SydneyCoastal Stormwater Study
ReportNo WSAA 45
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.
Thebroad strategic and management implications drawn from this report are:
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