25 years on, have Sydney's deepwater ocean outfalls created problems in adjacent sediment?

Approximately 80% of Sydney’s sewage receives primary treatment at the North Head, Bondi and Malabar wastewater treatment plants. Since 1990-91 treated wastewater has been released through three deepwater outfalls located 2... [ view full abstract ]

Approximately 80% of Sydney’s sewage receives primary treatment at the North Head, Bondi and Malabar wastewater treatment plants. Since 1990-91 treated wastewater has been released through three deepwater outfalls located 2 to 4 km offshore at depths of 60 to 80 m. Wastewater is discharged through small jets, situated along diffuser arrays, ensuring wastewater mixes rapidly with the coastal offshore waters. Over the 1999 to 2014 period Sydney Water monitored ocean sediments under a NSW EPA design. This design addresses two questions. Have chronic impacts occurred, and is any impact spreading south? Soft bottom sediment samples were collected from: each deepwater outfall location; three reference locations; and three gradient locations. Gradient locations were situated 3 km, 5 km and 7 km to the south of the Malabar deepwater outfall in the direction of the predominant current. In every third year a suite of chemicals were assessed and benthic invertebrates were identified to the family level and counted. Over the 16 years of sediment monitoring there is no evidence of accumulation of total organic carbon or fine sediment (particles <0.063mm) from the outfalls. This suggests wastewater discharges have not been a major source of organic enrichment or toxic metal build-up in the ocean sediments around the outfalls. Corresponding analyses of benthic community samples from outfall and reference locations found no measureable impact. That is, no temporally consistent chronic impact was detected. There was also no pattern to imply a measureable impact was spreading south of the Malabar outfall. Rather benthic invertebrate communities at each location were shown to be highly variable through time and a broad change in community structure from the most northern reference location to the most southern reference location was recorded. These sediment indicator results are supported by ongoing oceanographic modelling. Modelling indicated that the diffuser arrays operate better than the original design criteria. Particles present in the wastewater disperse and sink to the sea floor within 5 km in most years. In some years they spread over 10 km. Considering all the evidence, 25 years of deepwater outfall operation has not created measurable problems in ocean sediments.