Distinguishing wastewater contamination from on-site systems in mixed land use watersheds

Part of a large estuary along the eastern Australian coastline (150 kms north of Sydney, NSW) used for shellfish production has been closed to harvesting for over 18 months. Monitoring has shown viral contamination of oyster tissue on a number of occasions and surveys of drainage channels and the estuary indicate regular exceedances of bacterial standards set for shellfish growing waters, particularly following heavy rainfall. The contamination has been attributed to urban runoff, agricultural wastes and possibly failing on-site wastewater systems. Investigations have been undertaken to distinguish and estimate contributions from various sources of fecal contamination in this part of the estuary, and whether individual on-site wastewater systems can be linked to the recorded surface and groundwater contamination. Fecal biomarkers such as sterol compounds (e.g. coprostanol), along with microbiological indicators, have been used to determine the sources of the contamination. Hydrological investigations involving both surface and groundwater monitoring have also been completed to determine likely hydraulic pathways between the wastewater systems and the estuary. Apart from an isolated illegal on-site wastewater discharge, the principal sources of contamination appear to relate to other landuse activities within the watershed, principally from herbivores.