Heavy metal pollution indices in sediments and bio accumulation factor in the root tubers grown in selected wetlands of Lake Victoria Basin

Abstract

This study assessed Heavy metal pollution indices in sediments and bioaccumulation in root tubers grown in selected wetlands of Lake Victoria basin. Sediment samples at the surface (at depth < 0.2m) were taken. Plant materials were hand-pulled, and tubers separated, washed and kept at 40C until analysis. Heavy metals were determined using a Perkin Elmer model 1100B AAS, and data were analysed using the analysis of variance (ANOVA) in Excel at a 0.05 significance level (p<0.05). Highest concentrations of Cu and Pb were observed in Murchison Bay sediments ranging from (172.16±2.9) mg/kg Cu and (107.68±2.32) mg/kg Pb in cassava garden to (143.39±2.68) mg/kg Cu and (97.82±2.21) mg/kg Pb in cocoyam gardens. In root tubers, the concentrations were in the order: cassava ˃ sweet potatoes ˃ cocoyam. Nalusojjolo wetland registered the lowest concentrations in both the sediments and tubers. Ni & Cr were relatively low in sediments and root tubers in all the wetlands, except cadmium in Murchison Bay sediments, which exceeded the USEPA permissible limit value of 1.2mg/kg. Murchison Bay recorded the highest mean concentration of metal with Cu > Cr >Pb > Ni > Cd, and Nalusojjolo the least. The Cf and PLI values for the metals observed in this study displayed, Nalusojjolo and Kyetinda wetlands were uncontaminated for all metals except Cd, with moderate contamination in Kyetinda wetland. Ni and Cr in both Murchison Bay and Namanve registered Cf to low contamination. Murchison Bay and Namanve wetland had considerable Cf values of 4.012 and 5.103 for Cu, Pb and 3.075 and 4.900 for Pb, Cd, respectively. Nalusojjolo and Kyetinda PLI < 1, while the other two wetlands had PLI ˃ 1. The I-geo rating for Cu, Pb, Ni and Cr were < 0 for Nalusojjolo and Kyetinda wetlands, except Cd in Kyetinda. Therefore, Cu, Pb, Ni and Cr levels in sediments were not from anthropogenic activity but may be from parent material that formed the soil. The Igeo ˃ 0 for Murchison Bay and Namanve for Cu, Pb and Cd implied that most of the metal pollution originates from anthropogenic activities. The BAF values for all plants were < 0 despite of high concentration of metal in roots, suggesting that tubers can absorb but not bioaccumulate the metal.