Metal contamination of sediments associated with deepwater ocean sewage outfalls, Sydney, Australia

The New South Wales Environment Protection Authority has recently completed a programme to evaluate the impact on near-shore shelf sediments, following the commissioning of three new deepwater sewage outfalls off Sydney, Australia. These outfalls discharge a total of around 1300 ML day⁻¹ of primary treated sewage derived from domestic and industrial sources. The study compared three locations that were close to the outfalls (likely impacted) and three locations far from the outfalls (likely non-impacted). Four zones were sampled within each location to provide estimates of spatial variability. Three samples of surface sediment were collected from each zone at six monthly intervals over a three year period which encompassed the commissioning of the outfalls. The results of the study suggest that, associated with the commissioning of the three deepwater outfalls, there has not been a significant effect on the concentration of trace metals in the sediments surrounding the outfalls. The concentrations of trace metals in the whole sediment generally were close to the world-wide background levels quoted in the literature, and generally reflected the distribution pattern of the sedimentary fines (< 62.5 mm). When compared to a number of sediment quality guidelines, the concentrations of trace metals were found to be below a level considered to have the potential to cause biological effects or (in the case of As, Ni, Cr) at a level that could be tolerated by the majority of benthic organisms.