Geochemical and Statistical Approach to Assessing Trace Metal Accumulations in Lagos Lagoon Sediments, South Western, Nigeria

Abstract

The study areas is located in the Northern, central and the southern part of  Lagos Lagoon, South western Nigeria, longitude 3° 22' 27.97" to 3° 28' 58.60" East and latitude 6° 27' 41.44" to 6° 35' 42.60" North. Sediment samples were collected from bottom sediments in twelve sampling stations, with the aid of van-veen grab during the wet-season period, from May to July 2014, on a monthly basis. Sediment samples were air dried and disaggregated, 70 grams each of sediment samples were oven dried for 8 hours, and its grain size fractions determined. The result of the grain size analysis range from; coarse to very fine sand, moderate to well sorted, finely skewed to mesokurtic, while the visually described major clay fractions range from; sandy, plastic, whitish brown clay to brownish, shaly, plastic clay with occasional silt. The bi-modal peaks on the particle size plots suggest multiple source of sediment contaminants in; Unilag waterfront, Ijora and Ibeshe, and a unimodal peaks; single source of contaminant to the sediments of; Atlascove, Apapa and Ikorodu. The sieved sediments were further leached with Nitric/Hydrochloric acid (1:3), aqua regia, using APHA method and its trace metal contents analysed with Argillent 200 A model, Atomic Absorption Spectrophotometer (AAS). The analysed concentrations in mg/kg showed; Ni(Nd-17.55), Mn, (12.50-1180.25), Pb(Nd-15.37), Zn(51.68-659.55), Cu(Nd-35.55) and Cr(Nd-53.00). The major element (Fe) used as the normalizer showed a concentration of 832.64-25206.00 mg/kg. Potential contamination benchmarks; contamination factor (CF), enrichment factor (EF), geoaccumulation factor (Lgeo) and pollution load index (PLI), were used to assess whether, the observed concentrations represent background or contaminated levels. The result affirmed the elevation of Zn, Pb  and Mn and moderate contamination of zinc metals at; Iddo, Okobaba, Majidun and Ijora stations and the crustal influence in the deposition of; Cu, Cr and Ni. Multivariate statistical analysis employed also affirmed these potential contamination benchmarks. Based on the results it can be concluded that zinc metal represented a contaminated level, but, the overall toxicity level of the Lagos Lagoon sediments to the aquatic ecosystem is low.