Determination of Concentration of Selected Heavy Metals in Tilapia Fish, Sediments and Water from Mbagathi and Ruiru Athi River Tributaries, Kenya
Mwangi, John Muiruri
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Biomagnifications of toxic heavy metals to man through aquatic life occurs via industrial, runoff, agricultural, municipal and urban waste. Heavy metals such as lead (Pb), nickel (Ni) manganese (Mn) and cadmium (Cd) have World Health Organization (WHO) limits above which they are toxic. Some of the toxic effects include; reduced growth and development, cancer, damage to the nervous system, and in extreme cases death. Athi River,the second largest river in Kenya has its source in a densely populated area which has industrial and agricultural activities like coffee plantations.Some of its tributaries flow through Ruiru town, Athi River town and Nairobi City and hence the river risks untreated discharge and runoff from the activities. Nonetheless, water from Athi River is used for small scale farming and domestic purposes, while a variety of fish in this river including tilapia and mudfish are edible which could lead to accumulation of toxic metals to mankind. These pose a threat to human life hence the need to assess levels of heavy metals in Athi River. This study aimed at determininglevels of Pb,Ni, Mn, Zn, Cd and Cr in tilapia fish gills, sediments and water from Ruiru and Mbagathi tributaries of Athi River during wet and dry seasons in 2011.Purposive sampling was done to determine 4 sampling stations for fish and 10 for sediment and water, while statistical sampling was done to get fish (N=16), sediments (N=40) and water (N=40) samples. Atomic absorption spectroscopy was used for measurement of the levels of elements. The mean concentrations (mg/kg DW) of the heavy metals in tilapia fish gills ranged as follows: Pb (1.42-4.48), Ni (0.12-1.75), Mn (81.50-158.92), Zn (28.00-76.33), Cd (0.71-1.77) and Cr (ND-0.2) . The mean concentrations (mg/kg DW) of the heavy metals in sediments ranged from: Pb (23.19-51.08), Ni (0.15-2.21), Mn (280.88-1294.01), Zn (10.98-50.79), Cd (0.05-1.81) and Cr (0.69-7.33). In water, the range of mean concentration (mg/l) of heavy metals were as follows: Pb (0.004-0.047), Ni (0.007-0.062), Mn (0.187-1.048), Zn (0.002-0.695), Cd (ND - 0.01) and Cr (ND-0.068). The levels of the heavy metals increasedform water, fish and sediments. In the wet season, Zn was found to be significantly high in the fish gills, while Pb and Mn were significantly high in the sediments (P<0.0001). In the dry season, Zn and Cr were found to be significantly high in the sediment (P<0.0001). As far as the WHO’s set limits are concerned, Pb was found to be high in all samples with Mn being high in fish and water while Ni and Cd were high in fish and sediments, the results show evidence of bioaccumulation of heavy metals in the fish with alarming levels that are higher than WHO limits, therefore posing potential risk for inhabitants that depend on the river. While the findings are geared towards providing baseline data on the current pollution status of this river, constant monitoring of the levels of contamination to assess the impact of the heavy metals is deemed necessary.