Heavy Metal Pollutions and Its Associated Ecological Risks in Lagos Lagoon Sediments, South-western Nigeria

This study investigate the origin of heavy metal pollution and its associated ecological risks to the surrounding aquatic ecosystems in the Lagos Lagoon. Sediments samples from twelve selected stations, that covers the; Southern, central and North-Eastern segments of Lagos Lagoon were analysed for levels of selected heavy metals i.e, Fe, Cu, Mn, Zn, Pb, Cr and Ni over a 3 month period. Six Pollution indices were applied in assessing the pollution status of heavy metals in sediments. These comprised; three single pollution indices (The Contamination Factor, Ecological Risk Factor and Enrichment Factor) and three integrated pollution indices (Average Pollution Index, Ecological Risk Index and Nemerow Pollution Index). Heavy metal concentrations in the Lagos Lagoon sediments range from; Ni (Bdl-17.55 mg/kg), Mn (12.5-1180.25 mg/kg), Pb (Bdl-27.04 mg/kg), Zn (Bdl-543.33 mg/kg), Cu (Bdl-35.55 mg/kg), Cr (Bdl-220.53 mg/kg) and Fe (832.64-19722.80 mg/kg) respectively. The respective nutrients values in the sediments ranges are; nitrate (0.10-1.16 mg/kg), phosphate (1.61-6.61 mg/kg), silicate (1.77-63.55 mg/kg), total organic matter (0.27-4.35 mg/kg) and total organic carbon (0.15-2.45 mg/kg).The average metal concentrations were compared with its respective background values, using the Average shale contents (ASC) thus; Zn, Pb and Mn exhibited elevated concentrations above the ASC at; Iddo, Okobaba, Ijora and Majidun segments of Lagos Lagoon and are moderately to severely contaminated. It also demonstrates a dominant anthropogenic origin to all analysed heavy metals with the exception of Iron. However, the integrated pollution indices affirmed a low ecological risk index to the aquatic ecosystem of the Lagos Lagoon. The calculated Total Organic Carbon (TOC) displayed a high percentage values in the aforementioned stations and correlated significantly with nitrate concentrations in the sediments of Lagos Lagoon. The Nemerow pollution index revealed that the heavy metals conditions in the Lagos Lagoon sediments are in a precautionary state. This call for a quick and efficient control measures to be put in place to safeguard the aquatic biota in the Lagos Lagoon.