ASSESSMENT OF LEVELS AND DISTRIBUTION OF HEAVY METALS IN LAKE CHILWA CATCHMENT

Abstract

The aim of this study was to assess the levels and distribution of heavy metals in Lake Chilwa and its catchment. Water and sediment samples were collected from the lake and main inflowing rivers. The samples were analyzed for pH, Conductivity and heavy metals (Cd, Pb, Zn, Cr, Hg, Ni, and As). Six metal elements, in order of decreasing concentration, were detected in the water; Zn > Cu > Pb > Cr > Ni > Cd. Ranges for the mean values were; pH (7.87 – 10.17), Conductivity (97 – 390 µS/cm), Zinc (6.24 - 1168.7 µg/L), Copper (0 – 47.83 µg/L), Lead (0 – 49.94 µg/L), Chromium (0.22 – 33.05 µg/L), Nickel (0.40 – 8.20 µg/L), Cadmium (0 – 0.53 µg/L). Mercury and arsenic were not detected in all sampling locations. Positive correlations were observed between cadmium and Lead (r = 0.70), Copper and Zinc (r = 0.70), Cadmium and Nickel (r = 0.50) and Lead and Nickel (r = 0.41), suggesting similar sources of input. Concentrations in the lake sediments were detected in the following order; Zn > Cr > Ni > Cu > Pb > As. Mercury and cadmium were not detected. Average concentrations in the sediments were; Zn (64.15 and 68.11 mg/Kg), Pb (7.36 and 8.13 mg/Kg), Ni (35.03 and 35.74 mg/Kg), Cu (24.04 and 16 mg/Kg), Cr(53.72 55.89 mg/Kg), and As (1.27 and 0.73 mg/Kg). The heavy metal pollution index ranged from 2.24 to 114.45. All points except Kachulu harbor had values far below the critical pollution index value of 100. Concentrations were low in the upstream locations located in the highlands and high in the lowland areas close to the lake. This indicates the effects of anthropogenic activities that include agriculture and settlements activities.