Monitoring of Heavy Metal Loading into the Wetlands South of Lake Victoria Basin, Nothern Tanzania

Abstract

The paper discusses sources, concentrations and physicochemical parameters of heavy metals of environmental concern in the wetlands of Lake Victoria basin in Tanzania. The metals investigated include copper (Cu), lead (Pb), zinc (Zn), cadmium (Cd), chromium (Cr), nickel (Ni), arsenic (As), manganese (Mn), cobalt (Co) and mercury (Hg). These metals are released into the wetlands via weathering and erosion of rock formations, mining operations and industrial activities, discharge of municipal and domestic waste, and use of agrochemicals. The levels of these heavy metals in soil, sediment, water, and biota differ. Wetlands impacted by gold mining activities in the South Lake Victoria basin show elevated heavy metal contents in soil and sediment, particularly Cu (13-415 mg/kg), Pb (24-94 mg/kg), Zn (9-80 mg/kg), Cr (19-77 mg/kg), Ni (12-37 mg/kg) and Hg (0.19-1.76 mg/kg), contrary to non-impacted wetlands, which contain relatively low concentration of these metals (Cu 1-18 mg/kg, Pb 0-27 mg/kg, Zn 5-22 mg/kg; Cr 5-25 mg/kg, Ni 2-8 mg/kg, Hg <0.075 mg/kg). Elevated metal concentrations in soil and sediment are correlated with high metal contents in some plants within the affected wetlands. Oxidation and leaching of sulfide-bearing gold mine tailings resulted in acid mine drainage (AMD) with pH 3.25 - 4.10) and enhancement of heavy metal load and dissolved solids in streams draining into Nungwe Bay wetland on the southwestern shore of the Lake Victoria. Water samples from Bulyankhulu wetland drainage system were found to contain abnormally high Cu concentrations in the order of 270 mg/l. Samples from Mwakitolyo-Isanga and Nungwe Bay had higher values of Cr concentrations than those from Bulyankhulu and Luchili wetlands. Manganese was recorded to be 2.04 mg/L in a sample from Nungwe Bay, the reason being its mobilization from deeply weathered terrain associated with iron oxides.

Keywords: Heavy metals; wetlands; Lake Victoria gold field; mining.