Distribution, sources and ecological risks of metals in surficial sediments of the Forcados River and its Estuary, Niger Delta, Nigeria

Abstract

The concentrations of eleven metals (Cd, Pb, Cr, Ni, Cu, Co, Ba, Mn, Zn, Fe and Al) were measured in surficial sediments of the Forcados River and its estuary situated in the Niger Delta, Nigeria. The aim of the study was to provide information on the influence of seasonal changes on the distribution patterns and sources of these metals and the ecological risks associated with exposure to metals in the river sediment system. Samples from nine locations were collected monthly for a period of 6 months covering both the wet and dry seasons. The sediment samples were digested with aqua regia, and the metal concentrations in the digests were quantified by means of inductively coupled plasma-atomic emission spectrometry (ICP-AES). The concentration ranges of metals in the sediments were: Cd 0.78–2.16, Pb 11.5–72.0, Cr 21.9–49.6, Ni 4.75–11.7, Cu 4.48–13.7, Co 5.03–11.7, Ba 2.04–3.34, Mn 153–545, Zn 11.7–35.1, Fe 9350–18,400 and Al 9560–25,000 mg kg−1. The results showed significant spatial and seasonal variations (p < 0.05) in the metal distribution patterns. The ecological risk indices indicate that low ecological risks are associated with exposure to these metals but with significant impact from Cd. The enrichment factors, and geoaccumulation and contamination/pollution indices, suggested that Cd is the main metal contaminant in the riverbed. Principal component analysis found that the sources of metals in the sediments include natural inputs, vehicular emissions and discharges from oil industries, waste dumps and riparian farmlands.