Distribution,fractional composition and release of sediment-bound heavy metals in tropical reservoirs

The concentrations of heavy metals in bottom sediments from urban receiving waters, Kranji Reservoir and MacRitchie Reservoir in Singapore, were investigated. Distribution of the heavy metals in the bottom sediment, interstitial water and overlying water was analysed. The concentration of heavy metals in the interstitial water was found to be significantly high and exceed the water quality criteria by three to eleven times. The partitioning coefficient shows that the solubility of the metals are in the order: Mn > Zn > Cu > Pb > Fe > Al. Fractional composition of heavy metals in the sediments was determined using sequential extraction process. The results show that Cu was largely complexed by organics, 74% of Zn was in easily remobilised fractions, and 36% of Pb was in the easily reducible fraction and 47% in carbonate and ion-exchangeable fractions. Release of sediment-bound metals was studied. The results indicate that, besides pH and redox, the sediment buffering capacity is an important parameter affecting the remobilization of heavy metals from sediment.