Assessment of heavy metals in water, sediments and fish from lake Chilwa, Zomba,Malawi

Abstract

An assessment of heavy metals in water, sediments and fish was done in Lake Chilwa (2015 to 2016), Zomba, Malawi. Five sampling points namely Bonga, Chaone, Thunde, Mchenga and Kachulu were selected. These areas were chosen because of their easy accessibility and also are the fishing sites. Samples were analyzed for pH, electrical conductivity (EC), total dissolved solids (TDS), temperature, nitrates (NO3-), phosphates (PO42-), lead (Pb), cadmium (Cd), manganese (Mn), copper (Cu), zinc (Zn) and iron (Fe) using American Public Health Association (APHA) prescribed standard methods. The sample collection and analysis happened in dry season and wet (rainy) season. Generally, physicochemical parameters were higher in the dry season than the rainy season which was attributed to low water volumes. The results further revealed that Pb was undetectable in water, sediments and fish. However, Mn and Cu were detected in fish and sediment samples (Mnsediments= 2.25 – 10.66 mg kg-1 dry weight (dw); Mnfish = 0.015 – 1.18 mg kg-1 dw; Cusediments = n.d – 1.63 mg kg-1 dw; Cufish = 0.41 – 0.92 mg kg-1 dw). The concentrations of heavy metals in fish samples were below the maximum limits for edible fish recommended by FAO / WHO: Cd (0.1 mg kg-1), Cu (3 mg kg-1), Fe (43 mg kg-1), Pb (0.2 mg kg-1), Mn (2 – 9 mg kg-1) and Zn (60 mg kg-1). Generally, the levels of heavy metals were ranked in the order sediments > fish > water. The study further revealed significant differences in metal concentration between dry season and rainy season. Mn, Cu and Zn in sediments were significantly higher in the dry season than rainy season (p < 0.05). Conversely, Fe in sediments was significantly higher in the rainy season than the dry season (p < 0.05). Results of this study indicate that the main sources of heavy metals in Lake Chilwa are the use of fertilizers, burning of fossil fuels and disposal of metal products. Therefore, there is a need for an integrated approach towards controlling pollution activities in the area.