The coastal zone is influenced by human activities such as fishing, industries and shipping and can become vulnerable due to these persistence anthropogenic actions. Often enclosed and semi-enclosed marine areas such as harbours can become polluted, and this is detrimental to sediment quality. The basin of Lagos Harbour, Nigeria, is an area with restricted water circulation and typifies this situation. This study aimed to investigate the concentrations of heavy metals in sediments within the Lagos Harbour and Lagoon system. A total of twenty-six sampling stations were established, with more pristine Apese Lagoon serving as the control area. The different stations were selected according to their related anthropogenic activities and hence likely to have varying contaminant sources and types. Surface (1-5 cm) sediment samples were collected using a Van Veen grab on two occasions; during the wet (September 2013) and dry (February 2014) seasons. The sediment underwent two acid digestion (1M HCl or aqua regia) procedures according to the Canadian National Water Research Institute guidelines and the United States Environment Protection Agency (Method 3050B). The digests were analysed for the heavy metals: arsenic, chromium, copper, iron, lead and zinc. Some of these were selected due to their persistence, toxicity and potential to bioaccumulate in organisms. The resultant digests were analysed using both inductively coupled plasma mass spectrometry and optical emission spectrophotometry to give the bioavailable and extractable metal fractions. Total metal concentrations were measured using X-ray fluorescence. The concentration of metals was shown to be higher using the aqua regia method in the dry season compared with the wet season in all the locations including the control area. High concentrations for three metals (As = 23, Cr = 38, Zn = 144 mg/kg) were found. The concentrations were higher in the Lagos Harbour and urbanised part Lagos Lagoon, where there is high intensity of anthropogenic activities compared with the control zone. These data are useful for the policy makers to use as part of the baseline data for further investigation and in the development of guidelines for the protection of the marine environment in Nigeria.
Keywords: Heavy metals, Lagos Harbour, Nigeria, Sediment contamination, Marine pollution, Analytical techniques.