Characterization of organic matter at the air-sea interface, in subsurface water, and in bottom sediments near the Malabar sewage outfall in Sydney's coastal region

Abstract

The lipid and related-chemical compositions of samples from the air-sea interface, subsurface water and sediments collected adjacent to Sydney's Malabar nearshore sewage outfall during January and February 1990 were analysed in detail. A novel sampling scheme made use of a rotating-drum microlayer sampler, a towed Seastar sampler for filtration and extration of subsurface water in situ, and sediment collection in Teflon bags by divers. Particulate and dissolved organic matter was examined for four distinct aquatic environments: the surface microlayer in a no-slick zone (ML), the microlayer in a plume slick (PS), the microlayer in a banded slick (BS) and subsurface waters (SS). The concentrations of lipid classes and of many individual components in particulate matter from water samples generally followed the trend PS > ML > BS > SS, although in several instances the sequence began with ML > PS. A similar pattern was seen for the dissolved organic fractions. The composition of the ML sample differed from the compositions of the other water samples for several of the lipid classes analysed; the very high relative abundance of cholesterol and the presence of significant portions of long-chain saturated and monounsaturated fatty acids indicate a substantial marine origin for the lipids in the ML sample. The concentrations of most components were generally an order of magnitude higher in sediment 0.5 km from the Malabar outfall than in sediment 0.85 km away (Long Bay). The faecal indicator coprostanol was present in all samples at concentrations ranging from 0.1 to 7 µg L^-1 in water and from 0.1 to 1.1µg g^-1 in sediments. Petroleum contamination was also apparent in all samples, based on a number of distinctive features of the hydrocarbon profiles: the occurrence of a high abundance of unresolved complex material, little or no odd-over-even predominance in the distribution of n-alkanes, and the presence of hopanes and steranes characteristic of crude oil. Polycyclic aromatic hydrocarbons (PAHs) derived from combustion sources were detected in sediments at both distances from the outfall. PAH concentrations in water samples were at the limits of detection of this study. With the commissioning of deep-water outfalls to replace the nearshore ones, a decrease in nearshore contamination is expected. This study provides baseline chemical data for future comparative examination of the efficacy of Sydney's deep-water sewage outfalls.