Distribution of metals and speciation of sediment grabs in Lake Burragorang in Sydney, Australia

Lake Burragorang in South-West Sydney, holding 2.1 billion litres of water, is the principal source of drinking water supply for 4.5 million Sydney residents. The Wollondilly, Nattai, Kowmung and Coxs are the four major tributaries that provide approximately 83% of total inflow into the lake. Each of these tributaries has catchments with substantially different geology and hence the sediment loads contributed by each of these rivers have their own characteristics. As a part of the water quality analysis within Lake Burragorang, sediment grabs were extracted from the lake bottom (from pre-selected locations) with a view to determine spatial variation of sediment characteristics across the lake. A total of eleven sites were chosen for sequential extraction of sediment grabs. Samples were analysed to determine distribution and speciation of 14 selected heavy metals (from 11 sites) in bed sediments. The results were correlated with the geological and human histories of the local catchments and reservoir operations. A consistent general trend in metal concentration variation was observed. Some sites showed lower concentrations of metals than the background level whereas other sites indicated metal concentrations to be far higher than the background level. These were found to be related to previous mining activities. In general, no significant spatial variations were observed in the speciation trends.