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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 |
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