Investigations of fluvial and estuarine sediments have indicated stormwater is an important source of heavy metals to Port Jackson estuary and high concentrations of these sedimentary contaminants are a threat to the healthy functioning of the estuarine ecosystem. Stormwater remediation devices
have been installed in stormwater channels entering the estuary, however these devices are mainly for removing gross pollutants and are ineffective in removing heavy metals from stormwater.
A thorough characterisation of heavy metal inputs and behaviour has been undertaken by sampling, analysing and modelling heavy metals in stormwater entering Port Jackson estuary to provide a rigorous data base for future remediation efforts. A conceptual model of transport and fate of heavy
metals in stormwater entering Port Jackson estuary has also been developed to identify heavy metals, subcatchments and flow regimes requiring remediation, and to assist in designing remediation devices for optimum removal of heavy metals from stormwater.
Modelling of stormwater using the Model for Urban Stormwater Improvement Conceptualisation (MUSIC) indicated that the average annual discharge of stormwater from the Port Jackson catchment was 215,307 ML. Average annual loadings of arsenic, cadmium, chromium, copper, nickel, lead and zinc in stormwater discharging to Port Jackson estuary were 0.8, 0.5, 1.7, 3.2, 1.1,
3.6 and 17.7 tonnes per year, although comparison to other studies in the catchment suggests these values may be underestimations of actual loadings by 1.3 to 10 times. The proportion of heavy
metals discharged under low-flow conditions (<5mm of rainfall in 24 hours), medium-flow
conditions (between 5 and 50mm in 24 hours), and high-flow conditions (>50 mm of rainfall in 24 hours) was 6.5%, 62.5% and 31%, respectively.
The conceptual model indicates stormwater loadings of copper, lead and zinc pose a risk to the health of riverine and estuarine ecosystems in the catchment and these metals should be targeted for remediation. Stormwater channels which should be prioritised for remediation include the channels entering southern embayments west of Darling Harbour; Duck, Parramatta and Lane Cove Rivers; and the channels and rivers entering Neutral, Long and Sugarloaf Bays. Stormwater loadings of lead are predominantly associated with suspended particulates, whereas loadings of copper and zinc are equally partitioned between dissolved and particulate phases. Stormwater remediation strategies should target both dissolved and particulate phases to ensure effective removal of copper, lead and
Research suggests heavy metals in stormwater discharged to the estuary under high-flow conditions are rapidly exported seaward and bypass the estuary. Preliminary research also suggests that under medium-flow conditions, particulate heavy metals bypass the embayments of Port Jackson and are
deposited in the main channel. Once deposited in the main channel, particulate heavy metals are likely to be remobilised and removed from the estuary through multiple phases of resuspension.
Although further research is required in this area, this preliminary research suggests remediation should target low-flow conditions.
The findings of the current research could be used to identify appropriate remediation strategies for dissolved and particulate phase heavy metals in stormwater discharging to Port Jackson estuary. However, in designing stormwater remediation devices, consideration should also be given to the
range of contaminants that may be present in stormwater entering Port Jackson estuary (including suspended solids, nutrients, pesticides and organics).