Levels and spatial distribution of heavy metals in Lake Chilwa Catchment, Southern Malawi

Abstract

The aim of this study was to assess the levels and distribution of heavy metals in Lake Chilwa and its catchment, and to understand the associated level of pollution. Water and sediment samples were collected from the lake and main inflowing rivers. A total of 23 surface water samples were sampled and analysed for pH, EC, Cu, Cr, Zn, Cd, Pb, As and Hg. Conductivity and pH were measured on-site with a Hanna portable multi meter, while metals were analysed by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). The pH was found within the alkaline range (7.87-10.13), while conductivity ranged from 97-390μS/cm. The following metals were detected in the water samples; Zn (6.24–1168.70 μg/L), Cu (BDL–47.83 μg/L), Pb (BDL–49.94 μg/L), Cr (0.22–33.05 μg/L), Ni (0.40–8.20 μg/L) and Cd (BDL–0.53 μg/L). Hg and As were not detected in all sampling locations. Strong positive correlations were observed between Cd and Pb (r = 0.70), Cu and Zn (r = 0.70), while Cd and Ni (r = 0.50), Pb and Ni (r = 0.41) showed mild correlations, suggesting similar sources of input. Sediments were sampled from 2 locations in the lake and the following metals were detected; Zn (66.13 mg/Kg), Pb (7.74 mg/Kg), Ni (35.39 mg/Kg), Cu (20.02 mg/Kg), Cr (54.81 mg/Kg) and As (1.0 mg/Kg). Mercury and arsenic were not detected from both sampling locations. The heavy metal pollution index ranged from 2.24 to 114.45. All points except Kachulu harbour had values far below the critical pollution index value of 100. The values observed were well below the tolerable limits recommended by the World Health Organisation (WHO) standard for potable water, except for Pb at Kachulu Bay (P19) which was above the limit. Concentrations of the metals in the rivers were low upstream and increased downstream. Highest values for most of the elements were observed from the lake. Though levels are low now, the persistent and cumulative properties of these elements would render them unsafe in the near future if proper controls are not enforced.

Keywords: Heavy metals, Lake Chilwa, Water pollution, Water quality