Heavy metals are persistent in the environment, and can become toxic to marine life when present at elevated concentrations. Some metals also bioaccumulate to high levels in fish and shellfish – a risk for human consumers.
Levels of heavy metals in Derwent estuary water, sediments and seafood are among the highest recorded in Australia, and in many cases exceed national guidelines for zinc, mercury, lead, cadmium, copper and arsenic. Past industrial practices associated with zinc and paper production were the primary sources of metal contamination, as were lesser sources such as urban run-off containing leaded petrol residues. There has been a major metal reduction in metal discharges over the past few decades, following a series on on-ground projects to capture and treat contaminated ground and surface water at the Risdon zinc refinery, and further works are planned over the next few years.
The DEP coordinates and reports on a number of initiatives to monitor, investigate and manage heavy metal comtamination in the Derwent. These include:
- Monthly water quality monitoring for zinc
- Regular seafood safety and sediment surveys
- The Heavy Metal Water Quality Improvement Plan, described below
- Derwent-specific dredging guidelines
- On-ground projects to reduce heavy metal loads, including securing grant funding for groundwater remediation projects at Nyrstar and urban stormwater projects
The last 30 years has seen a 5 to 10-fold decrease in water column concentrations of zinc, and there also appears to be a gradual reduction in metal levels in sediments, particularly in some of the most highly contaminated mid-estuary sites. Nonetheless metal concentrations in the Derwent estuary sediments, shellfish and some fish remain high by national and international standards, and further work is needed to further reduce loads and manage contaminated sediments. In particular, nutrients and algal growth need to be carefully managed to avoid low oxygen conditions that could cause metals to be released from contaminated sediments.
Water Quality Improvement Plan
In 2007–2010, the Derwent Estuary Program received Commonwealth funding for a Water Quality Improvement Plan for heavy metals and nutrients. The aim of this project was to better understand and further reduce heavy metal contamination in the Derwent estuary, including how nutrients may influence metal availability.
The major elements of the WQIP included:
- Determining heavy metal inputs and existing levels
- Computer modelling of water and sediment transport processes controlling the movement of heavy metals
- Evaluating the processes that influence heavy metal availability in sediments
- Developing a biogeochemical model for the estuary to model nutrient inputs, processes and impacts
- Establishing heavy metal and nutrient targets and the loads required to achieve these
- Identifying and evaluating management options to meet these targets
- Recommending actions for implementation
The final report for this project sets out a number of recommendations, the majority of which have been implemented or are currently in progress. These include further management of contaminated groundwater and stormwater at the Nyrstar Hobart Smelter site, careful management of contaminated sediments, and better public information about seafood safety. The report also recommended that nutrients be carefully managed to avoid algal blooms and subsequent low oxygen conditions, that could cause metals to be released from contaminated sediments.