Heavy metals can enter waterways through natural processes such as the weathering of rocks and leaching from soils. Human activities such as mining, industrial discharges and car emissions also contribute to heavy metal loads in many waterways.
Heavy metals are persistent in the environment. They can become toxic to estuarine and marine mammals, and to humans if ingested through the consumption of seafood.
Major industry has been the main historic source of heavy metal contamination in the Derwent. The zinc smelter at Risdon and the newsprint mill at Boyer both historically discharged heavy metals into the river. In recent years, however, heavy metal discharges have largely been eliminated from the mill and dramatically reduced from the smelter.
The last 30 years has seen a 5 to 10-fold decrease in water column concentrations of zinc, cadmium and other metals. In addition there appears to be a gradual reduction in sediment heavy metal levels, particularly in some of the most highly contaminated mid-estuary sites.
Nonetheless metal concentrations in the estuarine sediment remain high by national and international standards.
Restoring and promoting the Derwent estuary