PRESENCE AND DISTRIBUTION OF TOXIC TRACE ELEMENTS IN WATER AND SEDIMENTS OF THE SOUTHERN TOGO RIVERS WATERSHED, WEST AFRICA

摘要

The Haho, Mono and Zio Rivers watershed covers about 3000 km~2 of southern Togo in West Africa and is important to both recreational and other activities such as fishery, irrigation in the region. In order to assess current water and sediment quality conditions, distribution of various trace metals and heavy metals were determined in river sediments at 23 sampling stations in the watershed. Concentrations of selected heavy metals (As, Cd, Co, Cr, Cu, Ni, Pb, Se, and Zn) in surface and core sediments sites in Lome coastal region were studied in order to understand metal contamination due to industrialization, urbanization, and economic development in Lome. Sediment and water samples were collected in summer 2008. Trace metals and heavy metal concentrations in surface sediments varied from 0.130-0.829 mg/kg for As, 16-125 for Ba, 0.016-0.121 mg/kg 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, 0.063-0.0425 mg/kg 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 showed a value of <2 mg/kg or little variation with regard to base line concentration. The concentration of arsenic in water samples was varied from 3.0 to 6.46 ppm for arsenic in Lake Togo, 3.0 to 4.9 ppm for Mono River samples, 3.4 to 4.0 ppm for Zio River, 3 to 5.92 ppm for Haho River. According to EPA arsenic permissible level [0.1 mg/1] these values show a 31% up to 65 % increase above MCL. The lead average concentrations in water samples were found to be high but they varied. Lead was recorded higher in samples from Haho and Mono Rivers (samples HI[0.04 mg/1], LT10 [0.1 mg/1] and M5[0.5 mg/l]and Z3 [0.07mg/l]). Comparing these values to EPA [MCL0.015 mg/1] the water samples showed a 25% up to 700 % increase above MCL in some locations. 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 Lome and other big cities along the rivers. The arsenic retention in the sediments was highly variable and controlled by local processes as a result of the natural weathering process of metamorphic bedrock. 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.