Heavy metal trends in the Calabar River, Nigeria

. Heavy metal concentrations in sediments of the Calabar River: Pb (0.6–30 ppm); Ni (1.2–22.5 ppm); Cr (0.6–3.3 ppm); Cu (0.3–48 ppm); Zn (0.8–27 ppm); Fe (0.2–2,880 ppm) and V (0 ppm) are generally below levels that are known to be harmful to aquatic biota. Metal levels displayed marked seasonal, spatial and tidal variations, which are attributed to both anthropogenic influences and natural processes in the environment. Iron and Cr increased during the dry season (January), while Cu, Zn, Pb and Ni had their peak values in the wet season (September). High values of metals during the wet season indicate inputs through human activities mediated by rainfall. Relatively higher concentrations of metals are obtained during high tide compared to low tide regime. This is attributed to redox processes in the sediment and subsequent displacement of sediment pore water rich in metals by seawater intrusion during high tide. Higher values of Fe observed in the dry compared to the wet season may be attributed to oxidation and precipitation of soluble forms brought into the river during the wet season.