A triad study of sediment quality associated with a major, relatively untreated marine sewage discharge

Sediment chemistry, toxicity and benthic community structure were used individually and together (i.e. the sediment quality triad) to assess the effects of screened but otherwise untreated sewage from a major city, discharged to the marine environment via an offshore, deep water outfall. Only two chemicals related to the outfall, mercury (probably primarily from hospitals and dental offices) and 1,4-dichlorobenzene (probably primarily from urinal deodorizers) were found in the sediments at concentrations of possible concern, up to 100 m away from the outfall terminus. Sediment toxicity tests indicated that survival was not a major problem, even at the outfall terminus; however, growth and development were reduced up to 100 m away from the outfall terminus. Benthic community structure followed the Pearson-Rosenberg model for organic enrichment, with stations 100 m away from the outfall terminus typical of moderately polluted conditions. Such minimal, highly localized impacts to the receiving environment are not unexpected, since the receiving environment is well flushed and mixed, and the city is not highly industrialized. The results of this study raise questions concerning generic decisions about sewage treatment where site- and situation-specific conditions do not indicate a major problem, and source control is a reasonable option.