Bioaccumulation of heavy metals from irrigant water in vigna unguiculata and other vegetables grown in Dar es Salaam

A baseline study involving analyses of physico-chemical parameters of irrigation water samples from various localities in Dar es Salaam where vegetables are grown was carried out. Water source classes namely tap water, well water and river water were examined. The study has revealed that tap water is comparatively the best water source for irrigation purposes followed by well water while river water ranks the last. Analysis of physico - chemical parameters of raw water from Upper Ruvu treatment works revealed that tap water is at its optimal quality at completion of that process. Observed contamination of tap water at consumer point is a result of re-contamination. It was also observed that bio-accumulation of heavy metals in vegetables showed a considerable variation in various vegetable species. This is due to differences in metal uptake efficiencies by the plants. Among the metals studied, Zn, Mn, and Fe were higher in all vegetables than Pb, Cr, Cd and Cu. The mobilization of heavy metals from the aquatic ecosystem to biota was studied by examining comparative levels in the two. The study has revealed that vegetables have ability to bio-accumulate much of the metal content in their bio-mass at levels greater than those found in the surroundings. Laboratory controlled experiments to examine bio-concentration of Cd, Pb and Cr in Vigna unguiculata revealed that Cd, and Pb were bio-concentrated more on the leaves of the plant while Cr levels were highest in the roots. The Bio-Concentration Ratio values as well as the metal ion concentration in the components of the vegetable increased with increasing concentration of the metal ions in irrigation solutions. The estimated daily intake for Cd, Pb and Cr levels in the popular local cowpea consumers is above the Recommended Dietary Daily Allowance (RDDA) of 0.001- 0.008, 0.007 - 0.03, and 0.005 - 0.02 µg per day respectively.