Heavy metals in water and tilapia fish from Athi-Galana-Sabaki tributaries, Kenya

The well-known long term toxic effects of heavy metals lead (Pb), nickel (Ni), manganese (Mn), zinc (Zn), cadmium (Cd) and chromium (Cr) to man are of both scientific and environmental concerns. The presence of these heavy metals in water creates a societal health risk in rivers that are otherwise useful for drinking water and fisheries as is of the case of Athi-Galana-Sabaki river in Kenya. Fish play an important role in human nutrition and therefore need to be carefully and routinely screened to ensure that there are no high levels of heavy metals being transferred to man through their consumption. We report levels of Pb, Ni, Mn, Zn, Cd and Cr in tilapia gills and water from tributaries of Athi River measured by Atomic Absorption spectroscopy. The concentration (mg/kg DW) found in the tilapia fish gills ranged; 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). In water, the ranges (mg/l) were 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). Seasonal variation showed that Zn was significantly high during wet season in fish and water (p<0.0001). There was no general trend in levels of heavy metals in water, but in fish gills the elements decreased in the order Mn>Zn>Pb>Cd>Ni>Cr.Results evidence bioaccumulation of heavy metals in the fish and also alarming levels that are higher than the 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. (author)