Trace metals in sediments from the Yarra River

Abstract

The distribution and phase association of iron, manganese, cadmium, copper, lead and zinc in bottom sediments taken from the freshwater, estuarine and bay regions of the Yarra River were investigated. The fraction of the estuarine sediments smaller than 20 µm contained substantially higher concentrations of cadmium, copper, lead and zinc than did the corresponding sediment fraction from the upper river and bay regions. Sequential chemical extraction revealed that most of the variability in the trace metal concentration of these sediments was associated with changes in the reducible phase. A substantial increase in trace metal concentrations occurred in going from river to estuarine sediments. For example, the lead concentration increased approximately eightfold (43-375 µg g^-1), the zinc fourfold (122-447 µg g^-1) and the copper almost twofold (64-106 µg g^-1). This was attributed to coagulation and sedimentation of trace-metal- enriched iron and manganese oxides in the estuarine region. The subsequent decrease in metal concentrations in the bay sediments was due to either physical dilution of the contaminated sedlment with material of lower metal concentration or remobilizatlon of the trace metals from sediments deposited in the bay.