No abstract

Concentrations of cadmium, cobalt, copper, chromium, iron, manganese, nickel, lead and zinc were determined in surface sediments of the Lagos Lagoon, Nigeria. The results revealed largely anthropogenic heavy metal enrichment and implicated urban and industrial wastes and runoff water transporting metals from land-derived wastes, as the sources of the enrichment. Higher levels (F < 0.05) of cadmium, iron, manganese, nickel and zinc occurred in sediment samples collected near industrialized-urban areas than in those from unindustrialized-rural areas only in the wet season. While iron constituted about 1% of dry sediment by weight, the other metals were present in trace amounts.

Analysis in the dry season gave the following mean values for organic matter, total alkalinity, nitrate, phosphate, silicate, and sulphate: 0.31, 53.10, 0.63, 0.24, 6.52 and 491.98 mg/1. Others were pH, 7.49; salinity, 13.68%o; and conductivity, 22.78 mmhos/cm. The corresponding wet season means were 0.70, 37.31, 0.55, 0.29, 10.09 and 245.47 mg/1, while pH was 7.20, salinity, 6.34%o and conductivity, 8.08 mmhos/cm. There were significant seasonal variations (P < 0.05) in salinity, conductivity, total alkalinity and silicate and sulphate levels. The levels of organic matter and nutrients were lower than expected in view of heavy loadings from urban and industrial wastes. The lagoon's ability in handling polluting wastes is attributable to a continual renewal of water, high dilution and sedimentation.