Deepwater ocean outfalls: A sustainable solution for sewage discharge for mega-coastal cities (Sydney, Australia): Influence of deepwater ocean outfalls on shelf sediment chemistry.

A cliff-face disposal system discharging approximately 940 ML/day, or 80% of sewage generated by the City of Sydney (Australia) (population 3.3 million) was replaced by three deepwater ocean outfalls in the early 1990s. Enrichment of anthropogenic chemicals from cliff-face discharges raised concerns regarding long-term accumulation of sewage particulates and associated contaminants in offshore sediments and for reduced beach water quality. The current post-commissioning investigation detected a southward gradient of sediment fining and increased total organic carbon in the study region. Deepwater ocean discharges have not contributed to an accumulation of fines, or to increased metallic/nonmetallic chemicals of concern with no elevated risk of adverse biological effects beyond pre-commissioning conditions. Instead, the best modelled relationship was recorded between benthic infauna and sedimentary fines and not to contaminants. Historic sea dumping prior to 1932 in the north of the study area has resulted in enrichment of some non-bioavailable sedimentary metals.