DISTRIBUTION OF HEAVY METALS IN MONO, HAHO AND ZIO RIVERS - AN ENVIRONMENTAL IMPACT ASSESSMENT OF THE SOUTHERN TOGO RIVERS WATERSHED-

Industrial and urban activities in the coastal zones are the main sources of inorganic and organic micro-pollutants input in the marine environment. Heavy metal input is caused by the dumping of trace metal rich phosphate tailings as well as industrial and household waste into the coastal waters of Togo. The purpose of this study is to assess the pollution in the coastal watershed sediments, which is mainly attributed to Zio, Haho and Mono Rivers flowing through concentrated population area of the Lomé, the capital of Togo. These rivers are served by various channels of domestic and industrial wastes carrying untreated effluent of various industries and ultimately drain into local lagoons and Atlantic Ocean. Concentrations of selected heavy metals in surface sediments from forty five sites in Lomé coastal region were studied in order to understand metal contamination due to industrialization, urbanization, and economic development in Lomé. Sediment samples were collected in summer 2008. We have found that heavy metal concentrations in surface sediments varied from 130-829 ug/g for As, 16-125 for Ba, 16-121 ug/g for Cd, 3 to 23 mg/kg for Co, 11 to 26 mg/kg for Cr, 3 to 23 mg/kg for Cu, 3-7 mg/kg for Pb, 5 to 44 mg/kg for Ni, 63-425 ug/g for Se, 10 to 35 for Va, and 6 to 26 mg/kg for Zn. The metalloids antimony as well as beryllium, silver, mercury, thallium, and molybdenum show a value of <2 mg/kg or little variation with regard to base line concentration. Enrichment factors (EFs) and Index of Geo-accumulation (IGeo) were calculated to assess whether the concentrations observed represent background or contaminated levels. The highest levels of metals were found to be at the confluence of the Mono and Haho River streams, indicating the impact of the effluents of the industrial activity as well as hazardous waste dumping in the vicinity of Lomé. The Arsenic retention in the sediments is highly variable and controlled by local processes as a result of natural weathering process of metamorphic bedrock. Furthermore, this study assessed heavy metal toxicity risk with the application of Sediment Quality Guideline (SQG) indices (effect range low/effect range median values, ERL/ERM). Results indicated that the potential toxicity of marine environment can cause adverse biological effects to the biota directly and the human health indirectly through food web chain.