Sydney Coastal Stormwater Study

This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.

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Sydney Coastal Stormwater Study

Report no. UWRAA 45

September 1992


The Sydney Water Board has undertaken studies of pollution of receiving waters from the discharge of sewage effluent and storm water. This is part of its holistic approach to water cycle management and its determination to address relevant environmental issues. The management of Sydney’s coastal environment is complex and involves many agencies, often with competing interests and responsibilities. The Board’s traditional interest in this zone has been with its sewage outfalls but now this is being extended to storm water. In the Sydney region (between Palm Beach and Port Hacking) there are approximately two hundred storm water outlets discharging to coastal receiving waters. Of these, there are only four outlets operated by the Water Board. The pollutants from these catchments are likely to confound the results of the environmental monitoring of the impact of deep water ocean sewage outfalls.

The interest in storm water to date has largely revolved around quantity issues. In recent times the focus has moved to quality issues and this has necessitated co-operative work with the local coastal councils to provide a better understanding of the nature and potential environmental significance of storm water discharge.

The Coastal Storm water Study was conducted by the Water Board’s Environment Management Unit with the aims of: (i) quantifying storm water quality and pollutant loads under varying flow conditions from a range of urbanised coastal catchments in the Sydney region; and (ii) relating these pollutant loads to impacts on bathing water quality.

The five coastal catchments chosen as study sites were Whale Beach, Greendale Creek at Brookvale, Bondi, Malabar and Shelly Beach at Cronulla. Drain discharge was sampled during both dry weather and storm events whilst bathing water was sampled only during dry weather. The key variables recorded to indicate pollution were pH, and concentrations of nutrients, dissolved oxygen, metals, pesticides, polycyclic aromatic hydrocarbons (PAHs) and faecal bacteria.

This Study also provided an opportunity to develop a software driven system to monitor also storm water drains. The Supervisory Control And Data Acquisition(SCADA) system continuously monitors storm water discharge for flow rate, temperature, pH, conductivity and dissolved oxygen. The SCADA system also incorporates an automatic water sampler at each of the remote sites.

There are no current standards or regulations for the discharge of urban runoff to coastal receiving waters in New South Wales. The Clean Waters Act Regulations,1972 (CWA) and the Water Quality Criteria for New South Wales Discussion Paper(WQC) (SPCC, 1990b) have been used as a reference for this study.

The water quality of urban runoff from each of the catchments was found to be poor. Specifically, faecal bacteria counts were high in the dry weather discharge at Whale Beach, Greendale Creek and Bondi, with counts exceeding the CWA limit (of1000 cfu/100 mL for restricted waters) in more than 98% of the samples from each catchment. Likewise, concentrations of nutrients were high in the discharge from each of the catchments.

The concentrations of metals, pesticides and polycyclic aromatic hydrocarbons were generally low in the discharge samples. However, the CWA limits were exceeded for some metals in certain catchments, as outlined in the separate catchment water quality sections.

No significant correlation was found between the water quality of dry weather discharge and the adjacent beach bathing water. Beach bathing water quality was not assessed during wet weather events.

A high degree of variability in pollutant levels was found in all drains under dry weather flow conditions. This highlights the danger of using only a few measurements of drain water quality (“Snap Shots”) to describe the status of a given storm water drain.

The results of this study and subsequent interpretation of the data have led to the following general recommendations.

· The high levels of faecal bacteria recorded in storm water discharges suggest that public health warnings should be posted by the owner/operators at the storm water drain outlets.

· Given the temporal variability of pollutant levels in the storm water discharges suggest that public health warnings should be posted by the owners/operators at the storm water drain outlets.

· The site-specific nature of the results highlights the need to exercise caution if these results are extrapolated to other catchments.

· Storm water drainage systems must address quality objectives, as well as quantity.

· The pollutant levels found in coastal storm water discharge are highly variable. Long term reference sites within the Sydney region need to be established so that both flow and pollutant levels are monitored continuously. Baseline data collected from these sites can be used by operators of drainage systems and regulatory authorities to assess the effectiveness of future remedial actions.

· Discharge criteria which reflect the range and variability of pollutants in storm water drains need to be developed for storm water drains which flow to marine and estuarine receiving waters.

The broad strategic and management implications drawn from this report are:

· Local government authorities, the community and the Water Board are responsible for the pollution of coastal waters.

· The Water Board alone cannot be responsible for the clean-up of the oceans by upgrading the performance of its coastal sewage treatment plants. Public education with respect to the causes, impacts and remedies of storm water pollution will be needed to assist in reducing the volume of discharge and the pollutant load.

· Monitoring programmes of ocean sewage effluent outfalls should be linked to storm water discharge monitoring programmes as both sources discharge a similar range of pollutant types.

· Effective catchment management will be expensive and will raise questions of equity. Storm water discharge is the product of all catchment management practices. The cost of remediation of the storm water system will be high and resolving the responsibility for remedial action will be difficult.

· Long term improvements in Sydney’s ocean environment will require careful management of both storm water and sewage systems.

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