Physicochemical parameters heavy metal residue levels and their speciation studies in the Victoria basin
ONGERI, DAVID M.K.
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The occurrence of heavy metal contaminants in excess of natural loads is a problem of increasing concern in aquatic ecosystems. This has been caused by anthropogenic, industrial and urban wastes as well as natural geochemical activities. Winam Gulf of Lake Victoria is no exception to these contamination processes even though it is an important resource for food, drinking water and communication to the local community and an important habitat for fish and other aquatic organisms of great biodiversity. This therefore, necessitated the study of the physical and chemical properties which influence water quality and ecosystem balance, the speciation and distribution of trace metals of environmental concern, which include Cd, Cu, Zn and Pb in addition to Fe, which is widely used in the construction industry. The study had a randomized complete block design and the sampling sites were located upstreams and river mouths of rivers Sio, Nyamasaria, Nyando and Sondu-Miriu, and in Port Victoria, Kisumu Car Wash, Dunga and Hippo Point beaches along the lake, as w.ell as Nyamasaria, Migosi and Nyawita Estates in Kisumu City. The samples were taken during the wet months of May and October, 2006 and the dry months of January and February, 2007. The samples were extracted in quadruplets for total and bioavailable metal concentration for solid samples and total metal concentration for water samples. The samples were then analysed using atomic absorption spectrophotometer, AAS (Shimadzu Corporation AA-6300 model) and the results confirmed by inductively coupled plasma mass spectrophotometer, X series 2 ICP-MS model. Physicochemical properties of water were determined. The pH ranged from 7.2-7.8 for estate waters and 6.5-8.8 for lake and upstream waters. Total Alkalinity, Conductivity, Dissolved oxygen, Turbidity, Acidity and Temperature for estate water samples averaged 47.0 mg/L, 333 IJS/cm, 5.4 mg/L, 1.0 NTU, NO and 26.3°C respectivel w . e)those o UN1VERS'TI(~ M/4.SEN --i'~~B·-R'ARY -:-;:sz ~:>. L\ of lake and upstream water samples averaged 47.1 rnq/L, 140.5 ~S/cm, 4.9 mg/L, 270.4 NTU, NO and 27.5°C respectively. The Cd, Pb, Cu, Zn and Fe metal concentrations in ~g/L in estate water samples averaged ND, 9.1, 44.6, 232.7 and 242.4 while those of lake and upstream river water samples averaged NO, 12.7, 53.0, 232.3 and 2,588, respectively. All the water samples analysed conformed to the set international standards for drinking water for the metals investigated except lead and iron in lake water samples. The amount of Fe in lake water samples was beyond the maximum allowable limits in fisheries and aquatic life internationally. The concentrations of the metals increased downstream for both water and sediment signifying that these rivers are among the major sources of the heavy metal load in Winam Gulf. The average total heavy metal concentrations in ~g/g for Cd, Pb, Cu, Zn and Fe in sediments on dry weight basis were 1.2, 32.5, 36.7, 163.3 and 45,990 respectively. The % average bioavailable Cd, Pb, Cu, Zn and Fe metal concentrations in the sediments on dry weight basis were 55.8, 66.9, 57.1, 73.3 and 65.6 .respectively. Statistical analysis showed significant variation (p < 0.05) between the sample sites, seasons and their interactions. These significant variations were attributed to the wide range of activities taking place in the vicinity of each sampling site in addition to the effects of each season especially on the rivers feeding the lake. Total heavy metal concentrations were also determined in the three most abundant fish species in the Winam Gulf (Lates niloticus, Oreochromis niloticus and Rastrineobola argentea). The fish samples were obtained from Ounga, Port Victoria, Sio Port and Hippo Point beaches. The average total metal concentrations determined on dry weight basis were 0.21 ~g/g Cd, 0.87 ~g/g Pb, 3.4 ~g/g Cu, 36.4 ~g/g Zn and 45. 7 ~g/g Fe for Lates niloticus, 0.21 ~g/g Cd, 0.61 ~g/g Pb, 2.7 I-Ig/g Cu, 35.9 I-Ig/g Zn and 48.0 I-Ig/g Fe for Oreochromis niloticus and 0.26 I-Ig/g Cd, 1.2 I-Ig/g Pb, 6.5 I-Ig/g Cu, 250.1 I-Ig/g Zn and 139.1 I-Ig/g Fe for Rastrineobola argentea. The % average bioavailable Cd, Pb, Cu, Zn and Fe metal concentrations in the three species were 67.1, 27.5, 46.3, 79.4 and 29.8 respectively for Lates niloticus, 66.7, 40.9, 41.7, 71.0 and 37.3 respectively for Oreochromis niloticus and 59.1, 47.4, 65.7, 76.2 and 44.5 respectively for Rastrineobola argentea. Statistical analysis showed significant variation (p < 0.05) between the fish species, sample sites, seasons and their interactions. This situation was attributed to the complex mobility trends of fish. The abundance trend of the metals in Lates niloticus and Oreochromis niloticus was Fe > Zn > Cu > Pb > Cd while that of Rastrineobola argentea species was Zn > Fe > Cu > Pb > Cd. The difference in the trends was attributed to the fact that Rastrineobola argentea was analysed whole while only muscle of Lates niloticus was determined after removing the internal organs. These parts were selected for analysis because they represent edible parts of the fish species by man. The fish samples of the three species conformed to the- limits set out by the international standards organisations, however, the concentration of Zn on wet weight basis for Rastrineobola argentea species went beyond the set out tnternational upper limits during the wet season and this was attributed to it being " consumed with its internal organs in place. The wet season generally registered higher metal concentrations in all types of samples analysed in this study. Sediments accumulated metals many more times compared to fish and water. The order of accumulation was found to be Sediment> Fish> Water. Metals in sediments are more bioavailable compared to the metals in fish and therefore could be desorped into the water much more readily than from fish.