Assessment of the sources, transport and fate of sewage-derived organic matter in Port Phillip Bay, Australia, using the signature lipid coprostanol

Abstract

To evaluate the distribution of faecal material in Port Phillip Bay, surface sediments from sites throughout the bay and inflowing water courses were analysed for fatty acids and sterols. Water samples were also collected to identify possible sources of faecal contamination. Bay sediments had total fatty acid concentrations between 4 and 183 µg g^–1 (dry weight) and total sterol concentrations between 0.6 and 39.2 µg g^–1. Creek sediments contained more sterols, ranging from 22.8–148 µg g^–1. These lipid distributions suggest that the bulk of the labile organic matter derives from marine microalgae, primarily diatoms. Coprostanol (5β-cholestan-3β-ol), a sterol often used as an indicator of faecal contamination, was also present. Concentrations ranged from <0.01–0.55 µg g^–1 in surface sediments, with values over 0.25 µg g^–1 and 5β-/5α-C₂₇ stanol ratios greater than 0.4, indicative of sewage inputs. A clearer picture of distributions of sewage-derived organic matter was obtained when coprostanol was normalized to total organic matter rather than sediment dry weight. Areas showing higher coprostanol concentrations included those adjacent to the main sewage treatment plant and several low-volume drains and creeks, indicating localized problems of sewage contamination.