals and reliably predicted their sources (i.e. anthropogenic and natural (rock or soil weathering), mobility, bioavailability, distribution and toxicity in the environment. Therefore, the orientation of this study is to study the geochemical fractionation and bioavailability of the following heavy metals Cd, Cr, Ni, Pb and Zn in the sediments of the outfalls of Iyalaro stream into Lagos Lagoon and their tendency of causing health hazard. II. Materials and Methods 2.1. Description of the Study Area 2.1.1. Study Area The study area is the outfall of Iyalaro stream into Lagos Lagoon with Latitude N6 0 33’53.91” and Longitude E3 0 24’04.21”. Iyalaro stream is an important stream due to the fact that it receives industrial effluents from the array of industries located along its course. It is a potential source of pollution to Lagos Lagoon. The stream flows into Ogudu canal, where it eventually emptied into Lagos Lagoon (Figure 1). The