SydneyCoastal Stormwater Study

ReportNo WSAA 45


September 1992




TheSydney Water Board has undertaken studies of pollution of receiving waters fromthe discharge of sewage effluent and stormwater. This is part of its holisticapproach to water cycle management and its determination to address relevantenvironmental issues. The management of Sydney’s coastal environment is complexand involves many agencies, often with competing interests andresponsibilities. The Board’s traditional interest in this zone has been withits sewage outfalls but now this is being extended to stormwater. In the Sydneyregion (between Palm Beach and Port Hacking) there are approximately twohundred stormwater outlets discharging to coastal receiving waters. Of these,there are only four outlets operated by the Water Board. The pollutants fromthese catchments are likely to confound the results of the environmentalmonitoring of the impact of deep water ocean sewage outfalls.


Theinterest in stormwater to date has largely revolved around quantity issues. Inrecent times the focus has moved to quality issues and this has necessitatedco-operative work with the local coastal councils to provide a betterunderstanding of the nature and potential environmental significance ofstormwater discharge.


TheCoastal Stormwater Study was conducted by the Water Board’s EnvironmentManagement Unit with the aims of: (i) quantifying stormwater quality andpollutant loads under varying flow conditions from a range of urbanised coastalcatchments in the Sydney region; and (ii) relating these pollutant loads toimpacts on bathing water quality.


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


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


Thereare no current standards or regulations for the discharge of urban runoff tocoastal 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.


Thewater 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 atWhale 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 fromeach catchment. Likewise, concentrations of nutrients were high in thedischarge from each of the catchments.


Theconcentrations of metals, pesticides and polycyclic aromatic hydrocarbons weregenerally low in the discharge samples. However, the CWA limits were exceededfor some metals in certain catchments, as outlined in the separate catchmentwater quality sections.


Nosignificant correlation was found between the water quality of dry weatherdischarge and the adjacent beach bathing water. Beach bathing water quality wasnot assessed during wet weather events.


Ahigh degree of variability in pollutant levels was found in all drains underdry weather flow conditions. This highlights the danger of using only a fewmeasurements of drain water quality (“Snap Shots”) to describe the status of agiven stormwater drain.


Theresults of this study and subsequent interpretation of the data have led to thefollowing general recommendations.


·        The high levels of faecal bacteria recorded in stormwater dischargessuggest that public health warnings should be posted by the owner/operators atthe stormwater drain outlets.


·        Given the temporal variability of pollutant levels in the stormwaterdischarges suggest that public health warnings should be posted by theowners/operators at the stormwater drain outlets.


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


·        Stormwater drainage systems must address quality objectives, as well asquantity.


·        The pollutant levels found in coastal stormwater discharge are highlyvariable. Long term reference sites within the Sydney region need to beestablished so that both flow and pollutant levels are monitored continuously.Baseline data collected from these sites can be used by operators of drainagesystems and regulatory authorities to assess the effectiveness of futureremedial actions.


·        Discharge criteria which reflect the range and variability ofpollutants in stormwater drains need to be developed for stormwater drainswhich flow to marine and estuarine receiving waters.


Thebroad strategic and management implications drawn from this report are:


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


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


·        Monitoring programmes of ocean sewage effluent outfalls should belinked to stormwater discharge monitoring programmes as both sources dischargea similar range of pollutant types.


·        Effective catchment management will be expensive and will raisequestions of equity. Stormwater discharge is the product of all catchmentmanagement practices. The cost of remediation of the stormwater system will behigh and resolving the responsibility for remedial action will be difficult.


·        Long term improvements in Sydney’s ocean environment will requirecareful management of both stormwater and sewage systems.


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